WO2009038530A1

WO2009038530A1 - Method for treatment of waste

Info

Publication number: WO2009038530A1
Application number: PCT/SE2008/051036
Authority: WO
Inventors: Estera Szwajcer Dey; Olof NORRLÖW; Joanna Wawrzynczyk
Original assignee: Kemira Oyj
Priority date: 2007-09-17
Filing date: 2008-09-16
Publication date: 2009-03-26
Also published as: SE0702104L; SE532194C2

Abstract

The present invention relates, in one aspect, to a method for treatment of waste comprising organic matter, wherein said waste is subjected to a thermal activation treatment at a temperature of about 50-750C for up to 1 hour, and thereafter the organic matter of the waste is allowed to degrade. The present invention relates in a further aspect to the above mentioned method for use in addition to or instead of conventional digestion.

Description

METHOD FOR TREATMENT OF WASTE

Field of the invention

The present invention relates to a novel method for treatment of waste comprising organic matter and the use of said method in addition to or instead of conventional digestion. Background of the invention

The processing and disposal of waste comprising organic matter are increasingly important topics of environmental, economic, social and technological concern. The waste volumes produced are increasing with industrialization and increasing population of the world. Organic waste is considered to refer to different industrial wastes, agricultural wastes and municipal wastes. The increasing amounts of wastes comprising organic matter, like waste waters and sludges, are a result of increases in the organic loading of wastewaters and sludges and environmental regulations that require a higher degree of waste treatment. Sludge from e.g. wastewater treatment may be used for land applications but stringent regulations regarding pathogens and heavy metals are major hinders, therefore the sludge is preferably combusted and then spread out onto fields as fertilizers. Since the legislations regarding fertilizing and releases from combustion processes are more and more restricted needs for new wastewater treatment methods arises.

Organic wastes like sludges contain valuable resources such as organic matter and nutrients like phosphorous, nitrogen and trace elements. Therefore, waste treatments that both decrease the resulting waste mass and results in value-adding (intermediate) products are desirable to attain. If a decrease of the waste mass could be done within a shorter time period compared to known processes, which may take several days or weeks, and/or with an increased yield, cost effective benefits and environmentally friendly methods gaining social acceptance would be the result of such a treatment. Thus, there still exists a need to find new ways to increase the degradation of waste comprising organic matter in order to meet strict regulations and achieve more cost effective and efficient methods that decreases the final waste mass. Also, there exists a desire to obtain value-adding products from the waste. Thus, the large amount of organic waste is in need of new treatment methods. Summary of the invention

The above-mentioned problems are dealt with in the present invention. The present invention relates in one aspect to a method for treatment of waste comprising organic matter, wherein said waste is subjected to a thermal activation treatment at a temperature of about 50-75°C for up to 1 hour, and thereafter the organic matter of the waste is allowed to degrade. According to the present invention the specified thermal activation treatment changes the biological activity of the treated waste, i.e. opens up the structure of the waste and allows it to become more accessible to microorganisms and enzymes, if present, in order to degrade the sludge to a higher extent and increase production of value-adding products, e.g. volatile fatty acids (VFA) which may be converted into e.g. methane. The thermal activation may be further improved by combination with cation binding agents and/or enzymes. The obtained value-adding VFA may appart from being used for conversion into methane (biogas) also be used as carbon source (enhancer) in treatments for phosphorous and nitrogen removal from e.g. sludge or in a process for synthesis of polyhydroxybutyrate, a biodegradable plastic.

The present invention relates in a further aspect to the method for use in addition to or instead of conventional digestion. Detailed description of the invention

An object of present invention is to provide novel methods for treat- ment of waste comprising organic matter. Said waste to be treated comprises organic matter and is also referred to as organic waste. The waste to be treated originates from e.g. industrial water works, sewage works, industrial digestion plants, food related industry and paper industry. The organic waste includes slurries such as industrial wastewaters or sludges like from the sugar industry, paper industry, food industry like fishing, meat, brewing and beverage industry etc or municipal waste waters or sludge e.g. sewage sludge. The present invention is useful for organic waste and slurries from agriculture and slaughterhouses, such as biomasses and manure from cattle or any other animals. Organic waste slurry is an aqueous sludge suspension or wastewater but is referred to as sludge in the present application. The said sludge to be treated comprises organic matter and is in that view considered an organic sludge. The present invention relates in one aspect to a novel method for treatment of organic waste such as sludge, resulting in increased sludge degradation and production of volatile fatty acids (VFA), by microorganisms, from the sludge subjected to a thermal activation, which optionally may be enhanced by presence of cation binding agents and/or selected enzymes. Increased accessability to and solubilisation of the sludge is done by thermal activation and optional use of cation binding agents and/or enzymes. The above mentioned method can be used in addition to or instead of conventional digestion that is currently used in e.g. wastewater purification. The method according to the present invention achieve a reduction of the sludge and production of VFA, as a key intermediate for methane synthesis or used in processes for biological phosphorous and nitrogen removal, polyhydroxybutyrate synthesis or for organic acids production.

The method according to the invention accelerates the release of biodegradable organic compounds that are converted into VFA with the help of endogenous microorganisms.

The method according to the present invention regards subjecting organic waste such as sludge to a thermal activation, i.e. a temperature increase to a desired temperature, and the sludge is then kept at said temperature for a specified time period.

The thermal treatment, i.e. the thermal activation, of the sludge imply an increase of the sludge temperature to 50-75⁰C, preferably 55-70⁰C, preferably 55-65°C, and more preferably 60-65⁰C. The thermal treatment of the sludge continues for up to 1 hour, preferably up to 30 minutes, preferably 0.5 to 15 minutes, preferably 0.5 to 10 minutes, and most preferably 0.5 to 5 minutes.

The thermal activation of the sludge makes the organic polymers contained in the sludge more accessable to digestion. The thermal activation opens the structure of the organic materials. If enzymes are present in the sludge, a thermal activation also increases the accessability for the enzymes to the organic matter. The thermal activation has a positive effect on the release of organic polymers as well as improves the action of enzymes and some bacteria. The thermal activation treatment may also destroy many undesirable non-sporulating pathogenic bacteria, like the ones from the Gram-negative group, that are present in the sludge. Examples of non- sporulating pathogenic bacteria from the Gram-negative group include, but is not limited to, Escherichia coli, Salmonella, and other Enterobacteriaceae, Pseudomonas, Moraxella, Helicobacter, Stenotrophomonas. Due to the thermal treatment sporulating bacteria are promoted and allowed to multiply.

Since the temperature for the thermal activation is chosen to be at most 75⁰C the heat treatment of the sludge do not affect the bacteria desired for the digestion of the sludge, thus these bacteria are allowed to grow and become more active.

Enzymes may also be used in the method according to the present invention. The specificity of the needed enzymes depends on the sludge composition. Enzymes act on specific substances present in the sludge and therefore can change the characteristics of the waste. The sludge becomes more amenable to further treatment and bioconversion to value-added products is facilitated.

Cation binding agents may also be used in the method according to the invention. Cation binding agents enhances the release of organic matter, thus resulting in easier access for enzymes and faster digestion of the sludge by microorganisms.

Also, a combination of both enzymes and cation binding agents may be used.

In one embodiment of the invention the sludge is provided with at least one cation binding agent. The sludge is provided with an at least one cation binding agent before and/or during the thermal activation, preferably before the thermal treatment. It is realized that any type of cation binding agent might be used in accordance with the invention as long as the desired result is achieved. Preferred cation binding agents are chosen from, but not limited to, citric acid, ethylenediaminetetraacetic acid (EDTA), tartaric acid and their salts, preferably their sodium and potassium salts, Zeolite A, sodium fluoride, sodium thiosulphate in combination with Zeolite A, sodium silicate, sodium silicate in combination with Zeolite A, and any other combination of the above, but other related compounds could also be used according to the invention. The above mentioned cationic binding agents are considered quite environmentally friendly agents. Preferably the agent is citric acid, potassium citrate, sodium citrate and/or ethylenediaminetetraacetic acid and its sodium or potassium salts. More preferably the cation binding agent is chosen from citric acid and sodium citrate, which are biodegradable. One or more cation binding agents are preferably added to the sludge in a total concentration of 0.1-200 mM, preferably about 0.1-75 mM, preferably about 0.1-50 mM, and more preferably about 0.25-5 mM, e.g. 0.5- 2 mM.

In another embodiment of the invention the sludge is provided with at least one enzyme before, during and/or after the thermal activation treatment. In the present invention if enzymes are provided this may be done by addition of an enzyme as it is. Said at least one enzyme is preferably chosen from enzymes capable of digesting natural polymeric materials, i.e. have the ability to digest the present natural polymeric materials. Thus, the enzymes are added in an effective amount to digest the present natural polymeric materials. In the context of the present invention the phrase "natural polymeric materials" that are hydrolysed by the enzymes are for instance proteins, polysaccharides, polyphenols, lignins, humic substances i.e. polymeric low molecular weight compounds. Naturally, the phrase includes any other material or component, not specifically mentioned here, that is present in the sludge and is also affected by the present enzymes. More preferably enzymes are chosen from, but not limited to, cellulases, amylases, lipases, poly-galactouronidases, pectinases, dextranases, proteases, endo- xylanase (e.g. Pulpzyme), carbohydrases e.g. Viscozyme (a multi-enzyme complex comprising a wide range of carbohydrases) and oxidases. Any enzyme being able to digest the sludge may of course be used in the present invention, e.g. as it is or in an enzyme mixture. A person skilled in the art may easily choose other variants of the enzymes. The choice of the enzymes used in an enzyme mixture is dependent upon the type and origin of the sludge suspension, e.g. domestic waste and/or industrial waste, the results being desired and on economy aspects.

Preferred enzymes are proteases such as Alcalase (product name of commercially available protease, which is genetically modified protease subtilisin Carlsberg) and Savinase (product name of commercially available subtilisin-like serine proteinase), and lipases such as Lipolase (product name of commercially available lipase). Alcalase is a protease that has a very broad specificity, namely high specificity for aromatic aminoacids such as phenylalanine (Phe), tryptophan (Trp) and tyrosine (Tyr), acidic aminoacids such as glycine (GIu), sulphur-containing aminoacids such as methionine (Met), and aliphatic amino acids such as leucine (Leu) and alanine (Ala). Above mentioned protease shares specificities which are separately associated with a few proteases. The same is valid for the commercially available Lipolase that has a broad substrate specificity. In other words, it promotes the hydrolysis of a wide variety of triglycerides, including mon- esters and waxes.

In a further embodiment of the invention the at least one enzyme according to the invention is a combination of for instance at least two enzymes, e.g. the combination which comprises for instance amylase (α- amylase and/or β-amylase) and cellulase (exo and/or endo-cellulase), protease and lipase, laccase and lipase, laccase and amylase, or any other combination of the above. A further combination of enzymes is for instance a protease such as Alcalase and a lipase such as Lipolase. A person skilled in the art realizes the required amount of enzymes needed to get an efficient degradation of sludge in view of the conditions of the process, i.e. temperature, type of sludge, and required efficiency and so on.

The amounts used of these enzymes for the method according to the invention are very low compared to amounts of enzymes for known sludge treatments. The reason for this is that the method according to the invention increases the activity and efficiency of the enzymes. The sludge is made more available for the enzymes by the thermal activation of the sludge and thus enabling better use of added enzymes. Since the efficiency of the enzymes is inceased according to the invention lower dosages of the enzymes are possible. The amount of enzymes needed for the method according to the invention are lower than the amounts needed for conventional sludge treatments. Enzymes already given in a lower dose (e.g. 12 mg/g TS (sludge total solids)) were found to be more effective after a heat/thermal activation according to the present invention and even more effective in the presence of cation-binding agents than at a full dose (60 mg/g TS) without the treatment according to the invention. As an example sludge treatment according to the present invention with a thermal activation and use of enzymes may only need a very low dosage in comparison with a full dose as is disclosed above. Such a low dose may be about 6 mg/g TS which can be recalculated to the enzyme activity from the specification sheet of the enzymes, different for each of the used commercial enzyme brand. The enzymatic activity can be recalculated based on the used amounts (e.g. Alcalase 2.4 = 2,4 A/g (A=Anson units). Celluloclast 1.5 L has declerad 700 EGU/g (EGU=endoglucanase units) and so on).

The term "enzyme" according to the present invention also includes the enzyme in the form of an enzyme mixture containing other components than just the desired enzyme. Commercial enzymes are often sold as mixtures with other components, thus the amount of the enzyme added according to the invention often referes to such an enzyme mixture.

In the context of the present invention the mass ratio of the enzyme mixture:sludge or sludge suspension has been defined to the range of 0.1- 30 mg/g dry solids. The enzyme mixture could, in addition to enzymes, comprise other amounts of constituents such as water, or suitable organic or inorganic solvents or other components. The enzyme mixture used in the method of the invention could for instance be a commercially available enzyme product containing the relevant enzyme(s) and amounts of solvent(s) and other components for these enzyme(s). It is naturally important that these other components and solvents do not disturb the important activity of the enzymes. The following commercially available enzyme products from Novozyme A/S: could be comprised in the enzyme mixture used in the method of the invention: Termamyl 300 L, Type DX (amylase) with a declared activity of 300 KNU/g, (37⁰C pH 5.6), Lipolase 100 L (lipase) 100 KLU/g (3O⁰C, pH 7.0), Celluclast 1.5 L (cellulase) 700 EGU/g. (3O⁰C pH 5.6), Pulpzyme HC (endo-Xylanase), 1000 AXU/g; and Dextranase Plus L (dextranase), Alcalase 2.4 LFG (protease), 2.4 AU-A/g. (37⁰C pH 8.5). In order to further explain the above, a non-limiting example of the invention is the case where a mass ratio of enzyme mixture: sludge total solids is 12 mg/g, which means, in this non-limiting example, 12 mg of one or mixture of several commercial enzyme products per 1 g sludge total solids. The enzyme mixture used in the method is not to be limited to the above mentioned specific examples of commercially available enzyme products. The mass ratio of the enzyme or enzyme mixture: sludge dry solids is from about 0,1-30 mg/g dry solids, preferably 0,1-15, preferably 0,5- 10 and preferably 1-5 mg/g total solids. The mass ratio of enzyme mixture:sludge dry solids could for example be as low as 2-6 mg/g total solids for economical reasons.

Further ingredients may be added to the enzymes to form an enzyme mixture such as emulsifiers, surfactants and suspending agents in order to facilitate the substrates to become more available to the bacteria being added afterwards. The use of cation binding agents and/or enzymes will result in better access to polymers contained in the sludge in order to digest the polymers according to the method of the invention. In one embodiment of the invention the method for treating sludge regards treatment of the incoming sludge by thermal activation under aerobic or anaerobic conditions, preferably anaerobic conditions, optionally combined with addition of cationic binding agents and/or enzymes according to the invention. The sludge is after the thermal activation treatment subsequently allowed to react under aerobic and/or anaerobic conditions, preferably anaerobic conditions, in order to further degrade the organic material and to enhance VFA formation according to the present invention. The conditions for the total treatment with thermal activation and subsequent reaction may occur under aerobic and/or anaerobic conditions, preferably anaerobic conditions or aerobic conditions during the thermal activation and then anaerobic conditions during the subsequent degradation period. The use of anaerobic or aerobic conditions during the thermal treament and the subsequent degradation reaction showed similar results after 4 h of degradation of organic matter after tested treatment with thermal activation. Also, VFA is with time converted into CO₂ under aerobic conditions and into CH₄ under anaerobic conditions and presence of methanogenic microorganisms. Due to this conversion the formation of VFA seems to stagnate and the dramatic increase of VFA declines with the retention time. In one embodiment of the present invention sludge comprising organic matter is degraded using thermal activation together with addition of enzyme(s) and cation binding agent(s) as disclosed above under anaerobic conditions.

In another embodiment of the invention sludge comprising organic matter is degraded using thermal activation together with addition of enzyme(s) as disclosed above under anaerobic conditions.

In yet another embodiment of the present invention sludge comprising organic matter is degraded using thermal activation together with addition of cation binding agent(s) as disclosed above under anaerobic conditions. In another embodiment of the present invention sludge comprising organic matter is degraded using thermal activation together with addition of enzyme(s) and cation binding agent(s) as disclosed above under aerobic conditions.

The method according to the invention involves a thermal activation and optionally additional cationic binding agents and/or enzymes and immidetely after said thermal activation the treated waste/sludge is either allowed to react (1) under anaerobic conditions to yield VFA which then may be converted into methane by fermentation with methanogenic bacteria or used as a carbon source for other treatments for nitrogen and phosphorous removal from e.g. wastewater or (2) under aerobic conditions to yield VFA which may be used as a carbon source for other treatments for nitrogen and phosphorous removal.

Tests of the method according to the invention have shown that during the subsequent degradation reaction VFA starts forming immidately and after a retention time of about 4 h a clear acculumlation of VFA is shown. During a retention time of about 12-18 h after the subsequent reaction begins, the highest increase of VFA is shown but the reaction continues with time even- though the amount of VFA stagnates. Thus, after about 12-18 h it is clearly shown the increased amount of VFA with the method according to the invention, but an indication of increased VFA is shown already after a few hours, e.g. 4 h. In another embodiment of the invention the method further comprises the step of adding at least one species of fermenting bacteria to the suspension, thereby fermenting the sludge suspension. The at least one species of fermenting bacteria is added to the sludge, thereby fermenting the sludge suspension after the solubilisation period. Thus, it is also possible to further increase the degradation of the sludge by adding to the sludge suspension fermenting bacteria present in the sludge, selected from other sludges or selected from a culture collection. The fermenting bacteria may for instance be chosen from acidogenic bacteria, acetogenic bacteria, and methane producing bacteria. Preferably, the fermenting bacteria are chosen from the group consisting of Gluconobacter oxydans, Acetobacter sp., Acetogenium kivui, B. macerans, polymyxa, B. coagulans, B. subtilis, Lactobacillus buchneri, Bifidobacterium sp., Clostridium thermoaceticus, Clostridium lentocellum, Clostridium formicoaceticu, Clostridium thermocellum and Pseudomonas sp. Naturally any other species of bacteria, not specifically mentioned, can be used in this embodiment of the invention. Furthermore, the methane producing bacteria are chosen from the group consisting of Methanosarcina barkeri, Methanosarcina mazeii, Methanosarcina soehngenii and Methanosarcina acetivorans, and Methanosaeta, and mixtures thereof. The amount of VFA is correlated to the amount of methane that is possible to obtain. A conversion of VFA into methane occurs under anaerobic digestion and presence of methanogenic bacteria. It is also possible according to the invention that the methane produced is separated from the sludge degradation step. As have been stated above, an additional effect is achieved in accordance with this particular embodiment in the form of a value-added product. The sludge is decomposed according to the method of present invention and the VFA level was chosen as indicator of the solubilisation by thermal activation, and optionally cation binding agents, followed by enzymatic decomposition, with added enzymes, of organic matter and it's fast utilisation by microbial consortia in the treatment reactors. The VFA may then further decompose under anaerobic conditions to methane as an end- product. The degradation of particulate organic matter results in an increase of VFA already during the solubilisation step which may be done under aerobic and/or anaerobic conditons. In the case of the process being combined with methane production VFA is immediately metabolised by methanogenic bacteria present in the sludge suspension, thus considered an attractive value adding product. VFAs are compounds that have been identified as e.g. acetic, propionic, isobutyric, butyric, isovaleric, and valeric acids.

On raising the temperature during a thermal activation treatment according to the present invention an increase in volatile fatty acids yield is observed. Said increase is mainly due to an improvement of the digestion of particulate organic matter.

It is sometimes required to adjust the pH of the sludge suspension in the method according to the invention to about 6-9, preferably about 7, after addition of the at least one cation binding agent by adding an acid or base, for instance by adding HCI or NaOH or any other suitable base or acid.

In an embodiment of the invention the treatment takes place in any anaerobic digester provided with methanogenic microorganisms. The end product of an anaerobic process is methane, which is a value-added product. Thus, one embodiment of the invention is performing the method of the invention in an anaerobic environment, whereby a value added product such as methane can be separated and remaining high quality bio-solids can be used as fertilizer and land reclamation material. However, the described solubilisation process and VFA production may be carried out in aerobic conditions but is preferably done under anaerobic conditions.

In order to further optimize the solubilisation and degradation method of the invention the sludge suspension may be subjected to agitation in the range from 0 to 200 rpm. The agitation is beneficial from an efficiency point of view. The enzymes are able to act more efficiently since the agitation causes the sludge to become more available. However, the further anaerobic VFA synthesis does not require agitation. In a further embodiment of the invention the sludge is pre-concentra- ted, prior to the addition of enzymes, cation binding agent and optionally bacteria, by gravitation or enhanced sedimentation to the range 10-80 g sludge solids per 1 I sludge suspension, e.g. 10-60 g or 10-40 g sludge solids per 1 I sludge suspension. In summary, according to the method of the present invention it has been shown, in view of the examples below, that in the use of a thermal activation treatment according to the present invention clearly enhances the degradation of sludge, which effect is increased by increasing treatment temperature. If the heat treatment is used in combination with presence of a cation binding agent such as citric acid or EDTA, and/or an enzyme such as protease or lipase the beneficial effects increased. Enzymes are more efficient together with cation binding agents than when used alone. This means that the dosage of enzymes can be reduced together with a cation binding agent compared to in the absence of a cation binding agent and lack of thermal activation, thereby decreasing costs for the added compounds.

The terms "heat treatment", "thermal treatment", "thermal activation", "heat activation" and "activation treatment" all refer to the specified temperature increase specified for the present invention.

In order to further explain the invention, the following non-limiting experiments are provided to show the beneficial effects of the invention. Examples

Surplus biological sludge, used in the experiments herein, was obtained from a local municipal wastewater treatment plant in Lund (Sweden). Each experiment was made in duplicates. Pre-concentrated biosludge (TS 2.2%, 700 ml) was transferred into 1 liter laboratory glass vessels, and further treated as described below. The treatments disclosed were performed under anaerobic conditions. Thereafter, the treated and untreated biosludge was used as a substrate in VFA and later methane production experiments. Also, as a reference the degradation of untreated biosludge is illustrated (herein also called inoculum)

Biosludge was used since it normally is considered a more difficult sludge to degrade compared to primary sludge. Thus, it is also beneficial if such sludge could be decomposed to a higer extent according to the present invention than conventional treated biosludge. Thermal treatment

Thermal activation is represented by the following treatment. A one liter glass vessel with pre-concentrated biosludge (TS 2.2%, 700 ml) was placed in water bath preheated to 80⁰C, for 5-10 min until the temperature in the sludge reached 50 or 65⁰C, depending on the desired temperature in the test. Then the glass vessel with the sludge was transferred to a water bath preheated to 50 or 65°C. Thermal activation treatment was performed for 5 min. The sludge was mixed with a propeller stirrer at 200 rpm during said treatment. After 5 min of thermal activation, the vessel with heated biosludge was placed in an ice bath. The sludge was rapidly cooled down to 37⁰C. The vessel with sludge was then placed in a water bath, at 37°C, for 4 h, and was mixed with a propeller stirrer at 200 rpm. Thereafter 100 ml samples were taken for analysis.

Treatment with cation binding agents

The one liter glass vessels with pre-concentrated biosludge (TS 2.2%, 700 ml) were placed in water bath, at 37°C. The cation binding agents EDTA (0.25 M) or sodium citrate (SC, 0.5M) were added to a pre-concentrated biosludge to a final concentration of 5 mM, which corresponded to 0.04 g sodium citrate /g TS and 0.09 g EDTA /g TS. After the addition of the cation binding agents the sludge was mixed. The sludge was then placed in a water bath, at 37°C, for 4 h, and was mixed with a propeller stirrer at 200 rpm. Thereafter 100 ml samples were taken for analysis. Enzymatic treatment

The one liter glass vessels with pre-concentrated biosludge (TS 2.2%, 700 ml) was placed in water bath, at 37°C. Two enzymes combinations were added to the sludge: either a mixture of protease (Alcalase® 2.4 FG, 12 mg/g TS, 2.4 AU/g) and lipase (Lipolase®, 12 mg/g TS, 100 KLU/g) or a mixture of cellulase (Celluloclast® 1.5L FG, 12 mg/g TS, 700 EGU/g) and alpha- amylase (Termamyl® 300L DX, 12 mg/g TS, 300 KNU/g). The sludge preparations were mixed for 4 h at 37⁰C with a propeller stirrer at 200 rpm. Thereafter 100 ml samples were taken for analysis. Combined treatments (a) Cation binding agents combined with thermal activation

The cation binding agents: EDTA (0.25 M) or sodium citrate (SC, 0.5M) were added to a pre-concentrated biosludge (TS 2.2%, 700 ml) to a final concentration of 5mM, which corresponded to 0.04 g sodium citrate /g TS and 0.09 g EDTA /g TS, respectively. After the addition of the cation binding agents the sludge was mixed and a glass vessel with biosludge and cation binding agent was placed in water bath preheated to 80⁰C, for 5-10 min until the temperature in the sludge reached 65⁰C. Then the vessel with sludge was transferred to a water bath preheated to 65⁰C. Heat activation treatment was performed for 5 min. The biosludge was mixed with a propeller stirrer at 200 rpm during said treatment. After 5 min of thermal activation, the vessel with heated sludge was placed in an ice bath. The sludge was rapidly cooled down to 37⁰C, occasionaly mixed. The vessel with sludge was placed in a water bath, at 37⁰C, for 4 h, and was mixed with a propeller stirrer at 200 rpm. Thereafter 100 ml samples were taken for analysis. (b) Cation binding agents combined with thermal activation and enzymes mixture addition The cation binding agents EDTA (0.25 M) or sodium citrate (SC, 0.5M) were added to a pre-concentrated biosludge (TS 2.2%, 700 ml) to a final concentration of 5 mM, which corresponded to 0.04 g sodium citrate /g TS and 0.09 g EDTA /g TS, respectively. After the addition of the cation binding agents the sludge was mixed and a glass vessel with pre-concentrated bio- sludge and cation binding agent was placed in water bath preheated to 80⁰C, for 5-10 min until the temperature in the sludge reached 65⁰C. Then the vessel with sludge was transferred to a water bath preheated to 65⁰C. Heat activation treatment was performed for 5 min. The sludge was mixed with a propeller stirrer at 200 rpm during said treatment. After 5 min of thermal activation, the vessel with heated sludge was placed in an ice bath. The sludge was rapidly cooled down to 37⁰C, occasionaly mixed. Thereafter, the enzymes were added and the sludge was mixed. Two enzymes combinations were tested: either a mixture of protease (Alcalase® 2.4 FG, 12 mg/g TS, 2.4 AU/g) and lipase (Lipolase®, 12 mg/g TS, 100 KLU/g) or a mixture of cellulase (Celluloclast® 1.5L FG, 12 mg/g TS, 700 EGU/g) and alpha- amylase (Termamyl® 300L DX, 12 mg/g TS, 300 KNU/g). The vessel with sludge was placed in a water bath, at 37⁰C, for 4 h, and was mixed with a propeller stirrer at 200 rpm. Thereafter 100 ml samples were taken for analysis. (c)Thermal activation combined with enzymes mixture addition

A one liter glass vessel with pre-concentrated biosludge (TS 2.2%, 700 ml) was placed in water bath preheated to 8O⁰C, for 5-10 min until the temperature in the sludge reached 65°C. Then the vessel with sludge was transferred to a water bath preheated to 65°C. Thermal activation was performed for 5 min. The sludge was mixed with a propeller stirrer at 200 rpm during said treatment. After 5 min of thermal activation, the vessel with heated sludge was placed in an ice bath. The sludge was rapidly cooled down to 37°C, with occasionaly mixing. Thereafter enzymes were added and the sludge was mixed. Two enzymes combinations were tested: either a mixture of protease (Alcalase® 2.4 FG, 12 mg/g TS, 2.4 AU/g) and lipase (Lipolase®, 12 mg/g TS, 100 KLU/g) or a mixture of cellulase (Celluloclast® 1.5L FG, 12 mg/g TS, 700 EGU/g) and alpha-amylase (Termamyl® 300L DX, 12 mg/g TS, 300 KNU/g). The vessel with sludge was placed in a water bath, at 37°C, for 4 h, and was mixed with a propeller stirrer at 200 rpm. Thereafter, 100 ml samples were taken for analysis. Volatile fatty acids (VFA) production The samples of the sludges after 4 h of treatment at 37⁰C under anaerobic conditions and mixing were centrifuged at 7000 x g, 4°C, for 10 min. The concentration of VFA was thereafter measured in the liquid phase, as disclosed below.

Then the sludges were after 4 h of treatment at 37⁰C under anaerobic conditions and mixing, further incubated under anaerobic conditions without mixing for several hours. An anaerobic treatment was performed in order to determine the effect of different treatments on VFA production. Treated sludges (500 ml) were transferred to blue cap bottles (1000 ml) and 200 ml of untreated biosludge (i.e. biosludge that has not been treated with heat, cation binding agents or enzymes according to the invention, herein also called inoculum) was added. The tested samples were a mixture of treated sludge and inoculum. This mix with inoculum was made to speed up the degradation since there are more microorganisms present in the untreated sludge to digest the organic matter in the sludge but an addition of untreated sludge to the treated is not necessary for the present invention. The bottles were tightly closed and incubated without mixing at 37°C for a desired retention time of about 14 h, 20 h or 86 h. Then, 100-200 ml of sludge was centrifuged at 7000 x g, 4°C, for 10 min. The concentration of VFA was thereafter measured in the liquid phase, as disclosed below. Note that the total retention/reaction time after the thermal activation and optional additions in the tested samples were 4 h, 18 h (4+14), 24 h (4+20 h) and 90 h (4+86). Anaerobic digestion experiments to produce methane

The possible conversion of VFA into methane, i.e. the methane potential, of the treated and untreated biosludge was tested in triplicate by laboratory-scale anaerobic batch tests as described in Hansen TL₁ Schmidt JE, Angelidaki I₁ Marka E, Jensen JIC₁ Mosbaek H, Christensen TH; 2004; Method for determination of methane potentials of solid organic waste; Waste Management 24:393-400. The tests were performed in 2-litre reactors with 500 ml working volume. Tests on samples with the following treatments were used: biosludges after (i) thermal activation, (ii) treatment with sodium citrate, (iii) thermal activation in presence of sodium citrate, (iv) thermal activation in presence of sodium citrate, followed by protease and lipase addition and (v) untreated sludge. Tested samples represented 40% of the total volatile solids (tVS) and anaerobic digestion inoculum (sludge from an anaerobic digester containing fermenting microorganisms) was about 60% tVS. In practice the ratio 40%:60% corresponded to about 100 ml of biosludge mixed with about 350 ml of anaerobic digestion inoculum, and then filled with water up to 500 ml. The mixture with anaerobic digestion inoculum was made to add the desired fermenting and methane producing bacteria for the degradation and conversion of VFA into methane. The reactors were kept at 35⁰C and methane production was monitored by gas chromatography until the gas production ceased and the accumulated gas production remained at a fixed level. Reference substrate in the form of cellulose was used to test the function of the anaerobic digestion inoculum. Since cellulose is very easily digested compared to sludge and formed VFA is converted into methane the reference test with cellulose is regarded as very good conversion of organic matter into methane. The cellulose was a 1 :1 mix of Avicel (Fluka, Sigma- Aldrich, Denmark) and cellulose powder (Bie & Berntsen, Denmark). Analytical methods

Total solids (TS) were measured according to a standard method, APHA (1995) - Standard methods for the examination of water and wastewater, American Waterworks Association; Water Environment Federation; Washington D. C₁ USA. VFA, i.e. in this case acetic acid and propionic acid, were analyzed in liquid phase. Supernatants were filtered through disposable filter (pore size 0.45 μm). Filtrated liquid (0.9 ml) was mixed with 10% phosphoric acid (0.1 ml). Acetic acid and propionic acid were measured by gas chromatography (Agilent 6850 series equipped with flame ionisation detector (FID) at 260⁰C, using a HP-FFAP column (30 m length, 0.32mm diameter and 0.25 mm thickness film) at 80-140⁰C, with an injector temperature of 180⁰C and nitrogen gas was used as carrier gas at a flow rate of 67 ml/min. 1 ml of this mixture was injected into pulsed splitless mode. The total VFA (∑VFA) represents mathematical sum of measured acetic acid and propionic acid.

The methane production was monitored by a gas chromatograph (Agilent 6850 series) equipped with the flame ionization detector (FID) and separated in column HP-1 (19091 Z-413E); 30 m length, 0.32mm diameter and 0.25 mm thickness film connected to Autosystem with HS40. Table 1.

Effect of thermal activation at 65°C or 5O⁰C during 5 minutes on VFA. (TS

activ.=thermal activation, heat treatment for 5 minutes

SC=sodium citrate (cation binding agent), 0.5M, pH 7.0, added to a final concentration of 5 mM lnoculum=untreated sludge, (also added as fresh to treated sludge after 4h treatment).

VFA=Σ (acetic acid and propionic acid)

Table 2.

Effect of cation binding agents combined with thermal activation on VFA roduction from slud e with TS 2.2%

activ.=thermal activation, heat treatment at 65°C for 5 minutes SC=sodium citrate, 0.5M, pH 7.0, added to a final concentration of 5 mM EDTA= ethylenediaminetetraacetic acid 0.25M, pH 7.0, added to a final concentration of 5 mM lnoculum=untreated sludge, (also added as fresh to treated sludge after 4h treatment). VFA=Σ (acetic acid and propionic acid) Table 3.

Effect of enzymes combined with thermal activation on VFA production from sludge with TS 2.2%

activ.=thermal activation, heat treatment at 65⁰C for 5 minutes P+L=enzyme mixture of protease (Alacalase) and lipase (Lipolase), 12 mg/g

TS each

C+A=enzyme mixture of cellulase (Celluclast) and amylase (Termamyl),

12mg/g TS each lnoculum=untreated sludge, (also added as fresh to treated sludge after 4h treatment).

VFA=Σ (acetic acid and propionic acid)

Table 4.

Effect of cation binding agent and enzymes combined with thermal activation

activ.=thermal activation, heat treatment at 65⁰C for 5 minutes SC=sodium citrate, 0.5M, pH 7.0, added to a final concentration of 5 mM P+L=enzyme mixture of protease (Alacalase) and lipase (Lipolase), 12 mg/g TS each

C+A=enzyme mixture of cellulase (Celluclast) and amylase (Termamyl), 12mg/g TS each lnoculum=untreated sludge, (also added as fresh to treated sludge after 4h treatment).

VFA=Z (acetic acid and propionic acid) Table 5.

VFA production during 18 h using the heat activation and optionally cation binding agent and enzymes. Sludge collected from two different waste water plants, in Malmό and Helsingborg (Sweden) .

activ.=thermal activation, heat treatment at 65⁰C for 5 minutes

SC=sodium citrate, 0.5M, pH 7.0, added to a final concentration of 5 mM

P+L=enzyme mixture of protease (Alacalase) and lipase (Lipolase), 12 mg/g

TS each

C+A=enzyme mixture of cellulase (Celluclast) and amylase (Termamyl),

VFA=E (acetic acid and propionic acid)

Table 6.

Accumulated methane production (in ml of CH4) from organic matter obtained by different treatments. (TS 2.2%)

In - anaerobic digestion inoculum, sludge from an anaerobic digester

US - untreated biosludge

TAS - thermal activated biosludge

TACS - thermal activated and citrate treated biosludge TACES - thermal activated, citrate and enzymes treated biosludge

Thermal activation=heat treatment at 65⁰C for 5 minutes

Citrate=sodium citrate, 0.5M, pH 7.0, added to a final concentration of 5 mM

Enzymes=enzyme mixture of protease (Alacalase) and lipase (Lipolase),

12mg/g TS each It is clearly shown in Table 1 that a heat treatment according to the present invention gives a better result than without treatment. Also, increasing temperature for the sludge during the thermal treatment is promoting the degradation of the sludge. Addition of a cation binding agent increases the degradation effect further. In Table 2 two different cation binding agents are tested with sodium citrate being the most efficient.

From Table 3 it is apparent that addition of enzyme increases the degradation further compared to only using heat treatment of the sludge.

In Table 4 the tests disclosed are run over a longer time period and it is apparent that with time a combination of heat treatment, cation binding agent and enzyme is the most efficient combination in view of degradation of the sludge. However, all treatments gives considerably better results compared to untreated sludge.

In Table 5 it is apparent when the process according to the invention is testet on sludges from different waste water treatment plants that a combination of heat treatment, cation binding agent and enzyme is the most efficient combination in view of degradation of the sludge.

The degradation of the tested sludge in the examples continues over time as is shown. In Table 6 the methane production is shown. Anaerobic digestion inoculum and water shows the degradation of the inoculum it self. Anaerobic digestion inoculum and cellulose show degradation of easily accessible organic matter, i.e. methane production. This is made as a test for methanogenic inoculum vitality. Anaerobic digestion inoculum combined with untreated sludge could be considered like a conventional digestion. Anaerobic digestion inoculum with thermal activated sludge (TAS) clearly show an increasing amount of methane compared to the untreated sludge. Anaerobic digestion inoculum with thermal activated and cationic binding agent treated sludge (TACS) show a considerably higher increase in methane production compared with the thermal activated sludge. Anaerobic digestion inoculum with thermal activated and cationic binding agent and enzyme treated sludge (TACES) show an even higher increase in methane production compared with the thermal activated sludge. It is clearly shown from the table that thermal activation according to the present invention of the sludge increases the methane production and an optional addition of cationic binding agents and/or enzymes contribute to even higher levels of methane. Note that it is not necessary to add anaerobic digestion inoculum (sludge from a digester comprising microorganisms) to achieve the methane production it is also possible to just add the desired fermenting and methane producing bacteria alone.

Claims

1. A method for treatment of waste comprising organic matter, wherein in said waste is subjected to a thermal activation treatment at a temperature of about 50-75⁰C for up to 1 hour, and thereafter the organic matter of the waste is allowed to degrade.

2. A method according to claim 1 , wherein the thermal treatment is at a temperature of 55-70⁰C, preferably 55-65⁰C and preferably 60-65⁰C.

3. A method according to claim 1 or 2, wherein the thermal activation treatment is done for up to 30 minutes, preferably 0.5 to 15 minutes, preferably 0.5 to 10 minutes, and most preferably 0.5 to 5 minutes.

4. A method according to any of the preceding claims, wherein the waste is provided with at least one enzyme before, during and/or after said thermal activation treatment.

5. A method according to any of the preceding claims, wherein the waste is provided with at least one cation binding agent before and/or during said thermal treatment, preferably before the thermal treatment.

6. A method according to any of the preceding claims, wherein the waste is provided with at least one enzyme before, during and/or after said thermal treatment and at least one cation binding agent before and/or during said thermal treatment, in optional order or simultaneously; preferably at least one cation binding agent is added before the thermal treatment and at least one enzyme is added after the thermal treatment.

7. A method according to claims 5 or 6, wherein the at least one cation binding agent is chosen from the group consisting of citric acid, ethylenedi- aminetetraacetic acid (EDTA), tartaric acid and their salts, preferably their sodium and potassium salts, Zeolite A₁ sodium fluoride, sodium thiosulphate in combination with Zeolite A, sodium silicate, sodiumsilicat in combination with Zeolite A, and any other combination of the above; preferably citric acid, ethylenediaminetetraacetic acid and their sodium or potassium salts; and preferably citric acid and sodium citrate.

8. A method according to any one of claims 5-7, wherein the at least one cation binding agent is present in a total concentration of about 0.1- 200 mM, preferably about 0.1-75 mM, preferably about 0.1-50 mM, and preferably about 0.25-5 mM.

9. A method according to claims 4 or 6, wherein said at least one enzyme is chosen from enzymes capable of digesting natural polymeric materials, preferably from the group consisting of cellulases, amylases, lipases, poly-galactouronidases, pectinases, dextranases, proteases, endo- xylanases, carbohydrases and oxidases, preferably lipases and proteases or amylase and cellulase.

10. A method according to any of the preceding claims, wherein the pH of the waste is adjusted to about 6 to 9, preferably about pH 7 by adding acid or base.

11. A method according to any of the preceding claims, wherein the waste is subjected to a thermal activation treatment under aerobic and/or anaerobic conditions, preferably anaerobic conditions.

12. A method according to any of the preceding claims, wherein at least one species of fermenting bacteria is added to the waste to ferment the waste under anaerobic conditions.

13. A method according to claim 12, wherein the fermenting bacteria are chosen from acidogenic bacteria, acetogenic bacteria and methane producing bacteria.

14. A method according to claim 13, wherein the fermenting bacteria are chosen from the group consisting of Gluconobacter oxydans, Acetobacter sp., Acetogenium kivui, B. macerans, polymyxa, B. coagulans, B. subtilis, Lactobacillus buchneri, Bifidobacterium sp., Clostridium thermoaceticus,

Clostridium lentocellum, Clostridium formicoaceticu, Clostridium thermocellum and Pseudomonas sp.

15. A method according to claim 13, wherein the methane producing bacteria are chosen from the group consisting of Methanosarcina barkeri, Methanosarcina mazeii, Methanosarcina soehngenii and Methanosarcina acetivorans, and Methanosaeta, and mixtures thereof.

16. A method according to any one of claims 13-15, wherein the methane produced is separated from the waste.

17. A method according to any of the preceding claims, wherein said waste is a slurry of wastewater and/or aqueous sludge, preferably sewage sludge.

18. A method according to claim 17, wherein said slurry is pre-concen- trated, prior to the thermal treatment and optional enzyme, cation binding agent and/or bacteria, by gravitation or enhanced sedimentation to a range of 10-80 g sludge solids per 1 I sludge suspension.

19. A method according to any one of claims 1-18, for use in addition to conventional digestion.

20. A method according to any one of claims 1-18, for use instead of conventional digestion.