Process Description

Influent wastewater is added to a partially filled reactor. The partially filled reactor contains biomass acclimated to the wastewater constituents during preceding cycles. Once the reactor is full, it behaves like a conventional activated sludge aeration basin, but without inflow and outflow. After biological reactions are completed, and aeration and mixing is discontinued, the biomass settles and the treated supernatant is removed. Excess biomass is wasted at any convenient time during the cycle. Frequent wasting results in holding the mass ratio of influent substrate to biomass nearly constant from cycle to cycle. In contrast, continuous flow systems hold the mass ratio of influent substrate to biomass constant by adjusting return sludge flow rates continually as influent flow rates and characteristics and settling tank underflow concentrations vary.

No specific SBR reactor shape is required. The width-to-length ratio is unimportant, although this is a concern with conventional continuous flow systems. Deep reactors improve oxygen transfer efficiency and occupy less land area. The SBR shown in Figure 11.18.1 uses an egg-shaped reactor that offers most of the advantages of a spherical reactor, and provides a deeper reactor. Along with improved oxygen transfer efficiency, deep reactors allow a higher fraction of treated effluent removal during decanting. Similar to a spherical reactor, the egg-shaped reactor has a minimum surface area to volume ratio resulting in lower heat loss, less material needed in reactor construction, and less energy required for mixing.

The small reactor top is easily enclosed to contain volatile organics, or to direct exhaust gases for removal in an absorber. During filling, floating materials are forced together towards the top center for easy removal. The con



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