Scum ‒ A Liability or an Asset?

Until recently, the 628 ML/d (166 mgd) City of Fort Worth Village Creek Water Reclamation Facility pumped scum from its primary and secondary clarifiers, gravity thickeners and filter backwash clarifier to either of two large concrete tanks. The two tanks were operated in parallel, using one tank at a given time. The scum was then concentrated by removing subnatant from the tank and allowing the scum, along with entrained solids, to accumulate at the surface. Once concentrated, the scum was pumped out using an outside contract company and hauled off to a landfill for disposal. Removal of the scum cost was around $400,000 per year, and the scum tanks were a significant source of odor in the plant.In its continued effort towards efficiency and energy conservation, the City decided to install a scum screening system to remove plastics and other trash solids from the scum. The two scum tanks were converted from parallel to series operation, with the first tank acting as a holding tank and preliminary concentrator for unscreened scum. The solids-laden scum is pumped from the first tank through a scum screen to the second tank, which further concentrates the screened scum. Screenings from the screen are bagged and sent to the landfill with the screenings from the plant’s headworks. Two screens were considered and evaluated that are typically used to screen primary sludge. Huber Strainpress® was the selected screen for the project.The screened scum is pumped to the plant’s existing co-digestion facility where it mixes with high-strength waste and subsequently goes to anaerobic digestion to boost biogas generation in the digesters; thereby making the scum an asset. Biogas fuels the plant’s combined heat and power system, which fulfills the majority of the plant’s energy demand. Although the impact on energy generation is nominal compared to the contribution from the high-strength waste, the project is anticipated to have about a two-year payback due just to the elimination of off-site scum disposal costs. This project is an outstanding example of how one facility has turned a liability into an asset through the innovative application of technology.