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

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If the proper conditions cannot be maintained, imbalances among the different
types of microorganisms in the process will occur. One of the most common
problems is the buildup of organic acids which kills the methanogenic
organisms <strong>and</strong> gives even more acidity to the process environment. Produced
acid can be controlled in a natural way by using inherent chemical buffers <strong>and</strong>
by the methanogens themselves as they consume acids to produce methane.
These controls cannot confront the difficulties if too much feed is added <strong>and</strong>
organic acids are produced faster than they are consumed, if inhibitory
compounds accumulate, or if the feed stream lacks natural pH buffers such as
carbonate <strong>and</strong> ammonium. (EPA.,2008)
Solids concentration higher than about 40% TS can also result in process
inhibition, likely due to the reduced contact area available to the AD
microorganisms. The content of TS in the MSW typically ranges from (30-60)
%, this is the reason why water needs to be added each time this problem
occurs. Process water can be used, but this may also result in the buildup of
inhibitory compounds. Thus, low-solids digesters require the addition of fresh
water. In higher temperatures which occur to smaller reactors a given waste
stream is needed to be processed. However, the micro-organisms themselves
are adapted to relatively narrow temperature ranges. Mesophilic <strong>and</strong>
thermophilic microbes are adapted to roughly 30-40°C <strong>and</strong> 50-60°C
respectively. (EPA., 2008)
2.2.5. AD stages
Generally the overall AD process (Figure 10.) can be divided into four stages:
Pretreatment, waste digestion , gas recovery <strong>and</strong> residue treatment.
Most digestion s y s t em s require pretreatment of waste to obtain
homogeneous feedstock. The preprocessing involves separation of nondigestible
materials <strong>and</strong> shredding. The waste received by AD digester is
usually source separated or mechanically sorted. The separation ensures
removal of undesirable or recyclable materials such as glass, metals, stones
etc. In source separation, recyclables are removed from the organic wastes
at the source. If source separation cannot be achieved, mechanical
separation could occur even though the final product hasn’t got the same
quality because it is not as clean as in source seperation.The waste is shredded
before it is fed into the digester. Inside the digester, the feed is diluted to
achieve desired solids content <strong>and</strong> remains in the digester for a designated
retention time. (Verma.,2002)