Solids Handling and Residuals Management
Solids and residuals
Solids handling begins with characterization: primary sludge is dense and digestible; waste activated sludge is light, high in volatile solids, and hard to thicken. Thickening reduces volume before stabilization; stabilization reduces volatile solids and pathogens; dewatering produces cake for land application, landfill, or incineration.
Mass balance is the core skill. Feed solids in lb/day = flow (MGD) x concentration (mg/L) x 8.34; a 1% slurry is 10,000 mg/L. Dewatering performance has two separate measures: cake solids (concentration of the cake) and solids capture (fraction of feed solids retained in cake). High cake solids with poor capture means high solids in the centrate or filtrate, recycling back to the headworks as ammonia and solids load.
Anaerobic digestion design uses volatile solids loading (lb VS/ft3/day), SRT (not HRT), and temperature. Mesophilic digestion at 35-37 C with 15-20 day SRT; gas production is roughly 12-18 ft3/lb VS destroyed, 60-65% methane.
Traps: confusing VS destruction with total mass reduction; reporting cake solids as capture; forgetting polymer dose is per dry ton; and ignoring centrate recycle load.
Worked example: 100,000 gpd of 1% feed = 8,340 lb/day solids. At 95% capture and 20% cake, cake solids = 7,923 lb/day and cake mass = 7,923 / 0.20 = 39,615 lb/day, about 4,750 gal/day at specific gravity 1.0. The 5% not captured, 417 lb/day, returns in the centrate.