Method for water purification and water treatment

Abstract

The present invention relates to a method and apparatus and the use of this method and apparatus for water purification and water treatment for converting biological waste into dischargeable water.

Description

[0001] METHOD FOR WATER PURIFICATION AND WATER TREATMENT

[0002] TECHNICAL FIELD

[0003] The invention relates to a method and an apparatus and the use of the aforementioned method and apparatus for water purification and water treatment for converting biological waste into dischargeable water.

[0004] PRIOR ART

[0005] Purification of water is often performed by biological processes with activated sludge, often preceded by a pre-settling step in a settling basin and followed by a postsettling step in a second settling basin.

[0006] In recent years, membrane bioreactor (MBR) processes have been implemented in many small and large wastewater treatment plants. These processes offer many advantages over conventional activated sludge processes, including a smaller footprint, no longer requiring settling basins, consistent effluent of higher quality that is suitable for reverse osmosis (RO) or nanofiltration, and improved biological processes that allow for better removal of nutrients.

[0007] CN106007201A describes, for example, a wastewater treatment technology wherein the aeration takes place by means of biological oxidation in a filter bed (by means of a so-called "Biological Aerated Filter").

[0008] US2023234871 describes a system for the treatment of wastewater originating from the production of printed circuit boards wherein after one or more settling steps the water is further treated by means of membrane bioreactor (MBR) processes.

[0009] The degradation of biological waste - such as manure, energy crops, vegetable waste, agriculture-related waste streams and secondary raw materials into dischargeable water ensures a circular economy wherein raw materials and materials are constantly reused or recycled.

[0010] Converting biological waste into dischargeable water, however, requires a more intensive process than the purification of wastewater.The degree of membrane fouling that occurs during treatment of biowaste is namely greater than during the processing of wastewater, inter alia due to a higher total suspended solids (TSS) in the biowaste. Membrane fouling and scaling in pressure-driven membrane processes are often a major problem because they can increase the operating and maintenance costs of the systems.

[0011] The present invention aims to find a solution for at least some of the above problems.

[0012] SUMMARY OF THE INVENTION

[0013] In a first aspect, the invention relates to a method for purifying and treating biological waste into dischargeable water. More particularly, the invention relates to a method comprising the following steps:

[0014] a. Digesting the biological waste into a digestate;

[0015] b. Separating the digestate into a thin fraction and a thick fraction, wherein the thin fraction has a dry matter content of less than 4%;

[0016] c. Aerating the thin fraction into a biological effluent with a nitrogen content lower than 2,000 mg / liter(L);

[0017] d. Lowering the total suspended solids (TSS) in the biological effluent from step c to a content lower than 40,000 mg / L, preferably lower than 35,000 mg / L, by means of one or more screening steps optionally combined with a flotation technique such as Dissolved Air Flotation (DAF);

[0018] e. Treating the purified biological effluent from step d in a cross-flow ultrafiltration (UF) unit, wherein the ultrafiltration is characterized by a filtration step and a backwashing step;

[0019] f. Treating the resulting permeate from step e in a reverse osmosis (reverse osmosis, RO) unit, wherein the resulting RO permeate is a dischargeable water characterized by:

[0020] i. a chemical oxygen demand (COD) value lower than 125 mg / L, ii. a biochemical oxygen demand (BOD205) value lower than 25 mg / L,

[0021] iii. a total nitrogen content (Ntotai) lower than 15 mg / L,

[0022] iv. a total phosphate content (Ptotai) lower than 2 mg / L and v. a suspended solids content lower than 10 mg / L.

[0023] The method of the present invention combines a suitable water purification with a subsequent water treatment and in this way enables a very efficient operation of theUF membranes and the RO membranes, wherein the advantages of a longer lifespan of the membranes and less energy are utilized. Moreover, in the present invention, use is made of a cross-flow ultrafiltration (UF) unit and the process flow of the ultrafiltration is characterized by a filtration step and a backwashing step, whereby the membrane fouling is also minimized.

[0024] In a second aspect, the invention relates to an apparatus for purifying and treating biological waste into dischargeable water according to claim 11.

[0025] In a final aspect, the invention relates to the use of the aforementioned method or the aforementioned apparatus for purifying and treating biological waste into dischargeable water.

[0026] DESCRIPTION OF THE FIGURES

[0027] Figures 1-3 show flow diagrams of methods for purifying and treating biological waste into dischargeable water according to embodiments of the present invention.

[0028] DETAILED DESCRIPTION

[0029] The invention relates to a method and an apparatus and the use of the aforementioned method and apparatus for water purification and water treatment for converting biological waste into dischargeable water. The present invention combines a suitable water purification with a subsequent water treatment and in this way enables a very efficient operation of the UF membranes and the RO membranes, wherein the advantages of a longer lifespan of the membranes and less energy are utilized. Converting biological waste into dischargeable water namely requires a more intensive process than the purification of wastewater and this due to the higher total suspended solids (TSS) in the biowaste. By means of a suitable water purification, inter alia by lowering the total suspended solids (TSS) in the biological effluent by means of one or more screening steps and optionally a flotation technique such as Dissolved Air Flotation (DAF), the membrane fouling and scaling in the subsequent water treatment can be reduced. Moreover, in the present invention, use is made of a cross-flow ultrafiltration (UF) unit and the process flow of the ultrafiltration is characterized by a filtration step and a backwashing step, whereby the membrane fouling is also minimized. As a result, the operating and maintenance costs can be minimized.Unless otherwise defined, all terms used in the description of the invention, including technical and scientific terms, have the meaning as commonly understood by a person skilled in the art to which the invention pertains. For a better understanding of the description of the invention, the following terms are explained explicitly.

[0030] "A", "an" and "the" refer in this document to both the singular and the plural unless the context clearly implies otherwise. For example, "one segment" means one or more segments.

[0031] When "around" or "about" is used in this document with respect to a measurable quantity, a parameter, a duration or point in time, and the like, this is intended to mean variations within ±20%, preferably within ±10%, more preferably within ±5%, even more preferably within ±1%, and still more preferably within ±0.1%, of the cited value, insofar as such variations are applicable to the invention described herein. However, it must be understood that the value of a quantity used where the term "about" or "around" is used, is itself specifically disclosed.

[0032] The terms "comprise", "comprising", "consist of", "consisting of", "provided with", "contain", "containing", "encompass", "encompassing", "include" and "including" are used herein as synonyms and are intended as inclusive or open terms indicating the presence of what follows, without excluding or preventing the presence of other components, characteristics, elements, members or steps known from or disclosed in the prior art.

[0033] The term "sludge" as used herein is a collective term for settleable substances that are separated during the purification of wastewater. This can occur as primary sludge during direct settling of wastewater or as surplus sludge originating from biological treatment processes. The sludge is characterized by a large fraction of organic material (approx. 50-80% of the total solids).

[0034] The term "biological waste" as used herein is a collective term for organic material of natural origin that is generated as a by-product or residual stream from industrial, agricultural, or natural processes. It consists mainly of biodegradable substances such as food residues, vegetable material and other organic components that can be subjected to biological or physicochemical treatments to degrade, stabilize and convert it into reusable or environmentally friendly outputs, such as dischargeable water.Quoting numeric intervals by the endpoints includes all integers, fractions, and / or real numbers between the endpoints, including those endpoints.

[0035] Detailed

[0036]

[0037] The degradation of biological waste - such as manure, energy crops, vegetable waste, agriculture-related waste streams and secondary raw materials into dischargeable water ensures a circular economy wherein raw materials and materials are constantly reused or recycled.

[0038] Converting biological waste into dischargeable water, however, requires a more intensive process than the purification of wastewater.

[0039] The degree of membrane fouling that occurs during treatment of biowaste is namely greater than during the processing of wastewater, inter alia due to a higher total suspended solids (TSS) in the biowaste. Membrane fouling and scaling in pressure-driven membrane processes are often a major problem because they can increase the operating and maintenance costs of the systems.

[0040] In a first aspect, the invention relates to a method for purifying biological waste into dischargeable water, the method comprising the following steps:

[0041] a. Digesting the biological waste into a digestate;

[0042] b. Separating the digestate into a thin fraction and a thick fraction, wherein the thin fraction has a dry matter content less than 4%; c. Aerating the thin fraction to a biological effluent with a nitrogen content lower than 2000 mg / liter (L);

[0043] d. Lowering the total suspended solids (TSS) in the biological effluent from step c to a content lower than 40,000 mg / L, preferably lower than 35,000 mg / L, by means of one or more screening steps optionally combined with a flotation technique such as Dissolved Air Flotation (DAF);

[0044] e. Treating the purified biological effluent from step d in a cross-flow ultrafiltration (UF) unit, wherein the ultrafiltration is characterized by a filtration step and a backwashing step;

[0045] f. Treating the resulting permeate from step e in a reverse osmosis (reverse osmosis, RO) unit, wherein the resulting RO permeate is a dischargeable water characterized by:

[0046] i. a chemical oxygen demand (COD) value lower than 125 mg / L,ii. a biochemical oxygen demand (BOD205) value lower than 25 mg / L,

[0047] iii. a total nitrogen content (Ntotai) lower than 15 mg / L,

[0048] iv. a total phosphate content (Ptotai) lower than 2 mg / L and v. a suspended solids content lower than 10 mg / L.

[0049] For filtration with UF membranes, the influent water generally has to be purified to remove particles larger than 1 mm, and particles with a high density must also be removed to minimize wear to the active separation layers. The TSS content may amount to a maximum of 40,000 mg / liter. It is also important that the nitrogen content is lower than 2,000 mg / liter. In the present invention, we disclose a combination of a (centrifugal) separator (step b), a denitrification step (the aerating in step c), followed by screening and optionally a flotation technique such as DAF, which delivers the desired water quality before the UF phase. In an alternative embodiment, the screening takes place before the denitrification step (the aerating in step c).

[0050] Moreover, in the present invention, use is made of a cross-flow ultrafiltration (UF) unit and the process flow of the ultrafiltration is characterized by a filtration step and a backwashing step, whereby the membrane fouling is also minimized. As a result, the operating and maintenance costs can be minimized.

[0051] In a second aspect, the invention also comprises an apparatus for purifying and treating biological waste into dischargeable water, the apparatus comprising:

[0052] a. One or more digesters for digesting the biological waste into a digestate;

[0053] b. One or more separators for separating the digestate into a thin fraction and a thick fraction;

[0054] c. One or more aerators for aerating the thin fraction into a biological effluent;

[0055] d. One or more screens and optionally a Dissolved Air Flotation (DAF) apparatus for lowering the total suspended solids (TSS) in the biological effluent;

[0056] f. A cross-flow ultrafiltration (UF) unit for treating the purified biological effluent;

[0057] g. A reverse osmosis (reverse osmosis, RO) unit for desalinating the resulting UF permeate into a dischargeable water.In a final aspect, the invention relates to the use of the aforementioned method or the aforementioned apparatus for purifying and treating biological waste into dischargeable water, wherein the dischargeable water is characterized by:

[0058] a. a chemical oxygen demand (COD) value lower than 125 mg / L, b. a biochemical oxygen demand (BOD205) value lower than 25 mg / L, c. a total nitrogen content (Ntotai) lower than 15 mg / L,

[0059] d. a total phosphate content (Ptotai) lower than 2 mg / L and

[0060] e. a total suspended solids (TSS) lower than 10 mg / L.

[0061] In a first step, the biological waste is digested in one or more digestion tanks or digesters into a digestate. The digestion tank comprises a digestate outlet. Digestate is a residual product that remains in the digestion tank after the biomass has been decomposed. Digestate is liquid. Digestate of biological waste, however, still contains a high dry matter content (typically 11-14%). The digestate outlet comprises an outlet tube. The digestate outlet preferably comprises a valve.

[0062] Subsequently, the digestate is separated into a thin fraction and a thick fraction, wherein the thin fraction has a dry matter content lower than 4%.

[0063] Separating the digestate into a thin and thick fraction is an essential step. It is namely crucial to obtain a fraction with a limited dry matter content (called "the thin fraction") which can be further purified and treated into dischargeable water. When the dry matter content of the thin fraction is too high, the subsequent steps in the water purification and water treatment process proceed with difficulty or no longer at all. Optimally separating the digestate into a thin fraction with a dry matter content lower than 4% and a thick fraction is therefore an important step.

[0064] In an embodiment, the thin fraction has a dry matter content between 3.5%-4.0%, 3.0%-3.5%, 2.5%-3.0%, 2.0%-2.5%, 1.5%-2.0%, 1.0%-1.5%, 0.5%-1.0% or 0.0%-0.5%.

[0065] In the context of this document, dry matter content is expressed as a ratio of the mass of the dry matter in a known mass of the suspension. This is represented as a dimensionless number or as a percentage. Alternatively, the dry matter content can also be expressed as a mass of dry matter in kg in a volume of 1 m3of the solution. The mass of dry matter is determined by taking a sample with a known volume, drying the sample until all liquid has completely evaporated and only dry matterremains, weighing the mass of dry matter and recalculating the mass of dry matter in the known volume of the solution to a volume of 1 m3of the solution.

[0066] After digestion of biological waste, a digestate is usually obtained with a dry matter content between 11-14%. Processing biological waste therefore results in a waste stream with a significantly higher dry matter content compared to a waste stream originating from polluted surface water.

[0067] The use of a high-performance separation technique for separating the digestate into a thin and a thick fraction is therefore of greater importance in purifying and treating biological waste than in processing polluted surface water.

[0068] The separator is, for example, a centrifuge, a belt press filter, a screw press or other suitable means. It will be apparent to one skilled in the art that a combination of several of said examples also forms part of the present invention.

[0069] In a preferred embodiment, the method comprises a centrifugal separation step.

[0070] The use of a separator, preferably a centrifuge, has clear technical, economic and operational advantages compared to a separation step by means of gravitational sedimentation (settling). By using a centrifuge instead of sedimentation, a rapid, controlled and compact separation of the digestate into a thin fraction is obtained with significantly fewer suspended solids than when, for example, the separation takes place by means of sedimentation. The use of a separator (preferably a centrifuge) results in less clogging in the subsequent aeration and membrane filtration steps, lower maintenance frequency and higher total process yields. Moreover, a centrifuge requires considerably less space and offers a continuous and automatically controllable process, which leads to higher operational reliability and lower operational costs compared to, for example, conventional sedimentation.

[0071] The digestate is preferably centrifugally separated into a thin fraction and a thick fraction, wherein the thin fraction has a dry matter content lower than 4%.

[0072] In an embodiment, the centrifugal separation step has a flow rate between 12-25 m3 / hour, preferably between 15-20 m3 / hour. In an embodiment, the centrifugal separation step is performed at a rotational speed between 2,000 and 3,000 revolutions / minute, preferably between 2,000-2700 revolutions / minute.In a preferred embodiment, the thick-to-thin fraction ratio at the end of step b is between 35%-65%.

[0073] Optionally, the separator comprises means configured for adding a polymer mixture (a flocculant) to the digestate. This is advantageous for obtaining a better separation into a thin fraction and a thick fraction.

[0074] In an embodiment, the digestate is injected into a centrifuge together with a polymer mixture. The polymer mixture causes flocculation of the solid particles in the digestate. Due to the centrifugal forces during the centrifugal separation step, the liquid is separated from the solid particles which cake together inside the drum of the centrifuge.

[0075] This embodiment is advantageous in that hereby the dry matter content in the thick fraction can be increased relative to the dry matter content in the digestate, whereby the thick fraction can be used as fertilizer and whereby it is in itself a valuable product. It can optionally be spread locally as fertilizer on a field or sold as a valuable raw material to, for example, a fertilizer producer. Because the thin fraction can be removed from the digestate, transport costs for transporting the thick fraction to, for example, a producer of fertilizers will be lower than the transport costs for transporting the digestate, whereby the first thick fraction is more valuable as raw material than the digestate.

[0076] The choice of polymer is dependent on the specific composition of the biological waste and the resulting digestate (in particular, the content of nitrogen, phosphorus, potassium, dry matter, organic matter, etc.).

[0077] In an embodiment, the polymer mixture is prepared by dissolving a powder polymer in a liquid. In an embodiment, a maximum of 5 grams, such as between 3 and 4 grams, of polymer is dissolved per liter of liquid. At more than 5 grams of powder polymer per liter of liquid, the polymer no longer mixes and becomes too viscous, thus difficult to pump. At an even higher dosage of the powder polymer, clumping occurs.

[0078] In an embodiment, between 4-12%, preferably between 6-8% polymer mixture is added to the digestate (m / m). In an embodiment, a polymer mixture is added in anamount of 5%, for example 1 m3of polymer per hour is added to 20 m3of digestate per hour.

[0079] In a preferred embodiment, phosphate remains mainly in the thick fraction, preferably at least 75%, such as for example between 75-80%, of the phosphate content that is present in the digestate remains in the thick fraction (taking the phosphate content in the digestate as 100%). The thin fraction therefore preferably has a phosphate content that amounts to a maximum of 25% of the phosphate content present in the digestate (taking the phosphate content in the digestate as 100%).

[0080] In an embodiment, after separating the digestate into a thin fraction and a thick fraction, one or more screening steps take place on the resulting thin fraction before the aeration step takes place. Such a screening step can be performed by means of any screening technique known from the prior art. In an embodiment, use is made of a screening step by means of a screen having perforations with a diameter of 0.7-1.5 mm, for example perforations with a diameter of approximately 1 mm. In a preferred embodiment, use is made of a rotary screening device, for example a rotary drum screen. Preferably, the rotary drum screen is an internally fed drum screen with perforations of 0.8-1.2 mm, for example 1 mm.

[0081] In an embodiment, the thin fraction is temporarily stored in a storage tank before the aeration step takes place. In a further embodiment, the centrifuge is in connection with the storage tank by means of transport lines.

[0082] In a subsequent step, the thin fraction is aerated to a biological effluent with a nitrogen content lower than 2,000 mg / liter (L), such as for example lower than 1,950 mg / liter, lower than 1,900 mg / liter, lower than 1,850 mg / liter or lower than 1,800 mg / liter. In an embodiment, the thin fraction is aerated to a biological effluent with a nitrogen content between 0-100 mg / L, 100-200 mg / L, 200-300 mg / L, 300-400 mg / L, 400-500 mg / L, 500-600 mg / L, 600-700 mg / L, 700-800 mg / L, 800-900 mg / L, 900-1,000 mg / L, 1,000-1,100 mg / L, 1,100-1,200 mg / L, 1,200-1,300 mg / L, 1,300-1,400 mg / L, 1,400-1,500 mg / L, 1,500-1,600 mg / L, 1,600-1,700 mg / L, 1,700-1,800 mg / L, 1,800-1,900 mg / L or between 1,900-2,000 mg / L.

[0083] Aeration tanks are optimized for purifying and treating large quantities of biological waste. The use of one or more aeration tanks makes it possible to treat a higher flowrate of waste and / or wastewater. In an embodiment, the flow rate of this aeration step amounts to at least 100 tons / 24 hours, preferably at least 200 tons / 24 hours, more preferably at least 300 tons / 24 hours, for example between 350-450 tons / 24 hours.

[0084] CN106007201A describes a wastewater treatment technology comprising an aeration step by means of a Biological Aerated Filter (BAF) process. In contrast to a method wherein the aeration step takes place by means of an aeration tank, in an aeration step making use of biological oxidation with a filter bed, the flow rate is limited by the risk of hydraulic short-circuiting or clogging of the filter bed. Such a filter-based system, moreover, works most efficiently at a low residual COD (e.g., < 150 mg / L). High loads clog the filter bed and cause oxygen depletion. An aeration step by means of biological oxidation in a filter bed is thus only suitable as a post-treatment, but impractical as a main biological step for raw or heavily loaded wastewater. In the current invention, where biological waste is used as starting material and high flow rates are required, an activated sludge process using an aeration tank is clearly more robust and scalable.

[0085] This aeration preferably takes place in one or more aeration tanks.

[0086] The aeration tank comprises an inlet configured for feeding thin fraction into the aeration tank. The aeration tank comprises an aeration system, such as, for example, but not limited to aeration discs, tube aerators and / or immersion aerators. The aeration system is configured to create fine air bubbles in the thin fraction. An aeration tank is advantageous for removing nitrogen from the first thin fraction.

[0087] Phosphate and nitrate are inorganic nutrients that play an important role in aquatic ecosystems, but their excessive presence can lead to serious environmental problems. These substances influence the oxygen content in the water because they contribute to the growth of algae and other aquatic plants. This process, known as eutrophication, can lead to oxygen shortages and suffocation of aquatic organisms, with far-reaching consequences for biodiversity and water quality.

[0088] In the dischargeable water obtained according to the present invention, the concentrations of these nutrients are strictly limited. The total nitrogen content (Ntotai) is lower than 15 mg / L, which contributes significantly to preventing excessive algal growth and the associated negative effects on the aquatic environment. In addition, the total phosphate content (Ptotai) is limited to less than 2 mg / L, whereby the risksof eutrophication are further minimized. These low levels ensure that the water complies with strict environmental standards and contributes to a sustainable and environmentally friendly discharge which maintains the ecological balance in water bodies.

[0089] In the aeration tank, the wastewater is mixed with previously formed activated sludge (a slimy raw material in which bacteria and protozoa live) and oxygen is supplied by means of aeration equipment. Under these conditions, the activated sludge can wholly or partially remove the organic and other contaminants from the wastewater.

[0090] In the activated sludge flocs, the living bacterial cells ensure the uptake and degradation of the contaminants, which ultimately lead to the (partial) purification of the wastewater. From the wastewater, small organic molecules (with fewer than 8 to 10 C atoms) can be taken up directly via the cell wall into the bacterial cell. The larger fragments must first be split by enzymes into smaller molecules, which can pass the cell wall.

[0091] Aerating activated sludge and wastewater in an aeration space has two functions: - introducing the (air-)oxygen required for purifying the wastewater

[0092] - ensuring sufficient turbulence (velocity) so that the sludge remains in good contact with the wastewater.

[0093] In a preferred embodiment, the aeration takes place via surface aeration. Herein, the aeration takes place by mechanical forces exerted on the liquid by horizontal rotors or vertical turbine or point aerators. The oxygen is introduced into the liquid by: the movement of the liquid surface, the air bubbles entrained in the liquid, the sprayed liquid and the air-liquid mixture at the location of the aerator, where the air is beaten into the liquid.

[0094] The amount of oxygen that is introduced into the liquid is influenced by the diameter of the rotating element, the rotational speed, the immersion depth and the shape and the placement of the teeth or blades. In addition to introducing oxygen, the aerators must generate circulation flows in the aeration zone in order to prevent settling of the activated sludge. Shape, dimension and volume of the aeration space must be such in relation to the aerator, that upon introducing the required amount of oxygen the circulation is sufficient.In the activated sludge process, it is possible to convert nitrogen biologically. Initially, this occurs via nitrification, or the oxidative conversion to nitrate; subsequently, the nitrate can be further reduced to nitrogen (denitrification). Via the nitrification / denitrification, the nitrogen in the thin fraction is converted. During the nitrification, bacteria convert, in the presence of oxygen, ammonia (NH3) to nitrate (NOT). During denitrification, nitrate is in the absence of oxygen in turn converted to the harmless nitrogen gas (N2).

[0095] The conversion of ammonium to nitrate proceeds in two steps with nitrite as intermediate product; the first step is the slowest and is brought about by the bacterial species Nitrosomonas. These bacteria grow slowly and will only be present in the sludge if the sludge age is sufficiently high (and the sludge load sufficiently low); in addition, they grow more slowly in winter than in summer. In addition, the oxygen concentration in the activated sludge must also be sufficiently high, at least 0.5 - 1.0 mg O2 / L.

[0096] A large number of bacterial species is capable, at a very low content of dissolved oxygen (in an anoxic environment), of bringing about an oxidative reaction to use the oxygen present in nitrate or nitrite for their own respiration processes. These bacteria are thus capable of switching from free dissolved (air-)oxygen to nitrate oxygen. The nitrate is thus reduced to nitrogen gas (N2) in the absence of dissolved oxygen and in the presence of organic matter (oxygen consumption). The nitrogen gas thus formed leaves the liquid in the form of bubbles. Organic matter, which leads to oxygen consumption, functions as a driving force for the occurrence of denitrification. Thus, oxygen-poor (anoxic) conditions and the presence of easily degradable organic material are necessary for this process.

[0097] In an embodiment, the aeration tank comprises two compartments. In a first compartment, there is aeration. The first compartment is suitable for nitrification. In a second compartment, there is no aeration. The second compartment is suitable for denitrification. Alternatively, the aeration tank comprises a single compartment and the aeration system comprises a timed control, suitable for timed switching on or off of the aeration system. This is advantageous for nitrification during switched on aeration and denitrification during switched off aeration in a single compartment.In an embodiment, the sludge / water mixture is pumped over thickeners after the nitrification-denitrification step in the one or more aeration tanks to cause a part of the water to evaporate.

[0098] As described above, the digestate from the digesters is separated on a centrifuge into a thick fraction and thin fraction. The thick fraction is discharged and the thin fraction is biologically purified. However, the separation on a centrifuge is not absolute; coarser, suspended particles such as organic fibers, pieces of plastic and other coarser particles still remain behind in the biological effluent. These particles can seriously foul the UF membranes and must therefore be removed before the UF.

[0099] The substances occur in widely varying dimensions in the biological effluent. Thus, there are visible particles or undissolved substances (also called suspended solids) at a dimension of 0.1 pm and larger. Examples are, for example, sand, sludge, clay, organic waste and micro-organisms such as algae. The total suspended solids content is represented by the TSS value (TSS="total suspended solids"). Substances with a particle size between 1 and 100 nm are called colloidal substances. Dissolved substances have dimensions of 1 nm or smaller. The total content of dissolved and undissolved substances is determined by evaporating a sample and subsequently drying the remainder. By means of filtration, the undissolved substances can be separated from the dissolved substances; from the material remaining on the filter, after drying and weighing, the filtration residue in g / L or mg / L (=undissolved substances) can be determined. The content of dissolved substances follows from the filtrate after evaporation.

[0100] It is important to reduce the total suspended solids (TSS) in the biological effluent from step c since these cause considerable membrane fouling and scaling in the pressure-driven membrane processes of the subsequent water treatment. The present invention provides a solution for this by implementing one or more screening steps, optionally combined with a flotation technique such as Dissolved Air Flotation (DAF), wherein suspended particles are removed from the biological effluent until a TSS content lower than 40,000 mg / liter is obtained.

[0101] To remove these coarser suspended particles, in the present invention the biological effluent is thus screened over a screen.As described above, in an embodiment one or more screening steps take place before the aeration takes place, wherein the screening steps occur on the thin fraction.

[0102] In an embodiment, step d of the method of the present invention comprises reducing the total suspended solids (TSS) in the thin fraction from step b and / or in the biological effluent from step c to a content lower than 40,000 mg / L by means of one or more screening steps optionally combined with a flotation technique such as Dissolved Air Flotation (DAF). In such an embodiment, the one or more screening steps can thus take place before the aeration (on the thin fraction) and / or after the aeration (on the biological effluent).

[0103] In an embodiment, the one or more screening steps take place only before the aeration takes place, wherein the screening steps thus occur on the thin fraction.

[0104] In an embodiment, the one or more screening steps take place only after the aeration has taken place, wherein the screening steps thus occur on the biological effluent. In an embodiment, the one or more screening steps after the aeration take place only on the part "biological effluent" that goes to the DAF unit.

[0105] In an embodiment, the screening steps take place both before the aeration has taken place and after the aeration has taken place.

[0106] Screens are applied as a fine screen in the purification of wastewater. There are different types in use: one of these is the drum screen.

[0107] A drum screen consists of a slowly rotating drum which is provided with fine perforations. The drum is driven by an electric motor via a reduction gearbox. The biological effluent to be treated is fed inside the drum and discharged to the outside through the perforated drum wall. The screened-out particles remain behind in the drum and are moved to the end of the screen drum and ejected by the rotation of the drum and the internal screw. Compared with course screen installations, small particles can be removed from the water to an even greater extent in this way.

[0108] In a preferred embodiment, use is made of a rotary drum screen. Preferably, the rotary drum screen is an internally-fed drum screen with perforations of 0.8-1.0 mm. The biological effluent falls through the perforations and the coarser particles are screened out.In an embodiment, the purified biological effluent is transported to the UF unit after the one or more screening steps.

[0109] The aeration tank comprises a discharge, configured to discharge the obtained biological effluent from the aeration tank.

[0110] In another embodiment, namely when the content of suspended particles remains too high after screening, use is made of a flotation technique such as Dissolved Air Flotation (DAF). In a further embodiment, this DAF installation is preceded by a screening step by means of a screen. In a further preferred embodiment, use is likewise made here of a rotary drum screen. Preferably, the rotary drum screen is an internally-fed drum screen with perforations of 0.8-1.0 mm. The biological effluent falls through the perforations and the coarser particles are screened out.

[0111] A DAF flotation installation is based on the principle of accelerated flotation of contaminant particles by introduction of minuscule air bubbles which adhere to the contaminant particles (= Dissolved Air Flotation).

[0112] The DAF installation comprises a tank with a supply, wherein the supply is configured to supply the biological effluent from the one or more aeration tanks into the tank of the DAF installation. The DAF installation comprises means for supply of one or more coagulants and / or polymers (also called flocculants) to the biological effluent. The DAF installation optionally comprises a supply for a base or an acid. This is advantageous for pH correction of the biological effluent treated with coagulant and / or polymer. Preferably, the DAF installation comprises a mixer, configured to mix the biological effluent and the coagulant / polymer.

[0113] In an embodiment, the DAF installation is preceded by a double stirred flocculator, for example subdivided into two compartments: a coagulation compartment and a flocculation compartment. In a further embodiment, the water flows gravitationally from the coagulation compartment to the flocculation compartment and then finally to the tank of the DAF installation.

[0114] Non-limiting examples of coagulant are FeCH and Fe2(SC>4)3. Coagulant is advantageous for forming flocs by destabilization of colloidal particles in the biological effluent. The DAF installation optionally comprises a supply for flocculant (polymer). Flocculant is advantageous for further enlarging the flocs formed by coagulant.Flocculant is added together with coagulant or after addition of coagulant. The DAF installation comprises a pressure vessel. The pressure vessel is configured to contain biological effluent under a pressure of at least 3 bar, preferably at least 4 bar and more preferably at least 5 bar. The pressure vessel comprises means for introducing air into the pressure vessel. The DAF installation comprises a line from the pressure vessel to the tank.

[0115] In an embodiment, a centrifugal pump pumps purified water to the pressure vessel and provides an optimal pressure for dissolving air in water. In this pressure vessel, compressed air is also dosed in a controlled manner so that water optimally saturated with air is obtained. From this pressure vessel, this air-saturated water is led back to the tank. Due to the pressure drop (or depressurization) of this water (from 5-6 bar to 0 bar), the dissolved air is released again in the form of minuscule air bubbles (30-50 pm, the so-called whitewater) in the tank. These air bubbles adhere to the contaminant particles and cause them to float. In an embodiment, the wastewater is distributed over the entire width of the installation via a distribution system and whitewater is also injected at this inlet. The air bubbles attach to the flocs and form a sludge layer at the surface.

[0116] The tank comprises an outlet configured to discharge settled floc particles from the tank. The DAF installation comprises means configured for removing the floating flocs. A non-limiting example is a scraper configured for scraping floating floc particles from the tank. In an embodiment, this layer is removed by a scraper. In an embodiment, the sludge is collected in a sludge compartment and the sludge is then pumped back from the sludge compartment to the digester where the biological waste is digested into a digestate which is subsequently separated again into a thin fraction and a thick fraction, and so on.

[0117] Heavier particles such as sand will not float, but will settle. In an embodiment, the bottom of the DAF installation comprises pneumatically controlled valves which can be opened to be able to remove the possible sediment. In an embodiment, these valves are opened in a timer-controlled manner. Due to the pressure of the water in the flotation, the bottom sludge is pushed out.

[0118] In an embodiment, the feed of the DAF installation comprises an electromagnetic flow measurement and a controlled valve so as to also be able to regulate the dosages of coagulant (and flocculant) as a function of the flow rate.In an embodiment, the DAF installation has a flow rate between 10-30 m3 / hour, for example 20 m3 / hour.

[0119] In an embodiment, the floating elements are sent back to the one or more digestion tanks after separation. The tank comprises a discharge, configured to discharge liquid, wherein the liquid is substantially purified of flocs.

[0120] In an embodiment, after the flotation technique, the purified biological effluent is fed again to the one or more aeration tanks, before it is treated in the UF unit.

[0121] In an embodiment, after the flotation technique, the purified biological effluent is fed to a storage tank before it is treated in the UF unit.

[0122] For filtration with UF membranes, the influent water generally has to be purified to remove particles larger than 1 mm, and particles with a high density must also be removed to minimize wear to the active separation layers. The TSS content may amount to a maximum of 40,000 mg / liter and is preferably situated below 35,000 mg / liter. It is also important that the nitrogen content is lower than 2,000 mg / liter, preferably lower than 1,800 mg / liter. In the present invention, we disclose a combination of a (centrifugal) separator (step b), a denitrification step (the aerating in step c), followed by screening and optionally a flotation technique such as DAF, which delivers the desired water quality before the UF phase.

[0123] The ultrafiltration unit comprises filter membranes. The filter membranes are installed dry. The ultrafiltration unit comprises a pump, configured to pump the liquid under pressure from a first side to a second side through the filter membranes. The ultrafiltration unit comprises a discharge of liquid at the second side. An ultrafiltration unit is advantageous for removing remaining undissolved matter and / or macromolecules in a liquid, whereby semipermeable membranes in a reverse osmosis installation must be replaced or cleaned less often. The remaining undissolved matter and / or macromolecules that has been filtered by the ultrafiltration unit out of the liquid originating from the aeration tank, is upgraded sludge from the ultrafiltration unit. The sludge from the ultrafiltration unit is also advantageous as biomass in the one or more aeration tanks. The purification installation comprises lines for feeding the liquid from the discharge at the second side of the ultrafiltration unit to the reverse osmosis unit.The ultrafiltration membranes are available in two types of geometries: inside-out filtration and outside-in filtration. In a preferred embodiment, the present method makes use of inside-out filtration, wherein the UF permeate is further treated by the RO unit. Besides these two geometries, there exist different membrane materials, such as PVDF, PAN and PES, which require very divergent operational parameters, such as flux or flow rates, transmembrane pressures (TMPs), filtration direction (inside the lumen or outside the lumen) and flushing directions. The configurations can vary considerably and are strongly dependent on the design of the system.

[0124] In a preferred embodiment, the ultrafiltration (UF) unit comprises more than one membrane module. The filter membranes of the UF unit preferably have a pore size between 10 and 50 nm, preferably between 10 and 30 nm, such as for example 20 nm.

[0125] The present invention makes use of a cross-flow ultrafiltration (UF) unit, wherein the process flow of the ultrafiltration consists of a sequential switching between filtration and (chemical) backwashing.

[0126] A cross-flow filtration unit is a filtration system wherein a liquid flows along the surface of a filter membrane instead of perpendicularly through it. This flow principle, known as cross-flow, minimizes the build-up of solids on the membrane surface and promotes a more efficient separation of the desired components, such as dissolved substances or pure water, from contaminants or solid particles. The liquid flow thus moves parallel to the membrane (in contrast to dead-end filtration, where the liquid flows perpendicularly through the membrane). A cross-flow filtration unit typically consists of a pump, lines, a membrane module, and a pressure regulation mechanism. The membrane divides the input stream (feed) into two streams, namely the liquid that passes through the membrane and contains the passed-through components (the permeate) and the liquid that flows along the membrane and contains the retained components (the concentrate). A cross-flow filtration unit has the advantage that a constant filtration speed is maintained because the cross-flow movement reduces contamination and fouling on the membrane surface.

[0127] During the filtration, biologically purified effluent water is fed to the inside of the membranes of the cross-flow UF unit. Water will permeate through the membrane to the outside and the suspended particles are retained by the membrane and are discharged with the concentrate.Membrane fouling is reduced to a minimum in a cross-flow arrangement by a high cross-flow velocity of the water over the membrane surface. Suspended particles are carried along with the water flow, thus the membrane surface is kept free.

[0128] In an embodiment, the transmembrane pressure (pressure across the membrane = measure for membrane fouling) is continuously monitored. At an increased transmembrane pressure, the water flow rate can be temporarily increased to realize a higher cross-flow velocity (2-3 m / s) to clean the membrane. Afterwards, the speed drops back to the normal speed.

[0129] Due to the accumulation of the contamination on the membrane surface, the transmembrane pressure rises and a backwashing is necessary to remove the particulate material and to restore the efficiency. During the backwashing, a part of the filtrate is sent back through the membrane from the filtrate side. This results in a removal of the contamination on the feed side of the membrane. In an embodiment, this backwashing step lasts between 30 seconds and 240 seconds, preferably between 30 seconds and 120 seconds, more preferably between 30 seconds and 60 seconds, such as 45 seconds.

[0130] In an embodiment, the method comprises a chemical backwashing step of the ultrafiltration unit. The efficiency of the backwashing is namely increased by regularly dosing disinfecting chemicals (for example sodium hypochlorite or bleach) into the discharge line of the backwash pump. In an embodiment, this chemical backwashing step lasts between 30 seconds and 360 seconds, preferably between 30 seconds and 240 seconds, more preferably between 60 seconds and 180 seconds, such as 120 seconds. In an embodiment, sodium hydroxide is also added to the sodium hypochlorite to increase the efficiency of the latter.

[0131] Sodium hypochlorite and sodium hydroxide are dosed to counteract biological growth. Occasionally, an amount of citric acid is also dosed by extended backwashing, whereby any inorganic precipitate is removed.

[0132] Due to these regular cleanings, it is important to select a chemically sufficiently resistant membrane for the UF unit. Preferably, the UF membranes used are manufactured from PVDF (polyvinylidene fluoride), known for its mechanical strength and chemical resistance.In a preferred embodiment, the cross-flow filtration unit comprises more than one membrane module. Preferably, each membrane module can be backwashed separately. Due to the separate backwashing of the membrane module, the ultrafiltration unit can remain operative and permeate is continuously available from the ultrafiltration step.

[0133] In an embodiment, a CIP (Cleaning-In-Place) cycle is performed, for example when the transmembrane pressure can no longer be restored during the (chemical) backwashing regime. During a CIP, the membranes are fed along the inside of the membrane with permeate containing a low concentration of bleach / base or a weak acid. This solution is circulated through the membrane to remove more stubborn fouling. In an embodiment, the installation is flushed and restarted after the CIP.

[0134] A combination of different 2-way valves (open / closed status) offers the necessary process control for different UF membranes. Extra valves enable the use of intermediate storage tanks for different processes involved in the maintenance of the UF filters. In an embodiment of the invention, a common pump is used for multiple purposes, such as UF backwashing, UF cleaning-in-place, RO flushing and RO cleaning-in-place.

[0135] In an embodiment, the UF installation has a flow rate between 5-20 m3 / hour, for example 10 m3 / hour.

[0136] In an embodiment, the UF installation has a membrane surface area between 150 and 300 m2.

[0137] In an embodiment, the concentrate of this UF step is recirculated back to the one or more aeration tanks. The reverse osmosis step will further purify the permeate of the ultrafiltration to dischargeable water.

[0138] The mechanism of reverse osmosis differs from filtration in the sense that in reverse osmosis there is no question whatsoever of physical separation. It is namely the case that water and substances with a smaller molecular weight are able to spread through the membrane polymer by moving between the segments of the chemical structure of the polymer. Dissolved salts and organic substances with a larger molecular weight, however, do not penetrate through the membrane due to their dimensionsand chemical properties. The reverse osmosis membrane is also able to bring about a complete removal of the suspended particles.

[0139] The success of the reverse osmosis technology is for the most part due to the cheap operation and the simplicity. Compared with other salt-removing technologies, it is relatively cheap in purchase and operation.

[0140] The reverse osmosis unit comprises a semipermeable membrane. The reverse osmosis unit comprises a supply for liquid and a first discharge for liquid on a first side of the semipermeable membrane. The reverse osmosis unit comprises a second discharge for liquid on a second opposite side of the semipermeable membrane. The reverse osmosis unit comprises a pump, configured for pumping the liquid under pressure from the discharge on the second side of the ultrafiltration unit into the supply for liquid. The pressure is at least higher than an osmotic pressure in the liquid. This causes water to migrate through the semipermeable membrane from the first side to the second side. Liquid with an increased concentration of minerals is discharged via the first outlet and purified water is discharged via the second outlet.

[0141] Due to the increased concentration of minerals in the mineral concentrate, the mineral concentrate is more valuable than a thin fraction, for example for a fertilizer producer.

[0142] Because purified water has been removed from the mineral concentrate, transport costs for the mineral concentrate are lower than for the thin fraction.

[0143] An RO membrane is designed to retain ions, such as Na+and Cl", whereby desalination of water becomes possible. Typically, these membranes are able to reject more than 99% of the monovalent and divalent ions, such as K+, Na+, Cl", Ca2+, Mg2+and SO42'. In a preferred embodiment, the membrane of the RO unit has a pore size between 0.1-1 nm. In an embodiment, the membranes are spiral-wound. In an embodiment, the RO unit comprises between 20 and 50 membranes, for example 30 membranes with a high salt tolerance. In an embodiment, the membranes have a surface area between 30-50 m2 / piece, for example 40.9 m2 / piece. In a preferred embodiment, these membranes have a salt retention of at least 95%, preferably at least 97%. In an embodiment, multiple membranes (for example 6 membranes) are placed in a pressure tube. In a preferred embodiment,the RO unit comprises 5 pressure tubes with 6 membranes each (30 membranes in total).

[0144] Most RO membranes require pressure to filter water through them. This pressure requirement is directly related to the amount of salt concentrations (or total dissolved solids, TDS) in the water to be processed. A higher TDS requires a higher feed pressure to overcome the osmotic pressure. Higher pressure means increased energy consumption by pumps to cause water to permeate through RO membranes.

[0145] For desalination, it is necessary to optimize the applied pressure for maximum efficiency (ratio of permeate to feed). Preferably, a pressure between 20 and 50 bar is applied during the RO step, preferably between 30 and 35 bar, such as 32 bar, while a permeate efficiency of 50-70% is achieved. In an embodiment, the RO permeate is produced at a flow rate of 5-15 m3 / hour, preferably at a flow rate between 4-10 m3 / hour. In an embodiment, the RO unit is fed at a flow rate of 10-25 m3 / hour.

[0146] In an embodiment, the RO unit comprises one or more dosing pumps for dosing, for example, antiscalants, biocides and / or acids.

[0147] Antiscalant (scale inhibitor) eliminates limescale and prevents deposition and fouling, which extends the service life of the RO elements. RO membrane biocides are chemical compounds that are designed to prevent biological fouling or to remove biological contaminants from RO membranes. The main goal of using an effective biocide program is controlling biological fouling in membranes, so that it can be relatively cost-efficient compared to cleaning programs. Acids (for example sulfuric acid) can be added to lower the pH. The pH is often lowered during reverse osmosis (RO) for various reasons related to optimizing the performance of the system and protecting the membranes against damage. Water flowing through an RO system often contains dissolved ions such as calcium, magnesium and bicarbonates. At a higher pH, these salts can precipitate and cause scaling on the membrane surface. This reduces the efficiency and service life of the membranes. By lowering the pH (usually to a level between 5 and 7), the formation of calcium carbonate (CaCOs) and other precipitates is inhibited, because these substances are more soluble in an acidic environment. Many RO membranes are sensitive to high pH values, which can lead to chemical degradation of the membrane material. A lower pH helps to prevent this and extends the service life of the membranes. A lower pH can inhibit the growth ofmicroorganisms, which helps to reduce biological fouling (biofouling). Although pH reduction alone is usually not sufficient to stop all microbial activity, it supports other biocide or cleaning programs. Some dissolved salts are discharged more effectively at a lower pH. This can contribute to a better overall salt rejection by the RO system. Anti-scaling agents and other chemicals that are used in RO systems often work better within a certain pH range. By adjusting the pH, the operation of these chemicals is optimized. In practice, an acid, such as sulfuric acid (H2SO4) or hydrochloric acid (HCI), is often added to the feed water to lower the pH and prevent scaling. It is important that the pH is carefully controlled, because too low values can cause corrosion in pipes and components.

[0148] The present method makes it possible to obtain an RO permeate that is dischargeable. The degree of pollution is determined by the amount of organic material present (biochemical oxygen demand (BOD) or chemical oxygen demand (COD)) and the nutrients (nitrogen and phosphate).

[0149] The oxygen-consuming substances can be distinguished into three groups:

[0150] - organic carbon compounds;

[0151] - ammonium nitrogen and organically bound nitrogen;

[0152] - other inorganic substances, such as divalent iron compounds, nitrites and sulfites.

[0153] To obtain an impression of the content of organic carbon compounds, the determination of the following is most suitable:

[0154] - the biochemical oxygen demand (BOD), i.e., by means of bacteria;

[0155] - the chemical oxygen demand (COD), with the aid of potassium dichromate.

[0156] The oxygen consumption by ammonium nitrogen and organically bound nitrogen is determined in the Kjeldahl determination.

[0157] The biochemical oxygen demand, BOD, is the amount of oxygen in mg that is necessary to convert, by means of bacteria, the biochemically oxidizable constituents present in 1 liter of water. A sample of wastewater is mixed with pure water with a known oxygen content and it is determined, after the mixture has been kept (usually) for 5 days in a dark place at 20°C, how much oxygen has been consumed for the oxidation of the organic matter. The test must be carried out in the dark, because then oxygen production by algae cannot occur simultaneously. Two oxygen measurements are necessary, namely one before and one after the test. As the content of biochemically oxidizable substances is greater, more oxygen will beconsumed. In practice, the BOD205is usually used, that is to say an oxidation that lasts 5 days at a temperature of 20°C. However, this does not mean that all biochemically oxidizable constituents have been completely oxidized by the bacteria after 5 days, because a much longer time is necessary for complete conversion. In oxidation by biochemical means, those biochemically oxidizable constituents are oxidized first, which are most easily taken up by bacteria as food.

[0158] The RO permeate obtained via the present invention has a biochemical oxygen demand (BOD205) (at 20°C and 5 days) value lower than 25 mg / L.

[0159] In the determination of the COD, most organic compounds are oxidized extensively by chemical means. Potassium dichromate is used as oxidizing agent. To determine the COD, the following is added to a sample:

[0160] - a known amount of potassium dichromate (K2Cr2O?);

[0161] - a certain amount of silver sulfate (Ag2SO4) that serves as a catalyst in the oxidation; - mercury(II) sulfate (HgS04) to prevent oxidation of chloride. After two hours of reflux boiling in a flask, the remaining amount of potassium dichromate is determined. From the difference between the original amount of potassium dichromate and the remainder, the consumed amount of oxygen can be calculated.

[0162] The RO permeate obtained via the present invention has a chemical oxygen demand (COD) value lower than 125 mg / L.

[0163] In an embodiment, the RO permeate has a 10-fold reduction of the COD value compared to the COD value of the biological effluent obtained after step c.

[0164] Nitrogen can occur in various forms, namely as organically bound nitrogen, such as in proteins or the degradation products thereof (amino acids), and in inorganic form, as NH3 (ammonia) or NH4+(ammonium) or in its oxidized form (NO2‘ or NOs'). During the mineralization process, the organically bound nitrogen is first converted into ammonium ions (or ammonia depending on the pH). The content of organically bound nitrogen and the ammonium nitrogen in mg / L is determined according to the Kjeldahl method. This is also referred to as Kjeldahl nitrogen. The oxidation of ammonium takes place in two stages. For the first nitrification stage, the bacterial species Nitrosomonas is necessary (nitrite formation), while in the second nitrification stage, the bacterial species Nitrobacter causes the formation of nitrate exclusively. The nitrifying bacteria develop only slowly and the reactions occur as long as thetemperature does not drop too far below 10°C. Nitrification stops when the oxygen content has dropped to approximately 1 mg / L or lower.

[0165] The RO permeate obtained via the present invention has a total nitrogen content (Ntotai) lower than 15 mg / L.

[0166] The RO permeate obtained via the present invention has a total phosphate content (Ptotai) lower than 2 mg / L.

[0167] The RO permeate obtained via the present invention has a total suspended solids (TSS) lower than 10 mg / L.

[0168] The obtained RO permeate is suitable as irrigation water. The purified water can also be discharged. The purified water can also be made suitable for use, for example, as process water in a food company, for example for rinsing and / or blanching vegetables.

[0169] In an embodiment, the resulting RO permeate is used in the flotation technique for reducing the suspended solids content. In an embodiment, specifically, the whitewater, which is important for good flotation, is continuously produced by recirculating a part of the purified water (the RO permeate produced during the last step of the method).

[0170] As described above, in an embodiment, a polymer mixture is used during the centrifugal separation step. In a further embodiment, the polymer mixture is prepared with the resulting RO permeate from the last step of the method and a powder polymer. Such RO permeate is sufficiently pure and moreover heated.

[0171] In the following, the invention is described by means of non-limiting examples illustrating the invention, and which are not intended or to be interpreted to limit the scope of the invention.EXAMPLES

[0172] EXAMPLE 1 (see Figure 1):

[0173] Biological waste (g), such as for example manure and / or vegetable waste and / or fruit waste, is digested in a digestion tank (1) by a thermophilic process, wherein a digestate (h) and biogas are formed. In a subsequent step, the digestate (h) is separated in a centrifuge (2) into a thin fraction (a) and a thick fraction, wherein the thin fraction has a dry matter content lower than 4%. During the separation process, a polymer is added. This polymer is prepared with RO permeate (f).

[0174] The phosphate remains primarily in the thick fraction, whereby the thin fraction has a phosphate (P2O5) content which amounts to at most 25% of the phosphate content in the digestate. Subsequently, the thin fraction (a) is introduced into one or more aeration tanks (3). In the aeration tank, nitrogen is removed from the thin fraction until a biological effluent (c) with a nitrogen content lower than 2,000 mg / liter is obtained. Because the thin fraction can be removed from the digestate, transport costs for transporting the thick fraction to, for example, a producer of fertilizers will be lower than the transport costs for transporting the digestate, whereby the first thick fraction is more valuable as raw material than the digestate.

[0175] Phosphate and nitrate are inorganic nutrients that play an important role in aquatic ecosystems, but their excessive presence can lead to serious environmental problems. These substances influence the oxygen content in the water because they contribute to the growth of algae and other aquatic plants. This process, known as eutrophication, can lead to oxygen shortages and suffocation of aquatic organisms, with far-reaching consequences for biodiversity and water quality.

[0176] Preceding water purification steps (in particular the centrifugal separation into a thin fraction and a thick fraction and the aerating of the thin fraction) ensure a significant reduction of these inorganic substances in the thin fraction which is further processed into dischargeable water.

[0177] However, the separation on a centrifuge is not absolute; coarser, suspended particles such as organic fibers, pieces of plastic and other coarser particles still remain behind in the biological effluent. These particles can seriously foul the UF membranes and must therefore be removed before the UF.It is important to lower the total suspended solids (TSS) in the biological effluent since these cause considerable membrane fouling and scaling in the pressure-driven membrane processes of the subsequent water treatment.

[0178] To remove these coarser suspended particles, filtration is performed in the biological effluent over a rotary drum screen with perforations of 0.8-1.0 mm (4). The biological effluent falls through the perforations and the coarser particles are screened out. The purified biological effluent (d) is treated in the UF unit (6).

[0179] When the content of suspended particles is too high, use is made of the flotation technique Dissolved Air Flotation (DAF) (5). Before the DAF installation, a screen (8) is also always arranged to prevent blockages. This screen (8) is likewise a rotary drum screen with perforations of 0.8-1.0 mm.

[0180] A DAF flotation installation is based on the principle of accelerated flotation of contaminant particles by introduction of minuscule air bubbles which adhere to the contaminant particles (= Dissolved Air Flotation).

[0181] The DAF installation comprises a tank with a supply, wherein the supply is configured for supplying biological effluent from the aeration tank into the tank of the DAF installation. Coagulant (for example FeCI3 and Fe2(SO4)3) is added to the thin fraction in the tank. pH correction of the biological effluent treated with coagulant takes place by addition of base or acid and the whole is mixed. Coagulant is advantageous for flocculation by destabilizing colloidal particles in the thin fraction. Flocculant is added. Flocculant is advantageous for further enlarging flocs formed by coagulant. Flocculant is added together with coagulant or after addition of coagulant. A centrifugal pump pumps purified water (in particular the recycled RO permeate obtained according to the present invention) to the pressure vessel and ensures an optimal pressure for dissolving air in water. In this pressure vessel, compressed air is also dosed in a controlled manner so that water optimally saturated with air is obtained. From this pressure vessel, this air-saturated water is led back to the tank. Due to the pressure drop (or depressurization) of this water (from 5-6 bar to 0 bar), the dissolved air is released again in the form of minuscule air bubbles (30-50 pm, the so-called whitewater) in the tank. These air bubbles adhere to the contaminant particles and cause them to float. The air bubbles attach to the flocs and form a sludge layer at the surface. This layer is removed by a scraper.Heavier particles such as sand will not float, but will settle. The underside of the DAF installation comprises pneumatically controlled valves which can be opened to be able to remove the possible sediment. These valves are opened in a timer-controlled manner. Due to the pressure of the water in the flotation, the bottom sludge is pushed out.

[0182] The feed of the DAF installation comprises an electromagnetic flow measurement and a controlled valve to thus also be able to regulate the dosages of coagulant and flocculant as a function of the flow rate.

[0183] After the flotation technique, the biological effluent is fed back to one or more aeration tanks (3).

[0184] Through these screening steps and DAF treatment, a suspended solids content (TSS) lower than 40,000, preferably lower than 35,000 mg / liter is obtained. Hereby, wear on the active separation layers of the UF unit is minimized.

[0185] Subsequently, the purified biological effluent (d) is treated in the cross-flow ultrafiltration (UF) unit (6). The ultrafiltration is characterized by a filtration step and a backwashing step.

[0186] During the filtration, biologically purified effluent water (d) is fed to the inside of the membranes of the cross-flow UF unit. Water will permeate through the membrane to the outside and the suspended particles are retained by the membrane and are discharged with the concentrate.

[0187] Membrane fouling is reduced to a minimum in a cross-flow arrangement by a high cross-flow velocity of the water over the membrane surface. Suspended particles are carried along with the water flow, thus the membrane surface is kept free.

[0188] The transmembrane pressure (pressure across the membrane = measure for membrane fouling) is continuously monitored. At an increased transmembrane pressure, the water flow rate is temporarily increased to realize a higher longitudinal flow velocity (2-3 m / s) to clean the membrane. Afterwards, the speed drops back to the normal speed.Due to the accumulation of the contamination on the membrane surface, the transmembrane pressure rises and a backwashing is necessary to remove the particulate material and to restore the efficiency. During the backwashing (±45 seconds), a part of the filtrate is sent back through the membrane from the filtrate side. This results in a removal of the contamination on the feed side of the membrane.

[0189] The efficiency of the backwashing is increased by regularly dosing disinfecting chemicals (sodium hypochlorite or bleach) in the discharge line of the backwash pump. This chemical backwashing step lasts for example 120 seconds.

[0190] The UF membranes used are manufactured from PVDF (polyvinylidene fluoride), known for its mechanical strength and chemical resistance.

[0191] The cross-flow filtration unit (6) has multiple membrane modules, which can each be backwashed individually. Due to the separate backwashing of the membrane module, the ultrafiltration unit can remain operative and permeate is continuously available from the ultrafiltration step.

[0192] The reverse osmosis step will further purify the permeate of the ultrafiltration (e) into dischargeable water. The reverse osmosis unit comprises a semipermeable membrane. The reverse osmosis unit (7) comprises a supply for liquid and a first discharge for liquid on a first side of the semipermeable membrane. The reverse osmosis unit comprises a second discharge for liquid on a second opposite side of the semipermeable membrane. The reverse osmosis unit comprises a pump, configured for pumping the liquid under pressure from the discharge on the second side of the ultrafiltration unit into the supply for liquid. The pressure is at least higher than an osmotic pressure in the liquid. This causes water to migrate through the semipermeable membrane from the first side to the second side. Liquid with an increased concentration of minerals is discharged via the first outlet and purified water is discharged via the second outlet.

[0193] Due to the increased concentration of minerals in the mineral concentrate, the mineral concentrate is more valuable than a thin fraction, for example for a fertilizer producer.

[0194] Because purified water has been removed from the mineral concentrate, transport costs for the mineral concentrate are lower than for the thin fraction.The membrane of the RO unit (7) has a pore size between 0.1-1 nm.

[0195] For desalination, it is necessary to optimize the applied pressure for maximum efficiency (ratio of permeate to feed). Preferably, a pressure between 30 and 35 bar is applied during the RO step, while a permeate efficiency of 50-70% is achieved. The RO permeate is produced at a flow rate of approximately 10 m3 / hour. The RO unit (7) is fed at a flow rate of approximately 17 m3 / hour.

[0196] The present method makes it possible to obtain an RO permeate (f) that is dischargeable. The obtained RO permeate has a biochemical oxygen demand (BOD205) (at 20°C and 5 days) value lower than 25 mg / L, a chemical oxygen demand (COD) value lower than 125 mg / L, a total nitrogen content (Ntotai) lower than 15 mg / L, a total phosphate content (Ptotai) lower than 2 mg / L and a total suspended solids (TSS) lower than 10 mg / L.

[0197] EXAMPLE 2 (see figure 2):

[0198] The method in example 2 is similar to that described in example 1, however, the one or more screening steps take place before the aeration takes place. This method is described in more detail below.

[0199] Biological waste (g), such as for example manure and / or vegetable waste and / or fruit waste, is digested in a digestion tank (1) by a thermophilic process, wherein a digestate (h) and biogas are formed. In a subsequent step, the digestate (h) is separated in a centrifuge (2) into a thin fraction (a) and a thick fraction, wherein the thin fraction has a dry matter content lower than 4%.

[0200] Subsequently, one or more screening steps are performed on the thin fraction (a) by means of a screen (9) with perforations of 1 mm. The screened thin fraction (i) can subsequently be stored in a storage tank (10).

[0201] Subsequently, the screened thin fraction (i) is introduced into one or more aeration tanks (3). In the aeration tank, nitrogen is removed from the thin fraction until a biological effluent (c) with a nitrogen content lower than 2,000 mg / liter is obtained.

[0202] Subsequently, the biological effluent (c) is treated in the cross-flow ultrafiltration (UF) unit (6).The reverse osmosis step in the reverse osmosis unit (7) will further purify the permeate of the ultrafiltration (e) into dischargeable water (f).

[0203] EXAMPLE 3 (see figure 3):

[0204] The method in example 3 is similar to that described in example 1, however, only a screening step (8) takes place before the DAF installation (5). This method is described in more detail below.

[0205] Biological waste (g), such as for example manure and / or vegetable waste and / or fruit waste, is digested in a digestion tank (1) by a thermophilic process, wherein a digestate (h) and biogas are formed. In a subsequent step, the digestate (h) is separated in a centrifuge (2) into a thin fraction (a) and a thick fraction, wherein the thin fraction has a dry matter content lower than 4%.

[0206] Subsequently, the thin fraction (a) is introduced into one or more aeration tanks (3). In the aeration tank, nitrogen is removed from the thin fraction until a biological effluent with a nitrogen content lower than 2,000 mg / liter is obtained.

[0207] However, the separation on a centrifuge is not absolute; coarser, suspended particles such as organic fibers, pieces of plastic and other coarser particles still remain behind in the biological effluent. These particles can seriously foul the UF membranes and must therefore be removed before the UF.

[0208] When the content of suspended particles is too high, use is made of the flotation technique Dissolved Air Flotation (DAF) (5). Before the DAF installation, a screen (8) is also always arranged to prevent blockages. This screen (8) is a rotary drum screen with perforations of 0.8-1.0 mm.

[0209] After the flotation technique, the biological effluent is fed back to one or more aeration tanks (3).

[0210] Subsequently, the biological effluent (d) is treated in the cross-flow ultrafiltration (UF) unit (6).

[0211] The present method makes it possible to obtain an RO permeate (f) that is dischargeable.

Claims

CLAIMS1. Method for purifying and treating biological waste into dischargeable water, the method comprising the following steps:a. Digesting the biological waste into a digestate;b. Separating the digestate into a thin fraction and a thick fraction, wherein the thin fraction has a dry matter content less than 4%; c. Aerating the thin fraction into a biological effluent with a nitrogen content lower than 2,000 mg / liter(L);d. Lowering the total suspended solids (TSS) in the thin fraction from step b or in the biological effluent from step c to a content lower than 40,000 mg / L by means of one or more screening steps optionally combined with a flotation technique such as Dissolved Air Flotation (DAF); e. Treating the purified biological effluent from step d in a cross-flow ultrafiltration (UF) unit, wherein the ultrafiltration is characterized by a filtration step and a backwashing step;f. Treating the resulting permeate from step e in a reverse osmosis (reverse osmosis, RO) unit, wherein the resulting RO permeate is a dischargeable water characterized by:vi. a chemical oxygen demand (COD) value lower than 125 mg / L, vii. a biochemical oxygen demand (BOD205) value lower than 25 mg / L,viii. a total nitrogen content (Ntotai) lower than 15 mg / L,ix. a total phosphate content (Ptotai) lower than 2 mg / L and x. a suspended solids content lower than 10 mg / L.

2. The method according to claim 1, wherein the cross-flow filtration unit comprises more than one membrane module, wherein each membrane module can be backwashed individually.

3. The method according to any one of the preceding claims, wherein the membrane of the RO unit has a pore size between 0.1-1 nm.

4. The method according to any one of the preceding claims, wherein the resulting RO permeate from step f is used in the flotation technique for reducing the suspended solids content.

5. The method according to any one of the preceding claims, wherein the one or more screening steps make use of a rotary drum screen with perforations smaller than 2.0 mm, preferably smaller than 1.0 mm.

6. The method according to any one of the preceding claims, wherein the thin fraction has a phosphate (P2O5) content that amounts to a maximum of 25% of the phosphate content in the digestate.

7. The method according to any one of the preceding claims, wherein the thick- to-thin fraction ratio at the end of step b lies between 35%-65%.

8. The method according to any one of the preceding claims, wherein a polymer mixture is used during the centrifugal separation step, wherein the polymer mixture is prepared with the resulting RO permeate from step f and a powder polymer.

9. The method according to any one of the preceding claims, wherein the RO permeate is produced at a flow rate of 5-15 m3 / hour.

10. The method according to any one of the preceding claims, wherein the RO permeate has a 10-fold reduction of the COD value compared to the COD value of the biological effluent obtained after step c.

11. Apparatus for purifying and treating biological waste into dischargeable water, the apparatus comprising:a. One or more digesters for digesting the biological waste into a digestate;b. One or more separators for separating the digestate into a thin fraction and a thick fraction;c. One or more aerators for aerating the thin fraction into a biological effluent;d. One or more screens and optionally a Dissolved Air Flotation (DAF) apparatus for lowering the total suspended solids (TSS) in the biological effluent;e. A cross-flow ultrafiltration (UF) unit for treating the purified biological effluent;f. A reverse osmosis (RO) unit for desalinating the resulting UF permeate into a dischargeable water.

12. Use of the method according to any one of claims 1-10 or the apparatus according to claim 11 for purifying and treating biological waste into dischargeable water, wherein the dischargeable water is characterized by: a. a chemical oxygen demand (COD) value lower than 125 mg / L, b. a biochemical oxygen demand (BOD205) value lower than 25 mg / L, c. a total nitrogen content (Ntotai) lower than 15 mg / L,d. a total phosphate content (Ptotai) lower than 2 mg / L ande. a total suspended solids (TSS) lower than 10 mg / L.

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