PROCEDURE FOR WATER PURIFICATION AND WATER TREATMENT

Abstract

The present invention concerns a process and apparatus and the use of such process and apparatus for water purification and water treatment for the conversion of biological waste into dischargeable water.

Description

2 The present invention aims to find a solution to at least some of the above-mentioned problems. SUMMARY OF THE INVENTION 5 In a first aspect, the invention concerns a method for purifying and treating biological waste into dischargeable water. More specifically, the invention concerns a method comprising the following steps: a. The fermentation of the biological waste into a digestate; b. The separation of the digestate into a thin fraction and a thick fraction, where the thin fraction has a dry matter content lower than 4%; c. The aeration of the thin fraction into a biological ffluent with a nitrogen content lower than 2000 mg / liter (L); d. Reducing the total suspended solids (TSS) content in the biological ffluent exit to less than 40,000 mg / L, preferably less than 35,000 mg / L, by means of one or more sieving steps, optionally combined with a flotation technique such as Dissolved Air Flotation (DAF); e.The treatment of the purified biological ffluent discharge in a cross-flow ultrafiltration (UF) unit, whereby the ultrafiltration is characterized by a filtration step and a backwash step; f. The treatment of the resulting permeate discharge in a reverse osmosis (RO) unit, whereby the resulting RO permeate and dischargeable water are characterized by: i. a chemical oxygen demand (COD) value lower than 125 mg / L, ii. a biochemical oxygen demand (BOD) value lower than 25 mg / L, iii. a total nitrogen content (Ntotal) lower than 15 mg / L, iv. a total phosphate content (Ptotal) lower than 2 mg / L and v. a suspended solids content lower than 10 mg / L. The method of the current invention combines suitable water purification with subsequent water treatments and in this way enables highly efficient operation of the UF membranes and RO membranes, whereby the benefits of a longer membrane lifespan and less energy are utilized.Moreover, the present invention makes use of a cross-flow BE2024 / 5933 3 ultrafiltration (UF) unit and the process flow of the ultrafiltration is characterized by a filtration step and a backwash step, whereby membrane fouling is also minimized. In a second aspect, the invention concerns a device for the purification and treatment of biological waste into dischargeable water in accordance with claim 11. In a final aspect, the invention concerns the use of the aforementioned method or device for the purification and treatment of biological waste into dischargeable water.10 DESCRIPTION OF THE FIGURES Figure 1 shows a flow diagram of a method for the purification and treatment of biological waste into dischargeable water in accordance with an implementation form of the present invention. DETAILED DESCRIPTION The invention concerns a method and a device and the use of the aforementioned method and device for water purification and water treatment for the conversion of biological waste into dischargeable water.The current invention combines suitable water purification with subsequent water treatment, thereby enabling highly efficient operation of the UF membranes and RO membranes, while utilizing the benefits of a longer membrane lifespan and reduced energy consumption. Converting biological waste into dischargeable water requires a more intensive process than wastewater purification due to the higher content of suspended solids (TSS) in the bio-waste. Through suitable water purification—including reducing the content of 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)—membrane fouling and flaking can be reduced in the subsequent water treatment.Moreover, the present invention makes use of a cross-flow ultrafiltration (UF) unit and the process flow of the ultrafiltration is characterized by a filtration step and a backwash step, whereby membrane fouling is also minimized. This allows operating and maintenance costs to be minimized. BE2024 / 5933 4 Unless otherwise defined, all terms used in the description of the invention, including technical and scientific terms, have the meaning as they are generally understood by the skilled professional in the technical field of the invention. For a better assessment of the description of the invention,5 the following terms are explicitly explained. “A”, “the” and “the” in this document refer to both the singular and the plural unless the context clearly implies otherwise. For example, “a segment” means one or more than one segment.10 When “approximately” or “around” is used in this document for a measurable quantity, a parameter, a duration or moment, and the like, variations are meant of + / - 20% or less, preferably + / - 10% or less, more preferably + / - 5% or less, even more preferably + / - 1% or less, and even more preferably + / - 0.1% or less than the quoted value, insofar as such variations apply to the described invention. However, this must be understood to mean that the value of the quantity for which the term “approximately” or “around” is used is itself specifically disclosed. 20 The terms “comprehensive”, “comprehensive”, “consist of”, “consisting of”, “provided with”, “contain”, “containing”, “encompass”, “containing”, “include”, “containing” are synonyms and are inclusive or open terms that indicate the presence of what follows, and that do not exclude or prevent the presence of other components, characteristics, elements, members, steps, known from or described in the standard technique.The term “sludge” as used herein is a collective name 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 resulting from biological treatment processes. Characteristic of the sludge is a large fraction of organic material (approx. 50-80% of the total substance). The term “biological waste” as used herein is a collective name 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, plant BE2024 / 5933 5 material and other organic components that can be subjected to biological or physico-chemical treatments to break them down, stabilize them, and convert them into reusable or environmentally friendly outputs, such as dischargeable water. Quoting numerical intervals through the endpoints includes all integers, fractions and / or real numbers between the endpoints, including these endpoints.Detailed description The breakdown of biological waste – such as manure, energy crops, plant waste, agriculture-related waste streams and secondary raw materials – into dischargeable water10 ensures a circular economy in which raw materials and materials are continuously reused or recycled. However, converting biological waste into dischargeable water requires a more intensive process than wastewater treatment.15 The degree of membrane fouling that occurs during the treatment of bio-waste is greater than during wastewater processing, partly due to a higher content of suspended solids (TSS) in the bio-waste. Membrane fouling and flaking in pressure-controlled membrane processes are often20 a major problem, because they can increase the operating and maintenance costs of the systems. In a first aspect, the invention concerns a method for purifying biological waste into dischargeable water, the method comprising the following steps: 25 a. The fermentation of the biological waste into a digestate; b.The separation of the digestate into a thin fraction and a thick fraction, where the thin fraction has a dry matter content lower than 4%; c. The aeration of the thin fraction into a biological ffluent with a nitrogen content lower than 2000 mg / liter (L); d. The reduction of the suspended solids (TSS) content in the biological ffluent outlet to a content lower than 40000 mg / L, preferably lower than 35000 mg / L, by means of one or more screening steps optionally combined with a flotation technique such as Dissolved Air Flotation (DAF); BE2024 / 5933 6 e. The treatment of the purified biological ffluent outlet in a cross-flow ultrafiltration (UF) unit, where the ultrafiltration is characterized by a filtration step and a backwash step; f. The treatment of the resulting permeate from the tape in a reverse osmosis (RO) unit, whereby the resulting RO permeate is dischargeable water characterized by: i. a chemical oxygen demand (COD) value lower than 125 mg / L, ii. a biochemical oxygen demand (BOD) value lower than 25 mg / L, iii.a total nitrogen content (Ntotal) lower than 15 mg / L, iv. a total phosphate content (Ptotal) lower than 2 mg / L and v. a suspended solids content lower than 10 mg / L. Before filtration with UF membranes, the influent water must generally be purified to remove particles larger than 1 mm, and high-density particles must also be removed to minimize wear on the active separation layers. The TSS content may not exceed 40,000 mg / liter. It is also important that the nitrogen content is lower than 2,000 mg / liter. In the present invention, we reveal a combination of a (centrifugal) separator (step b), a denitrification step (aeration in step c), followed by screen filtration and optionally a flotation technique such as DAF, which delivers the desired water quality for the UF phase. Furthermore, the present invention makes use of a cross-flow ultrafiltration (UF) unit, and the process flow of the ultrafiltration is characterized by a filtration step and a backwash step, whereby membrane fouling is also minimized.This allows operating and maintenance costs to be minimized. In a second aspect, the invention also comprises a facility for the purification and treatment of biological waste into dischargeable water, the facility comprising: 30 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 35 into a biological ffluent; BE2024 / 5933 7 d. One or more sieves and optionally a Dissolved Air Flotation (DAF) device for reducing the content of total suspended solids (TSS) in the biological ffluent; f. A cross-flow ultrafiltration (UF) unit for treating the purified biological ffluent; f. A reverse osmosis (RO) unit for desalinating the resulting UF permeate into dischargeable water.In a final aspect, the invention concerns the use of the aforementioned method or facility for the purification and treatment of biological waste into dischargeable water, where the dischargeable water is characterized by: a. a chemical oxygen demand (COD) value lower than 125 mg / L, b. a biochemical oxygen demand (BOD) value lower than 25 mg / L, c. a total nitrogen content (Ntotal) lower than 15 mg / L, d. a total phosphate content (Ptotal) lower than 2 mg / L and e. a suspended solids (TSS) content lower than 10 mg / L. In a first step, the biological waste is fermented in one or more fermentation tanks or digesters into a digestate. The fermentation tank contains a discharge for digestate. Digestate is a residual product that remains in the fermentation tank after the breakdown of the biomass. Digestate is liquid. The discharge for digestate comprises a discharge pipe. The discharge for digestate preferably includes a shut-off valve. Subsequently, the digestate is separated into a thin fraction and a thick fraction, where the thin fraction has a dry matter content lower than 4%.In one form, 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%. 30 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 presented as a dimensionless number or as a percentage. Alternatively, the dry matter content can also be expressed as a mass of dry matter in a volume of 1 m³ of the solution. The mass of dry matter is determined by taking a sample of a known volume, drying the sample until all liquid has completely evaporated and only dry matter remains, weighing the mass of dry matter, and converting the mass of dry matter in the known volume of the solution into a volume of 1 m³ of the solution. The separator is, for example, a centrifuge, a belt filter press, a screw press, or another suitable device.It is clear to a professional trained in the technical field5 that a combination of several of the mentioned examples also forms part of the current invention. In a preferred design, the method comprises a centrifugal separation step.10 The digestate is preferably centrifugally separated into a thin fraction and a thick fraction, where the thin fraction has a dry matter content lower than 4%. In a design, the centrifugal separation step has a flow rate between 12-25 m³ / hour, preferably between 15-20 m³ / hour. In a design,15 the centrifugal separation step is performed at a speed between 2000 and 3000 rpm, preferably between 2000-2700 rpm. In a preferred execution form, the ratio of thick to thin fractions at the end of step b lies between 35% and 65%.20 Optionally, the separators include devices configured for the addition of a polymer mixture (a flocculant) to the digestate. This is beneficial for achieving better separation into a thin fraction and a thick fraction.25 In one form, 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 that cake together inside the drum of the centrifuge.30 This form is advantageous because it allows the dry matter content in the thick fraction to be increased relative to the dry matter content in the digestate, whereby the thick fraction can be used as fertilizer and is a valuable product in itself. It can potentially be spread locally as fertilizer on a field35 or sold as a valuable raw material to, for example, a fertilizer producer.Because the thin fraction can be removed from the digestate BE2024 / 5933 9, transport costs for transporting the thick fraction to, for example, a fertilizer producer will be lower than the transport costs for transporting the digestate, making the former thick fraction more valuable as a raw material than the digestate. 5 The choice of polymer depends on the specific composition of the biological waste and the resulting digestate (in particular, the content of nitrogen, phosphorus, potassium, dry matter, organic matter, ...). In a single formulation, the polymer mixture is prepared by dissolving a powder polymer in a liquid. In a single formulation, 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. In a single formulation, between 4-12%, preferably between 6-8%, of polymer mixture is added to the digestate (m / m).In one configuration, 5% polymer mixture is added (for example, 1 m³ polymer per hour is added to 20 m³ digestate per hour).20 In a preferred configuration, phosphate remains mainly in the thick fraction; preferably, at least 75%, such as between 75-80%, of the phosphate content present in the digestate remains in the thick fraction (the phosphate content in the digestate taken as 100%). In the preferred configuration, the thin fraction therefore has a phosphate content that amounts to a maximum of 25% of the phosphate content present in the digestate (the phosphate content in the digestate taken as 100%). In one configuration, the thin fraction is temporarily stored in a storage tank30 before the aeration step takes place. In a further configuration, the centrifuge is connected to the storage tank by means of transport pipes. In a subsequent step, the thin fraction is aerated into a biological effluent with a nitrogen content lower than 2000 mg / liter (L), such as lower than 195035 mg / liter, lower than 1900 mg / liter, lower than 1850 mg / liter or lower than 1800 mg / liter.In one embodiment, the thin fraction is aerated into a biological effluent with BE2024 / 5933 10 and a nitrogen content between 0-100mg / L, 100-200mg / L, 200-300mg / L, 300-400 mg / L,400-500mg / L,500-600mg / L,600-700mg / L,700-800mg / L,800-900mg / L, 900-1000mg / L,1000-1100mg / L,1100-1200mg / L,1200-1300mg / L,1300-1400 mg / L, 1400-1500 mg / L, 1500-1600 mg / L, 1600-1700 mg / L, 1700-1800 mg / L, 1800-1900 mg / L or between 1900-2000 mg / L. This aeration preferably takes place in one or more aeration tanks. The aeration tank contains a feed configured for feeding the thin fraction into the aeration tank. The aeration tank contains an aeration system, such as, but not limited to, aeration plates, tube aerators and / or submersible aerators. The aeration system is configured for creating fine air bubbles in the thin fraction. An aeration tank is beneficial for removing nitrogen from the first thin fraction.Phosphate nitrate are inorganic nutrients that play an important role in aquatic ecosystems, but their excessive presence can lead to serious environmental problems. These substances affect the oxygen content in the water by contributing to the growth of algae and other aquatic plants. This process, known as eutrophication, can lead to oxygen deficiencies and suffocation of aquatic organisms, with far-reaching consequences for biodiversity and water quality. In the dischargeable water obtained under the current invention, the concentrations of these nutrients are strictly limited. The total nitrogen content (Ntotal) is lower than 15 mg / L, which contributes significantly to the prevention of excessive algae growth and the associated negative effects on the aquatic environment. In addition, the total phosphate content (Ptotal) is limited to less than 2 mg / L, whereby the risks of eutrophication are further minimized.These low levels ensure that the water meets strict environmental standards and contribute to a sustainable and environmentally friendly discharge that maintains the ecological balance in water bodies.30 In the aeration tank, the wastewater is mixed with previously formed activated sludge (a slimy material in which bacteria and protozoa live) and oxygen is supplied using aeration equipment. Under these conditions, the activated sludge can wholly or partially remove the organic and other contaminants from the35 wastewater. BE2024 / 5933 11 In inactive sludge flocs, the living bacterial cells ensure the uptake and breakdown 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 carbon atoms) can be directly absorbed into the bacterial cell via the cell wall. The larger fragments must first be split by enzymes into smaller molecules, which can pass through the cell wall.The aeration of activated sludge wastewater in an aeration chamber has two functions: - introducing the (air) oxygen required for the purification of the wastewater - ensuring sufficient turbulence (velocity) so that the sludge remains in good contact with the wastewater. In a preferred form of implementation, aeration is carried out via surface aeration. In this case, aeration is performed by mechanical forces exerted on the liquid by horizontal rotors or vertical turbine or point aerators. Oxygen is introduced into the liquid by: the movement of the liquid surface, air bubbles carried along by the liquid, sprayed liquids, and the air-liquid mixture at the location of the aerator, where air is forced into the liquid. 20 The amount of oxygen introduced into the liquid is influenced by the cross-section of the rotating element, the rotational speed, the immersion depth, and the shape and placement of the teeth or blades.In addition to the introduction of oxygen, circulation currents must be generated by the aerators in the aeration room to prevent the settling of the activated sludge. The shape, dimensions, and volume of the aeration room must be such in relation to the aerator that the circulation is sufficient when the required amount of oxygen is introduced. In the activated sludge process, it is possible to convert nitrogen by biological means. Initially, this takes place via nitrification, or the oxidative conversion to nitrate; subsequently, the nitrate can be further reduced to nitrogen (denitrification). Via nitrification / denitrification, the nitrogen in the liquid fraction is converted. During denitrification, bacteria convert ammonia (NH3) into nitrate (NO3-) in the presence of oxygen. During denitrification, nitrate is in turn converted into the harmless nitrogen gas (N2) in the absence of oxygen. The conversion of ammonium to nitrate takes place in two steps with nitrite as an intermediate product; the first step is the slowest and is brought about by the bacterial species Nitrosomonas.These bacteria grow slowly and will only occur 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. Furthermore, the oxygen concentration in the activated sludge must also be at a sufficient level, at least 0.5–1.0 mgO2 / l.10 A large number of bacterial species are able to effect an oxidative reaction at a very low level of dissolved oxygen (in an anoxic environment) in order to use the oxygen present in nitrate or nitrite for their own respiratory processes. These bacteria are therefore able to switch from free dissolved (atmospheric) 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 acts as the driving force for the occurrence of nitrification, which leads to oxygen consumption.For this process, oxygen-poor (anoxic) conditions are therefore required, as well as the presence of easily degradable organic material. In one configuration, the aeration tank comprises two compartments. In the first compartment, there is aeration. The first compartment is suitable for nitrification. In the second compartment, there is no aeration. The second compartment is suitable for nitrification. Alternatively, the aeration tank comprises a single compartment and the aeration system includes a time-switched control, suitable for time-switched switching on or off of the aeration system. This is advantageous for nitrification during switched-on aeration and nitrification during switched-off aeration in a single compartment. In one configuration, the sludge / water mixture is pumped after the nitrification step into one or more aeration tanks or thickeners to evaporate part of the water. BE2024 / 5933 13 As described above, the digestate from the digesters is separated on a centrifuge into a thick fraction and a thin fraction.The thick fraction is discharged and the thin fraction is biologically purified. However, separation in a centrifuge is not absolute; coarser suspended particles remain behind in the biological ffluent, such as organic fibers, pieces of plastic, and other coarser particles. These particles can severely contaminate the UF membranes and must therefore be removed before the UF. The substances occur in widely varying sizes in the biological ffluent. Visible particles or undissolved substances (also called suspended solids) are present when the size is 0.1 µm or larger. Examples include sand, silt, clay, organic waste, and microorganisms such as algae. The total suspended solids content is indicated 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 µm or smaller. The total content of dissolved and undissolved substances is determined by evaporating a sample and then drying the residue.By 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, the concentration of undissolved substances (mg / l) can be determined. After evaporation, the dissolved solids content follows from the filtrate. It is important to reduce the content of suspended solids (TSS) in the biological ffluent outlet, as these cause significant membrane fouling and flaking in the pressure-controlled 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), whereby suspended particles are purified from the biological ffluent until a TSS content lower than 40,000 mg / liter is achieved. To remove these suspended particles, the biological ffluent is therefore filtered through a screen in the present invention. Screens are used as fine mesh screens in wastewater treatment.There are various types in use: one of them is the drum screen.35 BE2024 / 5933 14 A drum screen consists of a slowly rotating drum equipped 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 jacket. The screened particles remain behind in the drum and are moved and ejected to the end of the screen drum by the rotating movement of the drum and the internal screw. Even more small particles can be removed from the water in this way than with screen installations. In a preferred configuration, a rotating drum screen is used. Preferably, the rotating drum screen is an internally fed drum screen with perforations of 0.8-1.0 mm. The biological ffluent passes through the perforations and the coarser particles are screened off. In one configuration, the purified biological ffluent is transported to the UF unit after one or more screening steps.The aeration tank contains a discharge, configured for discharging the obtained biological ffluent from the aeration tank. In another configuration, namely when the suspended particulate matter content remains too high after screening, a flotation technique such as Dissolved Air Flotation (DAF) is used. In a further configuration, this DAF installation is preceded by a screening step using a screen. In a further preferred configuration, a rotating drum screen is also used here. Preferably, the rotating drum screen is an internally fed drum screen with perforations of 0.8-1.0 mm. The biological ffluent falls through the perforations and the coarser particles are screened off. A DAF flotation system is based on the principle of accelerated propulsion of 30 dirt particles by the introduction of tiny air bubbles that adhere to the dirt particles (= Dissolved Air Flotation).The DAF installation comprises a bath with an inlet, where the inlet is configured for feeding the biological ffluent from one or more aeration tanks into the bath of the DAF installation. The DAF installation comprises means for feeding one or more coagulants and / or polymers (also referred to as flocculants) to the biological ffluent. The DAF installation optionally comprises an inlet for a base or an acid. This is beneficial for pH correction of the biological ffluent treated with coagulants and / or polymers. Preferably, the DAF installation comprises a mixer, configured for mixing the biological effluent and the coagulant / polymer. In one design form, the DAF installation is preceded by a double agitator flocculator, for example divided into two compartments: a coagulation compartments and a flocculation compartment. In a further configuration, the water flows by gravity from the coagulation compartment to the flocculation compartment and then finally to the bath of the DAF installation.Non-exhaustive examples of coagulants are FeCl3 and Fe2(SO4)3. Coagulants are beneficial for floc formation due to the stabilization of colloidal particles in the biological ffluent. The DAF installation optionally includes a supply for 15% flocculant (polymer). Flocculants are beneficial for further increasing the size of the flocs formed by the coagulant. Flocculant is added together with the coagulant or after the addition of the coagulant. The DAF installation includes a pressure vessel. The pressure vessel is configured to contain biological ffluent under a pressure of at least 3 bar, preferably at least 4 bar, and more preferably at least 520 bar. The pressure vessel includes means for introducing air into the pressure vessel. The DAF installation includes a pipe from the pressure vessel to the bath. In a single design, a centrifugal pump pumps purified water to the pressure vessel, which ensures optimal pressure for dissolving air in the water. In this pressure vessel, compressed air is also dosed in a controlled manner so that optimally air-saturated water is created. From this pressure vessel, this air-saturated water is led back to the bath.Due to the pressure drop (or relaxation) of this water (from 5-6 bar to 0 bar), the dissolved air is released back into the bath in the form of tiny air bubbles (30-50 μm, the so-called sparkling water or 'milk water'). These air bubbles attach themselves to the dirt particles and cause them to float. In one configuration, the wastewater is distributed across the full width of the installation via a distribution system, and sparkling water is also injected at the level of this inlet. The air bubbles adhere to the flakes and form a sludge layer on the surface.35 BE2024 / 5933 16 The bath contains a drain configured for the discharge of settled flakes from the bath. The DAF installation contains means configured for the removal of the floating flakes. A non-exhaustive example is a scraper configured for scraping floating flakes from the bath. In one configuration, this layer is removed by a scraper.In one configuration, 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 fermented into a digestate that is subsequently separated again into a thin fraction and a thick fraction, and so on. Heavier particles such as sand will not float, but will settle. In one configuration, the underside of the DAF installation comprises pneumatically controlled valves that can be opened to remove any sediment. In another configuration, these valves are opened by timer control. The bottom sludge is pushed out by the pressure of the water in the flotation. In another configuration, the feed of the DAF installation comprises electromagnetic flow measurement and a controlled valve to regulate the dosages of coagulant (flocculant) as a function of the flow rate. 20 In a configuration, the DAF installation has a flow rate between 10-30 m³ / hour, for example 20 m³ / hour.In a single configuration, the floating elements are returned to one or more fermentation tanks after separation. The bath contains a drain, configured for the discharge of liquid, whereby the liquid is substantially purified of flocs. In a single configuration, after the flotation technique, the purified biological effluent is again fed to one or more aeration tanks, before it is treated in the UF unit. Before filtration with UF membranes, the influent water must generally be purified to remove particles larger than 1 mm, and high-density particles must also be removed to minimize wear on the active separation layers. The TSS content may not exceed 40,000 mg / liter and is preferably 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 this invention we reveal a combination of a (centrifugal) separator (step b), a denitrification step (aeration in step c), followed by sieve filtration and optionally a flotation technique such as DAF, which delivers the desired water quality for the UF phase.5 The ultrafiltration unit comprises filter membranes. The filter membranes are installed dry. The ultrafiltration unit comprises a pump, configured for pumping the liquid under pressure from a first side to a second side through the filter membranes. The ultrafiltration unit comprises a liquid discharge at the second side. An ultrafiltration unit is beneficial for removing residual undissolved substances and / or macromolecules in a liquid, whereby semipermeable membranes in a reverse eosomosis installation need to be replaced or cleaned less frequently. The residual undissolved substances and / or macromolecules that 15% of the liquid originating from the aeration tank is filtered by the ultrafiltration unit, upgraded sludge from the ultrafiltration unit.The sludge from the ultrafiltration unit is also beneficial as biomass in one or more aeration tanks. The purification plant includes piping for transporting the liquid from the discharge on the second side of the ultrafiltration unit to the reverse eososis unit. The ultrafiltration membranes are available in two types of geometries: inside-outside filtration and outside-inside filtration. In a preferred configuration, the current method uses inside-outside filtration, in which the UF permeate is further treated by the RO unit. In addition to the three geometries, there are various membrane materials, such as PVDF, PAN, and PES, which require very diverse 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 highly dependent on the design of the system. 30 In a preferred design, 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 20 nm. 35 BE2024 / 5933 18 The current invention makes use of a cross-flow ultrafiltration (UF) unit where the process flow of the ultrafiltration consists of a successive circuit between filtration and (chemical) backwashing. A cross-flow filtration unit is a filtration system in which a liquid flows along the surface of a filter membrane instead of perpendicularly through it. This flow principle, known as cross-flow, minimizes the buildup of solids on the membrane surface and promotes a more efficient separation of the desired components, such as dissolved substances or pure water, from impurities or solid particles. The liquid flow therefore 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, piping, a membrane module, and a pressure control mechanism.The membrane divides the feed into two streams, namely the liquid passing through the membrane containing the permeable components (the permeate) and the liquid flowing along the membrane containing the retained components (the concentrate). A cross-flow filtration unit has the advantage that a constant filtration rate is maintained because the cross-flow movement reduces fouling on the membrane surface. During filtration, biologically treated effluent water is fed to the inside of the membranes of the cross-flow unit. Water will permeate through the membrane to the outside, and suspended particles are retained by the membrane and discharged with the concentrate. In a cross-flow setup, membrane fouling is reduced to a minimum by a high longitudinal flow velocity of the water over the membrane surface. Suspended particles are carried along with the water flow, thus keeping the membrane surface clear.30 In one design, the transmembrane pressure (pressure across the membrane = measure of membrane fouling) is continuously monitored. In the event of elevated transmembrane pressure, the water flow rate can be temporarily increased to achieve a higher longitudinal flow velocity (2-3 m / s) to clean the membrane. Afterwards, the velocity drops back to the normal velocity.35 BE2024 / 5933 19 Due to the accumulation of fouling on the membrane surface, the transmembrane pressure rises and backwashing is necessary to remove the particulate material and restore efficiency. During backwashing, a portion of the filtrate is returned through the membrane from the filtrate side. This results in the removal of fouling on the feed side of the membrane. In one design, 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. In one implementation form, the method comprises a chemical backwashing step of 10 ultrafiltration units.The efficiency of the backwash is increased by regularly dosing disinfecting chemicals (e.g., sodium hypochlorite or bleach) into the discharge line of the backwash pump. In one execution form, this chemical backwashing step lasts between 30 seconds and 360 seconds, preferably between 30 seconds and 240 seconds, or preferably between 6015 seconds and 180 seconds, such as 120 seconds. In one execution form, sodium hydroxide is also added to the sodium hypochlorite to increase the efficiency of the zelate. Sodium hypochlorite and sodium hydroxide are dosed to inhibit biological growth. Occasionally, a quantity of citric acid is also dosed through extended backwashing; this removes any inorganic precipitate. Due to these regular cleanings, it is important to select a sufficiently chemically strong membrane for the UF unit. Preferably, the UF membranes used are made of PVDF (polyvinylidene fluoride), known for their mechanical strength and chemical resistance.In a preferred configuration, the cross-flow filtration unit comprises more than one membrane module. Preferably, each membrane module can be backwashed separately. By backwashing the membrane module separately, the ultrafiltration unit can remain operational and continuous permeate is available from the ultrafiltration step. 35 In one implementation form, a C.IP (Cleaning-In-Place) cycle is performed, for example when the transmembrane pressure can no longer be restored BE2024 / 5933 20 during the (chemical) backwash regime. In 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 one implementation form, the installation is flushed and restarted after the C.IP.5 A combination of various 2-way valves (open / closed status) provides the necessary process control for various UF membranes.Additional valves enable the use of intermediate storage tanks for various processes involved in the maintenance of the UF filters. In one configuration of the invention, a common pump is used for multiple purposes, such as UF backwashing, in-situ UF cleaning, RO flushing, and in-situ RO cleaning. In one configuration, the UF installation has a flow rate between 5 and 2015 m³ / hour, for example 10 m³ / hour. In one configuration, the UF installation has a membrane surface area between 150 and 300 m². In one configuration, the concentrate from this UF step is recirculated back to one or more aeration tanks. The reverse eosomosis step will further purify the permeate from the ulftrofiltration into dischargeable water. The mechanism of reverse osmosis differs from filtration in the sense that in reverse osmosis there is no physical separation whatsoever.The point is that waters and substances with a smaller molecular weight are able to spread through the membrane polymer by moving between the segments of the polymer's chemical structure. However, dissolved salts and organic substances with a larger molecular weight do not penetrate the membrane due to their size and chemical properties. The reverse osmosis membrane is also capable of achieving complete removal of suspended particles. The success of reverse osmosis technology is largely due to its low cost and simplicity. Compared with other salt-removing technologies, it is relatively inexpensive to purchase and operate. BE2024 / 5933 21 The reverse osmosis unit comprises a semipermeable membrane. The reverse osmosis unit comprises an inlet for liquids and a first outlet for liquid on a first side of the semipermeable membrane. The reverse osmosis unit comprises a second outlet for liquid on a second opposite side5 of the semipermeable membrane.The reverse eosomosis unit comprises a pump configured to pump the liquid under pressure from the discharge at the second side of the ultrafiltration unit into the liquid inlet. The pressure here is at least higher than the osmotic pressure in the liquid. As a result, water migrates through the semipermeable membrane from the first side to the second side. Liquid with an increased mineral concentration is discharged via the first discharge, and purified water is discharged via the second discharge. Due to the increased mineral concentration in the mineral concentrate, the mineral concentrate is more valuable than a thin fraction, for example, for a fertilizer producer. Because purified water is removed from the mineral concentrate, transport costs for the mineral concentrate are lower than for the thin fraction. 20 An RO membrane is designed to retain ions, such as Na+ and Cl-, thereby enabling water desalination. Typically, these membranes are capable of retaining more than 99% of monovalent and divalent ions, such as K+, Na+, Cl-, Ca2+, Mg2+ and SO4 2-.In a preferred design, the membrane of the RO unit has a pore size between 0.1-1 nm. In design 25, the membranes are spiral-wound. In a design, the RO unit comprises between 20 and 50 membranes, for example 30 membranes with high salt tolerance. In a design, membranes have a surface area between 30-50 m² / piece, for example 40.9 m² / piece. In a preferred design, these membranes have a salt retention of at least 30%, preferably at least 97%. In a design, multiple membranes (for example 6 membranes) are placed in a pressure tube. In a preferred design, the RO unit comprises 5 pressure tubes, each containing 6 membranes (30 membranes in total). 35 Demeester RO membranes require pressure to filter water through them. This pressure requirement is directly related to the salt concentrations (or total dissolved substances, TDS) in the water to be processed. A higher TDS requires a higher inlet pressure to overcome the osmotic pressure.Higher pressure entails increased energy consumption by pumps to allow water to permeate through RO membranes. 5 For desalination, it is necessary to optimize the applied pressure for maximum efficiency (ratio of permeate to input). 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 one configuration, the RO permeate is produced at a flow rate of 10.5-15 m³ / hour, preferably at a flow rate between 4-10 m³ / hour. In another configuration, the RO unit is fed at a flow rate of 10-25 m³ / hour. In one configuration, the RO unit comprises one or more dosing pumps for dosing, for example, antiscalants, biocides, and / or acids.15 Antiscalant (scale inhibitor) eliminates limescale deposits and prevents fouling and soiling, which extends the service life of the RO elements. RO membrane biocides are chemical compounds designed to prevent biological fouling or to remove biological contamination from RO membranes.The most important goal of using an effective biocide program is to control biological fouling in membranes, so that it can be relatively cost-effective 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 system performance and protecting the membranes from 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 lifespan of the membranes. By lowering the pH to 30 (usually to a level between 5 and 7), the formation of calcium carbonate (CaCO₃) 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 prevent this and extends the lifespan of the membranes. A lower pH can inhibit the growth of microorganisms, 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 removed more effectively at a lower pH. This can contribute to better total salt removal by the RO system. Anti-scaling agents and other chemicals used in RO systems often work better within a specific pH range. By adjusting the pH, the action of these chemicals is optimized. In practice, an acid, such as sulfuric acid (H₂SO₄) or hydrochloric acid (HCl), is often used. added to the inlet water to lower the pH and prevent scaling. It is important that the pH is carefully monitored, because low values ​​can cause corrosion in pipes and components.10 The current method makes it possible to obtain an RO permeate that is dischargeable.The degree of pollution is determined by the amount of organic matter present (biochemical oxygen demand (BOD) or chemical oxygen demand (COD)) and the nutrients (nitrogen and phosphate).15 The oxygen-consuming substances can be distinguished into three groups: -organic carbon compounds; -ammonium nitrogen and organically bound nitrogen; -other inorganic substances, such as divalent iron compounds, nitrites and20 sulfites. To get an impression of the content of organic carbon compounds, the most relevant methods are the determination of: -biochemical oxygen demand (BOD), i.e. by means of bacteria;25 -chemical oxygen demand (COD), using potassium dichromate. The oxygen demand by ammonium nitrogen and organically bound nitrogen is determined in the Kjeldahl assay. The biochemical oxygen demand, BOD, is the amount of oxygen in mg required to convert the biochemically oxidizable components present in 1 liter of water by means of bacteria.A sample of wastewater is mixed with pure water with a known oxygen content, and the amount of oxygen consumed for the oxidation of the organic substance is determined after the mixture has (usually) been stored for 5 days in a dark place at 20°C. The experiment must be performed in the dark,35 because oxygen production by algae cannot occur simultaneously then. Two oxygen measurements are required, namely one before and one after the experiment. The higher the content of biochemically oxidizable substances, the more oxygen will be consumed. In practice, the BVZ20 5 is usually used, which means an oxidation that lasts 5 days at a temperature of 20°C. However, this does not mean that all biochemically oxidizable components are completely oxidized by the bacteria after 5 days, because a much longer time is required for complete conversion. During oxidation along 5 By the biochemical pathway, those biochemically oxidizable components are oxidized first, which are most easily absorbed by bacteria as food.The RO permeate obtained via the current invention has a biochemical oxygen demand (BOD) value lower than 25 mg / L (at 20°C and 5 days).10 In the determination of COD, most organic compounds are extensively oxidized chemically. Potassium dichromate is used as an oxidizing agent. To a sample, for the determination of COD, the following are added: - a known quantity of potassium dichromate (K2Cr2O7);15 - a certain quantity of silver sulfate (Ag2SO4) that serves as a catalyst for oxidation; - mercury(II) sulfate (HgSO4) to prevent oxidation of chloride. After two hours of boiling under reflux in a flask, the remaining quantity of potassium dichromate is determined. From the difference between the original quantity of potassium dichromate and the remainder, the amount of oxygen consumed can be calculated. The RO permeate obtained via the current invention has a chemical oxygen demand (COD) value lower than 125 mg / L.25 In an execution form, the RO permeate has a tenfold reduction in the COD value compared to the COD value of the biological ffluent obtained after step c.30 Nitrogen can occur in different forms, namely as organically bound nitrogen, such as in proteins or their breakdown products (amino acids), and in inorganic form, as NH3 (ammonia) or NH4+ (ammonium) or in its oxidized form (NO2- or NO3-). During the mineralization process, the organically bound nitrogen first passes over ammonium ions (or ammonia, depending on the pH). The content of organically bound nitrogen and ammonium nitrogen in mg / l is determined according to the Kjeldahl method. This is therefore also referred to as BE2024 / 5933 25 Kjeldahl nitrogen. The oxidation of ammonium occurs in two stages. For the first nitrification stage, the bacterial species Nitrosomonas is required (nitrite formation), while In the second nitrification stage, the bacterial species Nitrobacter exclusively causes the formation of nitrate. Denitrifying bacteria develop only slowly; the reactions occur as long as the temperature does not drop below 5-10°C. Denitrification ceases when the oxygen level has dropped to approximately 1 mg / l or lower.The RO permeate obtained via the current invention has a total nitrogen content (Ntotal) lower than 15 mg / L.10 The RO permeate obtained via the current invention has a total phosphate content (Ptotal) lower than 2 mg / L. The RO permeate obtained via the current invention has a content of suspended solids (TSS, total suspended solids) lower than 10 mg / L. The obtained RO permeate is suitable as irrigation water. The purified water is also dischargeable. The purified water can also be made suitable for use as process water in a food processing plant, for example for rinsing and / or blanching vegetables. In one implementation form, the resulting RO permeate is used in the flotation technique to reduce the suspended solids content. In one implementation form, the sparkling water, which is important for proper flotation, is continuously produced by circulating a part of the purified water (the RO permeate produced during the last step of the procedure).As described above, in one execution form a polymer mixture is used during the centrifugal separation step. In a further execution form30 the polymer mixture is prepared with the resulting RO permeate from the last step of the process and a powder polymer. Such RO permeate is sufficiently purified and, moreover, heated. In what follows, the invention is described by means of non-limiting examples35 which illustrate the invention, and which are not intended or should not be interpreted to limit the scope of the invention. BE2024 / 5933 26 EXAMPLES EXAMPLE 1 (see figure 1): 5 Biological waste (g), such as manure / or vegetable waste / or fruit waste, is digested in a digestion tank (1) by a thermophilic process, in which 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, where the thin fraction has a dry matter content lower than 4%. During the separation process10, a polymer is added. This polymer is produced using RO permeate (f).The phosphate remains mainly in the thick fraction, so that the thin fraction has a phosphate (P2O5) content that amounts to a maximum of 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 ffluent (c) with a nitrogen content lower than 2000 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 fertilizer producer will be lower than the transport costs for transporting the digestate, making the first thick fraction more valuable as a raw material than the digestate. 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 affect the oxygen content in the water by contributing to the growth of algae and other aquatic plants.This process, known as eutrophication, can lead to oxygen deficiencies and suffocation of aquatic organisms, with far-reaching consequences for biodiversity and water quality.30 Previous wastewater treatment steps (in particular centrifugal separation into a thin fraction and a thick fraction and aeration of the thin fraction) ensure a significant reduction of these inorganic substances in the thin fraction, which is further processed into dischargeable water.35 BE2024 / 5933 27 However, separation by centrifugation is not absolute; coarser suspended particles such as organic fibers, pieces of plastic, and other coarser particles remain in the biological ffluent. These particles can severely contaminate the UF membranes and must therefore be removed before the UF. 5 It is important to reduce the content of total suspended solids (TSS) in the biological ffluent, as these cause significant membrane fouling and flaking in the pressure-controlled membrane processes of the subsequent water treatment.10 To remove the coarser suspended particles, the biological effluent is filtered through a rotating drum screen with perforations of 0.8-1.0 mm (4). The biological effluent passes through the perforations and the coarser particles are sieved off. The purified biological effluent (d) is treated in the UF unit (6). 15 When the suspended particle content is too high, the Dissolved Air Flotation (DAF) (5) flotation technique is used. A screen (8) is also always installed in front of the DAF installation to prevent clogging. This screen (8) is also a rotating drum screen with perforations of 0.8-1.0 mm. 20 A DAF flotation system is based on the principle of accelerated buoyancy of dirt particles by the introduction of tiny air bubbles that adhere to the dirt particles (=Dissolved Air Flotation). The DAF system comprises a bath with a feeder, where the feeder is configured to feed biological ffluent from the aeration tank into the bath of the DAF system. Coagulant (for example FeCl3 and Fe2(SO4)3) is added to the liquid fraction in the bath.pH correction of the biological effluent treated with coagulant takes place by adding a base or acid, and the whole is mixed. Coagulant is beneficial for the formation of flocs by destabilizing colloidal particles in the thin fraction. Flocculant is added. Flocculant is beneficial for further enlarging flakes formed by the coagulant. Flocculant is added together with the coagulant or after the addition of the coagulant. A centrifugal pump pumps purified water (specifically the recycled RO permeate obtained according to the current invention) to the pressure vessel35 and ensures optimal pressure for dissolving air in water. In this pressure vessel, compressed air is also dosed in a controlled manner so that optimally air-saturated water is created BE2024 / 5933 28. From this pressure vessel, this air-saturated water is led back to the bath. Due to the pressure drop (or relaxation) of this water (from 5-6 bar to 0 bar), the dissolved air is released back into the bath in the form of minuscule air bubbles (30-50 μm, the so-called sparkling water or 'milk water').These air bubbles attach to the dirt particles and cause them to float. The air bubbles adhere to the flakes and form a layer of sludge on 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 contains pneumatically controlled valves that can be opened to remove any sediment. These valves are opened by timer control. The bottom sludge is pushed outwards by the pressure of the water in the flotation. The feed of the DAF installation contains an electromagnetic flow meter and a controlled valve to regulate the dosages of coagulants and flocculants as a function of the flow rate. After the flotation technique, the biological effluent is returned to one or more aeration tanks(3).20 Through these screening steps and DAF treatment, a suspended solids (TSS) content lower than 40000, preferably lower than 35000 mg / liter is obtained. This minimizes wear on the active separation layers of the UF unit.25 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 backwash step. 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 suspended particles are retained by the membrane and discharged with the concentrate. Membrane fouling in a cross-flow setup is reduced to a minimum by a high longitudinal flow velocity of the water over the membrane surface. BE2024 / 5933 29 Suspended particles are carried along with the water flow, thus keeping the membrane surface clear. The transmembrane pressure (pressure across the membrane = measure of membrane fouling) is continuously monitored. In the event of elevated transmembrane pressure, the water flow rate is temporarily increased to achieve a higher longitudinal flow velocity (2-3 m / s) in order to clean the membrane. Afterwards, the velocity drops back to the normal velocity.Due to the accumulation of fouling on the membrane surface, the transmembrane pressure rises, and backwashing is necessary to remove the particulate material and restore efficiency. During backwashing (±45 seconds), a portion of the filtrate is returned through the membrane from the filtrate side. This results in the removal of fouling on the feed side of the membrane. The efficiency of backwashing is increased by regularly dosing disinfecting chemicals (sodium hypochlorite or bleach) into the discharge line of the backwash pump. This chemical backwashing step lasts, for example, 120 seconds. The UF membranes used are manufactured from PVDF (polyvinylidene fluoride), known for its mechanical strength and chemical resistance. The cross-flow filtration unit (6) has multiple membrane modules, which can be backwashed individually. By backwashing the membrane module individually, the ultrafiltration unit can remain operational and there is continuous permeate available from the ultrafiltration step.The reverse eosmosis step will further purify the permeate from the ulftrafiltration(e) into dischargeable water. The reverse eosmosis unit contains a 30 semipermeable membrane. The reverse eosmosis unit(7) contains a liquid inlet and a first liquid outlet on a first side of the semipermeable membrane. The reverse eosmosis unit contains a second liquid outlet on a second opposite side of the semipermeable membrane. The reverse eosmosis unit contains a pump, geco.