Basin Design


Ideally, plant designs should have a minimum of two SBR basins and one flowequalization basin; however, every design is unique and one configuration does not fit all situations. All SBR designs should have a minimum of two basins to allow for redundancy, maintenance, high flows, and seasonal variations. Two basins allow for redundancy throughout the plant. If one basin is off line, the plant is still able to treat
influent wastewater because of the equalization basin. If the basin microbiology becomes depleted in one basin, the biomass from the remaining basin can be used to restock the basin with depleted biomass. For this to happen, a means of transferring sludge between the two basins must be provided.

During storm events and high-flow periods, instead of bypassing the basins or blending the stormwater, an additional basin can act as storage, or certain cycles can be shortened. In particular, the react cycle can be shortened under wet-weather conditions because of the diluted flow and the reduced time needed to treat the BOD. With higher flows, the fill phase and the idle cycle can also be shortened. A two-basin design also allows the plant to take one basin off line for draining and cleaning while the pre-flow basin and the one online basin remain fully operational.

For plants that have seasonal flow variations, a design that includes two treatment basins and an influent-flow-equalization basin allows one basin to be taken off line during the off season. This is important for seasonal plants, as it can save money by cutting electricity costs and reducing staff hours (fewer hours are spent on overall basin maintenance). The basin that remains on line is able to reseed the biomass in the off-line basin when the influent flow pattern peaks.

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