The Ohio city of London brought its new activated sludge wastewater treatment plant online in 2009 and soon encountered issues with solids pumping. Waste activated sludge (WAS) is pumped to a gravity belt thickener where it is concentrated to about 2.5 percent solids before being pumped to anaerobic digesters from a holding tank.
Wasting to the thickener is not continuous, so the thickening process operates only when sludge needs to be wasted. As a result, thickened sludge is kept in the holding tank until 15,000 to 20,000 gallons accumulate. Any septic tank waste or grease delivered by contractors is emptied directly
