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Methane production by anaerobic digestion of wastewater and solid wastesT.Z.D. de Mes, A.J.M. Stams, J.H. Reith and G. Zeeman1
Anaerobic conversion of organic materials andpollutants is an established technology for envi-ronmental protection through the treatment ofwastes and wastewater. The end product is biogasa mixture of methane and carbon dioxide,which is a useful, renewable energy source.Anaerobic digestion is a technologically simpleprocess, with a low energy requirement, used toconvert organic material from a wide range ofwastewater types, solid wastes and biomass intomethane. A much wider application of the tech-nology is desirable in the current endeavourstowards sustainable development and renewableenergy production. In the 1980s several projectswere initiated in The Netherlands to produce bio-
gas from wastes. Many projects were terminateddue to insufficient economic viability. Currently,the production of methane from wastes is recei-ving renewed attention as it can potentially redu-ce CO2 emissions via the production of renewableenergy and limit the emission of the greenhousegas methane from especially animal manure. Thistrend is supported by the growing market demandfor green energy and by the substantial optimisa-tion of anaerobic digestion technologies in thepast decades, especially the development ofmodern high rate and co-digestion systems.
The aim of this chapter is to review and evaluatethe various anaerobic digestion technologies toestablish their potential for methane production,aimed at broadening the range of waste streams
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1Corresponding author: see list of contributors
AbstractAnaerobic digestion is an established technology for the treatment of wastes and wastewater. The finalproduct is biogas: a mixture of methane (55-75 vol%) and carbon dioxide (25-45 vol%) that can be usedfor heating, upgrading to natural gas quality or co-generation of electricity and heat. Digestion installa-tions are technologically simple with low energy and space requirements. Anaerobic treatment systemsare divided into 'high-rate' systems involving biomass retention and 'low-rate' systems without biomassretention. High-rate systems are characterised by a relatively short hydraulic retention time but longsludge retention time and can be used to treat many types of wastewater. Low-rate systems are general-ly used to digest slurries and solid wastes and are characterised by a long hydraulic retention time, equalto the sludge retention time. The biogas yield varies with the type and concentration of the feedstockand process conditions. For the organic fraction of municipal solid waste and animal manure biogasyields of 80-200 m3 per tonne and 2-45 m3 per m3 are reported, respectively. Co-digestion is an impor-tant factor for improving reactor efficiency and economic feasibility. In The Netherlands co-digestion isonly allowed for a limited range of substrates, due to legislation on the use of digested substrate in agri-culture. Maximising the sale of all usable co-products will improve the economic merits of anaerobictreatment. Furthermore, financial incentives for renewable energy production will enhance the compe-titiveness of anaerobic digestion versus aerobic composting. Anaerobic digestion systems currently ope-rational in Europe have a total capacity of 1,500 MW, while the potential deployment in 2010 is esti-mated at 5,300-6,300 MW. Worldwide a capacity up to 20,000 MW could be realised by 2010.Environmental pressures to improve waste management and production of sustainable energy as well asimproving the technologys economics will contribute to broader application.
used for biogas production. The principles ofanaerobic digestion are outlined in Section 4.2. InSection 4.3 anaerobic digestion technologies andtheir application for specific waste streams are dis-cussed. An overview of solid wastes and wastewa-ter streams available for anaerobic digestion inThe Netherlands is presented in Section 4.4. InSection 4.5 the utilisation of biogas as a renewableenergy source is highlighted, including the current and potential share of bio-methane in TheNetherlands. The economics of anaerobic diges-tion are discussed in Section 4.6. The status ofinternational developments is presented inSection 4.7. Conclusions and perspectives forfurther development are presented in Section 4.8.
4.2 Basic principles of anaerobic digestion
4.2.1 Principle of the processAnaerobic microbiological decomposition is aprocess in which micro-organisms derive energyand grow by metabolising organic material in anoxygen-free environment resulting in the produc-tion of methane (CH4). The anaerobic digestionprocess can be subdivided into the following fourphases, each requiring its own characteristicgroup of micro-organisms:
Hydrolysis: conversion of non-soluble biopolymers to soluble organic compounds
Acidogenesis: conversion of soluble organic compounds to volatile fatty acids (VFA) and CO2
Acetogenesis: conversion of volatile fatty acids to acetate and H2
Methanogenesis: conversion of acetate and CO2 plus H2 to methane gas
A simplified schematic representation of anaero-bic degradation of organic matter is given asFigure 1. The acidogenic bacteria excrete enzymesfor hydrolysis and convert soluble organics tovolatile fatty acids and alcohols. Volatile fattyacids and alcohols are then converted by acetoge-nic bacteria into acetic acid or hydrogen and carbon dioxide. Methanogenic bacteria then useacetic acid or hydrogen and carbon dioxide toproduce methane.
For stable digestion to proceed it is vital thatvarious biological conversions remain sufficientlycoupled during the process, to prevent the accu-mulation of intermediate compounds. For exam-ple, an accumulation of volatile fatty acids willresult in a decrease of pH under which conditionsmethanogenesis cannot occur anymore, which
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Amino acids sugars Free long chainfatty acids + glycerol
Volatile fatty acids, alcohol
Suspended, colloidal organic matter
protein carbohydrate lipid
Hydrogen carbon dioxide
Figure 1. Simplified schematic representation of the anaerobic degradation process .
results in a further decrease of pH. If hydrogenpressure becomes too high, further reduced vola-tile fatty acids are formed, which again results ina decrease of pH.
4.2.2 Environmental factors affecting anaerobic digestion
As anaerobic digestion is a biological process, it isstrongly influenced by environmental factors.Temperature, pH and alkalinity and toxicity areprimary control factors.
Controlled digestion is divided in psychrophilic(10-20 C), mesophilic (20-40 C), or thermophilic(50-60 C) digestion. As bacterial growth and con-version processes are slower under low tempera-ture conditions, psychrophilic digestion requires along retention time, resulting in large reactorvolumes. Mesophilic digestion requires less reac-tor volume. Thermophilic digestion is especiallysuited when the waste(water) is discharged at ahigh temperature or when pathogen removal is animportant issue. During thermophilic treatmenthigh loading rates can be applied. Anaerobicdigestion can occur at temperatures as low as 0C,but the rate of methane production increases withincreasing temperature until a relative maximumis reached at 35 to 37 C . At this temperaturerange mesophilic organisms are involved. Therelation between energy requirement and biogasyield will further determine the choice of tempe-rature. At higher temperatures, thermophilic bac-teria replace mesophilic bacteria and a maximummethanogenic activity occurs at about 55C orhigher.
The first steps of anaerobic digestion can occur ata wide range of pH values, while methanogenesisonly proceeds when the pH is neutral . For pHvalues outside the range 6.5 - 7.5, the rate ofmethane production is lower. A sufficient amountof hydrogen carbonate (frequently denoted asbicarbonate alkalinity) in the solution is impor-tant to maintain the optimal pH range requiredfor methanogenesis.
Several compounds exhibit a toxic effect at exces-sive concentrations such as VFA's, ammonia,cations such as Na+, K+ and Ca++, heavy metals,
sulphide and xenobiotics, which adversely affectmethanogenesis.
4.2.3 Methane production potentialThe Chemical Oxygen Demand (COD) is used toquantify the amount of organic matter in wastestreams and predict the potential for biogas pro-duction. The oxygen equivalent of organic matterthat can be oxidised, is measured using a strongchemical oxidising agent in an acidic medium. During anaerobic digestion the biodegradableCOD present in organic material is preserved inthe end products, namely methane and the newlyformed bacterial mass. In case an organic compound (CnHaObNd) is com-pletely biodegradable and would be completelyconverted by the anaerobic organism (sludge yieldis assumed to be zero) into CH4, CO2 and NH3,the theoretical amount of the gases produced canbe calculated according to the Buswell equation (1):
CnHaObNd + (n-a/4 - b/2 +3d/4) H2O (n/2 +a/8 -b/4 -3d/8) CH4 + (n/2-a/8+b/4+3d/8) CO2 + dNH3 (equation 1)
The quantity of CO2 present in the biogas gene-rally is significantly lower than follows from theBuswell equation. This is because of a relativelyhigh solubility of CO2 in water and part of theCO2 may become chemically bound in the waterphase.Another widely used parameter of organic pollution is the Biological Oxygen Demand(BOD). This method involves the measurement ofdissolved oxygen used by aerobic microorganismsin biochemical oxidation of organic matter during5 days at 20 C. A very useful parameter to evaluate substrates foranaerobic d