Environmental Biotechnology - Theory and Application

Integrated Environmental Biotechnology 239

Hydrogenotrophes Acetoclasts

Figure 10.2 The methanisation of biowaste

Many of these have been based more on landfill gas (LFG) generation than truly
representative anaerobic bioreactors, which does lead to some confusion at times.
However, it is generally accepted that the linked, interdependent curves for cel-
lulose decomposition and gas evolution can be broadly characterised as having
five principal stages, outlined below.

• Stage I– Peak biowaste cellulose loadings; dissolved oxygen levels fall to
  zero; nitrogen, and carbon dioxide tend to atmospheric levels.


• Stage II– Carbon dioxide, hydrogen and free fatty acids levels peak; nitrogen
  levels fall to around 10%; cellulose begins to be broken down.


• Stage III– Carbon dioxide decreases and plateaus, to hold at around 40%;
  methane production commences and achieves steady state at around 60%; free
  fatty acids decrease to minimum levels; cellulose breakdown continues at a
  linear rate with respect to time; nitrogen levels fall to near zero.


• Stage IV – Carbon dioxide and methane continue in steady state at c. 40%
  and c. 60% respectively; cellulose component reduces steadily.