Liquid filtration process and system

Abstract

A liquid filtration process (100, Fig. 3) for filtering a liquid, e.g. wastewater, to remove waste material therefrom, the process includes filtering the liquid using a first filter 11 and discharging a first filtered liquid from the first filter, the first filter accumulating waste material filtered from the liquid. The first filter is washed using a washing liquid to dislodge at least some of the accumulated waste material from the first filter. The washing liquid and at least some of the dislodged waste material is discharged from the first filter. The washing liquid discharged from the first filter is filtered using a second filter 31 and a second filtered liquid is discharged from the second filter, the second filter accumulating waste material filtered from the washing liquid. The second filter comprises an open cell filter media 33, the open cell filter media comprising a plurality of filter elements (43, Fig. 11A) which form a static filter pack FP1, the filter elements each having one or more filter cells (45, Fig. 11B), the static filter pack filtering the washing liquid discharged from the first filter The present disclosure also relates to a liquid filtration system 1.

Description

TECHNICAL FIELD The present disclosure relates to a liquid filtration process and system. Aspects of the invention relate to a filtration process and a filtration system for filtering a liquid, such as water. BACKGROUND It is known to filter a liquid, such as water, to remove waste material. A filter is used to separate waste material from the liquid. The waste material may comprise or consist of solids which are filtered by the filter. The waste material accumulates on the filter which causes the filter to become blocked. It is known to supply a washing liquid to wash the filter. The washing liquid may, for example, be a backwashing liquid which is introduced to reverse the flow direction through the filter. The washing liquid may be supplied at high pressure to disperse the accumulated waste material. The washing liquid dislodges at least some of the accumulated waste material from the filter. The washing liquid and the dislodged waste material are discharged to waste. The frequency with which the filter is washed is dependent on the particle load of the liquid being filtered. A large quantity of washing liquid may be required to maintain the filter clear to perform effective filtration. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a liquid filtration process, a liquid treatment process, a liquid filtration system and a liquid treatment process as claimed in the appended claims. According to an aspect of the present invention there is provided a liquid filtration process for filtering a liquid to remove waste material, the liquid filtration process comprising: filtering the liquid using a first filter and discharging a first filtered liquid from the first filter, the first filter accumulating waste material filtered from the liquid; washing the first filter using a washing liquid to dislodge at least some of the accumulated waste material from the first filter and discharging the washing liquid and at least some of the dislodged waste material from the first filter; and filtering the washing liquid discharged from the first filter using a second filter and discharging a second filtered liquid from the second filter, the second filter accumulating waste material filtered from the washing liquid; wherein the second filter comprises an open cell filter media, the open cell filter media comprising a plurality of filter elements which form a static filter pack, the filter elements each having one or more filter cells, the static filter pack filtering the washing liquid discharged from the first filter. A liquid supplied to the first filter (i.e., the influent) may be referred to as a raw or unfiltered liquid. The liquid comprises waste material. The waste material may comprise or consist of solids (i.e. solid matter which is insoluble in the liquid; or solid matter which has come out of solution, for example in the form of precipitates). The waste material may comprise or consist of one or more of the following: particles, particulates, grains, granules, flocs, agglomerated material or other forms of solid material. The waste material is transported by the liquid and may, for example, be suspended in the liquid. The first filter filters the waste material from the liquid during the filtration process. The waste material filtered during the filtration process accumulates on the first filter. The accumulated waste material may form a filter cake or a precipitate on the first filter. The first filter may perform mechanical filtration of the liquid to trap solid waste material or particulate matter. The first filter may comprise one of the following: a bed filter, a sand filter, an activated carbon filter, a multimedia filter or a particulate filter. The first filter may comprise a first filter media. The first filter media may form a first filter bed. The first filter media may, for example, comprise particles, granules or beads. The first filter media may comprise or consist of filter elements which have a closed structure, i.e. at least substantially without internal cavities or apertures. The first filter media may, for example, comprise sand, gravel or glass beads. The liquid is passed through the first filter bed. The waste material suspended in the liquid is trapped in the cavities between the first filter media. The first filter bed may thereby separate the waste material from the liquid. A first filter bed of this type, for example, be used to filter clean water. In a variant, the first filter media may comprise an open cell filter media. The open cell filter media may comprise a plurality of filter elements which form a static filter pack. The filter elements each have one or more filter cells. The static filter pack may filter the liquid. Alternatively, the first filter may comprise a first filter screen. The first filter screen may comprise or consist of a filter mesh. The first filter screen may be cylindrical. The first filter may, for example, be a drum filter. The first filter is washed using the washing liquid to maintain operation of the first filter. At least in certain embodiments, the washing liquid is used to disperse the waste material accumulated on the first filter. The liquid filtration process may comprise supplying the washing liquid to wash the first filter. The washing liquid may be introduced into the first filter to dislodge at least some of the accumulated waste material from the first filter. The washing liquid may pass through the first filter in the same direction as the liquid during filtration. For example, the washing liquid may be supplied at high pressure to break down the accumulated waste material. Alternatively, the washing liquid may pass through the first filter in the opposite direction to the liquid during filtration. The washing liquid may be a backwashing liquid which reverses the flow direction through the first filter. The washing liquid may be supplied at a high pressure (i.e. greater than atmospheric pressure). The washing liquid may be supplied through at least one washing liquid inlet. The or each washing liquid inlet may comprise a nozzle. The or each nozzle may direct the washing liquid onto a localised region of the first filter. Alternatively, or in addition, the flow direction through the first filter may be reversed to perform backwashing. The washing liquid may comprise or consist of a backwashing liquid. The backwashing liquid may comprise or consist of the liquid filtered by the first filter. The first filter may be drained to discharge the used (soiled) washing liquid and the dislodged waste material. The first filter may be drained through a first filter waste outlet. As the first filter is drained, the liquid may function as a washing liquid to dislodge the accumulated waste material and / or to transport at least some of the accumulated waste material from the first filter. The liquid present in the first filter may be used as the washing liquid. A gas, such as air, may be introduced into the first filter to dislodge the accumulated waste material from the first filter. The gas may be introduced at high pressure. The gas may be directed onto the first filter in the form of one or more high-pressure jet. The gas may be directed through the first filter in the same direction as, or in the opposite direction to, the flow direction of the liquid through the first filter. As described herein, the first filter may comprise a filter screen or a filter mesh comprising a plurality of openings. The one or more high-pressure jet may be directed through the openings in the first filter. The washing liquid is discharged from the first filter to the second filter. The washing liquid transports at least some of the accumulated waste material from the first filter to the second filter. The waste material may, for example, be suspended in the washing liquid. The washing liquid discharged from the first filter may be referred to as used (soiled) washing liquid. At least some of the waste material transported from the first filter with the washing liquid is filtered by the second filter. The open cell filter media is effective to filter particulate matter from the washing liquid introduced from the first filter. The washing liquid is filtered and discharged from the second filter as the second filtered liquid (i.e. a second filtrate). The second filtered liquid discharged from the second filter may be used to wash the first filter during the current washing process and / or a subsequent washing process. The waste material removed from the washing liquid accumulates in the second filter. The filter elements in the second filter have a high retention capacity and, at least in certain embodiments, enable the waste material to be collected in the second filter. The used washing liquid is filtered by the second filter to remove at least some of the waste material. The washing liquid is discharged as a second filtered liquid from the second filter. The waste material accumulates in the second filter and may periodically be discharged from the second filter for further processing, for example drying or de-watering, or may be discharged for disposal. The waste discharged from the second filter is concentrated, i.e. has a lower water content than the waste material discharged from the first filter. At least in certain embodiments, the water filtration process is effective to reduce the liquid content of the waste. The liquid filtration process enables at least some of the washing liquid to be recovered. The recovered water may be re-used or recycled. The process may comprise supplying the second filtered liquid from the second filter to the first filter. At least some of the second filtered liquid may be supplied to the first filter. The second filtered liquid may be supplied directly to the first filter or may be supplied upstream of the first filter. The second filtered liquid may then be filtered by the first filter. The first filtered liquid may be discharged from the first filter to one or more system outlets. Alternatively, or in addition, at least some of the second filtered liquid may be discharged from the second filter to the one or more system outlet. Alternatively, or in addition, at least some of the second filtered liquid discharged from the second filter may be returned to the first filter. The second filtered liquid may be used as the washing liquid to wash the first filter. The waste material filtered from the washing liquid may accumulate in the static filter pack and / or the filter cells of the open cell filter media, The liquid filtration process may comprise purging the second filter to expel at least some of the waste material accumulated in the second filter. The washing liquid supplied from the first filter may be used as a purging liquid to expel at least some of the accumulated waste material from the second filter. For example, at least some of the washing liquid supplied from the first filter may be drained to purge the second filter. The waste material accumulated in the second filter may be expelled along with at least some of the washing liquid supplied from the first filter. Alternatively, or in addition, a separate purging liquid may be introduced into the second filter. The supply of the washing liquid from the first filter to the second filter may be stopped while the second filter is purged. For example, a fluid connection between the first filter and the second filter may be closed. The purging of the second filter may comprise introducing a fluid into the second filter to break up the static filter pack and / or to dislodge waste material from the filter cells. The fluid may be referred to as a pack disruption fluid. A pump may be provided to pump the fluid into the second filter under pressure. Alternatively, the fluid may be drawn into the second filter as the washing liquid therein is drained. The second filter may be sealed such that the pack disruption fluid is drawn into the second filter as the washing liquid is drained. The fluid may comprise a gas, such as air; or may comprise a liquid, such as water. The liquid filtration process may comprise a filtration process and a washing process. The first filter filters the liquid during the filtration process. The first filter may be washed during the washing process. The filtration process and the washing process may be performed sequentially or concurrently. The second filter may be purged while performing the filtration process and / or performing the washing process. The purging of the second filter may be performed concurrent with the liquid being filtered by the first filter. The purging of the second filter may be performed after the washing process. Alternatively, the purging of the second filter may be performed as part of the washing process. The purging of the second filter may be performed concurrent with the filtration process. At least in certain embodiments, the filtration process and the washing process are performed sequentially. The liquid filtration process may alternate between the filtration process and the washing process. At least in certain embodiments, the filtration process may be re-started after completing the washing process. The liquid filtration process may comprise establishing a first flow rate per unit cross-sectional area through the first filter; and a second flow rate per unit cross-sectional area through the second filter. The first flow rate per unit cross-sectional area may be larger than the second flow rate per unit cross-sectional area. The first and second filters may comprise first and second open cell filter media, for example open cell filter media of the type described herein. The first filter open cell filter media may form a first static filter pack; and the second open cell filter media may form a second static filter pack. The first static filter pack may have a first volume; and the second static filter pack may have a second volume. The first volume may be greater than the second volume. The liquid filtration process may comprise establishing a first flow rate per unit cross-sectional area through the first static filter pack; and a second flow rate per unit cross-sectional area through the second static filter pack. The first flow rate per unit cross-sectional area may be larger than the second flow rate per unit cross-sectional area. The first flow rate per unit cross-sectional area may two, three, four or more times greater than the second flow rate per unit cross-sectional area. The flow rate per unit cross sectional area of the filter media in the first static filter pack may be greater than or equal to 30 m3 / m2 / h, 50 m3 / m2 / h or 70 m3 / m2 / h. The flow rate per unit cross sectional area of the filter media in the second static filter pack may be in the range 5 m3 / m2 / h to 35 m3 / m2 / h, 10 m3 / m2 / h to 30 m3 / m2 / h or 20 m3 / m2 / h to 25 m3 / m2 / h. The liquid may be water. At least in certain embodiments, the liquid may be clean water, for example comprising or consisting of potable (drinking) water. The water may be suitable for human consumption. Alternatively, the liquid may be wastewater, for example municipal or industrial wastewater. According to a further aspect of the present invention there is provided a liquid treatment process comprising the liquid filtration process as described herein. According to a further aspect of the present invention there is provided a liquid filtration system for filtering a liquid to remove waste material, the liquid filtration system comprising: a first filter unit comprising: at least one first filter inlet for receiving the liquid to be filtered; a first filter for filtering waste material from the liquid; at least one first filter outlet for discharging a first filtered liquid; a washing liquid supply for supplying a washing liquid to wash the first filter; and at least one first filter waste outlet for discharging the washing liquid; a second filter unit comprising: at least one second filter inlet for receiving the washing liquid discharged from the first filter unit; a second filter for filtering waste material from the washing liquid; and at least one second filter outlet for discharging a second filtered liquid; wherein the second filter comprises an open cell filter media, the open cell filter media comprising a plurality of filter elements for forming a static filter pack, the filter elements each having one or more filter cells. The liquid filtration system may be configured selectively to operate in a filtration mode and a washing mode. When operating in the filtration mode, the liquid filtration system may be configured to supply a liquid to the at least one first filter inlet, the liquid is filtered by the first filter and a first filtered liquid is discharged from the first filter unit through the at least one first filter outlet. When operating in the washing mode, the liquid filtration system may be configured to use the washing liquid to wash the first filter. The used (soiled) washing liquid is discharged through the at least one first filter waste outlet, thereby displacing accumulated waste material from the first filter. The washing liquid discharged from the first filter unit is supplied to the second filter unit. The washing liquid is filtered by the open cell filter media in the second filter unit. The static filter pack is operative to separate waste material from the washing liquid. A second filtered liquid may be discharged from the second filter unit. At least in certain embodiments, the washing liquid is filtered by the second filter unit and discharged as the second filtered liquid. The first filter unit may comprise a first filter chamber. The first filter may be disposed in the first filter chamber. The at least one first filter inlet and the at least one first filter outlet may be in fluid communication with the first filter chamber. The first filter chamber may be formed in a first filter tank. The second filter unit may comprise a second filter chamber. The second filter may be disposed in the second filter chamber. The plurality of filter elements may form the static filter pack in the second filter chamber. The at least one second filter inlet and the at least one second filter outlet may be in communication with the second filter chamber. The second filter chamber may be formed in a second filter tank. The second filter unit may comprise a second filter waste outlet for discharging accumulated waste material. Alternatively, one of the at least one second filter inlet and the at least one second filter outlet may be used as a waste outlet for discharging accumulated waste material. The liquid in the first filter unit may function as a washing liquid to discharge at least some of the accumulated waste material from the first filter. As the water is drained from the first filter unit, the accumulated waste material may be flushed from the first filter. Alternatively, or in addition, the liquid filtration system may be configured to supply a washing liquid to the first filter in the washing mode. The washing liquid may be introduced into the first filter unit to transport the accumulated waste material from the first filter. The washing liquid supplied to the first filter unit may dilute the mixture of liquid and waste material in the first filter. The liquid filtration system may comprise at least one washing liquid inlet for introducing the washing liquid into the first filter unit. The washing liquid inlet may be a separate inlet, or one of the at least one first inlet and the at least one first outlet may be re-configured to function as a washing liquid inlet. The washing liquid inlet may comprise one or more nozzles. In use, the liquid to be filtered is supplied to the first filter unit. The liquid is filtered by the first filter. The first filter accumulates the waste material and a first filtered liquid is discharged from the first filter unit. The first filter is washed by the washing liquid to wash at least some of the accumulated wate material from the first filter. The used (soiled) washing liquid is discharged from the first filter unit to the second filter unit along with at least some of the accumulated waste material from the first filter. The second filter is operative to filter at least some of the waste material transported with the washing liquid. The discharged washing liquid is filtered by the open cell filter media and a second filtered liquid (i.e. a second filtrate) is discharged form the second filter unit. The washing liquid may, for example, be supplied to the at least one second filter inlet. The washing liquid is filtered by the open cell filter media in the second filter chamber. The waste material accumulates in the static filter pack. The waste material filtered by the first filter typically comprises or consists of solids (for example, solid matter which is insoluble in the liquid or which has precipitated out of solution). The waste material may comprise or consist of one or more of the following: particles, particulates, grains, granules, flocs, agglomerated material or other forms of solid material. The waste material is transported by the liquid and may, for example, be suspended in the liquid. In use, the first filter is operative to filter the waste material from the liquid during the filtration process. The filtered waste material accumulates in the first filter. The liquid filtration system may be configured selectively to wash the first filter to expel at least some of the accumulated waste material. The liquid filtration system is configured to discharge the washing liquid from the first filter to the second filter. The liquid in the first filter unit may function as a washing liquid to transport at least some of the accumulated waste material from the first filter. As the water is drained, the accumulated waste material may be flushed from the first filter unit. Alternatively, or in addition, the washing process may comprise supplying a washing liquid to wash the first filter. The liquid filtration system may be configured to supply a washing liquid to the first filter. The washing liquid may be introduced into the first filter to dislodge or disperse the accumulated waste material from the first filter. The washing liquid supplied to the first filter may dilute the mixture of liquid and waste material in the first filter. The used (soiled) washing liquid discharged from the first filter unit transports at least some of the accumulated waste material from the first filter unit to the second filter unit. The waste material may be suspended in the washing liquid. The second filter is effective in filtering at least some of the waste material transported by the washing liquid. The filtered washing liquid is discharged from the second filter as a second filtered liquid. The filtered waste material collects in the second filter. The filter elements have a high retention capacity and, at least in certain embodiments, enable the waste material to be collected in the second filter. At least in certain embodiments, the second filter unit may have a smaller volume than the first filter unit. The second filter operates to filter the washing liquid to remove at least some of the waste material from the washing liquid. The waste material accumulates in the second filter and may be discharged periodically for further processing, for example drying or de-watering, or disposal. The waste material discharged from the second filter is concentrated, i.e. has a lower water content than the waste material discharged from the first filter. The liquid filtration system may comprise a supply line for supplying the second filtered liquid from the second filter unit to the first filter unit. The supply line may be connected directly to the first filter unit or may be connected upstream of the first filter unit. Alternatively, or in addition, the at least one second filter outlet may be connected to the one or more system outlet. In use, the second filtered liquid may be discharged to the one or more system outlet. The second filter unit may be configured to be purged to expel at least some of the waste material accumulated in the static filter pack. The second filter unit may comprise at least one second filter waste outlet. The waste material accumulated in the second filter unit may be purged through the at least one second filter waste outlet. Alternatively, or in addition, the second filter may be purged through one of the second filter inlet and the second filter outlet. The liquid contained in the second filter unit may be used as a purging liquid to expel the waste material. Alternatively, or in addition, a purging liquid may be introduced into the second filter. The second filter unit may comprise one or more inlet for introducing the purging liquid. The second filter unit may comprise at least one fluid inlet for introducing a fluid to dislodge waste material from the filter cells. The fluid may be introduced to disrupt (i.e., to break up) the static filter pack formed by the filter elements. The fluid may be referred to as a pack disruption fluid. The at least one fluid inlet may be configured to introduce a liquid or a gas into the second filter. The at least one fluid inlet may be configured to introduce air into the second filter. A pump may be provided for pumping the fluid into the second filter chamber. Alternatively, or in addition, the fluid may be drawn into the second filter chamber as the second filter chamber is drained. The second filter chamber may be sealed such that air is drawn into the second filter chamber as the washing liquid is drained. The liquid filtration system may be configured selectively to operate in a filtration mode and / or a washing mode. The liquid filtration system may be configured to operate in the filtration mode and the washing mode sequentially. In use, the liquid filtration system may switch between the filtration mode and the washing mode. The liquid filtration system may be configured to purge the second filter unit when operating in the filtration mode and / or the washing mode. The purging of the second filter may be performed concurrent with the liquid being filtered by the first filter. The purging of the second filter may be performed after the washing process. Alternatively, the purging of the second filter may be performed during the washing mode. The purging of the second filter may be performed when operating in the filtration mode. The first filter unit may comprise one or more first filter inlet, one or more first filter outlet and one or more first filter waste outlet. The liquid filtration system may comprise one or more valve to control the liquid flow through one or more of the following: the first filter inlet, the first filter outlet and the first filter waste outlet. A first filter inlet valve may be provided to control the supply of water to a first filter chamber. A first filter outlet valve may optionally be provided to control the discharge of the first filtered water from the first filter unit. A first filter waste valve may be provided to control the discharge of the used (soiled) washing liquid (and the entrained waste material) from the first filter chamber to the second filter chamber. The second filter unit may comprise one or more second filter inlet, one or more second filter outlet and one or more second filter waste outlet. The liquid filtration system may comprise one or more valve to control the liquid flow through one or more of the following: the second filter inlet, the second filter outlet, and the second filter waste outlet. A second filter inlet valve may be provided to control the supply of the used (soiled) washing liquid from the first filter unit. A second filter outlet valve may optionally be provided to control the discharge of the second filtered liquid from the second filter. In use, the waste material filtered from the washing liquid accumulates in the static filter pack formed in the second filter unit. The liquid filtration system may be configured periodically to purge the second filter to expel the accumulated waste material. The waste material may be expelled to a system waste outlet or may be output for further processing. A second filter waste valve may be provided to control the discharge of the waste material from the second filter unit. The second filter inlet valve may be operated to close the second filter inlet. The second filter outlet valve may be operated to close the second filter outlet. The liquid filtration system may be configured to introduce a pack disruption fluid to disrupt (i.e. to break up) the static filter pack. The waste material trapped between individual filter elements and / or settled in the filter cells formed in the filter elements may be dislodged by the pack disruption fluid. The pack disruption fluid introduced into the second filter chamber may be a liquid or a gas. A second filter fluid control valve may be provided to control the introduction of the pack disruption fluid. The pack disruption fluid may comprise air. A pump may be provided to pump the pack disruption fluid through one or more fluid inlet. The one or more fluid inlet may be provided in a lower portion of the second filter chamber. In a variant, the fluid may be drawn into the second filter chamber as the washing liquid is drained from the second filter chamber. Alternatively, or in addition, a mechanical disruptor may be provided to disrupt the filter elements forming the static filter pack. The mechanical disruptor may comprise one or more movable member which is operable to disrupt the filter elements forming the static filter pack. The one or more movable member may rotate or sweep within the second filter unit. The second filter waste valve may be operated to control the discharge of waste material through the second filter waste outlet. The water and accumulated waste material may be drained from the second filter chamber to waste. In use, the second filter is operative to concentrate the waste material discharged from the first filter. The resulting waste material expelled from the second filter may be a sludge, for example. At least one electronic control unit may be provided to control operation of the one or more first filter valve and / or the one or more second filter valve. The first electronic control unit comprises one or more electronic controller and a memory device. A set of computational instructions may be stored on the memory device. When executed by the one or more electronic controller, the computational instructions may cause the at least one electronic control unit to control the liquid filtration system to operate in accordance with the method(s) described herein. The computational instructions may cause the at least one electronic control unit to control the liquid filtration system selectively to operate in a filtration mode and a washing mode. The liquid may be water. At least in certain embodiments, the liquid may be clean water, for example comprising or consisting of potable (drinking) water. The water may be suitable for human consumption. Alternatively, the liquid may be wastewater, for example municipal or industrial wastewater. The one or more filter cells may each have a cross-sectional area in the range 1mm2 to 10 mm2, or 1mm2 to 5mm2. The one or more filter cells may each have a length greater than or equal to 6mm, 8mm, 10mm or 12mm. The one or more filter cells may each have a length less than 30mm. The one or more filter cells may each have an internal volume greater than 10mm3, 20mm3, 30mm3 or 40mm3. The filter elements may have positive buoyancy, neutral buoyancy or negative buoyancy in water. Each filter cell may be defined by one or more sidewall which is impermeable. According to a further aspect of the present invention there is provided a liquid treatment system comprising one of more of the liquid filtration system described herein. The liquid treatment system may be configured to treat clean water, for example drinking (potable) water. The liquid treatment system may comprise one or more of the following: at least one reservoir, a screening system, a chemical treatment system, a settlement system and a sterilisation system. The chemical treatment system is configured to perform coagulation and / or flocculation of the water to cause particles or solids in the water to agglomerate. The sterilisation system is configured to sterilise the water prior to distribution, for example to a domestic (mains) water supply. The one or more liquid filtration system may be disposed between the chemical treatment system and the sterilisation system. The settlement system may be disposed between the chemical treatment system and the one or more liquid filtration system. The liquid filtration system may be configured to discharge the second filtered liquid upstream of the first filter unit. The second filtered liquid may be introduced upstream of the chemical treatment system. The second filtered liquid may be introduced upstream of the chemical treatment system and the settlement system. The sterilisation system may be provided downstream of the liquid filtration system. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a schematic representation of the liquid filtration system in accordance with an embodiment of the present invention; Figure 2A shows a partial cut-away view of a filter for the liquid filtration system according to embodiments of the present invention; Figure 2B shows a schematic representation of the filter shown in Figure 2A operating in a filtration mode to filter drinking water in accordance with an embodiment of the present invention; Figure 2C shows a schematic representation of the filter shown in Figure 4A operating in a purging mode to expel accumulated solid material; Figure 3 shows a flow diagram illustrating operation of the liquid filtration system shown in Figure 1; Figure 4 shows a schematic representation of a liquid filtration system in accordance with a further embodiment of the present invention; Figure 5 shows a flow diagram illustrating operation of the liquid filtration system shown in Figure 4; Figure 6 shows a schematic representation of a liquid treatment system incorporating a liquid filtration system in accordance with an embodiment of the present invention; Figure 7 shows a flow diagram illustrating operation of the liquid treatment system shown in Figure 6; Figure 8 shows a schematic representation of a liquid filtration system in accordance with a further embodiment of the present invention; Figure 9 shows a flow diagram illustrating operation of the liquid filtration system shown in Figure 8; Figure 10 shows a schematic representation of a liquid filtration system in accordance with a further embodiment of the present invention; Figure 11A shows a perspective view of a mechanical filter element for use in the second filter of the liquid filtration system described herein; Figure 11B shows an end view of the mechanical filter element shown in Figure 11 A; Figure 11C shows a perspective view of a variant of the mechanical filter element shown in Figure 11A; and Figures 12A to 12E shows examples of filter tanks suitable for the second filter of the liquid filtration system(s) according to the present invention. DETAILED DESCRIPTION A liquid filtration system 1 and a liquid filtration process 100 in accordance with one or more embodiment of the present invention is described herein with reference to the accompanying Figures. The liquid filtration system 1 and a liquid filtration process 100 are described herein with particular reference to filtering water. However, it will be understood that the liquid filtration system 1 and a liquid filtration process 100 may be used to filter liquids other than water. The liquid filtration system 1 according to a first embodiment of the present invention is shown in Figure 1. The liquid filtration system 1 in the present embodiment is configured to perform a filtration process and a washing process. The filtration process comprises or consists of a mechanical filtration to remove waste material from the water. The waste material may comprise particles, agglomerated particles or other debris which are suspended in the water. The washing process is performed to wash the filter to remove accumulated waste material. The washing process may, for example, be performed on a predetermined schedule or in dependence on a determination that the liquid filtration system 1 is partially or completely blocked, for example when a back pressure increases above a threshold pressure. As shown in Figure 1, the liquid filtration system 1 comprises a filtration system inlet 3, a filtration system outlet 5, a filtration system waste outlet 7, a first filter unit FT1 and a second filter unit FT2. The first filter unit FT1 is configured to filter the unfiltered water supplied to the filtration system inlet 3. In use, the first filter unit FT1 discharges a first filtered liquid (i.e., a first filtrate); and the second filter unit FT2 discharges a second filtered liquid (i.e., a second filtrate). It will be appreciated that the first and second filtered liquids are typically the same liquid. For example, in the present embodiment the first and second filtered liquids both comprise or consist of water. As described herein, the first and second filters 11, 31 are configured to filter waste material comprising or consisting of solids from the unfiltered water. The solids may comprise or consist of solid matter which is insoluble in the liquid and / or solid matter which has come out of solution, for example in the form of precipitates. The first filter unit FT1 discharges a first filtered liquid. The first filtered liquid comprises or consists of filtered water in the present embodiment. The first filtered liquid is discharged from the first filter unit FT1 to the filtration system outlet 5. The first filter unit FT1 comprises a first filter 11. The first filter 11 in the present embodiment comprises a first filter media 13. The first filter media 13 is disposed in a first filter chamber 15, as shown in Figure 1. The first filter chamber 15 has at least one first filter inlet 17, at least one first filter outlet 19 and at least one first filter waste outlet 21. The first filter media 13 may have a closed structure which does not include internal filter cells. For example, the first filter media 13 may comprise granules, grains or beads. Other types of filter media may be used in the first filter unit FT 1. The first filter media 13 in the present embodiment comprises sand. The first filter media 13 in the present embodiment forms a static filter bed SB1. The unfiltered (raw) water is passed through the static filter bed SB1. The first filter comprises sand which forms a sand bed SB1 in the first filter chamber 15. The water is filtered as it passes through the sand bed SB1. The waste material suspended in the water is trapped in the cavities between the sand in the sand bed SB1. The filtered water from the first filter unit FT1 is discharged to the at least one first filter outlet 19. The water is typically pumped by a water pump (not shown) through the 14 sand bed SB1 under pressure. Other types of filter are contemplated for the first filter unit FT 1. For example, the first filter unit FT 1 may comprise one or more of the following: sand, activated carbon, multimedia or particulate filter. The first filter media 13 may be omitted. For example, the first filter unit FT1 may comprise a settlement tank in which the waste material is filtered by settlement. Alternatively, the first filter media 13 may comprise an open cell filter media of the type described herein. The second filter unit FT2 comprises a second filter 31. The second filter 31 comprises a second filter media 33. The second filter media 33 is disposed in a second filter chamber 35. A partial cut-away of a second filter unit FT2 in accordance with an embodiment of the present embodiment is shown in Figure 2A (with the second filter media 33 omitted). The filter chamber 35 has at least one second filter inlet 37, at least one second filter outlet 39 and at least one second filter waste outlet 41. The at least one second filter waste outlet 41 may be separate from the at least one second filter inlet 37 and the at least one second filter outlet 39. Alternatively, the at least one second filter waste outlet 41 may be combined with one of the at least one second filter inlet 37 and the at least one second filter outlet 39. In the arrangement shown schematically in Figures 2B and 2C, the at least one second filter waste outlet 41 is combined with the at least one second filter inlet 37. The second filter media 33 is in the form of an open cell filter media 33. The second filter media 33 comprises a plurality of filter elements 43, the filter elements 43 each comprising one or more filter cells 45. A suitable open cell filter media 33 is described herein in more detail. The second filter media 33 forms a static filter pack FP1 in the second filter chamber 35, as shown schematically in Figure 2B. The water is filtered as it passes through the static filter pack FP1. The solids suspended in the water are deposited or settle in the filter cells 45 formed in the second filter media 33. The water may be pumped through the static filter pack FP1 under pressure. Alternatively, the water may flow through the static filter pack FP1 under gravity. The water discharged from the at least one first filter waste outlet 21 is introduced into the second filter unit FT2 through the at least one second filter inlet 37. The water flows through the static filter pack FP1 to the at least one second filter outlet 39. The static filter pack FP1 is effective in filtering the water supplied from the reservoir 11. The filtered water from the second filter unit FT2 is discharged to the at least one second filter outlet 39. The filtered water may be discharged for further filtration, re-use or discharge. As described herein, the second filter unit FT2 is periodically cleaned. A schematic representation of the cleaning of the second filter unit FT2 is shown in Figure 2C. The cleaning of the second filter unit FT2 comprises purging the liquid to expel the accumulated waste. The accumulated waste is expelled from the second filter unit FT2 through the at least one second filter waste outlet 41. The expelled waste may undergo further treatment, for example comprising de-watering or drying. One or more valve PFV-n is provided to control the flow through one or more of the first filter inlet 17, the first filter outlet 19 and the first filter waste outlet 21. A first filter inlet valve PFV-1 is associated with the first filter inlet 17. The first filter inlet valve PFV-1 is opened to supply the unfiltered (raw) water from the filtration system inlet 3 to the first filter chamber 15. A first filter outlet valve PFV-2 is associated with the first filter outlet 19. The first filter outlet valve PFV-2 is opened to discharge the filtered water from the first filter chamber 15 to the filtration system outlet 5. A first filter waste valve PFV-3 is associated with the first filter waste outlet 21. The first filter waste valve PFV-3 is opened to discharge the waste material from the first filter chamber 15. An electronic control unit ECU1 is provided to control operation of the one or more first filter valve PFV-n. The electronic control unit ECU1 comprises one or more electronic controller EC1 and a memory device MD1. The electronic control unit ECU1 outputs control signals to control the or each first filter valve PFV-n. In a variant, the one or more first filter valve PFV-n may be controlled manually, for example by a system operator. The water filtered by the first filter unit FT1 is discharged to the filtration system outlet 5. As described herein, the first filter unit FT1 discharges a first filtered liquid (i.e. a first filtrate). The waste material filtered from the water by the first filter media 13 is retained in the first filter unit FT1. In use, the first filter unit 11 separates the waste material from suspension. The accumulated waste material may form a filter cake or a precipitate on the first filter unit 11. The accumulated waste material comprises or consists of the retained particles and any other impurities present in the liquid. The first filter unit FT1 is periodically cleaned to remove at least some of the accumulated waste material. In the present embodiment, a washing process is performed to clean the first filter unit FT1. The washing process comprises introducing a washing fluid into the first filter chamber 15 to displace the accumulated waste material from the first filter media 13. The washing fluid is introduced into the first filter chamber 15 through one or more washing inlet 23. The one or more washing inlet 23 is disposed in a lower portion of the first filter chamber 15. A pump 25 is provided for pumping the washing fluid into the first filter chamber 15. The washing fluid is introduced under pressure (i.e. at a pressure greater than atmospheric pressure) and disrupts or breaks up the sand bed SB1. The washing fluid may comprise a gas, such as air; or a liquid, such as water. In the present embodiment, the washing fluid is a washing liquid. The liquid present in the first filter unit FT1 may be discharged from the first filter chamber 15, thereby washing the first filter media 13 and transporting accumulated waste material out of the first filter 11. Alternatively, or in addition, a separate washing liquid may be introduced into the first filter chamber 15 to wash the first filter media 13. The pump 25 may be operated to pump the washing fluid into the first filter chamber 15 to wash the first filter unit FT 1. Alternatively, or in addition, the flow direction of the liquid may be reversed during the washing process to wash the first filter unit FT1. The washing liquid may comprise a backwashing liquid which passes through the first filter 11 in the opposite direction. The washing liquid is preferably the same as (or compatible with) the liquid supplied to the liquid filtration system 1 to be filtered. In the present embodiment, the washing liquid is water. The washing liquid may comprise unfiltered (raw) water (for example from the filtration system inlet 3) or filtered water (for example from downstream of the first filter unit FT1). Alternatively, or in addition, the washing liquid may be obtained from a different source. The washing liquid is introduced into the first filter chamber 15 and disrupts or breaks up the sand bed SB1. At least some of the accumulated waste material is dispersed from the first filter media 13. The waste material is held in suspension in the washing liquid in the first filter chamber 15. The used (soiled) washing liquid is then expelled through the at least one first filter waste outlet 21. The waste material suspended in the washing liquid is thereby expelled from the first filter chamber 15. The used (soiled) washing liquid is discharged to the second filter unit FT2. At least some of the waste material from the first filter unit FT1 is flushed from the first filter chamber 15 into the second filter chamber 35. The second filter unit FT2 is configured to filter the used (soiled) washing liquid discharged from the first filter unit FT1 during the washing process. The second filter unit FT2 comprises a second filter media 33 disposed in a second filter chamber 35. The second filter chamber 35 has at least one second filter inlet 37, at least one second filter outlet 39 and at least one second filter waste outlet 41. The second filter media 33 is in the form of an open cell filter media 33. The second filter media 33 comprises a plurality of filter elements 43, the filter elements 43 each comprising one or more filter cells 45. The open cell filter media 33 is described herein in more detail. The second filter media 33 forms a static filter pack FP1 in the second filter chamber 35. The washing liquid is filtered as it passes through the static filter pack FP1. The waste material suspended in the washing liquid is deposited in the filter cells 45 formed in the second filter media 33. The second filter 31 separates the waste material from suspension. The separated waste material accumulates in the second filter 31. The accumulated waste material comprises or consists of the retained particles and any other impurities in the washing liquid. The accumulated waste material may form a filter cake or a precipitate on the second filter media 33. The washing liquid may be pumped through the static filter pack FP1 under pressure. Alternatively, the washing liquid may flow through the static filter pack FP1 under gravity. The washing liquid discharged from the at least one first filter waste outlet 21 is introduced into the second filter unit FT2 through the at least one second filter inlet 37. The washing liquid flows through the static filter pack FP1 to the at least one second filter outlet 39. The static filter pack FP1 is effective in filtering the washing liquid supplied from the first filter unit FT1. The second filter unit FT2 discharges a second filtered liquid. The second filtered liquid comprises or consists of the filtered washing liquid in the present embodiment. The second filtered liquid is discharged from the second filter chamber 35 to the at least one second filter outlet 39. In the present embodiment, the second filtered liquid is discharged for further filtration. The liquid filtration system 1 comprises a supply line 47 connected to the at least one second filter outlet 39. The supply line 47 is configured to introduce the second filtered liquid to the first filter unit FT 1. The supply line 47 introduces the second filtered liquid upstream of the first filter chamber 15. The second filtered liquid is thereby recycled in the liquid filtration system 1 rather than sent to waste. The second filtered liquid is mixed with the unfiltered (raw) water introduced at the one filtration system inlet 3. In a variant, the second filtered liquid may be introduced directly into the first filter chamber 15. The second filtered liquid may mix with the unfiltered (raw) water in the first filter chamber 15. A one-way valve (not shown) may be provided in the supply line 47 to reduce or prevent unfiltered (raw) water bypassing the first filter unit FT1. One or more valve SFV-n is provided to control the flow through one or more of the second filter inlet 37, the second filter outlet 39 and the second filter waste outlet 41. A second filter inlet valve SFV-1 is associated with the second filter inlet 37. The second filter inlet valve SFV-1 is opened to supply the washing liquid discharged from the first filter chamber 15 to the second filter chamber 35. A second filter outlet valve SFV-2 is associated with the second filter outlet 39. The second filter outlet valve SFV-2 is opened to discharge the filtered water from the second filter chamber 35 to the filtration system outlet 5. A second filter waste valve SFV-3 is associated with the second filter waste outlet 41. The second filter waste valve SFV-3 is opened to discharge the waste material from the second filter chamber 35. The electronic control unit ECU1 is provided to control operation of the one or more second filter valve SFV-n. The electronic control unit ECU1 outputs control signals to control the or each first filter valve SFV-n. It will be understood that two or more electronic control units may be provided to control the primary and secondary valves PFV-n, SFV-n. In a variant, the one or more second filter valve SFV-n may be controlled manually, for example by a system operator. The waste material filtered from the washing liquid accumulates in the static filter pack FP1 formed in the second filter chamber 35. The second filter unit FT2 is periodically purged to expel the accumulated waste material to the filtration system waste outlet 7. The second filter inlet valve SFV-1 is operated to close the second filter inlet 37; and the second filter outlet valve SFV-2 is operated to close the second filter outlet 39. A pack disruption fluid is introduced into the second filter chamber 35 to disrupt (i.e. to break up) the static filter pack FP1. The waste material trapped between individual filter elements 43 and / or settled in the filter cells 45 formed in the filter elements is dislodged. The pack disruption fluid introduced into the second filter chamber 35 may be a liquid or a gas. A second filter fluid control valve SFV-4 is provided to control the introduction of the pack disruption fluid. In the present embodiment, the pack disruption fluid is air. An air pump 51 is provided to pump the air through one or more fluid inlet 53 provided in a lower portion of the second filter chamber 35. The second filter waste valve SFV-3 is operated to open the second filter waste outlet 41. The water and accumulated waste material is thereby drained from the second filter chamber 35 to waste. The second filter unit FT2 effectively concentrates the waste material discharged from the first filter unit FT1. The resulting waste material expelled from the second filter unit FT2 comprises a sludge. In a variant, the fluid may be drawn into the second filter chamber 35 as the washing liquid is drained from the second filter chamber 35. Alternatively, or in addition, a mechanical disruptor may be provided to disrupt the filter elements 43 forming the static filter pack FP1. The mechanical disruptor may, for example, comprise one or more movable member which is operable to displace the filter elements 43 forming the static filter pack FP1. The one or more movable member may rotate or sweep within the second filter chamber 35. A water filtration process 100 in accordance with an embodiment of the present invention will now be described with reference to a first flow diagram FD1 shown in Figure 3. The water is received as influent water in the liquid filtration system 1 (BLOCK 105). The liquid filtration system 1 selectively operates in a filtration mode and a washing mode. In the filtration mode, the unfiltered (raw) water is supplied to the first filter unit FT 1 (BLOCK 110). The unfiltered (raw) water may, for example, comprise particles or other debris which is held in suspension. The particles may aggregate together to form larger clumps or flocs which may also be suspended in the unfiltered (raw) water. The first filter 11 mechanically filters the unfiltered (raw) water to remove the waste material held in suspension (BLOCK 115). A first filtered water (filtrate) is discharged from the first filter unit FT1 to the one or more filtration system outlet 5 (BLOCK 120). The liquid filtration system 1 is periodically switched to a washing mode to clean the first filter unit FT1 (BLOCK 125). In the washing mode, the first filter unit FT1 is cleaned by introducing a washing liquid (BLOCK 130). The washing liquid dislodges the waste material accumulated in the first filter unit FT1. In the present embodiment, the washing liquid is water. At least some of the waste material is held in suspension in the washing liquid. The washing liquid and the suspended waste material are discharged from the first filter unit FT1 to the second filter unit FT2 (BLOCK 135). The second filter unit FT2 is operative to filter the washing liquid to remove the suspended waste material (BLOCK 140). A second filtered liquid comprising or consisting of the second filtered liquid is discharged from the second filter unit FT2 (BLOCK 145). At least some of the second filtered liquid is recycled (BLOCK 150). In the present embodiment, the second filtered liquid is recycled by returning at least some of the liquid to upstream of the liquid filtration system 1. The second filter unit FT2 is periodically cleaned (BLOCK 155). The cleaning of the second filter unit FT2 in the present embodiment is performed while the liquid filtration system 1 is operating in the washing mode. However, it will be appreciated that the second filter unit FT2 may be cleaned while the liquid filtration system 1 is operating in the filtration mode. For example, the liquid filtration system 1 may switch from the washing mode to the filtration mode before or during cleaning of the second filter unit FT2. The second filter unit FT2 is cleaned by introducing a fluid, such as air, to disrupt the static filter pack FP1 (BLOCK 160). The accumulated solid waste material is dislodged from the static filter pack FP1. The water and the waste material is discharged from the second filter unit FT2 to the filtration system waste outlet 7 (BLOCK 165). For example, the second filter waste outlet 41 may be opened to discharge the water and the accumulated waste material, thereby purging the second filter unit FT2. The material discharged from the second filter unit FT2 is typically in the form of a concentrated sludge. The water filtration process continues switching between the filtration mode and the washing mode, as required. At least in certain embodiments, the second filter unit FT2 may reduce water consumption of the liquid filtration system 1. The second filter unit FT2 enables recovery of at least some of the washing liquid used to clean the first filter unit FT1. The waste material discharged from the second filter unit FT2 is concentrated. The accumulated waste material may be disposed of in conventional manner or may undergo further treatment, for example to perform drying to reduce the water content. A liquid filtration system 1 and a liquid filtration process 300 in accordance with a further embodiment of the present invention will now be described with reference to Figures 4 and 5. The liquid filtration system 1 and the liquid filtration process 300 are variants of the embodiment described above with reference to Figures 1, 2 and 3. Like reference numerals are used for like features. The operation of the first filter unit FT 1 is unchanged from the above embodiment. A first filtered liquid (i.e. a first filtrate) is discharged from the first filter 11 to the filtration system outlet 5. The first filter 11 is periodically cleaned by performing washing to expel at least some of the accumulated waste. A washing liquid is introduced into the first filter chamber 15. The washing liquid is expelled through the at least one first filter waste outlet 21, thereby transporting at least some of the accumulated waste out of the first filter chamber 15. The washing liquid is discharged to the second filter unit FT2. The second filter 31 is configured to filter the washing liquid. As described herein, the second filter 31 comprises the second filter media 33 which is in the form of open cell filter media. The second filter 31 filters the washing liquid and discharges a second filtered liquid (i.e. a second filtrate). The liquid filtration system 1 and the liquid filtration process 100 in the above embodiment return the second filtered liquid from the second filter unit FT2 to the first filter unit FT1 for filtration. The second filtered liquid comprises or consists of the filtered washing liquid discharged from the second filter unit FT2. In the present embodiment, the second filtered liquid is discharged from the second filter unit FT2 to the one or more filtration system outlet 5. At least in certain embodiments, it has been determined that the second filter unit FT2 provides sufficient filtration to enable the second filtered liquid to be discharged from the liquid filtration system 1. The second filtered liquid may be mixed with the first filtered liquid discharged from the first filter unit FT1. The supply line 47 may be omitted in this arrangement. Alternatively, a flow control valve (not shown) may be provided selectively to supply the second filtered liquid to the first filter unit FT1 or the one or more filtration system outlet 5. The liquid filtration process 200 in accordance with the present embodiment of the invention will now be described with reference to a second flow diagram FD2 shown in Figure 5. The clean water is received as influent water in the liquid filtration system 1 (BLOCK 205). The liquid filtration system 1 selectively operates in a filtration mode and a washing mode. In the filtration mode, the unfiltered (raw) water is supplied to the first filter unit FT1 (BLOCK 210). The unfiltered (raw) water may, for example, comprise particles or other debris which is held in suspension. The particles may aggregate together to form larger clumps or flocs which may also be suspended in the unfiltered (raw) water. The first filter 11 mechanically filters the unfiltered (raw) water to remove the waste material held in suspension (BLOCK 215). The resulting filtered water (filtrate) is discharged from the first filter unit FT1 as a first filtered liquid to the one or more filtration system outlet 5 (BLOCK 220). The liquid filtration system 1 is periodically switched to a washing mode to clean the first filter unit FT1 (BLOCK 225). The switch from the filtration mode to the washing mode may, for example, be performed on a predetermined (time) schedule, or in dependence on a pressure change across the first filter. In the washing mode, the first filter 11 is cleaned by introducing a washing liquid (BLOCK 230). The washing liquid dislodges the waste material accumulated in the first filter unit FT1. At least some of the waste material is held in suspension in the washing liquid. The washing liquid and the suspended waste material are discharged from the first filter unit FT1 to the second filter unit FT2 (BLOCK 235). The second filter unit FT2 is operative to filter the washing liquid to remove at least some of the suspended waste material (BLOCK 240). The filtered washing liquid is discharged from the second filter unit FT2 as a second filtered liquid (BLOCK 245). In the present embodiment, at least some of the second filtered liquid is discharged to the filtration system outlet 5 (BLOCK 250). The second filter unit FT2 is periodically cleaned (BLOCK 255). The cleaning of the second filter unit FT2 in the present embodiment is performed while the liquid filtration system 1 is operating in the washing mode. However, it will be appreciated that the second filter unit FT2 may be cleaned while the liquid filtration system 1 is operating in the filtration mode. For example, the liquid filtration system 1 may switch from the washing mode to the filtration mode before or during cleaning of the second filter unit FT2. The second filter unit FT2 is cleaned by introducing a fluid, such as air, to disrupt the static filter pack FP1 (BLOCK 260). The accumulated solid waste material is dislodged from the static filter pack FP1. The waste material is discharged from the second filter unit FT2 to the filtration system waste outlet 7 (BLOCK 265). For example, the second filter waste outlet 41 may be opened to discharge the water and the accumulated waste material, thereby purging the second filter unit FT2. The material discharged from the second filter unit FT2 is typically in the form of a concentrated sludge. The water filtration process continues switching between the filtration mode and the washing mode, as required. The liquid filtration system 1 in the above embodiments is described with reference to a first filter unit FT1 comprising a sand bed filter. Other types of first filter unit FT1 are contemplated for the liquid filtration system 1. For example, the first filter unit FT1 may comprise or consist of a biological filter. The biological filter may be periodically cleaned in a washing process comprising introducing a washing liquid. The biological filter may be periodically cleaned by washing. A washing liquid may be introduced into the biological filter to perform cleaning. The second filter unit FT2 may be provided to filter at least some of the washing liquid used to clean the biological filter in accordance with the method(s) described herein. Alternatively, the first filter unit FT 1 may comprise a drum filter or a screen filter. The first filter unit FT 1 may provide a continuous or substantially continuous supply of wastewater to the second filter unit FT2. A balancing tank 49 may optionally be provided in the liquid filtration system 1. The balancing tank 49 is represented by a broken line in Figure 4 and is provided between the first filter unit FT1 and the second filter unit FT2. The balancing tank 49 may be appropriate for batch processing systems. The second filter unit FT2 is suitable for use with other types of first filter unit FT1 which are cleaned using washing. The first filter unit FT1 could comprise a static filter pack FP1 composed of a plurality of the open cell filter elements 43 of the type described herein. The liquid filtration system 1 may comprise first and second filters 11, 31 of the same type. The first filter unit FT1 may be periodically washed using a washing liquid. The washing liquid may be discharged to the second filter unit FT2 and filtered in the second filter unit FT2. The first and second filters 11, 31 may have different volumes. For example, the first filter unit FT1 may comprise a first static filter pack FP1 having a first volume, and the second filter unit FT2 may comprise a second static filter pack FP2 having a second volume. The first volume may be greater than or less than the second volume. A liquid treatment system 500 for treating a liquid is shown in Figure 6. The liquid treatment system 500 may, for example, be in the form of a liquid treatment plant or a liquid treatment facility. The liquid treatment system 500 comprises a liquid filtration system 1 according to an embodiment of the present invention. A liquid treatment process 600 for treating a liquid is shown in Figure 7. The liquid treatment process 600 comprises a liquid filtration process 100 according to an embodiment of the present invention. As described herein, the liquid filtration system 1 and the liquid filtration process 100 may be used independently of the liquid treatment system 500 and the liquid treatment process 600 described herein. The liquid treatment system 500 and the liquid treatment process 600 are configured to treat water in the present embodiment. The liquid treatment system 500 may be referred to as a water treatment system 500; and the liquid treatment process 100 may be referred to as a water treatment process 100. The water could be wastewater, for example municipal wastewater or industrial wastewater. The liquid treatment system 500 may be referred to as a wastewater treatment system 500; and the liquid treatment process 100 may be referred to as a wastewater treatment process 100. In the present embodiment, the liquid treatment system 500 and the liquid treatment process 600 are suitable for treating clean water. The term “clean water” is used herein to refer to water which is not harmful to humans. The clean water may comprise or consist of drinking (potable) water and / or water for domestic use. The liquid treatment system 500 may be referred to as a clean water treatment system 500; and the liquid treatment process 100 may be referred to as a clean water treatment process 100. The liquid treatment system 500 and the liquid treatment process 600 for treating water will now be described. A schematic representation of the liquid treatment system 500 is shown in in Figure 6. The liquid treatment system 500 comprises one or more system inlet 510; one or more system outlet 520; and one or more system waste outlet 530. The system inlet 510 is configured to receive influent water for treatment. The water introduced through the system inlet 510 is treated as it is conveyed through the liquid treatment system 500. To help differentiate from the water within the liquid treatment system 500, the water received at the system inlet 510 is referred to as untreated (raw or unfiltered) water. The system outlet 520 is configured to discharge the water treated by the liquid treatment system 500. The water discharged at the system outlet 520 is referred to herein as treated water. It will be understood that the treated water discharged from the system outlet 520 may undergo additional treatment or filtration downstream of the liquid treatment system 500. The system waste outlet 530 discharges waste material removed from the water. The waste material discharged from the liquid treatment system 500 through the system waste outlet 530 may undergo further treatment. For example, the discharged waste material may undergo a drying process or de-watering to reduce the water content. The liquid treatment system 500 comprises at least one reservoir 540, a screening system 550, a chemical treatment system 560, a settlement system 570 and a sterilisation system 580. The liquid filtration system 1 is disposed between the settlement system 570 and the sterilisation system 580. The at least one reservoir 540 is configured to store the water introduced at the system inlet 510. The or each reservoir 540 may comprise one or more of the following: a storage vessel, a storage tank, a lagoon or a pond. Other types of reservoir 540 are also contemplated. The screening system 550 is configured to perform preliminary screening to remove debris, such as solids or leaves. The chemical treatment system 560 is configured to perform coagulation and / or flocculation of the water to cause particles or solids in the water to agglomerate. The chemical treatment system 560 may, for example, comprise one or more chemical dosing system for introducing a coagulating agent; and / or one or more electrocoagulation unit. The settlement system 570 may comprise one or more settlement tank to promote settlement of solids in the water. The water is supplied from the settlement system 570 to the liquid filtration system 1 described herein. The water discharged from the settlement system 570 is introduced to the filtration system inlet 3. As described herein, a cleaning water is used to clean the liquid filtration system 1. At least some of the cleaning water is filtered and returned for processing in the liquid treatment system 500. The cleaning water is thereby recycled within the liquid treatment system 500. The recycled water is illustrated as being returned upstream of the liquid filtration system 1. In particular, the recycled water is introduced between the screening system 550 and the chemical treatment system 560. The liquid filtered by the liquid filtration system 1 is output to the sterilisation system 580 which is operative to sterilise the water. The sterilised water is discharged from the sterilisation system 580 to the one or more system outlet 520. The treated water is output for distribution. The one or more system outlet 520 may, for example, be connected to a mains water supply. The liquid treatment system 500 may comprise one or more pump (not shown) for pumping the water. The liquid treatment process 600 is illustrated in a third block diagram FD3 shown in Figure 7. The liquid filtration process 100 is implemented within the liquid treatment process 600. The liquid treatment process 600 comprises storing the untreated liquid in one or more reservoir (BLOCK 605). The water is screened to remove debris (BLOCK 610). The water then undergoes chemical treatment (BLOCK 615). The chemical treatment may comprise or consist of coagulation and / or flocculation, for example. The treated water is introduced into a settlement tank to promote settlement of debris (BLOCK 620). After settlement, the water is filtered (BLOCK 625). The filtration corresponds to either the first liquid filtration process 100 or the second liquid filtration process 200 described herein. The liquid filtration process 100, 200 comprises using a washing liquid or a washing liquid to clean the filter apparatus. The washing liquid may, for example, be a washing liquid. The washing liquid is filtered and a portion of the second filtered liquid returned for further processing. As described herein, at least some of the 24 washing liquid may be recycled. The filtered water is then sterilised (BLOCK 630). The sterilised water is then discharged as potable water (BLOCK 635). The potable water is then discharged, for example as a mains water system. A liquid filtration system 1 and a liquid filtration process 300 in accordance with a further embodiment of the present invention will now be described with reference to Figures 8 and 9. The liquid filtration system 1 and the liquid filtration process 300 are variants of the embodiment described above with reference to Figures 1, 2 and 3. Like reference numerals are used for like features. The liquid filtration system 1 and the liquid filtration process 300 in the present embodiment are configured to filter wastewater. The wastewater may be municipal or industrial wastewater. The filtration process comprises or consists of a mechanical filtration to remove waste material from the wastewater. The waste material may comprise particles, agglomerated particles or other debris which are suspended in the water. As shown in Figure 8, the liquid filtration system 1 comprises a filtration system inlet 3, a filtration system outlet 5, a filtration system waste outlet 7, a first filter unit FT1 and a second filter unit FT2. The first filter unit FT1 is configured to filter the unfiltered water supplied to the filtration system inlet 3. The unfiltered water comprises waste material. The waste material may comprise solids or impurities that are suspended in the unfiltered water. The first filter unit FT1 is configured to separate (i.e. remove) the waste material from suspension in the unfiltered. The waste material separated from the water accumulates in the first filter unit FT1. The accumulated waste material comprises or consists of the retained particles and any other impurities present in the water. The accumulated waste material may form a filter cake or a precipitate on the first filter 11. In use, the first filter unit FT 1 discharges a first filtered liquid (i.e., a first filtrate); and the second filter unit FT2 discharges a second filtered liquid (i.e., a second filtrate). As described herein, the first and second filters 11, 31 are configured to filter waste material comprising or consisting of solids from the unfiltered water. The solids may comprise or consist of solid matter which is insoluble in the liquid and / or solid matter which has come out of solution, for example in the form of precipitates. The first and second filtered liquids comprise or consist of filtered water in the present embodiment. The first filtered liquid is discharged from the first filter unit FT1 to the filtration system outlet 5. The second filtered liquid discharged from the second filter unit FT2 may also be discharged to the filtration system outlet 5. The first filter unit FT1 in the present embodiment is a drum filter. The first filter unit FT1 comprises a first filter 11. The first filter 11 comprises a filter screen 55. The filter screen 55 in this arrangement is cylindrical and forms a first filter chamber 15, as shown in Figure 8. The first filter unit FT1 has at least one first filter inlet 17, at least one first filter outlet 19 and at least one first filter waste outlet 21. The filter screen 55 is configured to rotate about a central longitudinal axis. The wastewater is pumped into the first filter chamber 15. The first filtered water (i.e., the first filtrate) passes through the filter screen 55 and exits the first filter outlet 19. A section of the filter screen 55 disposed in a lower position is effective to filter the wastewater. The first filtered water is discharged to the filtration system outlet 5. The waste material is retained in the first filter chamber 15. When the first filter unit FT1 becomes blocked, a washing liquid is supplied under pressure to clean the filter screen 55. The washing liquid is delivered as a high-pressure jet to dislodge or break down the waste material. The washing liquid may be applied to an inside of the filter screen 55. Alternatively, or in addition, the washing liquid may be applied to an outside of the filter screen 55. As the washing liquid is supplied, the filter screen 55 is rotated, for example through 90° or 180°. The washing liquid thereby impinges on a sector of the filter screen 55. The rotation of the filter screen 55 displaces a clean section to a lower portion to perform filtration of the wastewater supplied to the first filter unit FT1. The washing liquid is typically supplied periodically, for example during a cleaning process. The cleaning process may be performed in dependence on a determination that the filter screen 55 is blocked or outside normal operating conditions. The cleaning process may be performed continuously or substantially continuously, for example if the first filter unit FT1 is operating at capacity. The washing liquid and the accumulated waste material is expelled from the first filter chamber 15 through the first filter waste outlet 21. The used (soiled) washing liquid is discharged from the first filter unit FT 1 to the second filter unit FT2. The second filter unit FT2 comprises a second filter 31. The second filter 31 comprises a second filter media 33. The second filter media 33 is disposed in a second filter chamber 35. The second filter unit FT2 is unchanged from the arrangement described herein with reference to Figures 2A, 2B and 2C. The filter chamber 35 has at least one second filter inlet 37, at least one second filter outlet 39 and at least one second filter waste outlet 41. The at least one second filter waste outlet 41 may be separate from the at least one second filter inlet 37 and the at least one second filter outlet 39. Alternatively, the at least one second filter waste outlet 41 may be combined with one of the at least one second filter inlet 37 and the at least one second filter outlet 39. In the arrangement shown schematically in Figures 2B and 2C, the at least one second filter waste outlet 41 is combined with the at least one second filter inlet 37. The second filter media 33 is in the form of an open cell filter media 33. The second filter media 33 comprises a plurality of filter elements 43, the filter elements 43 each comprising one or more filter cells 45. The open cell filter media 33 is described herein in more detail. The second filter media 33 forms a static filter pack FP1 in the second filter chamber 35, as shown schematically in Figure 2B. The water is filtered as it passes through the static filter pack FP1. The solids suspended in the water are deposited or settle in the filter cells 45 formed in the second filter media 33. The second filter unit FT2 separates the waste material from suspension. The separated waste material accumulates in the second filter unit FT2. The accumulated waste material comprises or consists of the retained particles and any other impurities in the washing liquid. The accumulated waste material may form a filter cake or a precipitate on the second filter media 33. The water may be pumped through the static filter pack FP1 under pressure. Alternatively, the water may flow through the static filter pack FP1 under gravity. The water discharged from the at least one first filter waste outlet 21 is introduced into the second filter unit FT2 through the at least one second filter inlet 37. The water flows through the static filter pack FP1 to the at least one second filter outlet 39. The static filter pack FP1 is effective in filtering the water supplied from the reservoir 11. The filtered water discharged from the second filter unit FT2 is discharged to the at least one second filter outlet 39 as filtered water. The filtered water may be discharged for further filtration, re-use or discharge. As described herein, the second filter unit FT2 is periodically cleaned. A schematic representation of the cleaning of the second filter unit FT2 is shown in Figure 2C. The cleaning of the second filter unit FT2 comprises purging the liquid to expel the accumulated waste. The accumulated waste is expelled from the second filter unit FT2 through the at least one second filter waste outlet 41. The expelled waste may undergo further treatment, for example comprising de-watering or drying. As described herein, the first filter unit FT1 discharges a first filtered liquid (i.e. a first filtrate). The first filtered liquid comprises or consists of the waste water filtered by the first filter unit FT 1. The first filtered liquid is discharged from the first filter unit FT1 to the filtration system outlet 5. The waste material filtered from the water by the first filter media 13 is retained in the first filter unit FT1. The first filter unit FT1 is cleaned to remove the accumulated waste material. In the present embodiment, a washing process is performed concurrent with the first filter unit FT1 filtering the wastewater. The washing process comprises introducing a washing fluid into the first filter chamber 15 to displace the waste material. The washing fluid is introduced into the first filter chamber 15 through one or more cleaning inlet 23. A pump 25 is provided for pumping the washing fluid into the first filter chamber 15. The washing fluid may optionally be introduced under pressure (i.e. at a pressure greater than atmospheric pressure) to disrupt or break up the accumulated waste. The washing fluid may comprise a gas, such as air; or a liquid, such as water. In the present embodiment, the washing fluid is a washing liquid. The washing liquid is preferably the same as (or compatible with) the liquid supplied to the liquid filtration system 1 to be filtered. In the present embodiment, the washing liquid is in the form of water. The washing liquid may comprise unfiltered (raw) water (for example from the filtration system inlet 3) or filtered water (for example from downstream of the first filter unit FT1). Alternatively, or in addition, the washing liquid may be obtained from a different source. The washing liquid is introduced into the first filter chamber 15 and the accumulated waste material and the cleaning water is expelled from the first filter chamber 15 through the first filter waste outlet 21, typically in the form of a sludge. The accumulated waste material is thereby expelled from the first filter chamber 15. At least some of the accumulated waste material may be flushed from the first filter chamber 15 into the second filter chamber 35. The second filter unit FT2 is configured to filter the washing liquid discharged from the first filter unit FT1 during the cleaning process. The second filter unit FT2 comprises a second filter media 33 disposed in a second filter chamber 35. The second filter chamber 35 has at least one second filter inlet 37, at least one second filter outlet 39 and at least one second filter waste outlet 41. The second filter media 33 is in the form of an open cell filter media 33. The second filter media 33 comprises a plurality of filter elements 43, the filter elements 43 each comprising one or more filter cells 45. The open cell filter media 33 is described herein in more detail. The second filter media 33 forms a static filter pack FP1 in the second filter chamber 35. The washing liquid is filtered as it passes through the static filter pack FP1. The waste material suspended in the washing liquid is deposited in the filter cells 45 formed in the second filter media 33. The washing liquid may be pumped through the static filter pack FP1 under pressure. Alternatively, the washing liquid may flow through the static filter pack FP1 under gravity. The washing liquid discharged from the at least one first filter waste outlet 21 is introduced into the second filter unit FT2 through the at least one second filter inlet 37. The washing liquid flows through the static filter pack FP1 to the at least one second filter outlet 39. The static filter pack FP1 is effective in filtering the washing liquid supplied from the first filter unit FT1. The second filter unit FT2 discharges a second filtered liquid. The second filtered liquid comprises or consists of the filtered washing liquid in the present embodiment. The second filtered liquid is discharged from the second filter chamber 35 to the at least one second filter outlet 39. In the present embodiment, the second filtered liquid may be discharged for further filtration or may be re-circulated through the first filter unit FT1 (represented by a broken line in Figure 8) or used as the washing liquid for the first filter unit FT1. The liquid filtration system 1 comprises a supply line 47 connected to the at least one second filter outlet 39. The supply line 47 is configured to introduce the second filtered liquid to the first filter unit FT1. The supply line 47 may be configured to introduce the second filtered liquid upstream of the first filter unit FT1. The second filtered liquid is thereby recycled in the liquid filtration system 1 rather than sent to waste. The second filtered liquid is mixed with the unfiltered (raw) water introduced at the one filtration system inlet 3. In a variant, the second filtered liquid may be introduced directly into the first filter chamber 15. The second filtered liquid may mix with the unfiltered (raw) water in the first filter chamber 15. A one-way valve (not shown) may be provided in the supply line 47 to reduce or prevent unfiltered (raw) water bypassing the first filter unit FT1. The waste material filtered from the washing liquid accumulates in the static filter pack FP1 formed in the second filter chamber 35. The second filter unit FT2 is periodically purged to expel the accumulated waste material to the filtration system waste outlet 7. The second filter inlet valve SFV-1 is operated to close the second filter inlet 37; and the second filter outlet valve SFV-2 is operated to close the second filter outlet 39. A pack disruption fluid is introduced into the second filter chamber 35 to disrupt (i.e. to break up) the static filter pack FP1. The waste material trapped between individual filter elements 43 and / or settled in the filter cells 45 formed in the filter elements is dislodged. The pack disruption fluid introduced into the second filter chamber 35 may be a liquid or a gas. A second filter fluid control valve SFV-4 is provided to control the introduction of the pack disruption fluid. In the present embodiment, the pack disruption fluid is air. An air pump 51 is provided to pump the air through one or more fluid inlet 53 provided in a lower portion of the second filter chamber 35. The second filter waste valve SFV-3 is operated to open the second filter waste outlet 41. The water and accumulated waste material is thereby drained from the second filter chamber 35 to waste. The second filter unit FT2 effectively concentrates the waste material discharged from the first filter unit FT1. The resulting waste material expelled from the second filter unit FT2 comprises a sludge. In a variant, the fluid may be drawn into the second filter chamber 35 as the washing liquid is drained from the second filter chamber 35. Alternatively, or in addition, a mechanical disruptor may be provided to disrupt the filter elements 43 forming the static filter pack FP1. The mechanical disruptor may, for example, comprise one or more movable member which is operable to displace the filter elements 43 forming the static filter pack FP1. The one or more movable member may rotate or sweep within the second filter chamber 35. A liquid filtration process 300 in accordance with an embodiment of the present invention will now be described with reference to a fourth flow diagram FD4 shown in Figure 9. The liquid filtration process 300 in this embodiment is a wasteliquid filtration process 300. The wastewater is received as influent water in the liquid filtration system 1 (BLOCK 305). The liquid filtration process 300 comprises concurrent filtration and cleaning processes. The unfiltered (raw) wastewater is supplied to the first filter unit FT1 (BLOCK 310). The unfiltered (raw) water may, for example, comprise particles or other debris which is held in suspension. The particles may aggregate together to form larger clumps or flocs which may also be suspended in the unfiltered (raw) water. The first filter 11 mechanically filters the unfiltered (raw) water to remove the waste material (BLOCK 315). A first filtered water (filtrate) is discharged from the first filter unit FT1 to the one or more filtration system outlet 5 (BLOCK 320). A washing liquid is supplied to clean the first filter unit FT 1 (BLOCK 325). The first filter unit FT 1 is cleaned by the washing liquid (BLOCK 330). The washing liquid dislodges the waste material accumulated in the first filter unit FT1. In the present embodiment, the washing liquid is water. At least some of the waste material is held in suspension in the washing liquid. The washing liquid and the suspended waste material is discharged from the first filter unit FT1 to the second filter unit FT2 (BLOCK 335). The second filter unit FT2 is operative to filter the washing liquid to remove the suspended waste material (BLOCK 340). A second filtered liquid comprising or consisting of the second filtered liquid is discharged from the second filter unit FT2 (BLOCK 345). At least some of the second filtered liquid may be recycled (BLOCK 350). The second filter unit FT2 is periodically cleaned (BLOCK 355). The cleaning of the second filter unit FT2 in the present embodiment is performed while the liquid filtration system 1 is operating in the washing mode. However, it will be appreciated that the second filter unit FT2 may be cleaned while the liquid filtration system 1 is operating in the filtration mode. For example, the liquid filtration system 1 may switch from the washing mode to the filtration mode before or during cleaning of the second filter unit FT2. The second filter unit FT2 is cleaned by introducing a fluid, such as air, to disrupt the static filter pack FP1 (BLOCK 360). The accumulated solid waste material is dislodged from the static filter pack FP1. The water and the waste material is discharged from the second filter unit FT2 to the filtration system waste outlet 7 (BLOCK 365). For example, the second filter waste outlet 41 may be opened to discharge the water and the accumulated waste material, thereby purging the second filter unit FT2. The material discharged from the second filter unit FT2 is typically in the form of a concentrated sludge. The water filtration process continues switching between the filtration mode and the washing mode, as required. A liquid filtration system 1 in accordance with a further embodiment of the present invention will now be described with reference to Figure 10. The liquid filtration system 1 is a variant of the embodiment described above with reference to Figures 3, 4 and 5. Like reference numerals are used for like features. The first filter unit FT1 comprises a first filter 11. The first filter 11 in the present embodiment comprises a first filter media 13. The first filter media 13 is disposed in a first filter chamber 15, as shown in Figure 10. The first filter chamber 15 has at least one first filter inlet 17, at least one first filter outlet 19 and at least one first filter waste outlet 21. The first filter media 13 is in the form of an open cell filter media 13. The first filter media 13 comprises a plurality of filter elements 43, the filter elements 43 each comprising one or more filter cells 45. A suitable open cell filter media 13 is the filter media 33 described herein with reference to the second filter unt FT2. The first filter media 13 forms a first static filter pack FP1 in the first filter chamber 15 (corresponding to the arrangement shown schematically in Figure 2B). The water is filtered as it passes through the first static filter pack FP1. The second filter unit FT2 comprises a second filter 31. The second filter 31 comprises a second filter media 33. The second filter media 33 is disposed in a second filter chamber 35. The filter chamber 35 has at least one second filter inlet 37, at least one second filter outlet 39 and at least one second filter waste outlet 41. The at least one second filter waste outlet 41 may be separate from the at least one second filter inlet 37 and the at least one second filter outlet 39. Alternatively, the at least one second filter waste outlet 41 may be combined with one of the at least one second filter inlet 37 and the at least one second filter outlet 39. The second filter media 33 is in the form of an open cell filter media 33. The second filter media 33 comprises a plurality of filter elements 43, the filter elements 43 each comprising one or more filter cells 45. A suitable open cell filter media 33 is described herein in more detail. The second filter media 33 forms a second static filter pack FP2 in the second filter chamber 35, as shown schematically in Figure 4B. The water is filtered as it passes through the static filter pack FP1. The solids suspended in the water are deposited or settle in the filter cells 45 formed in the second filter media 33. The water may be pumped through the static filter pack FP2 under pressure. Alternatively, the water may flow through the static filter pack FP2 under gravity. The water discharged from the at least one first filter waste outlet 21 is introduced into the second filter unit FT2 through the at least one second filter inlet 37. The water flows through the static filter pack FP2 to the at least one second filter outlet 39. The static filter pack FP2 is effective in filtering the water supplied from the reservoir 11. The filtered water from the second filter unit FT2 is discharged to the at least one second filter outlet 39 as filtered water. The filtered water may be discharged for further filtration, re-use or discharge. As described herein, the second filter unit FT2 is periodically cleaned. A schematic representation of the cleaning of the second filter unit FT2 is shown in Figure 4C. The cleaning of the second filter unit FT2 comprises purging the liquid to expel the accumulated waste. The accumulated waste is expelled from the second filter unit FT2 through the at least one second filter waste outlet 41. The expelled waste may undergo further treatment, for example comprising de-watering or drying. The first filter chamber 15 of the first filter unit FT1 has a larger volume than the second filter chamber 35 of the second filter unit FT2. The filtration process of the first and second filter units FT1, FT2 corresponds closely to the filtration process 100 described herein with referenced to Figures 4 and 5. However, in this embodiment, the flow rate per unit cross sectional area of the filter media in the first static filter pack FP1 is higher than the flow rate per unit cross sectional area of the filter media in the second static filter pack FP2. By reducing the flow rate in the second static filter pack FP2, the filtration of the washing liquid discharged from the first filter unit FT1 may be improved. The flow rate per unit cross sectional area of the filter media in the first static filter pack FP1 may be two, three, four or more times greater than the flow rate per unit cross sectional area of the filter media in the second static filter pack FP2. The flow rate per unit cross sectional area of the filter media in the second static filter pack FP2 may be in the range 10 m3 / m2 / h to 30 m3 / m2 / h, or 15 m3 / m2 / h to 25 m3 / m2 / h. The flow rate per unit cross sectional area of the filter media in the first static filter pack FP1 may be greater than or equal to 30 m3 / m2 / h, 50 m3 / m2 / h or 70 m3 / m2 / h. At least in certain embodiments, the second filter unit FT2 may reduce water consumption of the liquid filtration system 1. The second filter unit FT2 enables recovery of at least some of the washing liquid used to clean the first filter unit FT1. The waste material discharged from the second filter unit FT2 is concentrated. The accumulated waste material may be disposed of in conventional manner or may undergo further treatment, for example to perform drying or dewatering to reduce the water content. The present embodiment has been described with reference to a first filter unit FT1 in the form of a drum filter. Other types of filter, such as a screen filter, may have a similar arrangement in which a washing liquid is supplied to prevent blocking of a screen or mesh. It will be appreciated that the liquid filtration system 1 and a liquid filtration process 300 according to the present embodiment may be modified to accommodate different types of filter. For example, the first filter unit FT 1 may comprise a screen filter in place of the drum filter. The second filter unit FT2 in each of the above embodiments is configured to perform mechanical filtration of the water to remove at least some of the solid particles suspended in the water discharged from the first filter unit FT1. The second filter chamber 35 is formed by a filter tank 57. The at least one second filter inlet 37, the at least one second filter outlet 39 and the at least one second filter waste outlet 41 are formed in the filter tank 57. The at least one second filter inlet 37 is disposed in a lower region of the second filter unit FT2; and the at least one second filter outlet 39 is disposed in an upper region of the second filter unit FT2. This arrangement is appropriate for the second filter unit FT2 in the present embodiment in which an up-flow of water is established. The relative position of the at least one second filter inlet 37 and the at least one second filter outlet 39 may be reversed if the second filter unit FT2 is configured to establish a down-flow of water. As described herein, the second filter unit FT2 performs mechanical filtration of the washing liquid discharged from the first filter unit FT1. The washing liquid supplied to the second filter unit FT2 is filtered by the filter elements 43 to remove at least some of the waste material transported with the washing liquid discharged from the first filter unit FT1. The second filtered liquid is discharged from the second filter unit FT2 through the at least one second filter outlet 39. The second filter media 33 comprises a plurality of the filter elements 43. As described herein, the first filter media 33 may optionally also a plurality of the filter elements 43. The filter elements 43 in the present embodiment comprise an open cell structure. A schematic 32 representation of one of the filter elements 43 is shown in Figures 11A and 11B. A perspective view of a variant of one of the filter elements 43 is shown in Figure 11C. The dimensions of the filter elements 43 are shown in Figure 11C by way of example. The indicated dimensions have a tolerance of ± 1mm. The internal walls of the mechanical filter element 71 have a thickness of approximately 0.75mm to 1mm. A plurality of external ribs are formed around the outer perimeter of the filter elements 43. The ribs have a radial length of approximately 0.5mm to 1mm. It will be understood that the filter elements 43 having different dimensions may be used in the liquid filtration system 1 described herein. The filter elements 43 have a non-porous structure and each comprise one or more filter cells 45. The filter elements 43 comprise walls (internal and external) which form the one or more filter cells 45. The walls are impermeable and prevent the flow of liquid between adjacent filter cells 45. The one or more filter cells 45 each have a substantially uniform profile along the length of the filter elements 43. The one or more filter cells 45 are open at each end. In the present embodiment, the filter cells 45 each have a cross-sectional area in the range one (1) to five (5) square millimetres and a length greater than or equal to six (6) millimetres. The filter elements 43 collectively form a static filter pack FP1 in the second filter chamber 35. The filter elements 43 in the present embodiment have a positive buoyancy in water. The filter elements 43 float in the water in the second filter chamber 35 and form a static filter pack FP1 in an upper region of the second filter chamber 35. In a variant, the filter elements 43 may have a negative buoyancy and may form the static filter pack FP1 in a lower region of the second filter chamber 35 35. The filter elements 43 are effective in performing filtration by promoting settlement of the solid particles suspended in the water. The solid particles settle within the filter cells 45 and on the surfaces of the filter elements 43. The filter elements 43 are formed by extrusion moulding a polymer in the present embodiment. Other techniques may be used to form the filter elements 43. The second filter unit FT2 is periodically cleaned to discharge the solids accumulated in the second filter chamber 35. The cleaning of the second filter unit FT2 comprises purging the second filter chamber 35 to expel the accumulated waste material. The second filter unit FT2 may be cleaned while the first filter unit FT1 operates in the filtration mode. The cleaning of the second filter unit FT2 comprises closing the at least one second filter inlet 37 and the at least one second filter outlet 39. The second filter waste outlet 41 is closed during the cleaning operation and then opened to purge (or drain) the second filter unit FT2. The second filter inlet valve SFV-1 is operated selectively to open and close the at least one second filter inlet 37; and the second filter outlet valve SFV-2 is operated selectively to open and close the at least one second filter outlet 39. The second filter waste valve SFV-3 is controlled selectively to open and close the second filter waste outlet 41. During cleaning of the second filter unit FT2, the second filter inlet valve SFV-1 and the second filter outlet valve SFV-2 are closed. The pack disruption fluid is then introduced into the second filter chamber 35 to disrupt the static filter pack FP1. In the present embodiment, the second filter fluid control valve SFV-4 is opened to introduce the pack disruption fluid into the second filter chamber 35. The pressurised air is introduced into the second filter chamber 35 and breaks up the static filter pack FP1. The pressurised air is effective to agitate the filter elements 43. The accumulated waste material is thereby dislodged from the surface of the filter elements 43 and / or from within the filter cells 45. The supply of air continues for a predetermined period of time. The period of time may be determined through empirical analysis, for example. The second filter waste valve SFV-3 is then operated to open the second filter waste outlet 41. The washing liquid is discharged from the second filter chamber 35 and functions as a purging liquid which transports the accumulated waste material in the second filter chamber 35 to waste. The supply of the pressurised air may optionally continue after the second filter waste outlet 41 is opened. After the second filter chamber 35 has been drained, the second filter waste valve SFV-3 is closed to close the second filter waste outlet 41. The supply of air to the second filter chamber 35 is stopped by closing the second filter fluid control valve SFV-4. The at least one second filter inlet 37 remains closed when the liquid filtration system 1 is operating in the filtration mode. At least some of the second filtered water discharged from the second filter chamber 35 may optionally be returned for further filtration within the liquid filtration system 1. For example, a return line (not shown) may be provided to return at least some of the second filtered water discharged from the second filter unit FT2. In use, the open cell filter elements 43 form a static filter pack FP1 which is operative to filter solids from the water. The filter elements 43 may have positive, negative or neutral buoyancy. The filter elements 43 have an open cell structure which provides a high retention capacity. The flow rate per unit cross sectional area of the static filter pack FP1 may be 20m3 / m2 / h. The flow rate per unit cross sectional area of the static filter pack FP1 may be less than 20m3 / m2 / h exclusive, for example in the range 0.1m3 / m2 / h to 19.9m3 / m2 / h; 5m3 / m2 / h to 19.5m3 / m2 / h; or 11m3 / m2 / h to 19m3 / m2 / h. The flow rate per unit cross sectional area of the static filter pack FP1 may be greater than 20m3 / m2 / h, for example greater than 50m3 / m2 / h. A flow rate per unit cross sectional area of the static filter pack FP1 less than or equal to 20m3 / m2 / h may be appropriate to perform single pass filtration consisting of a single filtration cycle. A flow rate per unit cross sectional area of the static filter pack FP1 greater than 20m3 / m2 / h may be appropriate to perform multi-pass filtration comprising a plurality of filtration cycles. It has been recognised that these flow rates per unit cross-sectional area are particularly effective in the removal of flocs (also known as flocculant masses) from the water. The flocs comprise or consist of loosely aggregated particles or soft flakes. At least in certain embodiments, the flocs can settle within the filter cells 45 of the filter elements 43 or on an exterior thereof. The relatively low flow rate through the static filter pack FP1 helps to reduce or avoid disrupting the flocs. A higher flow rate could potentially cause certain types of flocs to break down or disintegrate into smaller flocs or individual particles. At least in certain embodiments, the liquid filtration system 1 is capable of capturing very fine particles suspended in the water. The flow rate per unit cross-sectional area of the second filter media 33 influences the filtration of the water. The retention capacity of the second filter unit FT2 is dependent on the volume of the second filter media 33 in the second filter chamber 35. To facilitate cleaning of the second filter media 33, the second filter media 33 preferably occupy approximately 40% to 60% by volume of the second filter chamber 35. In a preferred embodiment, the second filter media 33 occupy approximately 50% by volume of the second filter chamber 35. The second filter unit FT2 may comprise different sizes of filter tank. The mechanical filter tank 57 may comprise or consist of a cylindrical section having a circular cross-section. The mechanical filter tank 57 may have different cross-sections, for example polygonal, rectangular or square. The mechanical filter tank 57 may be oriented such that the central longitudinal axis extends vertically or horizontally. A first mechanical filter tank 57A is shown in Figure 11A by way of example. The first mechanical filter tank 57 comprises a cylindrical section. The first mechanical filter tank 57 comprises a second filter inlet 37, a second filter outlet 39 and a second filter waste outlet 41. The cylindrical section of the first mechanical filter tank 57A has a diameter of 1.2m. A second mechanical filter tank 57B is shown in Figure 11B by way of example. The second mechanical filter tank 57 comprises a cylindrical section. The second mechanical filter tank 57B comprises a second filter inlet 37, a second filter outlet 39 and a second filter waste outlet 41. The cylindrical section of the first mechanical filter tank 57B has a diameter of 0.9m. A third mechanical filter tank 57C is shown in Figure 11C by way of example. The third mechanical filter tank 57 comprises a cylindrical section. The third mechanical filter tank 57C comprises a second filter inlet 37, a second filter outlet 39 and a second filter waste outlet 41. The cylindrical section of the third mechanical filter tank 57C has a diameter of 0.75m. A fourth mechanical filter tank 57D is shown in Figure 11D by way of example. The fourth mechanical filter tank 57 comprises a cylindrical section. The fourth mechanical filter tank 57D comprises a second filter inlet 37, a second filter outlet 39 and a second filter waste outlet 41. The cylindrical section of the fourth mechanical filter tank 57D has a diameter of 0.6m. A fifth mechanical filter tank 57E is shown in Figure 11E by way of example. The fourth mechanical filter tank 57 comprises a cylindrical section. The fourth mechanical filter tank 57E comprises a second filter inlet 37, a second filter outlet 39 and a second filter waste outlet 41. The cylindrical section of the fourth mechanical filter tank 57E has a diameter of 0.5m. The second filter unit FT2 is sized in dependence on the flow rate of water to be filtered. The filtration of the unfiltered (raw) water is described in the above embodiments as comprising a single filtration stage which is performed by a single first filter unit FT1. It will be appreciated that the filtration may comprise a plurality of filtration stages, for example performed by two or more of the first filters 11. The first filters 11 may be arranged in series or in parallel with each other. The first filters 11 may have like configurations or may have different configurations. Furthermore, the liquid filtration system 1 may comprise two or more different types of first filter unit FT1 for filtering the unfiltered liquid. The secondary filtration of the unfiltered (raw) water is described in the above embodiments as comprising a single filtration stage which is performed by a single second filter unit FT2. It will be appreciated that the secondary filtration may comprise a plurality of filtration stages, for example performed by two or more of the second filters 31. The second filters 31 may be arranged in series or in parallel with each other. The plurality of second filters 11 may have like configurations or may have different configurations. The liquid filtration system 1 and the liquid filtration process 100, 200, 300 have been described with reference to filtering clean water and wastewater. The clean water may, for example, comprise or consist of drinking (potable) water. 6the liquid filtration system 1 and the liquid filtration process 100 may be used to filter water which is used in, or discharged from, an industrial process or a chemical process. The water may not have been biologically filtered before being supplied to the liquid filtration system 1. The water may optionally have undergone chemical filtration before being supplied to the liquid filtration system 1. It will be understood that the liquid filtration systems 1 and the liquid filtration processes 100, 200, 300 described herein may be used to filter liquids other than water. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application. FIRST FLOW DIAGRAM LABELS 105 INFLUENT LIQUID RECEIVED 110 OPERATE IN FILTRATION MODE 115 FIRST FILTER REMOVES WASTE MATERIAL FROM LIQUID 120 DISCHARGE FILTERED LIQUID FROM FIRST FILTER 125 OPERATE IN WASHING MODE 130 INTRODUCE WASHING LIQUID TO WASH FIRST FILTER 135 DISCHARGE USED WASHING LIQUID AND WASTE MATERIAL TO SECOND FILTER 140 USED WASHING LIQUID IS FILTERED BY SECOND FILTER HAVING OPEN CELL FILTER MEDIA 145 DISCHARGE FILTERED LIQUID FROM SECOND FILTER 150 SUPPLY FILTERED LIQUID FROM SECOND FILTER TO FIRST FILTER FOR FILTRATION 155 CLEAN SECOND FILTER 160 INTRODUCE FLUID TO DISRUPT FILTER PACK IN SECOND FILTER 165 DISCHARGE CONCENTRATED WASTE MATERIAL FROM SECOND FILTER SECOND FLOW DIAGRAM LABELS 205 INFLUENT LIQUID RECEIVED 210 OPERATE IN FILTRATION MODE 215 FIRST FILTER REMOVES WASTE MATERIAL FROM LIQUID 220 DISCHARGE FIRST FILTERED LIQUID FROM FIRST FILTER 225 OPERATE IN WASHING MODE 230 INTRODUCE WASHING LIQUID TO WASH FIRST FILTER 235 DISCHARGE USED WASHING LIQUID AND WASTE MATERIAL TO SECOND FILTER 240 USED WASHING LIQUID IS FILTERED BY SECOND FILTER HAVING OPEN CELL FILTER MEDIA 245 DISCHARGE FILTERED LIQUID FROM SECOND FILTER 250 AT LEAST SOME FILTERED LIQUID FROM SECOND FILTER DISCHARGED TO SYSTEM OUTLET(S) 255 CLEAN SECOND FILTER 260 INTRODUCE FLUID TO DISRUPT FILTER PACK IN SECOND FILTER 265 DISCHARGE CONCENTRATED WASTE MATERIAL FROM SECOND FILTER THIRD FLOW DIAGRAM LABELS 605 WATER FOR CLEAN WATER TREATMENT STORED IN RESERVOIR 610 SCREEN WATER SUPPLIED FROM RESERVOIR 615 CHEMICAL TREATMENT OF WATER 620 SETTLEMENT OF SUSPENDED WASTE MATERIAL 625 FILTER WATER 630 STERILISE WATER 635 DISCHARGE DRINKING (POTABLE) WATER FOURTH FLOW DIAGRAM LABELS 305 INFLUENT LIQUID RECEIVED 310 LIQUID SUPPLIED TO FIRST FILTER 315 FIRST FILTER REMOVES WASTE MATERIAL FROM LIQUID 320 DISCHARGE FIRST FILTERED LIQUID FROM FIRST FILTER 325 SUPPLY WASHING LIQUID TO FIRST FILTER 330 WASHING LIQUID USED TO WASH FIRST FILTER 335 DISCHARGE USED WASHING LIQUID AND WASTE MATERIAL TO SECOND FILTER 340 USED WASHING LIQUID IS FILTERED BY SECOND FILTER HAVING OPEN CELL FILTER MEDIA 345 DISCHARGE FILTERED LIQUID FROM SECOND FILTER 350 OPTIONALLY RECYCLE SECOND FILTERED LIQUID 355 CLEAN SECOND FILTER 360 INTRODUCE FLUID TO DISRUPT FILTER PACK IN SECOND FILTER 365 DISCHARGE CONCENTRATED WASTE MATERIAL FROM SECOND FILTER

Claims

1. A liquid filtration process for filtering a liquid to remove waste material, the liquid filtration process comprising:filtering the liquid using a first filter and discharging a first filtered liquid from the first filter, the first filter accumulating waste material filtered from the liquid;washing the first filter using a washing liquid to dislodge at least some of the accumulated waste material from the first filter and discharging the washing liquid and at least some of the dislodged waste material from the first filter;filtering the washing liquid discharged from the first filter using a second filter and discharging a second filtered liquid from the second filter, the second filter accumulating waste material filtered from the washing liquid;wherein the second filter comprises an open cell filter media, the open cell filter media comprising a plurality of filter elements which form a static filter pack, the filter elements each having one or more filter cells, the static filter pack filtering the washing liquid discharged from the first filter.

2. A liquid filtration process as claimed in claim 1 comprising introducing at least some of the second filtered liquid discharged from the second filter into the first filter.

3. A liquid filtration process as claimed in claim 2, wherein the at least some of the second filtered liquid introduced directly into the first filter or is introduced upstream of the first filter.

4. A liquid filtration process as claimed in any one of claims 1, 2 or 3, wherein at least some of the first filtered liquid and / or at least some of the second filtered liquid is discharged to one or more system outlet.

5. A liquid filtration process as claimed in any one of claims 1 to 4, wherein the waste material filtered from the washing liquid accumulates in the static filter pack and / or the filter cells of the open cell filter media, wherein the process comprises purging the second filter to expel at least some of the accumulated waste material from the second filter.

6. A liquid filtration process as claimed in claim 5, wherein purging the second filter comprises introducing a fluid into the second filter to break up the static filter pack and / or to dislodge waste material from the filter cells.

7. A liquid filtration process as claimed in any one of the preceding claims comprising afiltration process and a washing process; wherein the first filter filters the liquid during thefiltration process and the first filter is washed during the washing process; wherein the filtration process and the washing process are performed sequentially or concurrently.

8. A liquid filtration process as claimed in any one of the preceding claims, wherein theliquid comprises or consists of water.

9. A liquid filtration process as claimed in any one of the preceding claims, wherein the liquid is clean water.

10. A liquid filtration process as claimed in any one of claims 1 to 8, wherein the liquid is wastewater.

11. A liquid treatment process comprising the liquid filtration process as claimed in any one of the preceding claims.

12. A liquid filtration system for filtering a liquid to remove waste material, the liquid filtration system comprising:a first filter unit comprising:at least one first filter inlet for receiving the liquid to be filtered;a first filter for filtering waste material from the liquid;at least one first filter outlet for discharging a first filtered liquid;a washing liquid supply for supplying a washing liquid to wash the first filter; and at least one first filter waste outlet for discharging the washing liquid;a second filter unit comprising:at least one second filter inlet for receiving the washing liquid discharged from the first filter unit;a second filter for filtering waste material from the washing liquid; and at least one second filter outlet for discharging a second filtered liquid;wherein the second filter comprises an open cell filter media, the open cell filter media comprising a plurality of filter elements for forming a static filter pack, the filter elements each having one or more filter cells.

13. A liquid filtration system as claimed in claim 12 comprising at least one supply line for supplying the second filtered liquid to the first filter.

14. A liquid filtration system as claimed in claim 13, wherein the at least one supply line is connected directly to the first filter unit or is connected upstream of the first filter unit.

15. A liquid filtration system as claimed in any one of claims 12, 13 or 14, wherein the at least one first filter outlet and the at least one second filter outlet are connected to one or more system outlet for discharge the first and second filtered liquids.

16. A liquid filtration system as claimed in any one of claims 12 to 15, wherein the second filter unit is configured to be purged to expel at least some of the waste material accumulated in the open cell filter media.

17. A liquid filtration system as claimed in claim 16, wherein the second filter unit comprises at least one fluid inlet for introducing a fluid to break up the static filter pack and / or to dislodge waste material from the filter cells.

18. A liquid filtration system as claimed in any one of claims 12 to 17, wherein the liquid filtration system is configured to operate selectively in a filtration mode and a washing mode; wherein, when operating in the filtration mode, the first filter filters the liquid; and, when operating in the washing mode, the first filter is washed.

19. A liquid filtration system as claimed in any one of claims 12 to 18, wherein the liquid is water.

20. A liquid filtration system as claimed in any one of the claims 12 to 19, wherein the liquid is clean water.

21. A liquid filtration system as claimed in any one of the claims 12 to 19, wherein the liquid is wastewater.

22. A liquid treatment system comprising one or more liquid filtration system as claimed in any one of claims 12 to 21.Application No: GB2417127.4Examiner: Dr Alun OwenClaims searched: 1-22Date of search: 20 May 2025Patents Act 1977: Search Report under Section 17Documents considered to be relevant:Category Relevant to claims Identity of document and passage or figure of particular relevance X 1-22 JP Hl 1253985 A (HITACHI) See whole document X 1-22 JP Hl 119690 A (NIPPON) See whole document X 1-22 JP H05269482 A (KAT AG AI) See whole document v A 1-22 WO 2017 / 061872 Al (BIOWATER) See whole document, especially the figures, line 27 of page 8 to line 4 of page 9 &lines 12-25 of page 13 A - EP 0423403 Al (NISHIHARA) See whole document A - JPH0947771 A (EBARA) See whole documentCategories:X Document indicating lack of novelty or inventive step A Document indicating technological background and / or state of the art. Y Document indicating lack of inventive step if P Document published on or after the declared priority date but combined with one or more other documents of same category. before the filing date of this invention. & Member of the same patent family E Patent document published on or after, but with priority date earlier than, the filing date of this application.Field of Search:International Classification:Subclass Subgroup Valid From C02F 0001 / 00 01 / 01 / 2023 BOID 0024 / 16 01 / 01 / 2006 BOID 0024 / 46 01 / 01 / 2006 BOID 0029 / 66 01 / 01 / 2006 BOID 0033 / 06 01 / 01 / 2006 BOID 0033 / 48 01 / 01 / 2006 BOID 0036 / 02 01 / 01 / 2006 BOID 0036 / 04 01 / 01 / 2006 C02F 0001 / 463 01 / 01 / 2023 C02F 0001 / 52 01 / 01 / 2023 C02F 0003 / 00 01 / 01 / 2023 C02F 0009 / 00 01 / 01 / 2023

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