Santa Rosa Fine Tunes its Flexible Biosolids Program

Beginning in 1992, Santa Rosa, California has developed a uniquely flexible biosolids disposal/reuse program consisting of a combination of reuse by land application and by composting/marketing, and landfill disposal. Land application was implemented after a demonstration program and subsequent permitting of 1,200 acres for Class B dewatered biosolids. In 1996 the 8 ton/day agitated bin in-vessel composting system was constructed. By the year 2002 the distribution of biosolids was 48% land application, 28% composting, 12% landfill disposal, and 12% storage. In 2003 a master plan was prepared to guide the program over the next 20 years.The master plan included a non-linear programming based economic optimization model to determine both the nature and timing of improvements, which include additional storage, centrifuges to replace belt presses, temperature-phased anaerobic digestion (TPAD), compost process improvements, and land application alternatives. The model also incorporated a means for evaluating the effects of potential future changes in regulations or the biosolids end products markets.Thicker sludge was determined to provide substantial economic and capacity benefits to all downstream processes, especially the composting process. Pilot tests of co-thickening on gravity belt thickeners are planned for 2004. Centrifuge dewatering is recommended when belt presses need replacement in approximately 2008 to further increase solids concentrations. Current mesophilic anaerobic digestion is slated for upgrade to temperature-phased anaerobic digestion (TPAD) for increased volatile solids destruction and to provide the potential for TPAD with heated batch tanks to achieve Class A product.