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

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Another important design parameter is the total solids (TS) concentration in
the reactor, expressed as a fraction of the wet mass of the prepared feedstock.
The remainder of the wet mass is water by definition. The classification scheme
for solids content is usually described as being either highsolids or low-solids.
High-solids systems are also called dry systems <strong>and</strong> low-solids systems may be
referred to as wet systems. A prepared feedstock stream with less than 15% TS is
considered wet <strong>and</strong> feedstocks with TS greater than 15-20 percent are
considered dry (although there is no established st<strong>and</strong>ard for the cutoff point).
Feedstock is typically diluted with process water to achieve the desirable solids
content during the preparation stages.
Before AD became an accepted technology for treating MSW, single-stage wet
digesters were used for treating agricultural <strong>and</strong> municipal wastewater.
However, MSW slurry behaves differently than wastewater sludge. Because of
the heterogeneous nature of MSW, the slurry tends to separate <strong>and</strong> form a
scum layer which prevents the bacteria from degrading these organics.
The scum layer tends to evade the pump outlets <strong>and</strong> can clog pumps <strong>and</strong> pipes
when it is removed from the reactors. To prevent this, pretreatment to remove
inert solids <strong>and</strong> homogenize the waste is required. Solids can also short circuit
to the effluent pipe before they have broken down completely, therefore design
modifications were made to allow longer contact time between bacteria <strong>and</strong>
dense, recalcitrant material.
Furthermore, MSW tends to contain a higher percentage of toxic <strong>and</strong> inhibitory
compounds than wastewater. In diluted slurry, these compounds diffuse
quickly <strong>and</strong> evenly throughout the reactor. In high enough concentrations, this
can shock the microorganisms, whereas in a dry system the lower diffusion rate
protects the microbes.
Because of these constraints, dry systems have become prevalent in Europe,
making up 60 percent of the single-stage digester capacity installed to date. Dry
digesters treat waste streams with (20-40) % total solids without adding
dilution water. However, these systems may retain some process water or add
some water either as liquid or in the form of steam used to heat the incoming
feedstock. Furthermore, as organic matter breaks down, the internal MC of the
digester will increase. Nonetheless, heavy duty pumps, conveyors, <strong>and</strong> augers
are required for h<strong>and</strong>ling the waste, which adds to the systems’ capital costs.
Some of this additional cost is offset by the reduction in pretreatment
equipment required. Most dry digesters operate as plug flow digesters, but due