Primary Sedimentation

The treatment performed prior to this step of the process is intended to remove materials that could damage equipment and impair the downstream processes. Primary settling represents the first step of treatment intended to abate water pollution. Primary settling is merely a physical separation of solids from the wastewater. After grit removal, the wastewater flows into the primary settling tanks where the flow velocity is further reduced and the suspended material is allowed to settle to the bottom of the tanks. Approximately 50% of the suspended solids and 35% of the BOD are removed in this unit process. The settled material, referred to as primary sludge, is pushed by automatic sludge collection equipment into a hopper from which the sludge is pumped to the sludge thickeners. The treatment of this sludge is discussed separately. As part of the primary treatment, floatable materials such as oil and grease that rise to the surface of the tanks are skimmed and commingled with screenings and grit for removal to an out-of-state landfill. Lime is used for odor control, if necessary. There are currently sixteen tanks with two bays each, ranging in length from 100’ to 115’ long and 29’ to 33’ in width. Each tank is 7.5’ deep.

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