A biological wastewater treatment method and apparatus

Abstract

A method and apparatus for biological treatment of water, the method comprising: a) transporting wastewater into a bioreactor through an inlet fluid conduit; b) transporting the wastewater, in a plug flow pattern, through a mass of carrier elements, said mass of carrier elements having a constant degree of filling of 70% or more of a volume of the bioreactor; wherein the constant degree of filling of the carrier elements restricts movement of the carrier elements, urging the wastewater to move through the reactor in the plug flow pattern; c) treating the wastewater as the wastewater is transported through the carrier elements, the carrier elements having a biofilm layer formed thereon the carrier elements; d) transporting treated water to an outlet fluid conduit; e) repeating steps a) to d) on a continuous basis during an operational state of the bioreactor; f) during steps a) to e), cleaning the carrier elements by ejecting fluid at the mass of carrier elements from a cleaning unit disposed inside the reactor to shear off biofilm from the carrier elements; wherein the sheared off biofilm is suspended in the bioreactor and flow to the outlet fluid conduit with the transported treated water; and wherein a constant liquid volume is maintained in the bioreactor throughout steps a) to f).

Description

[0001] A BIOLOGICAL WASTEWATER TREATMENT METHOD AND APPARATUS

[0002] Background

[0003] The present invention relates to biofilm bioreactors. Biofilm bioreactors may be employed to treat municipal wastewater such as sewage and other municipal wastewater streams, industrial wastewater, and can also be employed for removal of nutrients like nitrogen and phosphorus. A biofilm bioreactor is a type of bioreactor used in wastewater treatment where microorganisms grow on surfaces to form a biofilm in order to degrade pollutants. Biofilm bioreactors employ carriers to provide surfaces for biofilm formation. These can include plastic media, sand, gravel, or specially designed biofilm carriers with high surface areas. The biofilm is an aggregation of microorganisms growing on a solid surface.

[0004] Types of biofilm bioreactors include trickling filters and Moving Bed Biofilm Reactors (MBBRs). In trickling fluid biofilm reactors, wastewater trickles over a bed of media where biofilm grows, allowing microorganisms to degrade organic matter as the water flows through. In MBBRs, free-moving biofilm carriers circulate in the wastewater. The carrier element filling ratio of MBBRs has a maximum ratio of 65%. In these types of biofilm reactors, regular monitoring and control of biofilm thickness is not normally conducted, however, based on the natural sloughing process of biofilm, this potentially causes clogging of media and reduces treatment efficiency.

[0005] One type of biofilm reactor for controlling biofilm thickness is a Continuous Flow Intermittent Cleaning (CFIC) reactor. In a CFIC reactor, wastewater flows continuously through the reactor with carrier media tightly packed with over a 65% filling ratio, when the CFIC is in a normal operational state. This allows for a relatively stagnant media bed. The consistent treatment of incoming wastewater is through the media bed with a plug-flow pattern during the normal operational state. This continuous flow ensures that the process is ongoing and efficient. However, cleaning of the reactor or the biofilm is done periodically and normal operation of the CFIC is transitioned to a MBBR stage with a higher water level and a lower filling ratio during cleaning of the carrier elements. This periodic cleaning helps in maintaining the efficiency of the biofilm by preventing excessive buildup and clogging of biofilm without disrupting the treatment process.

[0006] CFIC reactors have advantages compared to other types of wastewater treatment systems since they generally consume less energy. This is because CFIC biofilm systems have a relatively stagnant carrier media bed in the reactor and typically utilise less aeration energy compared to MBBR systems. Due to high mass transfer rates in a plug-flow pattern, less air flow is required than MBBR systems. CFIC often requires less space compared to conventional wastewater treatment methods like activated sludge systems.

[0007] A particular known CFIC reactor utilises a biofilm process where the growth surface for micro-organisms consists of carrier elements that, in normal operation, are packed so closely that they cannot move freely and have no, or a hindered, movement. The degree of filling of the carrier elements at normal operation constitutes an amount corresponding to 70% to 100% of the reactor liquid volume. During a cleaning phase, however, the carrier elements are fluidised. During said cleaning phase, excess sludge (excess biofilm) can be removed from the bioreactor. The carrier elements are fluidised in that the water level in the CFIC reactor is temporarily increased so that the degree of filling of the carrier elements becomes less, such as less than 70%, of the reactor liquid volume. Turbulence in the reactor tears an amount of excess sludge (excess biofilm) off the carrier elements and sedimented sludge (feared off biofilm) is suspended in the bioreactor liquid volume. Inlet water is led into the reactor through one or more inlets and thus brings sludge out of the reactor through one or more outlets. After the cleaning phase, the water level in the reactor is reduced again so that the degree of filling for the carrier elements returns to that during normal operation. More particularly, during the cleaning phase of the CFIC, a treated water outlet pipe is closed. Water is still inflowing through the inlet pipe which raises the water level to a sludge outlet pipe positioned higher than the treated water outlet pipe on the bioreactor. Thus, the CFIC bioreactor has two differentiated states: a normal operation state and a cleaning state (phase). During normal operation, the carriers are tightly packed, the water level is lower, treated water flows out through a first lower outlet pipe, and the carriers are not being cleaned. During the cleaning state (phase), the carriers are fluidised, the water level is higher, sludge flows out through a second higher outlet pipe, and the carriers are cleaned by turbulence forces.

[0008] WO2015088353A1 discloses a method for biological purification of water, the method comprising: leading the water into a reactor through one or more inlet pipes or inlet zones; leading the water and substrate through carrier elements for biofilm growth which have a high protected surface area (>200 m2 / m3carrier elements) and a large pore volume (>60%). One or more membrane units are submerged in the water in the reactor. Permeate is pulled out of the reactor through the pores of the membranes. Oxygen-containing gas is supplied in the reactor through an aeration system. During normal operation, the water level in the reactor is maintained below one or more outlet pipes or outlet zones that are dedicated for excess sludge removal. During washing operation, strong turbulence is created for removal of excess sludge as the water level in the reactor is temporarily raised to the level of the outlet pipes or outlet zones that are dedicated for excess sludge removal.

[0009] NO329665B1 discloses a method for the biological purification of water comprising introducing the water into a reactor through one or more inlet pipes or inlet zones and directing water and substrate through biofilm carrier elements. The filling degree of the elements during normal operation constitutes an amount corresponding to 70% to 100% of the reactor's wet volume. The elements, which have a high protected surface area and large pore volume, are kept approximately stationary or in restricted movement, with the elements having a specific gravity in the range of 0.8 to 1 .4. The treated water is directed to one or more outlet zones and one or more outlet pipes. NO329665B1 also relates to a reactor for aerobic, anoxic, or anaerobic purification of wastewater, comprising one or more inlet pipes and one or more inlet zones, and one or more outlet zones and outlet pipes for water and substrate, and one or more outlet pipes for sludge, and one or more mixing mechanisms for the transport of water and substrate. The filling degree of elements is so large during normal operation that it prevents free movement of the elements, which have a large specific surface area and large pore volume, and such that when the water level is raised sufficiently for the elements to move freely with the help of one or more of the mentioned mixing mechanisms, the sludge is removed through pipes.

[0010] Disadvantages of the above-described CFIC reactor system are that the system requires a more complex arrangement with multiple outlets, as well as requiring hydraulic complexity between cleaning and non-cleaning states. Further disadvantages are that normal operation of the bioreactor must be interrupted to clean the carrier elements and that the carrier elements with excess biofilm growth and other solids accumulated from the wastewater cannot be cleaned during the normal operation.

[0011] It is an object of the invention to provide a method and system for continuous processing of wastewater which addresses problems with the above-described known systems and methods. In particular, it is an object of the invention to provide a method and apparatus where normal operation, i.e. treatment of wastewater, is not interrupted for cleaning of the carrier elements.

[0012] Summary of the Invention

[0013] According to a first aspect of the invention, there is provided a method for biological purification of water, comprising: a) transporting wastewater into a bioreactor through an inlet fluid conduit; b) transporting the wastewater, in a plug flow pattern, through a mass of carrier elements, said mass of carrier elements having a constant degree of filling of 70% or more of a volume of the bioreactor; wherein the constant degree of filling of the carrier elements restricts movement of the carrier elements, urging the wastewater to move through the reactor in the plug flow pattern; c) treating the wastewater as the wastewater is transported through the carrier elements, the carrier elements having a biofilm layer formed thereon; d) transporting treated water to an outlet fluid conduit; e) repeating steps a) to d) on a continuous basis during an operational state of the bioreactor; f) during steps a) to e), cleaning the carrier elements by ejecting fluid at the mass of carrier elements from a cleaning unit to shear off biofilm from the carrier elements; wherein the sheared off biofilm is suspended in the bioreactor and flows to the outlet fluid conduit with the transported treated water; and wherein a constant liquid volume is maintained in the bioreactor throughout steps a) to f).

[0014] The constant degree of filling of said mass of carrier elements may be in the range of 75% to 90% of the reactor volume.

[0015] Ejecting fluid from the cleaning unit may comprise releasing liquid or gas bubbles upwards, in an in-use orientation, through the bioreactor from the cleaning unit disposed inside the bioreactor.

[0016] The method may further comprise ejecting fluid from the cleaning unit at a predetermined pulse frequency and flow rate.

[0017] The method may further comprise selecting a pulse frequency and flow rate to achieve a shearing efficacy of the carrier elements suitable for maintaining a layer of biofilm at 1 mm or less on the carrier elements.

[0018] According to a second aspect of the invention there is provided a biological wastewater treatment apparatus for performing the method of the first aspect of the invention, comprising: a bioreactor containing: a constant liquid volume; a mass of carrier elements, said mass of carrier elements having a constant degree of filling of 70% or more of a volume of the bioreactor; and a cleaning unit configured to eject fluid at the mass of carrier elements during wastewater treatment; an inlet fluid conduit fluidly connected to the bioreactor for continuous inflow of wastewater into the bioreactor; and an outlet fluid conduit fluidly connected to the bioreactor for continuous outflow of treated water or treated water comprising sheared biomass. The outlet fluid conduit may consist of a single outlet pipe.

[0019] The degree of filling for the mass of carrier elements may be in the range of 75% to 90% of the reactor volume.

[0020] The cleaning unit may be configured to eject fluid at the mass of carrier elements at a predetermined pulse frequency and flow rate.

[0021] The cleaning unit may be configured to eject a fluid at the mass of carrier elements at a predetermined frequency and flow rate suitable for providing a predetermined shearing efficacy of the carrier elements, said shearing efficacy for maintaining a layer of biofilm at 1 mm or less.

[0022] The cleaning unit may be an array of liquid jets.

[0023] The cleaning unit may be an array of gas jets.

[0024] The cleaning unit may be disposed inside the bioreactor floor and configured to release liquid or gas bubbles upwards through the bioreactor in the in-use orientation.

[0025] The apparatus may further comprise a sieve aligned with the outlet fluid conduit, wherein the cleaning unit facilitates cleaning of the sieve.

[0026] Brief Description of the Drawings

[0027] Fig. 1a is a schematic diagram of a biological wastewater treatment apparatus according to an example of the invention;

[0028] Fig. 1b is a schematic diagram of the biological wastewater treatment apparatus of Fig. 1a during cleaning of the carrier elements; and

[0029] Fig. 2 is a flowchart of a method of biological treatment of wastewater according to an example of the invention. Detailed Description

[0030] Unless otherwise defined, all terms of art, notations and other scientific terms or terminology used herein are intended to have the meanings commonly understood by those of skill in the art to which this invention pertains. In some cases, terms with commonly understood meanings are defined herein for clarity and / or for ready reference, and the inclusion of such definitions herein should not necessarily be construed to represent a substantial difference over what is generally understood in the art.

[0031] The present invention relates to an improved continuous flow biofilm reactor and improved continuous flow method for biological purification of water. In particular, the present invention relates to a biofilm reactor of the continuous flow type with enhanced cleaning which addresses some of the disadvantages of the biofilm reactors described above. The invention provides a method and apparatus in which normal water treatment operation of inflowing wastewater continues even during cleaning of the carrier elements.

[0032] The present invention relates to a biological wastewater treatment apparatus of the continuous flow type wherein, during normal operation, wastewater is continuously transported (led) into a bioreactor, through a mass of carrier elements for water treatment, and treated water is continuously transported (led) out through an outlet. During normal operation, the bioreactor has a constant liquid volume and a constant degree of filling of a mass of carrier elements, the degree of filling being 70% or more of the reactor volume. The degree of filling creates a plug flow pattern of wastewater through the reactor throughout normal operation. A cleaning unit is installed the apparatus wherein the cleaning unit is configured to eject a fluid at the mass of carrier elements. In an example, the cleaning unit is disposed within the bioreactor. An inlet fluid conduit is fluidly connected to the bioreactor for inflow of wastewater (influent containing contaminated water) into the bioreactor, and an outlet fluid conduit is fluidly connected to the bioreactor for outflow of treated water (effluent and effluent comprising sheared biomass). During normal operation, inflow of wastewater and outflow of treated water is preferably continuous.

[0033] The cleaning unit is configured to eject fluid at the mass of carrier elements during normal operation of the apparatus. In this way, the constant liquid volume and the constant degree of filling of carrier elements is maintained even during cleaning. The cleaning can be continuous or intermittent. Continuous cleaning can be defined as expulsion of fluid from jets in a constant flow or pulsed manner throughout the entire duration of the apparatus’s normal operational state. Whereas intermittent enhanced cleaning can be defined as periods of fluid expulsion in a constant flow or pulsed manner interrupted by periods of no fluid expulsion.

[0034] The invention will now be described with reference to the figures in an illustrative manner.

[0035] Figure 1a shows an example apparatus 100 for carrying out biological treatment (purification) of water. The apparatus 100 for carrying out biological treatment (purification) of water can also be referred to as a continuous flow enhanced cleaning (CFEC) apparatus 100. The CFEC apparatus 100 has a bioreactor 3, a wastewater inlet (an inlet fluid conduit) 1 , a purified water outlet (an outlet fluid conduit) 2, and a cleaning unit 4. The bioreactor 3 is of the continuous flow type and is densely packed with a mass of carrier elements 6. The mass of carrier elements 6 provides surfaces for biofilm formation. The carrier elements 6 are held in place in the bioreactor with the help of an outlet arrangement. The degree of filling of the carrier elements in the bioreactor is so large that they are not free to move during normal operation, i.e. they display hindered movement. The carrier elements 6 filling is calculated based on the height of the carrier elements filling compared to the reactor and divided by the water height in the reactor. This degree of filling is around 70% or more of the reactor liquid volume. In some examples, the degree of filling is 75% to 90%, more preferably 80% to 85% of the reactor liquid volume. The carrier elements filling degree results in increased system compaction and restricted movement of the carrier elements. Microorganisms grow on the carrier elements forming a biofilm. These microorganisms consume contaminants in the wastewater which thereby treats (purifies) the wastewater as it flows through the mass of carrier elements.

[0036] The bioreactor may be an aerobic bioreactor or an anaerobic (anoxic) bioreactor. In an aerobic bioreactor, the microorganisms are aerobic microorganisms. These microorganisms require oxygen to break down organic matter (such as the contaminants in the wastewater), resulting in the production of carbon dioxide, water, and biomass. For aerobic bioreactors, aeration in the form of air or oxygen can be supplied into the reactor, such as from the bottom of the reactor. This aeration may be supplied by the cleaning unit, or by a separate aeration device 8.

[0037] All known types of carrier elements with a specific weight relatively near the specific weight of water can be used. Preferably, the carrier elements have a specific weight similar to water (i.e. approximately 0.9 to 1.2 g / cm3). Preferably, the carrier elements have a high protective area of more than approximately 300 m2 / m3. Preferably, the carrier elements are made from a material which is configured to withstand exposure to pollutants, chemicals, biological degradation, and mechanical factors (i.e. mixing, compression, etc). The process is based on the principle that biomass is established on a carrier element for the formation of a biofilm. Ideal carrier elements are those which have a large, protected surface area and a large pore volume so that water can flow through the carrier elements and good contact between water, substrate and biofilm is achieved.

[0038] As a result of the degree of filling of the carrier elements in the reactor, water to be treated by the CFEC apparatus 100 (i.e. wastewater) moves through the bioreactor 3 in a plug flow pattern, as demonstrated by the arrows 7 in Figure 1a. This is because the mass of carrier elements occupy a large space of the reactor which enables the plug flow pattern. The plug flow pattern is a flow regime where the water (i.e. wastewater) moves through the bioreactor with minimal mixing in the direction of flow and substantially maintains its original flow profile as it travels. In this way, contaminants in the wastewater are treated sequentially as they pass through the reactor, with the treatment processes occurring progressively along the length of the reactor. Thus, the time that a given contaminant spends in the bioreactor (residence time) is more predictable and uniform in the plug flow system of the present invention, compared to systems with significant mixing that can shortcut the treatment process. The plug flow pattern of wastewater, and air / oxygen in aerobic bioreactors, flow through the carrier elements enhances mass transfer, such as wastewater organic, nutrient and oxygen and reduces energy consumption. In the example of Figure 1a, wastewater flows from the bottom to the top of the bioreactor and the head of the flow moves upwards continuously and homogeneously. The progressive reduction in contaminant concentration can lead to more efficient treatment. In some examples of the invention, wastewater flows from the top to the bottom of the bioreactor and the head of the flow moves downwards continuously and homogeneously. In yet further examples of the invention, wastewater flows from a first side to a second side of the bioreactor and the head of the flow moves across continuously and homogeneously, in the bioreactor’s in-use configuration.

[0039] A filter element 5 is disposed over the outlet 2. Said filter element 5 aids retention of the carrier elements in the bioreactor 3. In the example of Figure 1a, the cleaning unit 4 is disposed along an internal side of the bioreactor 3. The cleaning unit 4 is configured to eject fluid into the densely packed mass of carrier elements 6. In the example of Figure 1a, the cleaning unit 4 is disposed along a bottom surface of the bioreactor 3 in an in-use orientation of the CFEC apparatus and is configured to eject fluid from the bottom of the apparatus upwards into the densely packed mass of carrier elements 6. In some examples, the cleaning unit 4 is installed along a top surface of the bioreactor, in an in-use configuration of the CFEC apparatus 100, and is configured to eject fluid from the top of the apparatus downwards into the densely packed mass of carrier elements 6. In yet further examples of the invention, the cleaning unit can be installed on a side of the bioreactor, in an in-use configuration of the CFEC apparatus, and is configured to eject fluid from the side of the apparatus across into the densely packed mass of carrier elements 6. In a first example, the cleaning unit 4 is configured to eject a liquid such as water. Thus, the cleaning unit 4 may be a series or array of liquid jets. In a second example, the cleaning unit 4 is configured to eject gas, such as air. Thus, the cleaning unit 4 may be a series or array of gas jets. Figure 1a shows the CFEC apparatus 100 when the cleaning unit 4 is not expelling cleaning fluid.

[0040] Figure 1 b shows the CFEC apparatus 100 when the cleaning unit 4 is expelling cleaning fluid during cleaning of the carrier elements 6. The cleaning unit 4 may be activated, in order to expel fluid and clean the carrier elements, continuously during normal operation of the CFEC apparatus 100. Alternatively, the cleaning unit 4 may be activated intermittently during normal operation of the CFEC apparatus 100. The cleaning unit 4 is configured to encourage detachment of excess biofilm formed on the carrier elements in order to maintain a layer of biofilm below a predetermined thickness. The fluid ejected from the cleaning unit 4 passes through the mass of carrier elements and shears off excess biofilm attached on the carrier elements. During the continuous or intermittent cleaning, wastewater is still continuously flowing through the bioreactor 3. In contrast to previous continuous flow bioreactors, during cleaning, the outlet for treated water is not closed off, nor is the water level changed. Thus, the degree of filling of the carrier elements is unchanged during cleaning of the carrier elements.

[0041] Since the degree of filling of the carrier elements is unchanged, the wastewater continues to exhibit plug flow behaviour through the apparatus 100, even when the apparatus 100 is performing cleaning of the carrier elements. This is demonstrated by arrows 7 in Figure 1b. In this way, all of the above-described benefits of continuous plug flow are retained even during cleaning. Furthermore, the CFEC apparatus 100 can continue to process wastewater and perform water treatment by the CFEC apparatus 100 which is uninterrupted by the cleaning process.

[0042] The cleaning unit 4 is configured to supply the ejected fluid at a strength and / or frequency to maintain a biofilm layer on the carrier elements of preferably less than 1 mm. A flow rate of the fluid ejection may be in the range of 1 m3 / h to 4000 m3 / h. In some examples, the cleaning unit 4 is configured to eject fluid as a continuous stream during operation. In other examples, the cleaning unit 4 is configured to pulse fluid ejection at a predetermined frequency. In some examples, the predetermined frequency of fluid ejection is in the range of 1 to 60 times a minute.

[0043] In examples wherein cleaning is intermittent, the apparatus 100 is configured to switch between the intermittent non-cleaning period and intermittent cleaning period after a predetermined duration, such as a duration in the range of 5 to 10 minutes as an illustrative example.

[0044] A user of the apparatus 100 can tailor the predetermined flow rate, pulse frequency, and intermittency based on specific operational needs and the type of wastewater being treated. The apparatus allows for the selection of a flow rate that ensures the fluid ejection strength is sufficient to maintain the desired biofilm thickness. The pulse frequency of fluid ejection can be adjusted to provide flexibility to achieve the necessary shearing efficacy for different wastewater compositions. For intermittent cleaning, the user can set the duration of cleaning and non-cleaning periods to optimise the balance between biofilm maintenance and energy consumption. This customisation ensures that the bioreactor operates efficiently, preventing excessive biofilm buildup and clogging while adapting to the varying characteristics different wastewater types, such as municipal wastewater, industrial effluents, or agricultural runoff.

[0045] The sheared-off biofilm is suspended in the bioreactor 3 and can flow with the effluent liquid to the outlet fluid conduit 2. Preferably, the sheared off biofilm is caught in the sieve (filter) 5. The sieve (filter) can then be periodically removed and cleaned.

[0046] The apparatus 100 can be arranged for aerobic, anaerobic and anoxic purification of municipal and industrial wastewater, processing water, water from aquaculture installations and drinking water. With reference to Figure 2, the invention further comprises a method 200 for biological treatment (purification) of water. At step 102, wastewater (water containing contaminants to be removed) is led (transported) into a bioreactor through an inlet fluid conduit. At step 104, the wastewater (water containing contaminants) is led (transported) through a densely packed mass of carrier elements for biofilm growth, wherein the degree of filling for carrier elements constitutes an amount corresponding to 70% or more of the bioreactor volume. This ensures that the carrier elements stay approximately stationary or are prevented from moving throughout execution of the method 200. In a particular example wherein an inlet pipe is at a bottom of the bioreactor and an outlet pipe at a top of the bioreactor, wastewater passes through the mass of carrier elements (substratum) from the bottom to the surface of the bioreactor in an upwards plug flow pattern. In another example wherein the inlet pipe is at a top of the bioreactor and the outlet pipe is at a bottom of the bioreactor, the wastewater passes through the mass of carrier elements from the top to the bottom of the bioreactor in a downward plug flow pattern. Yet further, the wastewater may flow from side to side through the mass of carrier elements from an inlet in the side wall of the bioreactor to an outlet in an adjacent side wall of the bioreactor. Other configurations are possible which give rise to a wastewater plug flow through at least a majority of the mass of carrier elements. Microorganisms grow on the carrier elements, forming biofilm by consuming contaminants in the wastewater, thereby purifying the wastewater. Depending on the type of wastewater and needs of organism cultivation, aeration in the form of air or oxygen can be supplied into the bioreactor, such as from the bottom of the bioreactor. This aeration may be supplied by the cleaning unit described above, or by a separate aeration device. At step 106, the treated water is led to the outlet.

[0047] Steps 102 to 106 are repeated on a continuous basis 108 during normal operation of the biological purification of the wastewater. As demonstrated by step 110, cleaning is carried out during normal operation of the biological purification method, i.e., steps 102 through 106. In particular, gas and / or liquid jets, such as those described with reference to figure 1a and 1 b, are installed in the bioreactor to supply gas- and / or liquid flow. In an example, the gas is air. In an example, the liquid is water. Such gas- and / or liquid flow will pass through the mass of carrier elements and shear off extra (undesired) biofilm(biomass) on the mass of carrier elements. The fluid flow enhances detachment of extra biofilm formed or accumulated in the mass of carrier elements and maintains a thinner layer of biofilm than would accumulate in the absence of the cleaning. According to the method 200, the cleaning can be continuous cleaning or intermittent cleaning.

[0048] Continuous cleaning can be defined as expulsion of fluid from jets in a constant flow or pulsed manner throughout the entire duration of steps 102 to 106. Whereas intermittent cleaning can be defined as periods of fluid expulsion in a constant flow or pulsed manner interrupted by periods of no fluid expulsion. The periods of fluid expulsion and periods of no fluid expulsion may be an order of magnitude longer than the pulse duration. For example, pulse frequency may be in the range of approximately 1 to 60 times per minute, whereas intermittency may be in the range of approximately every 5 to 10 minutes, as an illustrative example.

[0049] In both the continuous cleaning and the intermittent cleaning, an optional step 111 may be taken wherein the flow rate and pulse frequency of the fluid expulsion is selected. Selection of the flow rate and pulse frequency may be selected based on a predetermined desired thickness of the biofilm layer on the carrier elements. In some examples, the predetermined desired thickness of the biofilm layer is 1 mm or less. A biofilm layer thickness of 1 mm or less may require a pulse frequency of fluid ejection in the range of approximately 1 to 60 times per minute. In some examples, the fluid flow rate is in the range of approximately 1 m3 / / i to 4000 m3 / h.

[0050] In intermittent cleaning, an optional step 112 may be taken wherein the duration of intermittence is selected. The duration of intermittence is the period duration of fluid expulsion and period duration of no fluid expulsion. In some examples, the duration of intermittence is an order of magnitude longer than the pulse duration of the cleaning. In some examples, duration of intermittence is in the range of approximately every 5 to 10 minutes. Throughout normal operation, including during continuous or intermittent cleaning, a constant liquid volume in the reactor is maintained, as demonstrated by step 116. This has the effect that a constant degree of filling of carrier elements is also maintained throughout normal operation, including during continuous or intermittent cleaning.

[0051] The sheared-off organisms will be suspended in the bioreactor and flow with the liquid to the bioreactor outlet.

[0052] The method 200 can be carried out by the apparatus 100. All features described with respect to the apparatus 100 can be combined with the method 200 except where clearly mutually exclusive.

[0053] In the previously known continuous fluid biofilm bioreactor, the water level in the tank varies during the cleaning and non-cleaning phases of operation. Increasing the water level reduces the filling ratios and leads to more forces in the tank to dislodge excess biofilm mass from the carrier elements. However, increasing the water level leads to hydraulic differences between operational phases of the apparatus, which may be undesirable. The apparatus and method of the present invention allow a constant water level in the tank throughout its entire operation, in all states and phases. The cleaning unit effectively washes the carrier elements even at a degree of carrier elements filling around 75% to 90% and avoids use of different hydraulic levels.

[0054] The apparatus described herein uses a high microbial density leading to efficient degradation of organic matter and pollutants. The biofilms-based apparatus and method are more resilient to fluctuations in wastewater composition, temperature changes, and toxic shocks compared to other water treatment systems such as suspended growth systems. The apparatus and method of the invention generally produces less excess sludge, reducing the need for sludge handling and disposal. Furthermore, the apparatus of the present invention requires less space than conventional activated sludge systems, MBBR and the previously known continuous fluid biofilm bioreactor, making it suitable for locations with limited space. This results in a more compact and less energy demanding installation.

[0055] Having described preferred examples of the invention it will be apparent to those skilled in the art that other embodiments incorporating the invention may be used.

Claims

CLAIMS1 . A method for biological treatment of water, comprising: a) transporting wastewater into a bioreactor through an inlet fluid conduit; b) transporting the wastewater, in a plug flow pattern, through a mass of carrier elements, said mass of carrier elements having a constant degree of filling of 70% or more of a volume of the bioreactor; wherein the constant degree of filling of the carrier elements restricts movement of the carrier elements, urging the wastewater to move through the reactor in the plug flow pattern; c) treating the wastewater as the wastewater is transported through the carrier elements, the carrier elements having a biofilm layer formed thereon; d) transporting treated water to an outlet fluid conduit; e) repeating steps a) to d) on a continuous basis during an operational state of the bioreactor; f) during steps a) to e), cleaning the carrier elements by ejecting fluid at the mass of carrier elements from a cleaning unit to shear off biofilm from the carrier elements; wherein the sheared off biofilm is suspended in the bioreactor and flows to the outlet fluid conduit with the transported treated water; and wherein a constant liquid volume is maintained in the bioreactor throughout steps a) to f).

2. The method of claim 1 , wherein the constant degree of filling of said mass of carrier elements is in the range of 75% to 90% of the reactor volume.

3. The method of claim 1 or 2, wherein ejecting fluid from the cleaning unit comprises releasing bubbles upwards, in an in-use orientation, through the bioreactor from the cleaning unit.

4. The method of any of claims 1 to 3, further comprising ejecting fluid from the cleaning unit at a predetermined pulse frequency and flow rate.

5. The method of claim 4, further comprising selecting a pulse frequency and flow rate to achieve a shearing efficacy of the carrier elements suitable for maintaining a layer of biofilm at 1 mm or less on the carrier elements.

6. A biological wastewater treatment apparatus for performing the method of any of claims 1 to 5, comprising: a bioreactor containing: a constant liquid volume; and a mass of carrier elements, said mass of carrier elements having a constant degree of filling of 70% or more of a volume of the bioreactor; and a cleaning unit configured to eject fluid at the mass of carrier elements during wastewater treatment; an inlet fluid conduit fluidly connected to the bioreactor for continuous inflow of wastewater into the bioreactor; and an outlet fluid conduit fluidly connected to the bioreactor for continuous outflow of treated water or treated water comprising sheared biomass.

7. The apparatus of claim 6, wherein the outlet fluid conduit consists of a single outlet pipe.

8. The apparatus of claim 6 or claim 7, wherein the degree of filling for the mass of carrier elements is in the range of 75% to 90% of the reactor volume.

9. The apparatus of any of claims 6 to 8, wherein the cleaning unit is configured to eject fluid at the mass of carrier elements at a predetermined pulse frequency and flow rate.

10. The apparatus of claim 9, wherein the cleaning unit is configured to eject a fluid at the mass of carrier elements at a predetermined frequency and flow rate suitable for providing a predetermined shearing efficacy of the carrier elements, said shearing efficacy for maintaining a layer of biofilm at 1 mm or less.11 . The apparatus of any of claims 6 to 10, wherein the cleaning unit is an array of liquid jets.

12. The apparatus of any of claims 6 to 10, wherein the cleaning unit is an array of gas jets.

13. The apparatus of any of claims 6 to 12, wherein the cleaning unit is configured to release bubbles upwards through the bioreactor in an in-use orientation.

14. The apparatus of any of claims 6 to 13, further comprising a sieve aligned with the outlet fluid conduit, wherein the cleaning unit facilitates cleaning of the sieve.

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