The operation of AD requires the co-operation of fermentative bacteria, hydrogen producing acetogenic bacteria, hydrogen consuming acetogenic bacteria; carbon-dioxide reducing methanogens and aceticlastic methanogens (Appels et al., 2008; Lee et al., 2008; Montero et al., 2008; Zinder, 1986; Archer and Kirsop, 1990). Organic polymers are first hydrolysed to simple soluble substrates which are then fermented to yield various organic acids including acetate, formate and reduced organics such as propionate and butyrate (Cantrell et al., 2008; Siller and Winter, 1998; Sarada and Joseph, 1993a,b). These acids are then metabolised to methanogenic precursors, hydrogen and carbon-dioxide (Montero et al., 2008; Tatara et al., 2008; Forster-Caneiro et al., 2008abc; Stams, 1994). About 76% of the metabolisable organics are degraded via reduced intermediate. Acetogenic bacteria metabolise long chain fatty acids to acetate, hydrogen and carbon-dioxide, assisted by hydrogen consuming methanogens. The latter helps to keep hydrogen concentration at a low level, thus making acetogenesis a key process in anaerobic digestion (methanogenesis) (Montero et al., 2008; Tatara et al., 2008; McHugh et al., 2003).
The synthrophic interaction of the variety of anaerobes that operate in the process is essential for methanogenesis, and success of the reaction. By this interaction, several anaerobes can share the energy available in the metabolism of a compound to methane and carbon-dioxide, and so drive endergonic reactions that would have been mechanistically/ energetically difficult under standard conditions (McHugh et al., 2003; Schink, 1997; Dolfing, 1992;). This synthrophy, which is the principal strength of AD, is also the major source of its weakness since various organisms with differing growth rates and requirements have to grow in a synchronised fashion. As a result, the pace of AD is determined by the growth rate of the slowest growing organism in this "symbiosis", which may be as low as 0.08-0.15 d (Verstreate et al., 1996). Pace of digestion and methanogenesis are also influenced by the flux of metabolites, and efficiency of transfer between producing and consuming partners.
AD often has retention times of more than seven days, but usually up to 30 days or more, to avoid washout of methanogens. Effective operation requires retention of viable sludge, and the maintenance of sufficient contact between sludge and wastewater. The system is very sensitive to drop in pH, hence acid consumption must be balanced with generation to avoid failure (Cantrell et al., 2008). This need for growth synchronisation sometimes makes AD precarious and unreliable, requiring highly trained personnel for its operation (Zinder, 1986). This also makes the multi-stage digesters preferred option to the single stage batch processes.
A variety of complex engineering designs have been developed to be able to uncouple biomass from hydraulic retention, and so reduce system fragility and stabilisation time, including anaerobic contact processes with sludge recycle. Some prominent variants of this are AD-ultra filtration, anaerobic filters and fluidised beds (Wheatley, 1990; Coombs, 1990; Hobson, 1990; Noone, 1990; Zinder, 1986;). By far the most popular variant is the upflow anaerobic sludge blanket, UASB (Lettinga et al., 1980), which involves slow upward pumping of fluid waste in a reactor under anaerobic conditions. Selection takes place resulting in the microorganisms growing as granules, which then act as the catalyst, converting organic waste to biogas (Verstreate 1996; Lettinga 1995; Lettinga and Hulshoff Poll 1991). Since its introduction, improved modifications of UASB have been developed, including expanded granular sludge blanket (EGSB), internal circulation reactor (ICR), tubular reactors, etc.
Anaerobic digestion requires careful and close monitoring and control, and usually produces effluents that require further aerobic polishing. This is because individual anaerobes have low substrate affinity (high Ks), resulting in high residual volatile fatty acid content in the effluent, even when stable synthrophic associations have developed. This tends to restrict the use of AD to treatment of highly concentrated wastes as a pre-process to be followed by aerobic polishing. AD cannot be relied upon to detoxify noxious chemicals being often poisoned by them, and has poor pathogen kill capacity (Verstreate et al., 1996; Lettinga, 1995), except when they are operated as thermophilic processes (Aitken et al., 2007; Ahn and Forster, 2000). Attempts to reduce the stabilisation time and increase process efficiency have led to the development of thermophilic anaerobic digestion.
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