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Assessment of the anaerobic biodegradability of macropollutants Article in Reviews in Environmental Science and Bio/Technology · January 2004 DOI: 10.1007/s11157-004-2502-3 · Source: OAI
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Reviews in Environmental Science and Bio/Technology 3: 117–129, 2004. 2004 Kluwer Academic Publishers. Printed in the Netherlands.
Assessment of the anaerobic biodegradability of macropollutants Irini Angelidaki1; * & Wendy Sanders2 1
Environment & Recourses DTU, Building 115, 2800 Lyngby, Technical University of Denmark; 2 Wageningen Agricultural University, Subdepartment of Environmental Technology, P.O. Box 8129, 6700 EV Wageningen (*author for correspondence, phone: +45-45251429; fax: +45-45932850; e-mail: [email protected]
Key words: anaerobic, biodegradation assays, hydrolysis, macropollutants, methane potential Abstract A variety of test procedures for determination of anaerobic biodegradability have been reported. This paper reviews the methods developed for determination of anaerobic biodegradability of macro-pollutants. Main focus is paid to the ﬁnal mineralization of organic compounds and the methane potential of compounds. Hydrolysis of complex substrates is also discussed. Furthermore, factors important for anaerobic biodegradation are shortly discussed.
1. Introduction Anaerobic degradation or digestion can be deﬁned as a biological conversion process without external electron acceptor such as oxygen as in aerobic processes or nitrate/sulphate as in anoxic processes. In the anaerobic process organic carbon is converted by subsequent oxidations and reductions to its most oxidized state (CO2 ), and its most reduced state (CH4 ). A wide range of microorganisms catalyze the process in the absence of oxygen (McInerney et al. 1980). The main products of the process are carbon dioxide and methane, but minor quantities (usually less than 1% of the total gas volume) of other products such as nitrogen, nitrogen oxides, hydrogen, ammonia, hydrogen sulphide and other volatile compounds are also generated (McInerney et al. 1980). The mixture of gaseous products is termed biogas and the anaerobic degradation process is often also termed the biogas process. As the result of the removal of carbon, organic bound non-carbon compounds are released to their soluble inorganic form. With the increasing application of the anaerobic digestion process there is an urgent need to review methods for estimation of the biodegradability and methane potential of wastes used in for anaerobic digestion.
A substance or a compound is biodegradable if it can be decomposed by the action of microorganisms. Microorganisms can use this compound as energy source and as carbon source. A compound can be mineralized i.e. converted besides to new microbial biomass to the end carbon products i.e. to carbon dioxide and methane. In some cases complete mineralization is not seen and the compound can be involved in microbial metabolism with only transformation (also called primary biodegradation) of the compound to intermediates, but without total conversion to end products. An organic compound can be processed during anaerobic degradation through metabolism (the compound is supplying energy and carbon source for the micro-organisms) or by cometabolism (the compound is converted only at the presence of another, usually easily degradable organic compound such as glucose, ethanol etc.), that is supplying micro-organisms with energy and carbon for their cell mass built up). In this paper biodegradability with reference to macro-pollutants only, will be treated, since micro-pollutants do not generate enough biogas in order to determine biodegradation through biogas production and there are also uncertainties related to adsorption.
118 2. Some general considerations concerning /inﬂuencing anaerobic biodegradation There are several physical, chemical and physiological factors in the environment that aﬀect biodegradation of organic compounds, such as availability of the compounds, the availability of electron donors and acceptors, oxygen concentration, temperature, pH, moisture, salinity, sorption of chemicals to particulate material, concentration of the chemicals. Diﬀerent factors might have diﬀerent inﬂuence according the speciﬁc characteristics of the compound. 2.1. Redox conditions One of the major factors governing biodegradation is the nature and availability of electron acceptors. From a thermodynamic point of view, oxygen is the most favourable electron acceptor. Under anoxic conditions, the biodegradation will often depend on the availability of electron acceptors, such as nitrate, iron, sulphate or carbon dioxide. In truly anaerobic conditions there is absence of inorganic electron acceptors other than CO2 , and small amount of energy is gained by consecutive oxidations reductions of the organic matter, or CO2 is used as electron acceptor. The energy released in a redox process as a result of electron transfer from one compound to another, is used for the maintenance and growth of the microbial population in the environment. 2.2. Temperature Temperature aﬀects survival and growth of microorganisms and it also inﬂuences their metabolic activities. In general, higher temperatures that do not kill microorganisms result in higher metabolic activities. Temperature is the most important variable in controlling the rate of microbial metabolism in anaerobic environments (Westermann et al. 1989). Anaerobic digestion is applied under three diﬀerent temperature ranges, i.e. the mesophilic (25–40 C), the thermophilic (45–60 C) and the psychrophilic (