Wastewater treatment plants using sludge streams containing higher solids concentrations than typically used in the past are becoming more common. In the UK, the wastewater treatment plants are working to maximize the capacity of existing anaerobic digestion facilities by using higher design loadings. Southern Water adopted a design volatile solids (VS) loading of up to 0.28 lb VS/ft3/day. The Nigg plant has a design loading of 0.25 lb VS/ft3/day. The Lowestoft
plant has an average design loading of 0.22 lb VS/ft3/day (Cumiskey, 2004). These loadings in excess of 0.22 lb VS/ft3/day compare with typical US design loadings in the range of 0.12 to 0.16 lb VS/ft3/day (WEF Manual of Practice 8, 1998).
The Orange County Sanitation District (OCSD) is expanding their wastewater treatment plant located in Fountain Valley, California to provide full secondary treatment. The expansion includes increasing the capacity of the existing anaerobic digestion facilities to match the future capacity for treating wastewater. Instead of building new digesters, feed sludge to the process will be thickened by centrifugation to 6 percent dry solids concentration. This will increase volatile solids loading to the digesters while maintaining a minimum 15-day detention time in accordance with EPA 40 CFR Part 503 Regulations for Class B biosolids. The current operation thickens primary sludge in the clarifiers and pumps primary sludge (PS) to the digesters at a solids concentration of about 4 percent. Waste activated sludge (WAS) is thickened using dissolved air flotation (DAF) to approximately 4 percent.