(best examples and good practices) on household organic waste ...

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to the viscosity of the feed, the incoming waste does not mix with the contents
of the digester. This prevents inoculation of the incoming waste which can lead
to local overloading. Therefore, most of the digester designs include an
inoculation loop in which the incoming OFMSW is mixed with some of the
exiting dig estate paste prior to loading.
Multi-stage systems: are designed to take advantage of the fact that different
portions of the overall biochemical process have different optimal conditions.
By optimizing each stage separately, the overall rate can be increased. Typically,
two-stage processes (Figure 11.) attempt to optimize the hydrolysis <strong>and</strong>
fermentative acidification reactions in the first stage where the rate is limited
by hydrolysis of complex carbohydrates. The second stage is optimized for
methanogenesis where the rate in this stage is limited by microbial growth
kinetics. Since methanogenic archaea prefer pH in the range of 7–8.5 while
acidogenic bacteria prefer lower pH, the organic acids are diluted into the
second stage at a controlled rate. Often a closed recirculation loop is provided
to allow greater contact time for the unhydrolyzed organic matter.
Figure 11.: Two stage process
(EPA., 2008)
Some multi-stage systems apply a microaerophilic process in an attempt to
increase the oxidation of lignin <strong>and</strong> make more cellulose available for
hydrolysis. Although adding oxygen to an anaerobic environment seems
counterintuitive, sludge granules can shield the obligate anaerobes from
oxygen poisoning <strong>and</strong> the practice has been shown to increase biogas yield in
some situations. In two-stage systems, because methanogens are more
sensitive to oxygen exposure than fermentative bacteria, the air may
preferentially inhibit methanogens, which could help maintain a low pH in the
hydrolysis stage. However, if the oxygen is not completely consumed <strong>and</strong> the
biogas contains a mixture of oxygen <strong>and</strong> hydrogen <strong>and</strong>/or methane, hazardous
conditions could be created.