Primary Settling Return to Process Page

What is primary settling and why is it done?     After the inorganic grit is removed from the wastewater, the next step is to remove the organic solids.  The organic solids are material that can be processed at the treatment facility through the anaerobic digestion process.  To separate these solids from the rest of the wastewater we use simple gravity.  The wastewater is split into four long tanks that allow the water to slow down to less than 0.5 feet per second.  At this speed, floatable material will separate from the water to the top, and the heavier solid material will sink to the bottom.  The floatable material (also called "scum") is skimmed off the surface of the water and also sent to the digester.  Inside each tank is a long series of sweeps or "drags" (see picture).  These drags pull the settled solids to a sump at the northern end of the tank where an air operated pump pulls the solids from the sump and pumps it to the anaerobic digesters. After the water reaches the end of the settling tank, it flows over a weir and proceeds to the secondary treatment stage.