Static fill introduces influent wastewater into the SBR with little or no mixing and contact with the settled biomass, resulting in a high substrate concentration when mixing first begins. High substrate concentrations result in high reaction rates. In addition, such concentrates control sludge bulking because they favor organisms that form more dense floc particles over filament forming organisms. Bulking sludge is a common problem in continuous flow systems where substrate concentrations are always low. Finally, static fill favors organisms that produce internal storage products during high substrate conditions, a requirement for biological phosphorous removal. However, static fill time should be limited if an influent biodegradable constituent is present at concentrations toxic to the organisms.
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