WO2008038860A1

WO2008038860A1 - Production method of methane gas from olive mill waste

Info

Publication number: WO2008038860A1
Application number: PCT/KR2006/005503
Authority: WO
Inventors: Eui-So Choi
Original assignee: Greentech Environment Consulting Co., Ltd; Kunhwa Consulting & Engineering Co., Ltd
Priority date: 2006-09-26
Filing date: 2006-12-15
Publication date: 2008-04-03
Also published as: KR20080028247A

Abstract

Disclosed herein is a method for producing methane gas from olive mill wastewater. The method comprises the steps of (a) obtaining secondary sludge containing nitrogen sources from a municipal sewage treatment plant and selectively culturing the secondary sludge under anaerobic conditions to produce concentrated anaerobic microbial sludge, (b) feeding another secondary sludge containing nitrogen sources obtained from the municipal sewage treatment plant and olive mill wastewater to the concentrated anaerobic microbial sludge, followed by mixing, and (c) anaerobically digesting the mixture to produce methane gas and effluent. According to the method, olive mill wastewater can be anaerobically digested without pretreatment, methane gas can be produced in high yield, and olive mill wastewater and secondary sludge can be simultaneously treated. In addition, anaerobic digestion can be performed in one reaction tank, which is advantageous in terms of space reduction.

Description

[DESCRIPTION] [Invention Title]

PRODUCTION METHOD OF METHANE GAS FROM OLIVE MILL WASTEWATER

[Technical Field]

The present invention relates to a method for producing methane gas from olive mill wastewater, and more specifically to a method for producing methane gas from olive mill wastewater wherein olive mill wastewater can be anaerobically digested without pretreatment, methane gas can be produced in high yield, and olive mill wastewater and secondary sludge can be simultaneously treated.

[Background Art]

The Mediterranean countries produce about 98% of olive oil demand in the world which is about 1.77 million tons per year. Since olive mill wastewater is produced in an amount of about two times the amount of olive oil, the Mediterranean region is responsible for the production of about 3.50 million tons of olive mill wastewater a year. Spain makes up the largest portion (602,000 tons) of the world production of olive oil. Italy (451,000 tons), Greece (332,000 tons), Tunisia (173,000 tons), Turkey (92,000 tons), Syria (81,000 tons), Morocco (46,000 tons) and other countries, including Australia, U.S., and Lebanon, share the world production of olive oil. These olive producing countries have a desert climate characterized by low annual rainfall and relatively high temperature. Olive oil produced from these countries is exported to all countries of the world. The production of olive oil in these countries is expected to gradually increase.

Olive oil is produced by pressing the olive fruit under high pressure to extract oil components and water and collecting the oil components only by centrifugation. Solid waste obtained during production of olive oil is used as feed for camels and sheep, causing no waste problem. However, the treatment of olive mill wastewater, which is the major waste produced from the extraction of olive oil, becomes a serious problem. Olive mill wastewater represents 150,000 mg/L organics and 10,000 mg/L polyphenols, as calculated on the basis of CODcr. Since olive mill wastewater has high organic contents, energy production with anaerobic digestion could be a solution, but it was found that anaerobic degradation is difficult without pretreatment of phenolic compounds, such as catechol, 4-hydoxybenzoic acid, 2-(3,4- dihydroxy)phenylethanol, 3, 4-dihydroxyphenyl acetic acid, and so forth.

Thus, many processes, such as UV radiation, fenton oxidation, ozonation and electrolysis, have been proposed to pretreat olive mill wastewater. However, since these pretreatment processes result in the removal of non-toxic organics as well as toxic organics, there is a problem in that the amount of gases produced by subsequent anaerobic fermentation is decreased. In addition, considerable costs are incurred to conduct the pretreatment processes. Olive mill wastewater is still generally stored in a lagoon to evaporate for a long period of time to leave residue, and the final residue is used for fuel. However, the problems associated with the lagoon storage are an unpleasant smell from a lagoon, and particularly, the possibility of contamination of underground water through the soil .

[Disclosure] [Technical Problem]

Therefore, it is an object of the present invention to provide a method for producing methane gas in high yield from olive mill wastewater by anaerobically digesting olive mill wastewater without pretreatment.

[Technical Solution]

In accordance with an aspect of the present invention for achieving the above object, there is provided a method for producing methane gas from olive mill wastewater, the method comprising the steps of (a) obtaining secondary sludge containing nitrogen sources from a municipal sewage treatment plant and selectively culturing the secondary sludge under anaerobic conditions to produce concentrated anaerobic microbial sludge, (b) feeding another secondary sludge containing nitrogen sources obtained from the municipal sewage treatment plant and olive mill wastewater to the concentrated anaerobic microbial sludge, followed by mixing, and (c) i anaerobically digesting the mixture to produce methane gas and effluent . In an embodiment of the present invention, step (a) may be carried out for 20 to 32 days.

In a further embodiment of the present invention, the secondary sludge used in steps (a) and (b) may have a total nitrogen content of 1.5 to 15 wt%, based on the solids content of the secondary sludge.

In another embodiment of the present invention, the method may further comprise the step of, prior to step (b) , stirring the secondary sludge containing nitrogen sources obtained from the municipal sewage treatment plant and the olive mill wastewater.

In another embodiment of the present invention, the method may further comprise the step of, after step (c) , returning a portion of the effluent to the mixture of the secondary sludge containing nitrogen sources obtained from the municipal sewage treatment plant and the olive mill wastewater.

In another embodiment of the present invention, the secondary sludge used in steps (a) and (b) may be liquid or dried sludge. In another embodiment of the present invention, in step

(b) , the secondary sludge containing nitrogen sources and the olive mill wastewater may be fed in a volume ratio ranging from 8.5 : 1 to 10.5 : 1.

In another embodiment of the present invention, steps (a) , (b) and (c) may be carried out in a sequencing batch reaction process .

In yet another embodiment of the present invention, the anaerobic digestion (step (c) ) may be performed at a hydraulic retention time (HRT) of 8 to 12 days, a mixed liquor suspended solid (MLSS) of 20,000 to 30,000 mg/L, a pH of 7 to 8 and a temperature of 25 to 35°C.

[Description of Drawings]

The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a process chart illustrating a method for producing methane gas from olive mill wastewater according to an embodiment of the present invention; and FIG. 2 is a graph showing the amounts of methane gas produced during operating periods of anaerobic digestion in a method for producing methane gas from olive mill wastewater according to an embodiment of the present invention.

[Best Mode]

Preferred embodiments of the present invention will now be described in greater detail with reference to the accompanying drawings . A method for producing methane gas from olive mill wastewater according to an embodiment of the present invention is illustrated in FIG. 1. As indicated by the solid lines in FIG. 1, the method of the present invention comprises the steps of (a) obtaining secondary sludge containing nitrogen sources from a municipal sewage treatment plant and selectively culturing the secondary sludge under anaerobic conditions to produce concentrated anaerobic microbial sludge, (b) feeding another secondary sludge containing nitrogen sources obtained from the municipal sewage treatment plant and olive mill wastewater to the concentrated anaerobic microbial sludge, followed by mixing, and (c) anaerobically digesting the mixture to produce methane gas and effluent .

Municipal sewage treatment largely involves the following steps: (1) filtration and removal of relatively large solids contained in wastewater using a screen; (2) sedimentation and removal of grit, which is a mixture of sand and dirt, in a sand basin; (3) sedimentation and removal of solid substances that can be settled in the wastewater in a primary sedimentation basin; (4) assimilation of microbial cells from organic substances that are not settled in the primary sedimentation basin in an aeration tank, which serves to facilitate the sedimentation of the assimilated microbial cells; (5) sedimentation and removal of the microbial cells in a secondary sedimentation basin; and (6) sterilization by feeding chlorine to the secondary sedimentation basin to kill pathogenic microbes. Sludge settled in the secondary sedimentation basin is referred to as 'secondary sludge' and is substantially composed of microbes. Secondary sludge obtained during treatment of activated sludge is also referred to as 'activated sludge' . Because the secondary sludge is substantially composed of microbes, it can be approximated by CsH₇θ₂N. That is, since the secondary sludge has a high nitrogen content, it can act as a nitrogen source for anaerobic bacteria upon anaerobic digestion. Further, various microbes present in the secondary sludge can act as seed bacteria for anaerobic reactions.

The secondary sludge used in step (a) is essentially composed of aerobic bacteria or facultative bacteria (i.e. microbes that can grow with or without oxygen) . Facultative and anaerobic organic acid producing bacteria and methane producing bacteria grow in the secondary sludge by anaerobic digestion, which occurs in a state in which no air is introduced. This growth of the bacteria, which is an ecological succession, utilizes a process wherein microbes suitable for growth under anaerobic environments become dominant species. That is, aerobic bacteria are dominant species in the secondary sludge obtained from the municipal sewage treatment plant, but anaerobic bacteria suitable for growth under anaerobic conditions become dominant species when anaerobic conditions are maintained by stopping the supply of oxygen by aeration. A detailed explanation of this process will be given below. Anaerobic digestion occurs by the action of various kinds of bacteria. Bacteria involved in anaerobic digestion are largely divided into organic acid producing bacteria and methane producing bacteria. Facultative and anaerobic organic acid producing bacteria degrade organic substances to form corresponding organic acids and alcohols, along with small amounts of carbon dioxide (CO₂) gas, acetone, hydrogen (H₂) gas, etc. This reaction is called a ^λfirst digestion step' . Methane producing bacteria, which are classified into four genera, i.e. Methanobacterium, Methanococcus, Methanosarcina and Methanospirillium, in terms of morphology, degrade organic acids formed in the first digestion step to form gaseous substances, including methane (CH₄) and carbon dioxide (CO₂) , along with small amounts of other gases, such as ammonia (NH₃) gas, hydrogen sulfide (H₂S) and mercaptan. An appropriate activity of organic acid producing bacteria and methane producing bacteria is required to convert organic substances to corresponding gases. That is, organic acids as nutrients necessary in a subsequent second digestion step are formed in the^' first digestion step, while the organic acids are degraded in a second digestion step to prevent pH reduction arising from the accumulation of the organic acids. In addition, organic acid producing bacteria not only produce nutrients necessary for the growth of methane producing bacteria but also consume oxides to form reductants, thus creating complete anaerobic conditions. In step (a) , secondary sludge containing nitrogen sources obtained from a municipal sewage treatment plant is selectively cultured to produce concentrated anaerobic microbial sludge, i.e. concentrated sludge of methane producing bacteria, etc. Thereafter, when another secondary sludge containing nitrogen sources obtained from the municipal sewage treatment plant and olive mill wastewater are fed to the concentrated anaerobic microbial sludge in step (b) , anaerobic microbes, such as methane producing bacteria, become dominant species to allow anaerobic digestion to sufficiently occur in step (c) , resulting in an increase in the amount of methane gas produced in step (c) . According to a report, olive mill wastewater is characterized by the following physical properties: pH of 4.5-5.9, chemical oxygen demand (COD) of 40-200 g/L, biochemical oxygen demand (BOD) of 20-110 g/L, phosphorus (P) of 800-1,100 mg/L, potassium (K) of 7,200 mg/L, calcium (Ca) of 700 mg/L, magnesium (Mg) of 400 mg/L, sodium (Na) of 900 mg/L, iron (Fe) of 70 mg/L, chlorine (Cl) of 300 mg/L, total sugar of 1-8 wt% (% dry matter), organic nitrogen of 0.28-2 wt% (% dry matter), organic acids of 0.5-1 wt% (% dry matter), polyalcohols of 1-1.5 wt% (% dry matter), tannins 0.37-1 wt% (% dry matter), polyphenols of 0.5-2.4 wt% (% dry matter), grease of 0.03-1 wt% (% dry matter) .

As a result of an actual measurement, olive mill wastewater contains 150 g/L CODcr, which represents that carbon necessary for the growth of anaerobic microbes is present in a sufficient amount, and 0.16 wt% (% dry matter) of organic nitrogen, which represents lack of nitrogen. In addition, olive mill wastewater contains large amounts (10 g/L) of toxic polyphenols (including catechol, 4-hydoxybenzoic acid, 2- (3, 4-dihydroxy) phenylethanol, 3, 4-dihydroxyphenyl acetic acid, and so forth) . In step (b) , secondary sludge containing the nitrogen sources obtained from a municipal sewage treatment plant, together with olive mill wastewater, fed to the concentrated anaerobic microbial sludge enables the supply of nitrogen, which is necessary for the growth of the anaerobic microbes but is insufficiently present in the olive mill wastewater. The toxicity of the polyphenols can be inhibited i) by diluting the high-concentration polyphenols with a large amount of moisture present in the secondary- sludge containing the nitrogen sources obtained from the municipal sewage treatment plant to a concentration of the polyphenols that can be sufficiently degraded by the microbes, ii) by degrading the polyphenols by the action of denitrifying microbes or sulfate-reducing bacteria present in the secondary sludge, or iii) by a combination of both functions of i) and ii) . Various microbes present in the secondary sludge from the municipal sewage treatment plant can act as seed bacteria for anaerobic digestion. That is, the secondary sludge containing nitrogen sources obtained from the municipal sewage treatment plant can be used for anaerobic digestion of the olive mill wastewater. In addition, since the secondary sludge containing nitrogen sources obtained from the municipal sewage treatment plant can be degraded by anaerobic digestion, it can be treated by the method of the present invention. Accordingly, according to the method of the present invention, secondary sludge, which is the waste produced from a municipal sewage treatment plant, and olive mill wastewater can be simultaneously treated. As a consequence, olive mill wastewater can be anaerobically digested by the method of the present invention, which enables the production of methane gas from olive mill wastewater. Step (a) may be carried out for 20 to 32 days. If step

(a) is carried out for less than 20 days, there may be the danger that growth of methane producing bacteria is insufficient for the production of methane gas by anaerobic digestion of the olive mill wastewater. Meanwhile, if step

(a) is carried out for more than 32 days, the time required for the production of methane gas is unnecessarily long, thus adversely affecting the operating conditions.

The secondary sludge used in steps (a) and (b) may have a total nitrogen content of 1.5 to 15 wt%, based on the solids content of the secondary sludge. A total nitrogen content lower than 1.5 wt% or greater than 15 wt% of the secondary sludge may be unsuitable for the growth of the anaerobic microbes . As indicated by the dotted line arrow shown in FIG. 1, the method of the present invention may further comprise the step of, prior to step (b) , stirring the secondary sludge containing nitrogen sources obtained from the municipal sewage treatment plant and the olive mill wastewater. This stirring step is advantageous for homogeneous reaction of the concentrated anaerobic microbial sludge with the secondary sludge containing nitrogen sources obtained from the municipal sewage treatment plant and the olive mill wastewater, thus achieving stabilized anaerobic digestion of the olive mill wastewater. As indicated by the dotted line arrow shown in FIG. 1, the method of the present invention may further comprise the step of, after step (c) , returning a portion of the effluent to the mixture of the secondary sludge containing nitrogen sources obtained from the municipal sewage treatment plant and the olive mill wastewater. Since the effluent undergoes anaerobic digestion, it contains various organic substances and microbes necessary for anaerobic digestion. Accordingly, the return of the effluent assists in the anaerobic digestion of the olive mill wastewater.

The secondary sludge used in steps (a) and (b) may be liquid or dried sludge. Since common secondary sludge has a moisture content as high as 95 to 99%, dried sludge may be preferably used in terms of convenience of use. The dried sludge is prepared by drying sludge on a sludge drying bed. Specifically, the dried sludge means sludge in which 20-90% of the initial moisture content is removed.

The secondary sludge containing nitrogen sources and the olive mill wastewater may be fed in a volume ratio ranging from 8.5 : 1 to 10.5 : 1. When the secondary sludge is used in an amount of less than 8.5 parts by volume per one part by volume of the olive mill wastewater, i.e. the concentration of the olive mill wastewater is relatively high, there is the risk that growth of the anaerobic microbes is limited. Meanwhile, when the secondary sludge is used in an amount of more than 10.5 parts by volume per one part by volume of the olive mill wastewater, i.e. the olive mill wastewater is excessively diluted, growth of the anaerobic microbes may be limited due to lack of carbon sources necessary for the growth of the anaerobic microbes and the reaction tank must be larger in volume than is necessary.

Steps (a) , (b) and (c) may be carried out in a sequencing batch reaction process. The sequencing batch reaction process basically consists of the following consecutive steps: filling of sewage wastewater, reaction, settlement and drawing. One or more rest periods may be provided between the respective steps. That is, secondary sludge containing nitrogen sources obtained from a municipal sewage treatment plant is filled in a sequencing batch ^' reaction tank where satisfactory anaerobic conditions are created in the absence of oxygen, as in step (a) of the method according to the present invention; stirring is conducted for reaction; settling occurs after passage of a specified time period; supernatant is drawn as an effluent; and methane gas is collected. This series of consecutive steps is carried out in the sequencing batch reaction tank. As a result, the hydraulic retention time (HRT) of the secondary sludge obtained from the municipal sewage treatment plant is maintained constant and concentrated sludge (i.e. concentrated anaerobic microbial sludge) in which anaerobic microbes, such as methane producing bacteria, become dominant species can be produced. The concentrated anaerobic microbial sludge enables the production of methane gas by anaerobic reactions even when the secondary sludge containing nitrogen sources and the olive mill wastewater are fed into the sequencing batch reaction tank. In addition, since the consecutive steps, i.e. filling, reaction, settling and drawing, are carried out in one reaction tank, the sequencing batch reaction is economically advantageous in terms of space reduction. Furthermore, since step (b) (i.e. feeding of the secondary sludge containing nitrogen sources obtained from the municipal sewage treatment plant and the olive mill wastewater to the concentrated anaerobic microbial sludge, followed by mixing) and step (c) (i.e. anaerobic digestion of the mixture) are consecutively carried out in the sequencing batch reaction tank, feeding of specified amounts of the secondary sludge containing nitrogen sources obtained from the municipal sewage treatment plant and the olive mill wastewater, anaerobic reactions with the solution for anaerobic digestion (i.e. the concentrated anaerobic microbial sludge) , settling of the anaerobic microbes after the anaerobic reactions and drawing of supernatant (effluent) after the settling consecutively occur in one reaction tank. As a result, the stream of the reaction tank is maintained at a constant level and the number of the anaerobic microbes, such as methane producing bacteria, settled in the reaction tank is increased, so that the amount of methane gas produced in the reaction tank can be increased to a permissible level of the system.

In step (c) , the anaerobic digestion can be performed at a hydraulic retention time (HRT) of 8 to 12 days, a mixed liquor suspended solid (MLSS) of 20,00^'0 to 30,000 mg/L, a pH of 7 to 8 and a temperature of 25 to 35°C. The operating conditions for the anaerobic digestion in step (c) are also applicable to the production of the concentrated anaerobic microbial sludge by selective culturing of the secondary sludge obtained from the municipal sewage treatment plant under anaerobic conditions .

When the hydraulic retention time is less than 8 days, the time period required for the anaerobic digestion is too short, causing the risk that the olive mill wastewater may be insufficiently degraded. Meanwhile, when the hydraulic retention time is more than 12 days, the overall operating period may be unnecessarily lengthened.

When the mixed liquor suspended solid (MLSS) is less than 20,000 mg/L, the number of the anaerobic microbes, such as methane producing bacteria, is small, causing the danger that the anaerobic digestion may insufficiently occur. Meanwhile, when the mixed liquor suspended solid (MLSS) is exceeds 30,000 mg/L, the number of the anaerobic microbes, such as methane producing bacteria, is excessively large, thus negatively affecting the growth of the microbes .

A pH lower than 7 or higher than 8 may make the growth of the anaerobic microbes, such as methane producing bacteria, difficult because the anaerobic microbes, such as methane producing bacteria, are sensitive to a pH variation. An optimum pH range for the anaerobic digestion is required to increase the production of methane gas.

A temperature lower than 25°C or higher than 35°C may also make the growth of the anaerobic microbes, such as methane producing bacteria, difficult.

[Mode for Invention]

The following examples serve to provide further appreciation of the invention but are not meant in any way to restrict the scope of the invention.

EXAMPLES Example 1

Sewage sludge (secondary active sludge collected from a Jungrang wastewater treatment plant, Seoul, Korea) was filled at a rate of 570 mL/day in a sequencing batch anaerobic digester. The anaerobic digester was operated at a hydraulic retention time (HRT) of 10 days with a 24 hour cycle for 28 days . The sewage sludge was selectively cultured under anaerobic conditions to produce concentrated anaerobic microbial sludge. The filling of the sewage sludge was conducted for 12 hours while the sequencing batch anaerobic digester was mixing. The mixing was continued for 6 hours after the filling was stopped, followed by settling for 6 hours. The filling was conducted for 30 minutes and stopped for 2.5 hours. This filling procedure was repeated. The amount of methane gas evolved from the sequencing batch anaerobic digester was measured. 29 to 50 days after the operation was initiated, 30 ml/d of olive mill wastewater was mixed with 570 mL/d of another sewage sludge collected from the Jungrang wastewater treatment plant in a stirring tank, and then the mixture was fed into the sequencing batch anaerobic digester using a pump to react the mixture with the concentrated anaerobic microbial sludge. The anaerobic digester was operated at a hydraulic retention time (HRT) of 10 days with a 24 hour cycle. The feeding of the mixture was conducted for 12 hours while the sequencing batch anaerobic digester was mixing. The mixing was continued for 6 hours after the feeding was stopped, followed by settling for β hours. The feeding of the mixture of the olive mill wastewater and the sewage sludge was conducted for 30 minutes and stopped for 2.5 hours . This feeding procedure was repeated. In the same manner as above, the amount of the olive mill wastewater fed was increased to 40 ml/d at 51 to 70 days after the operation was initiated, and increased to 60 ml/d at 71 to 90 days after the operation was initiated. The amount of gases produced for a total of 90 days, including the period (20 days) for the production of the concentrated anaerobic microbial sludge, was measured daily while continuously feeding the wastewater. 90 days after the operation was initiated, the production of methane gas was stabilized. At this time, the amounts of the sludge and the olive mill wastewater fed were 570 mL/d and 60 mL/d (9.5 : 1 (v/v) ) , respectively. The operating conditions in the respective periods are shown in Table 1. When the production of methane gas was stabilized (90 days after the operation was initiated) , the sequencing batch anaerobic digester was operated at a mixed liquor suspended solid (MLSS) of 24,000 mg/L, a pH of 7.6 and a temperature of 35⁰C. TABLE 1

Test Example 1

Analysis of composition of gases

The contents of the gases, i.e. methane, carbon dioxide, hydrogen, nitrogen and hydrogen sulfide, produced when the production of methane gas was stabilized (at 71 to 90 days after the operation was initiated) in Example 1, were measured by gas chromatography (Shimazue, Japan) . The analytical results show that the gases include, on average, 65 vol% of methane, 31 vol% of carbon dioxide, 3.6 vol% of hydrogen, 0.3 vol% of nitrogen and 0.06 vol% of hydrogen sulfide. Based on these results, it was concluded that methane gas could be produced in high yield by the method of the present invention.

Test Example 2

Analysis of methane gas The amount of methane gas produced during the operating periods was measured daily by gas volumetric analysis. The results are shown in FIG. 2. At the acclimation stage, the amount of methane gas produced tended to be decreased at 20 days and 28 days after the operation was initiated. This tendency is believed to be due to the lack of organic substances, particularly carbon sources, by an increased number of methane producing bacteria. FIG. 2 shows that methane gas was not produced any further at 50 and 70 days after the operation was initiated. The amount of the olive mill wastewater was increased at the time points (50 and 70 days) to increase the amount of methane gas produced. FIG. 2 also shows that methane gas was stably produced after 90 days of the operation. The reason why the production of methane gas was not increased is believed to be because the number of methane producing bacteria increased, causing lack of carbon sources. It is estimated that additional feeding of the olive mill wastewater after 90 days of the operation can increase the production of methane gas, resulting in stable production of methane gas. As is evident from these results, the method of the present invention enabled stable production of methane gas .

[Industrial Applicability]

As apparent from the above description, the method of the present invention provides the advantages that olive mill wastewater can be anaerobically digested without pretreatment, methane gas can be produced in high yield, and olive mill wastewater and secondary sludge can be simultaneously treated. In addition, according to the method of the present invention, anaerobic digestion can be performed in one reaction tank, which is advantageous in terms of space reduction. Furthermore, since effluent obtained after anaerobic digestion is free of toxic substances, it can be used in agricultural applications .

Claims

[CLAIMS]

[Claim 1]

A method for producing methane gas from olive mill wastewater, the method comprisirig the steps of:

(a) obtaining secondary sludge containing nitrogen sources from a municipal sewage treatment plant and selectively culturing the secondary sludge under anaerobic conditions to produce concentrated anaerobic microbial sludge; (b) feeding another secondary sludge containing nitrogen sources obtained from the municipal sewage treatment plant and olive mill wastewater to the concentrated anaerobic microbial sludge, followed by mixing; and

(c) anaerobically digesting the mixture to produce methane gas and effluent.

[Claim 2]

The method according to claim 1, wherein step (a) is carried out for 20 to 32 days.

[Claim 3]

The method according to claim 1, wherein the secondary sludge used in steps (a) and (b) has a total nitrogen content of 1.5 to 15 wt%, based on the solids content of the secondary sludge.

[Claim 4]

The method according to claim 1, further comprising the step of, prior to step (b) , stirring the secondary sludge containing nitrogen sources obtained from the municipal sewage treatment plant and the olive mill wastewater.

[Claim 5]

The method according to claim 4, further comprising the step of, after step (c) , returning a portion of the effluent to the mixture of the secondary sludge containing nitrogen sources obtained from the municipal sewage treatment plant and the olive mill wastewater.

[Claim 6]

The method according to claim 1, wherein the secondary sludge used in steps (a) and (b) is liquid or dried sludge.

[Claim 7] The method according to claim 1, wherein, in step (b) , the secondary sludge containing nitrogen sources and the olive mill wastewater are fed in a volume ratio ranging from 8.5 : 1 to 10.5 : 1.

[Claim 8] The method according to claim 1, wherein steps (a) , (b) and (c) are carried out in a sequencing batch reaction process .

[Claim 9]

The method according to claim ^'1, wherein the anaerobic digestion is performed at a hydraulic retention time (HRT) of

8 to 12 days, a mixed, liquor suspended solid (MLSS) of 20,000 to 30,000 mg/L, a pH of 7 to 8 and a temperature of 25 to 35°C.