dry anaerobic digestion of municipal solid waste and digestate ...

2.6.2 Single-stage continuous systems
In continuous digestion process, the waste is fed and withdrawn from the reactor
continuously. As the substrate is continuously added, all biochemical reactions involved in
the generation of biogas occur at a reasonably constant rate. The system receives its weight
little by little, spread over time, so that digestion takes place uninterrupted having no end
point. A reasonably constant rate of biogas production is resulted by this. Full-scale single
stage continuous digestion systems in Europe cover over 87% digestion capacity of
biowaste and sewage sludge (De Baere, 2006). Industrialists prefer one-stage systems
because of their simpler designs and lower investment costs. The continuous input
anaerobic digestion requires less land area and its operating cost can be comparable.
Importantly, the higher initial investment cost may be compensated from real state cost
reduction where the land is scarce.
However, a technical difficulty associated with pump has been encountered in loading the
feedstock in continuous manner (Sharma et al. 2000). Mixing is of pivotal importance in
all anaerobic digestion systems, continuous systems rely on pumping for its continuous
operation (Lissens et al , 2001). Further, the continuous system requires high internal
fluidity for the smooth feedstock intake and removal process. Such systems are, therefore,
principally suitable for low solid wastes. For higher solid content, transport and handling
of the waste is carried out with conveyor belts, screws and powerful pumps especially
designed for viscous streams. Such types of equipment are very expensive (Mata-Alvarez
et al., 2003).
There has been a shift of the research focus on semi-continuous mode of operation. Semicontinuous
digesters are fed at continuous intervals of time, as for example on daily basis,
or on more frequent intervals, with simultaneous removal of the digestate (Wang et al.,
2003; Misi and Forster, 2002). These systems are suited to regularly and steadily arising
waste stream. The biogas yield of semi-continuous processes is characteristically higher
and more regular. The higher production rate is attributed to the waste that is kept in their
original state, and not diluted with water (Ole szkiewicz and Poggi-Varaldo, 1997). The
distinction between continuous and semi-continuous system is rather subjective. Most of
the continuous digesters in large scales are not truly continuous. They are operated in semicontinuous
mode (Sharma et al., 2000). The term ‘continuous system’ is used in a broader
sense, which includes truly continuous and semi-continuous digestion systems where the
digesters are fed once or twice a day.
2.6.3 Multi-stage continuous systems
As anaerobic digestion process consists of four steps and each of these requires different
conditions for optimal growth and function of its respective microbial group involved in
the process. For instance, acidogens are more active at relatively low pH (i.e. 5.5–6.0)
while acetogens and methanogens require stable neutral pH and are sensitive to even low
concentrations of inhibitors (e.g., ammonia and VFA). Moreover, acetogens also need
close proximity with methanogens for efficient inter-species hydrogen transfer. However,
most full-scale digestion systems in use are single-stage digesters, in which all of the four
processes have to be carried out simultaneously. In this way, the metabolic activities of
different microbial groups are compromised and the performance of single-stage systems is
often suboptimal with a low reduction of VS. Thus, multi-stage system or more
appropriately, two-stage systems were introduced by the researchers to manage and
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