EP2297052A1

EP2297052A1 - Dewatering of anaerobically digested sludge

Info

Publication number: EP2297052A1
Application number: EP09770493A
Authority: EP
Inventors: Anders ADOLFSÉN; Jan Kastensson; Anna Svensson
Original assignee: Mercatus Engineering AB
Current assignee: Mercatus Engineering AB
Priority date: 2008-06-27
Filing date: 2009-06-25
Publication date: 2011-03-23
Also published as: SE0801525L; CN102076617A; SE532532C2; WO2009157868A1; EP2297052A4

Abstract

A method for treatment of sludge from biogas production plants, said sludge comprising at least partly suspended biological matter, is provided. Said method comprises the steps of elevating the temperature of the sludge; and filtrating the sludge in one or more ultra filtration and/or micro filtration units, whereby a concentrate and a permeate is obtained, said concentrate comprising substantially all said suspended biologica lmatter and said permeate comprising substantially no suspended biological matter.

Description

Dewatering of anaerobically digested sludge

Field of the Invention This invention pertains in general to the field of treatment of sludge, such as entirely or partly biological sludge. More particularly the invention relates to a method of treatment of sludge, such as entirely or partly biological sludge.

Background of the Invention Sludge, that is a byproduct from e.g. digesters for preparation of biogas, consists mainly of non-digestible biological matter. In the process of digestion, the digestible carbon is converted to methane gas, which may be used as bio-energy, and reduces the volume of the incoming matter.

Depending on how the sludge is treated after digestion, access to water during the process, costs for transport and storage etc. there may be many reasons for separation, purification and recycling of water from sludge. In certain processes, the water is separated from the sludge in order to be re-circulated in the digestion process. Such re-circulated water is called make-up water. Other reasons for dewatering of sludge may be to achieve sludge with higher concentration and thereby higher nutritional value, in order to enhance the economy of storage, transport and spreading. Dewatering may give water that needs purification to a sufficient degree, in order to let it out in the recipient/sewage system.

It is well known in the art that hydrolysis of the sludge before digestion increases the production of methane, since organic molecules and molecule complexes that are hard to break down, are decomposed into smaller molecules that are more easily broken down e.g. by bacteria. The sludge can be hydrolyzed by lowering the pH with a strong acid, by heating the sludge, or by a combination of acid treatment and heating. Digested biological sludge comprises suspended substances, dissolved phosphor and nitrogen, and organic carbon compounds. Conventional dewatering of sludge in screen band presses or centrifuges give water with a very high content of suspended substances, especially if the sludge has been thermally hydrolyzed before digestion. Not even very substantial addition of polymers as filtering aids give more than marginally improved water quality. Addition of polymer in combination with trivalent iron or lime, or a combination of polymer, trivalent iron and lime, improve water quality, but is expensive and increase the sludge amount substantially, which is negative. When using processes that re-circulate make-up water, it is important that the make-up water is sufficiently purified, in order to avoid concentration of non-digestible components. A common problem with state of the art technology, where the water is not sufficiently purified, is that the re-circulated non-digestible components will occupy increasingly more space in relation to the applied biomass and thereby the efficacy and profitability of the process will be lower.

Especially pollutants in suspended form lead to problems in the biological purification, since micro-particles comprising organic carbon tend not to be separated with conventional technology, and thereby lead to greatly increased carbon oxygen demand (COD) in the treated exit water. Also, recirculation of make-up water may result in concentration of other substances, e.g. chlorides and ammonia, which may disturb the digestion process if present in too high concentrations.

In other, equally probable processes, the water is separated from the sludge and then let out in the recipient/sewage system without being re-circulated. With conventional technology, e.g. as mentioned above, water that is separated to be re-circulated or let out in the sewage system will be heavily polluted and require subsequent purification.

Purification with traditional technology may in this way also cause problems, since traditional technology in special cases is insufficient to avoid that non-digestible matter is re-circulated and concentrated in the process of recycling extracted water from the process of digestion.

In the digested sludge that is separated with conventional de-watering, there are also pathogenic bacteria and virus. These pathogens may cause disease or damages on living organisms, e.g. when the sludge is used as fertilizer. In some settings, this is handled by hygienizing the sludge before digestion, but conventional methods may not be sufficient to completely eradicate all pathogens. These pathogens may then multiply in the digestion, which normally is performed at temperatures around 40 to 60°C. The thermal hygienization that is performed according to the art is costly, both from an investment and operational point of view. Treatment of chemical sludge, i.e. sludge comprising iron- or aluminum hydroxide, with ultra- or microfiltration at high temperatures is described in WO 03/099728. However, this document only considers recycling of iron or aluminum from sludge by initial acidification of sludge and does not concern improving production of biogas from biological sludge. In fact, the initial acidification makes this method unsuitable for digestion of sludge, since the low pH hinders the digestion process.

Hence, an improved treatment of sludge from digesters would be advantageous and in particular a method for dewatering sludge, which improves sludge properties in relation to recycling and/or outlet into in the recipient/sewage system of water from production of biogas, improved hydrolysis, and more efficient and cheaper hygienization of sludge.

Summary of the Invention Accordingly, the present invention preferably seeks to mitigate, alleviate or eliminate one or more of the above-identified deficiencies in the art and disadvantages singly or in any combination and solves at least the above-mentioned problems by providing a method for treatment of sludge comprising elevating the temperature of the sludge; and filtrating the sludge in one or more ultra filtration (UF) and/or micro filtration (MF) units, whereby a concentrate and a permeate is obtained, said concentrate comprising substantially all said suspended biological matter and said permeate comprising substantially no suspended biological matter.

Advantageous features of the invention are defined in the dependent claims.

Brief Description of the Drawings

These and other aspects, features and advantages of which the invention is capable of will be apparent and elucidated from the following description of embodiments of the present invention, reference being made to the accompanying drawings, in which Fig. 1 is a process scheme according to one embodiment of the invention;

Fig. 2 is a process scheme according to another embodiment of the invention;

Fig. 3 is a process scheme according to yet another embodiment of the invention; and

Fig. 4 is a part process scheme according to yet another embodiment of the present invention.

Fig. 5 is a process scheme according to a further embodiment of the invention.

Fig. 6 is a process scheme according to yet a further embodiment of the invention. Description of embodiments

The following description focuses on an embodiment of the present invention applicable to treatment of sludge and in particular to a method for treating sludge, wherein the dewatering of sludge is performed by ultra filtration (UF) and/or micro filtration (MF). Any combination of only UF units, only micro filtration units or UF units combined with micro filtration units may be used within the scope of the invention. However, ultra filtration, used by itself, is preferable. The ultra filtration units may be positioned in series and/or in parallel.

Usually, i.e. according to methods in prior art, UF is working at temperatures well below 100 ⁰C, such as from 20 to 50 ⁰C. In this temperature interval, sludge is viscous and its content of cells, proteins, fat and other macromolecules makes the flux low and the risk of fouling high. This means that the process must be run with low stress and frequent washes of the equipment and replacement of membranes in order to avoid problems. Traditional UF technology also demands open channels in order to work, which gives the process a low efficiency. The high temperature at which UF according to the present invention is working leads to sludge with low viscosity and good filtration properties, which gives a high flux and low risk of fouling even if the concentration of biological matter in the incoming sludge is high. The high temperature makes cell walls burst, proteins coagulate and macromolecules disintegrate. The heat formed in the UF step or steps according to embodiments of the present invention may also be advantageously heat exchanged with incoming sludge. The technology with UF may thereby give significant energy savings, even considering that ultra filtration in many cases in itself is an energy demanding process. Around 80 to 90% of the added electric effect may be recycled. The added heat energy contributes partly to increase the flux and content of dry substance (DS), and partly to sterilizing and coagulation of proteins. In embodiments of the invention, the temperature of the sludge for ultra filtration may be in the interval of 100 to 250 ⁰C, and preferably in the interval of 100 to 200 ⁰C, such as in the interval of 120 to 170 ⁰C.

The method of filtering the sludge at high temperatures, also has an advantage over prior art by making it possible to concentrate sludge with high capacity and high purity without having problems with stoppage and high costs for maintenance and cleaning. At high temperatures, it is also possible to use a smaller channel size, thereby achieve a higher efficiency in the plant, compared to traditional UF technology.

Thanks to the properties that are gained by the high temperature, a high content of DS may be achieved. The DS content in the concentrate is consistent with the content that is achieved with state of the art technology, but without the use of addition of aiding coagulants.

Ultra- or micro filtration of the sludge separate all suspended substances, which is not the case with state of the art technology, where the reject water has high amounts of suspended matter. Also, UF separates larger organic molecules.

A separation of all suspended matter may give a larger amount of organic carbon in the water, which in turn gives lower costs for treatment and a more simple purification process, if the separated water phase subsequently needs purification.

Additionally, in the case where water is re-circulated, no non-digestible suspension is re-entering the digester. This may give higher process efficiency. Moreover, part of the non-digestible matter in the sludge will be made digestible through hydrolysis at the high temperatures used in the filtration, and thus, as small compounds will pass the UF and be returned as digestible matter through the make-up water, efficiency of the digesting process may be enhanced. Compared to the systems used within the art, this may give a significant increase in capacity of existing plants where re-circulated water is used, which is not sufficiently pure.

Also, hydrolysis of sludge may improve the efficiency when water is separated from sludge and re-circulated.

In case the sludge is not hydro lyzed before the digester, large increases in the process efficiency may be achieved by hydrolyzation when dewatering, and re- circulating make-up water, comprising the decomposed, digestible biologic matter. This efficiency is not achievable through state of the art technology.

The process heat may also be used in the hygienization of sludge that preferably is performed when using sludge as fertilizer. Since the sludge is heated to above 100°C when filtering, a complete sterilization of the de-watered sludge is achieved. This may give significantly improved hygiene compared to state of the art methods. Alternatively, the heat energy in the process may be recycled in the hygienization even before digestion, which saves energy.

According to one embodiment, a system for treatment of digested sludge is described, where the sludge is ultra- or micro filtrated at high temperatures, which gives a permeate, free from suspended substances, and a concentrate, free from bacteria and virus.

Water that is separated during dewatering may be re-circulated in the digestion process or be purified for subsequent outlet, or a combination of re-circulation and outlet. If the content of DS is above about 3-7%, depending of composition, in the digested sludge, it may be an economical advantage to use a conventional pre- dewatering step. Reject water from the pre-dewatering that may normally have a DS content of about 2-5% may then be used in the process according to some embodiments of the invention. However, a drawback with this system is that not all sludge is sterilized.

Since neither ultra- nor microfϊltration of biological, digested sludge give complete separation of dissolved organic substance, further purification of the permeate is often necessary. An alternative to further biological purification is to treat UF or MF filtrate with nano filtration (NF) or reversible osmosis (RO). Nano filtration will separate all organic molecules larger than about 200-400 Dalton and the main part of polyvalent ions, while monovalent ions, e.g. ammonium and chloride, will pass through the membrane. RO will separate substantially all organic molecules and 90 to 95% of the dissolved salts. The most common and cheap NF and RO filters are spiral wound membranes. A prerequisite to be able to use this type of membrane is that the treated liquid is free from suspended substances. A liquid that is UF or MF filtered fulfills this requirement, compared to reject water from a conventional plant.

However, the permeate from UF or MF will still comprise dissolved proteins if the filtration has taken place at conventional, low temperatures. These proteins will cause fouling on the NF and RO membranes. This means that the membranes need to be cleaned often and also that the membranes are hard to clean. Post treatment of the UF or MF permeate with RO or NF will therefore be complicated and uneconomical. When UF or MF of biologically digested sludge is performed at high temperatures, according to some embodiments, the proteins in the sludge will coagulate and be separated. The permeate is free from protein and this makes a post treatment with NF or RO possible. Post treatment of UF or MF permeate with NF or RO offers new opportunities to separate salts, e.g. ammonium from the organic phase. Through purification with RO, ammonium and salts may be separated from the UF permeate and concentration of pollutants in the re-circulated make-up water may be avoided. Through purification with NF ammonium and chlorides is separated from the rest of the organic matter. The organic matter, largely digestible after the UF hydrolysis, may fully or partly be re- circulated in the digestion process and the NF permeate may be purified from ammonia through e.g. stripping. Purification from ammonia may be effectively performed because the major part of the buffering organic matter has been discarded.

In the case where outlet to sewage is desired, a purification step with NF/RO means that subsequent purification may be performed cheaper and more effectively. According to a first embodiment of the present invention a sludge, such as a sludge entirely or partly consisting of biological suspended matter, is treated in accordance with the flow chart of Fig. 1. The sludge has neutral or basic pH. A pump 111 pumps incoming sludge stream 110 into the system and increase the pressure so that it, in all parts of the system, exceeds the pressure of evaporation of water at the specific working temperature. The pump 111 may also be replaced by several pumps, in order to allow separate pressure regulation if the incoming sludge stream 110 is subdivided into several part streams. The electrical energy that is added to the circulation pump(s) is substantially transformed into heat and may be used for heating the sludge and for the thermal hydrolysis. The pressurized stream of sludge may thus be distributed over a first heat exchanger 112 and a second heat exchanger 113. In the first heat exchanger 112, the sludge is subject to heat exchange with hot UF permeate stream 122, exiting from a first 115, second 116 and third 117 UF unit, respectively. In the second heat exchanger 113, the sludge is subject to heat exchange with the hot UF concentrate stream 123, exiting from the first, second and third UF units 115, 116, and 117.

Of course, the first, second and third UF units 115, 116, and 117 may be replaced by one single UF unit, or other combinations of UF units and/or micro filtration units, such as UF units connected in parallel and/or series as described below. To recycle the maximum amount of heat, the incoming flow of sludge may be distributed e.g. proportional to the flow of permeate and concentrate, respectively. After heat exchange, heat energy 114 may be added to compensate for possible heat loss of the system. This heat energy 114 may for example be the addition of hot steam. Hereafter, the heated sludge is treated with ultra filtration (UF) and/or micro filtration. In this example, the first, second and third UF units 115, 116, and 117 are connected in series. A first 118, second 119 and third 120 UF pump are pumping sludge through the first, second and third UF units 115, 116, and 117, respectively.

To operate the UF units, the sludge is put under elevated pressure in one or more pressurizing pumps, the effect of which will be describe more extensively below. This pressure preferably exceeds or equals the pressure of evaporation of water in every point of the UF unit. In order to prevent fouling on membrane surfaces as far as possible, the flow rate through the membrane canals is preferably high.

The first, second and third UF pumps 118, 119, and 120 may have significantly more capacity compared to pump 111. From each UF unit there is an outflow of permeate stream, whereby the concentration of the suspended matter, such as biological suspended matter, in the sludge is increasing in the concentrate stream. The concentrate stream may be partly re-circulated over the membranes and partly transferred to the next membrane step, as illustrated in Fig. 1. Thus, in one embodiment of the invention, the concentrated sludge stream may be recycled under pressure in each UF step. Start of hydrolysis of the sludge partly depends on sludge retention time and partly on the working temperature. The hydrolysis of the sludge may start already in the UF step, i.e. the UF units, depending on the generated heat therein, and the heat added. In another embodiment of the invention according to fig. 3, positioning a tank 121 in one of the particular UF steps further induces hydrolysis. The concentrated flow from the last UF step is then led to the tank 121. In the tank 121 the hot concentrated sludge may get a retention time being sufficient for achieving optimal hydrolysis. In the mean time the sludge may be kept at an elevated temperature in tank 121, also for achieving optimal hydrolysis. Such a retention time and temperature may preferably be in the interval of 15 to 60 minutes and a temperature in the interval of 100 to 250 ⁰C, and preferably in the interval of 100 to 200 ⁰C, such as in the interval of 140 to 165 ⁰C. A UF permeate stream 124, exiting the UF units or the first heat exchanger 112, may then for example be used directly as make-up water with carbon source or further treated for discharge and a UF concentrate stream 125, exiting the UF units, tank 121, or the second heat exchanger 113, may be used as a fertilizer. Thus, an embodiment of a method for treatment of sludge from biogas production plants, said sludge comprising at least partly suspended biological matter, has been described. In this method the temperature of the sludge is elevated, where after the sludge is filtrated in one or more ultra filtration units and/or micro filtration units, whereby a concentrate and a permeate is obtained, said concentrate comprising substantially all said suspended biological matter and said permeate comprising substantially no suspended biological matter.

Factors that may be used to characterize the UF is setup of the filter(s), operating pressure, operating temperature, membrane material, membrane characteristics, transmembrane pressure and feed flow velocity. As indicated above, in an embodiment according to the invention, the method may be performed over one or more UF units and/or micro filtration units, such as ultra filtration membranes. More specifically, in one embodiment of the invention, the UF membranes are connected in series. In another embodiment of the invention, the UF membranes are connected in parallel. In yet another embodiment of the invention, the UF membranes are connected both in series and in parallel. In further embodiments according to the invention, one or more UF membranes of the abovementioned embodiments may be replaced by one or more micro filtration membranes.

To operate the UF units, the sludge is put under elevated pressure in one or more pressurizing pumps until a pressure is achieved. The splitting of the elevation of pressure on several pumps, such as a pump before each UF unit may be beneficial in respect of the stress on the pumps. Also, if one pump fails, other pumps may make up for the decrease in pump capacity, rendering the pressure on at least some of the UF units and/or micro filtration units satisfactory for achieving the demanded filtration effect. In one embodiment of the invention, this pressure exceeds or equals the pressure of evaporation of water in every point of the UF unit. In order to prevent fouling on membrane surfaces as far as possible, the flow rate through the membrane canals is preferably high, such as 6 m/s. In one embodiment of the invention, the flow rate through the membrane canals is in the order of 5 m/s. The flux is partly depending on the transmembrane pressure. A high pressure, above 5 bar, only gives a moderate increase of the flux while the risk of particles penetrating the active parts of the membrane increases. Thus, the transmembrane pressure is preferably in the interval of 2 to 5 bar. In one embodiment of the invention, the average transmembrane pressure is about 3 bar.

As already indicated above, according to one embodiment of the invention, the sludge is heated through heat exchange with e.g. hot UF permeate and concentrate, respectively, exiting from the UF unit(s). This is done to achieve a desired operating temperature. The heat exchange means that most of the heat brought to the UF may be recycled. In order to compensate for systemic heat loss, energy may also be added to the sludge. In one embodiment of the invention, this energy addition is made by injecting heat energy 114, such as hot steam. When the sludge has reached the working temperature desired for the following ultra filtration, the sludge may be lead to one or more UF steps.

The membranes used in the UF units and/or micro filtration units have to be able to withstand the heat and pressure produced. Suitable membranes, which satisfy these demands, are ceramic membranes.

In each filtration step, there is a production of permeate, which means that the sludge (concentrate) is having an increasingly higher content of dry substance after passing each filtration step. In one embodiment, the membrane surface in each UF step and the number of UF steps at a given flux may be adapted partly to the amount of sludge that is to be treated and partly to the content of DS that is desired after the final UF step. At the high temperature of UF, the same content of DS may be achieved as in conventional dewatering equipment, i.e. 10 to 20%. Suspended substances and molecules in the sludge, larger than the membrane cut-off, cannot pass the membranes. This makes the separation of contaminants using UF and/or micro filtration superior to using a centrifuge only. The cut-off does not only constitute a clear boundary for separation based on molecular weight, but also the shape of the molecule matters. The organic molecules that pass the membrane and are found in the permeate are molecules with a relatively low molecular weight compared to those in the concentrate. Because of this, the permeate will contain organic substances that are easy to break down biologically. The permeate may therefore serve as a source of carbon in the sewage treatment works where nitrogen reduction is a part of the purification process or as digestible carbon source in re-circulated make-up water. Since no suspended substances or larger molecules may pass the membrane, the permeate will be cleaner than the reject water from a conventional dewatering equipment. In one embodiment of the invention, the cut-off of the membrane should preferably be 5 to 500 kD.

According to one embodiment of the invention, as indicated above, the hot UF concentrate stream 123 is lead to a reactor tank after concentration with UF, where it is retained for a retention time. The total retention time for the sludge (including the retention time in the UF) depends on the temperature and may vary from 15 to 60 minutes. In one embodiment of the invention, a retention time of 30 minutes at 165⁰C gives a good thermal hydrolysis. After the retention time in the reactor tank, the hydrolyzed sludge may be subject to heat exchanged with a part stream of the incoming cold sludge before it may be led to a buffer tank for storage. The stored sludge may then be used e.g. as fertilizer.

In another embodiment of the present invention, according to Fig. 2, it is disclosed how phosphor may be re-circulated from the permeate exiting from the UF unit(s). Phosphor may be extracted from the sludge according to the following formula:

MePO₄(S) + 3 H⁺ → Me³⁺ + 3 H⁺ + PO₄ ^3"

wherein Me is a metal ion of choice.

In this embodiment sulfuric acid may be added to make the sludge acidic. Other acids may as well be used within the scope of the present invention. The acidification may take place before the first UF step. In this case, all sludge and permeate will be acidified. The pH is preferably decreased to a pH level in the interval of 1 to 2. If the sludge is heavily buffered, the amount of sulfuric acid needed to decrease the pH will increase. Therefore, in another embodiment of the invention, the sludge is acidified before one of the UF units being located subsequent of the first UF unit and not before the first UF unit, in case the UF units are serially arranged. The amount of sludge that will be acidified is then decreased and so is the consumption of sulfuric acid. Also, the non-acidified permeate may contribute to an increased pH if it is mixed with the acidified permeate. This may in turn contribute to decreasing the amount of base needed to later increase the permeate pH.

To increase the retention time and secure a satisfactory dissolution of the metal phosphate in the sludge and/or increase hydrolysis of the sludge, according to one embodiment of the invention, the tank 121 is a part of the last UF step instead of after the last UF step, as illustrated in Fig. 3. After the sludge has passed UF and reaction tank, and has been subject to heat exchange with the incoming sludge, the outgoing concentrated sludge may be neutralized with a suitable base. Such suitable bases may for example be selected from the group consisting of lye/caustic liquor, lime, soda, magnesium oxide/magnesia. The choice of base is preferably made in order to optimize the sludge composition for subsequent use as fertilizer. The pH of the acidified permeate is increased to about 3 by the addition of a suitable base. Such suitable base may for example be NaOH (sodium hydroxide), but other suitable bases may also be used without departing from the scope of the present invention. Before this, the permeate is cooled through a heat exchange process with incoming cold sludge (i.e. the sludge transported to the UF unit(s)). After or before the pH of the acidified permeate has been increased, a chemical coagulant is added.

A suitable chemical coagulant is a trivalent iron salt. At pH about 3, a precipitation of sparingly soluble iron phosphate will occur. The precipitation of iron phosphate is separated e.g. through sedimentation, filtration or centrifugation. Thus, the iron phosphate precipitated from the permeate will contain less amounts of heavy metals per kg phosphate than the untreated sludge, since co -precipitation of heavy metals does not occur at pH 3.

According to yet another embodiment of the invention the hot UF concentrate stream 123 is lead to a flash tank 410, i.e. according to Fig. 4. The flash tank 410 may be provided with an overpressure. The overpressure may for example be 0,1 to 0,5 bar, i.e. that the pressure in the flash tank 410 is 1,1 to 1,5 bar. Thus, water vapor 411 is released when the hot UF concentrate stream 123 enters the flash tank 410, since the pressure in the hot UF concentrate stream 123 is well above 1,1 to 1,5 bar, such as 3 to 8 bars. In the flash tank 410 the level of sludge may be kept substantially constant, such that a flash tank sludge concentrate stream 412 may be drawn off from the bottom of the flash tank 410 while a flash tank water vapor stream 411 may be drawn off at the top of the flash tank 410. The sludge concentrate stream 412 may then be led to a buffer tank for storage. The stored sludge concentrate may then be used e.g. as fertilizer.. The flash tank water vapor stream 411 may be led to, and thereby mixed with, the incoming sludge stream 110. In an embodiment according to fig. 4, the pump 111 is replaced by two pumps I l ia and 11 Ib, which are positioned in each of the part streams of the incoming sludge stream 110. Mixing of the flash tank water vapor stream 411 with the incoming sludge stream 110 may be performed on the suction side of the pump 11 Ib. According to another embodiment, said mixing may however also be performed in a setup with the pump 111 simply replacing the second heat exchanger 113. All of the incoming sludge stream 110 may be led to the first heat exchanger 112 and be subject to heat exchange with the hot UF permeate stream 122, or only a part of the incoming sludge 110 is subject to heat exchange in the first heat exchanger 112. The water vapor stream 411 may thus heat the incoming sludge stream 110, or a part of the incoming sludge stream 110. In the embodiment according to fig. 4, an example of a setup is shown, wherein the water vapor stream 411 heats part of the incoming sludge stream 110 and part of the incoming sludge stream 110 is subject to heat exchange in the first heat exchanger 112.

In an embodiment according to Fig. 5, incoming sludge 500 is lead to a digester 510. Digested sludge 511 is filtrated 512. The filtration 512 may be according to any of the previous embodiments. The filtration concentrate 513 is discarded and may be used as e.g. fertilizer. The filtration permeate may be re-circulated 514 or be lead to post treatment 515. Post treatment 515 may be NF or RO, or a combination between NF and RO. The post treatment concentrate 516 may be further treated and/or discharged. It may also be re-circulated 517. The post treatment permeate may be discharged/further processed 518 or re-circulated 519. Re-circulated filtration permeate 514, re-circulated post treatment concentrate 517 and/or re-circulated post treatment permeate 519 is lead to a make-up water tank 520. From there, it is added 521 to the incoming sludge 500. In on embodiment, wherein the post treatment 515 is NF, the further processing 518 is ammonia removal. In an embodiment according to Fig. 6, no re-cirulation of permeate/filtrate is performed. All further treatment permeate is discharged 518 into e.g. sewage or recipient.

The abovementioned embodiments will now be exemplified in the following, non-limiting examples:

Example 1

In an embodiment of the invention according to Fig. 1, one example of a setup for dewatering and hydrolysis of sludge according to the invention is described. The pump 111 pumps incoming sludge stream 110, with pH between about 5 to about 9, into the system and increase the pressure so that it, in all parts of the system, exceeds the pressure of evaporation of water at the specific working temperature. The pressurized incoming sludge stream 110 is distributed over a first heat exchanger 112 and a second heat exchanger 113. In the first heat exchanger 112, the sludge is subject to heat exchange with the outward-bound, hot UF permeate stream 122 and in the second heat exchanger 113, the sludge is subject to heat exchange with the outward- bound, hot UF concentrate stream 123. To recycle the maximum amount of heat, the incoming flow of sludge may be distributed e.g. proportional to the flow of permeate and concentrate, respectively. After heat exchange, heat energy is added 114 to compensate heat loss of the system, e.g. by addition of hot steam. Hereafter, the heated sludge is treated with ultra filtration (UF). In this example, three UF steps 115, 116 and 117 are connected in series. The UF pumps 118, 119 and 120 are pumping sludge through one UF respectively. The UF pumps 118, 119 and 120 have significantly more capacity compared to pump 111. From each UF step there is an outflow of permeate, whereby the concentration of sludge is increasing in the concentrate that partly is re- circulated over the membranes and partly are taken to the next membrane step. The outward-bound cold UF permeate stream 124 may then be used as make-up water with carbon source or further treated for discharge the outward-bound cold UF concentrate stream 125 may be used as a fertilizer.

Example 2

Further embodiments of the invention is illustrated in Figs. 2 and 3, which embodiments describe another setup for dewatering and hydrolysis of sludge according to the invention, including recovery of phosphor content. The basic setup is the same as in example 1. Before the first 115, second 116 and/or third 117 UF step, sulfuric acid 211 may be added to the sludge, to lower the pH. Consequently, bound phosphate (PO₄ ^") is set free from the sludge/concentrate. The phosphate that has been set free pass the UF membranes 115, 116, 117 and will be found in the UF permeate stream 122. After the UF permeate stream 122 has been cooled in the first heat exchanger 112, a chemical coagulant 212 is added. An example of such chemical coagulant is Fe³⁺, such as for example iron chloride. Before or after adding said chemical coagulant pH may be adjusted by adding a base 213, e.g. lye. The addition of the chemical coagulant and/or base may take place in a second tank 214. Alternatively, the chemical coagulant and/or the base may be added before said second tank 214 in the conduit leading the UF permeate stream 122 to the second tank 214. Preferably, said second tank 214 is provided with a stirrer. A sparingly soluble precipitation of iron phosphate will be formed. The precipitation is separated e.g. in a sedimentation tank 215, where a clear water phase 216 is separated from precipitated iron phosphate 217. The clear water phase 216 from the sedimentation tank 215 may further be neutralized or be used as make-up water with carbon source.

The outward-bound cold UF concentrate stream 218 is neutralized after cooling in the second heat exchanger 113 and is decompressed. In order to secure that a maximum of the precipitated phosphate is transferred into ion form, the tank 121 may be integrated into the last UF step, whereby the retention time for the production of the permeate is increased. The tank 121 is then placed inside the loop of the third UF 117 as shown in fig. 3.

Example 3 In an embodiment according to Fig. 5, incoming sludge 500 is lead to a digester 510, where it may be digested to form e.g. biogas. Digested sludge 511 is filtrated 512. The filtration 512 may be according to any of the previous examples. The filtration concentrate 513 is discarded and may be used as e.g. fertilizer. The filtration permeate may be re-circulated 514 or be lead to post treatment 515. Post treatment 515 may be NF or RO, or a combination between NF and RO. The post treatment concentrate 516 may be further treated and/or discharged. In case the post treatment 515 is NF, the post treatment concentrate 516 is polyvalent salts and organic substances. In case the post treatment 515 is RO, the post treatment concentrate 516 is ammonia, salts and organic substances. The post treatment concentrate may also be re-circulated 517. The post treatment permeate may be discharged/further processed 518 or re-circulated 519. Re-circulated filtration permeate 514, re-circulated post treatment concentrate 517 and/or re-circulated post treatment permeate 519 is lead to a make-up water tank 520. From there, it is added 521 to the incoming sludge 500. In on embodiment, wherein the post treatment 515 is NF, the further processing 518 is ammonia removal, e.g. through stripping or gas transfer membranes, techniques well known to a person skilled in the art.

Although the present invention has been described above with reference to specific embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the invention is limited only by the accompanying claims and, other embodiments than the specific above are equally possible within the scope of these appended claims, e.g. different setups of the equipment than those described above.

In the claims, the term "comprises/comprising" does not exclude the presence of other elements or steps. Additionally, although individual features may be included in different claims, these may possibly advantageously be combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. In addition, singular references do not exclude a plurality. The terms "a", "an", "first", "second" etc do not preclude a plurality. Reference signs in the claims are provided merely as a clarifying example and shall not be construed as limiting the scope of the claims in any way.

Claims

1. A method for treatment of sludge from biogas production plants, said sludge comprising at least partly suspended biological matter, characterized by elevating the temperature of the sludge to a temperature in the interval of 100 to 250 ⁰C; and filtrating the sludge in one or more ultra filtration and/or micro filtration units, whereby a concentrate and a permeate is obtained, said concentrate comprising substantially all said suspended biological matter and said permeate comprising substantially no suspended biological matter.

2. The method according to claim 1, further comprising arranging at least two ultra filtration and/or micro filtration units in series and/or in parallel.

3. The method according to claim 1 or 2, further comprising recycling the sludge over the ultra filtration and/or micro filtration unit(s).

4. The method according to claim 1 or 2, further comprising post treatment of the permeate with nano filtration and/or reverse osmosis.

5. The method according to claim 4, further comprising purification of ammonia from the nano filtration permeate.

6. The method according to claim 1 or 2, wherein the membrane of the ultrafilter and/or micro filter is a ceramic membrane.

7. The method according to claim 1 or 2, further comprising adding energy to said sludge before filtrating the sludge in one or more ultra filtration and/or micro filtration units.

8. The method according to claim 1 or 2, further comprising heat exchanging said permeate with said sludge before or after the elevation of the pressure of said sludge.

9. The method according to claim 1 or 2, further comprising heating the concentrate, to increase hydrolysis.

10. The method according to claim 9, wherein said heating is performed during a time period of 10 to 120 minutes.

11. The method according to claim 1, 2, 7, 8, or 10, further comprising heat exchanging said concentrate with said sludge before or after the elevation of the pressure of said sludge.

12. The method according to claim 1, wherein said concentrate is subjected to a decrease in pressure, whereby water vapor is released from said concentrate.