The circular primary sedimentation tanks have been installed and are arranged to operate in parallel. Either tank may be isolated for maintenance or repair purposes. The tanks are 60 feet in diameter with normal side water depths of 9 feet. Each tank has a mechanical scraper which rotates around the sloping bottom and pushes the sludge to a sludge thickening zone and a sludge sump. Sludge is withdrawn from the tanks by positive displacement plunger pumps located in the headworks and is transported to the anaerobic digestion tanks. Floating matter is mechanically skimmed into collecting hoppers in each tank and is then piped to a single scum sump located on the south edge of Sedimentation Tank No. 1. The scum is pumped from the sump to anaerobic digestion by a positive displacement plunger pump located in the headworks. Plant effluent can be returned from the effluent pump station to the primary sedimentation tanks to assist in sewage sludge separation.

The primary sedimentation process is basically a physical process utilizing gravitational forces. Settleable and suspended solids, which are the major components of sludge and are heavier than water, settle out of the sewage along with any grit carryover from the headworks. Scum, which is lighter than water, floats to the surface and is removed by skimming. Approximately 60 to 65 percent of the suspended solids will be removed by gravitational forces as part of primary sedimentation.

Effluent from the primary sedimentation tanks overflows into double sided circumferential launders and then flows into the effluent junction boxes. The effluent then flows to the aeration structure, where it is directed to the aeration basins.