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Corbel ^

Figure 13.3 Gas storage covers for anaerobic digesters: (a) floating cover; (b) gas holder cover.

Mixer

Mixer

that does accumulate. The waffle bottom digester is another configuration that facilitates grit and heavy solids removal. :

The purpose of anaerobic digestion is the stabilization of biodegradable particulate organic matter. Consequently, its performance can be quantified by the percent VS destruction. At an SRT of 15 to 20 days, 80 to 90%' of the influent biodegradable particulate organic matter will be converted to methane gas.5* This corresponds to destruction of about 60% of the VS contained in primary solids and 30 to 50% of the VS contained in waste activated sludges, as described further in Section 13.2.9.:""

Many reference works and textbooks discuss two-stage anaerobic digestion, in which two digesters are operated in series."' ? " Heating and mixing are provided in the first stage, where active digestion occurs, while quiescent conditions are provided in the second stage for liquid-solids separation. Supernatant from the second-stage is recycled to the liquid process train while thickened, settled solids are directed to further processing or ultimate disposal. Although of historical interest, use of the two-stage process has declined significantly in recent years for the following reasons:

• Experience indicates that while efficient liquid-solids separation will occur when treating primary solids or a mixture of primary solids and attached growth biomass, it can be quite poor when suspended growth biomass. either alone or mixed with primary solids, is digested. When suspended growth biomass is digested, the supernatant may be of poor quality, resulting in the recycle of significant quantities of suspended solids to the liquid treatment process train, thereby causing adverse impacts on its performance.

• Significant advances have been made in recent years in solids thickening technology, particularly for suspended growth biomass. This technology is mechanically reliable, allows the consistent production of a thickened solids with a concentration of 50 g/L or more, and is cost-effective.

• Thickening the feed solids prior to anaerobic digestion results in a significant reduction in required tank volume and associated capital cost. Operating costs are also reduced since the volume of feed that must be healed is significantly reduced.

• The recycle of poor quality digester supernatant is eliminated.

Consequently, current practice is to thicken the feed solids prior to single-stage, highrate anaerobic digestion, which is the process illustrated in Figure 13.2.

13.1.3 Low-Rate Anaerobic Processes

Low-rate anaerobic processes are slurry bioreactors that utilize a combination of solids sedimentation and accumulation to increase the SRT relative to the HRT. They often use earthen basins (Figure 13.5), although rectangular concrete vessels have also been used:i (Figure 13.6). Mixing is typically provided simply by the addition of influent wastewater and by gas evolution. As a consequence, well mixed conditions are not generally provided and suspended solids settle and accumulate in the bioreactor. Some systems have incorporated settled solids recycle from a downstream settling zone to an upstream reaction zone, as indicated in Figure 13.6. Historically, materials in the wastewater were allowed to float to the surface and form a scum mat that provided some insulation and odor control, although gas would pass through it and escape to the atmosphere. More recently, membranes and similar materials have been used to trap and collect the gas for use elsewhere. Heating is not typically provided. Consequently, either the process is used to treat wastewaters that are already warm or a sufficiently long SRT is maintained to allow treatment to occur at ambient temperature.

Environmental conditions within low-rate processes are not well regulated and, even though active biomass accumulates, accurate control of the SRT is not generally

Stored Gas Cover

Stored Gas Cover

Figure 13.5 Low rate anaerobic process using an earthen basin.
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