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

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Considerations such as the design <strong>and</strong> feedstock, give different technical
choices, such as batch or continuous flow, vertical or horizontal orientation,
<strong>and</strong> capacity, total solids content, number of stages, mixing <strong>and</strong> pre-treatment.
Digester processes are constructed for meeting specific conditions, design,
locations, types of waste, <strong>and</strong> the desired degree of autonomy <strong>and</strong> complexity.
Vertical tanks are gravity driven, the material flowing generally downward,
though the path can vary, depending on interior boundaries in the chamber. In
other cases, material is pumped into the bottom of the tank <strong>and</strong> removed from
the top, causing general upward flow accompanied by a lesser, downward,
gravity driven flow. Horizontal tanks require greater space <strong>and</strong> are closer to
plug flow than to perfect mixing. (Buekens.,2005)
2.2.6. Categories of Engineered AD Systems
(V<strong>and</strong>evivere et al.,2002) categorizes the (MSW AD) systems into 3 main
categories :
‣ One-stage Continuous Systems
Low-solids or ‘Wet’
High-solids or ‘Dry’
Dry-Wet
Wet-Wet
‣ Two-stage Continuous Systems
‣ Batch Systems
One Stage
Two Stage
(EPA., 2008)
Single-stage digesters: are considered to be simple in design, build, <strong>and</strong>
operation <strong>and</strong> are generally less expensive. The organic loading rate (OLR) of
single-stage digesters is limited by the ability of methanogenic organisms to
tolerate the sudden decline in pH that results from rapid acid production
during hydrolysis.
Two-stage digesters separate the initial hydrolysis <strong>and</strong> acid-producing
fermentation from methanogenesis, which allows for higher loading rates but
requires additional reactors <strong>and</strong> h<strong>and</strong>ling systems.