Although composting certainly accounts for the majority of biowaste treatment applications around the world, anaerobic digestion (AD) is an alternative option which has been receiving increasing interest over recent years. In many respects, it is a regulated version of the natural events of landfill, in that it results in the controlled release of methane-rich biogas, which offers the potential for a very real form of energy from waste. This technology is viewed in certain circles as rather novel, but this is not really the case. It has been used in the water industry for around a hundred years to treat sewage and, more recently, been successfully applied to the processing of agricultural and household wastes, most notably in Germany and the Netherlands. However, waste management tends to be a naturally cautious field and the relative lack of a proven track record with MSW-derived biowaste compared to composting has made the uptake of this approach slow.
The key to effective practical applications of AD technology lies in regulating and optimising the internal environment of an enclosed bioreactor vessel such that the ideal conditions for the process are produced and maintained. Under these circumstances, in the absence of free oxygen, anaerobic bacteria convert the large organic molecules mainly into methane CH4 and carbon dioxide CO2.
The actual progression of this breakdown is chemically very complex, poten-tially involving hundreds of intermediary reactions and compounds, many of which have their own additional requirements in terms of catalysts, enzymes or synergistic chemicals. Unlike composting, AD occurs at one of three distinct temperature ranges, namely:
· Cryophilic (<20 ◦C).
· Mesophilic (20 – 45 ◦C).
· Thermophilic (>45 ◦ C).
Since AD is very much less exothermic than composting, within a landfill or in bogs and swamps, it proceeds under cryophilic conditions. This largely accounts for the relatively protracted timescale and the irregular progress of breakdown typically encountered in these examples. In order to overcome these drawbacks, engineered anaerobic bioreactors are usually run at one or other of the higher ranges, with additional heat supplied by external means to elevate the temperature to the required level. A variety of technology vendors have developed commercial systems based around either thermophilic or mesophilic digestion, which have their own particular characteristics. Without entering into a lengthy discussion of the relative merits of these approaches, it is important to note that the internal conditions favour different bacterial complements and that certain aspects of the reaction details also differ. Consequently, for any given application, one or other may be particularly suited, dependent on the specifics of the material to be processed and the overall requirements for treatment.
Carbohydrates, cellulose, proteins and fats are broken down and liquefied by the extracellular enzymes produced by hydrolytic bacteria. The proteins are broken down into amino acids, fats into long-chain fatty acids and carbohydrates into simple sugars, while the liquefaction of complex biological polymers, especially cellulose, to simple, soluble substances is often the rate-limiting step in digestion. The rate of hydrolysis is governed by the nature and availability of the substrate, bacterial population, temperature and pH.
The monomers released by hydrolysis, together with volatile fatty acids (VFAs) derived from the protein, fat and carbohydrate components of the material being treated are converted to acetic, lactic and propionic acids, the pH falling as the concentrations of these rise. Methanol and other simple alcohols, carbon dioxide and hydrogen are also produced during acidogenesis, the exact proportions of the different byproducts being dependent on bacterial species and the environmental conditions within the reactor. Though we have considered these events as a single stage in the process, some authorities prefer to subdivide them into acidogenesis and acetogenesis to highlight the importance of acetic acid, which accounts for around 75% of the methane produced by the next step.
Relying on obligate anaerobes whose overall growth rate is slower than those responsible for the preceding stages, this final phase involves the production of methane from the raw materials previously generated. Of these, acetic acid and the closely related acetate are the most important, for the reason mentioned above. There are other potential substrates for methanogenic bacteria, but we will limit the scope of this discussion to the most usual examples, as represented in the following equations:
CH3COOH −−−→ CH4 + CO2
CH3OH + H2 −−−→ CH4 + H2O
Carbon dioxide and hydrogen
CO2 + 4H2 −−−→ CH4 + 2H2O
Methanogenic bacteria also play an important part in the wider overall breakdown process, since by converting volatile fatty acids (VFAs) into methane, they effec-tively act to limit pH decrease within the digester. With the acid/base equilibrium naturally regulated in this way, any potential bacterial inhibition by acidification is effectively overcome. This is particularly important for methanogens themselves,since they thrive in a relatively narrow pH threshold of 6.6 – 7.0, becoming pro-gressively more impaired as the pH falls below 6.4. In this event, the persistence of unmodified VFAs can have potentially serious implications for the final use or disposal of the material derived.
There are four main groups of bacteria involved in AD, as shown below, with some typical examples of each:
•Hydrolytic fermentative bacteria – Clostridium and Peptococcus.
•Acetogenic bacteria – Syntrophobacter and Syntrophomonas.
•Acetoclastic methanogens – Methanosarcina and Methanothrix.
•Hydrogenotrophic methanogens – Methanobacterium and Methanobrevibac-terium.
In reality, these are not the only species present in a digester and, though the stages previously described represent the main desired biochemical reactions, a number of additional bacterial types and biochemical pathways play a role in the overall breakdown process. As with composting, there is much interaction between these various organisms.