APPLYING RHEOLOGICAL TECHNIQUES TO UPGRADE ANAEROBIC DIGESTERS AND HANDLE HIGH SOLIDS CONCENTRATIONS

The Orange County Sanitation District (OCSD) is implementing a Capital Improvements Program that includes upgrading their wastewater treatment Plant No. 1 in Fountain Valley, California to full secondary treatment. The expansion will increase both primary and secondary solids production. OCSD prepared a Long-Range Biosolids Management Plan (LRBMP) to select the most cost-effective methods for treatment and disposal of the wastewater solids. The LRBMP concluded that the waste solids from the primary and secondary treatment processes should continue to be anaerobically digested to produce Class B biosolids, which can be land applied or further processed to Class A biosolids.The LRBMP also investigated the capacity of the existing anaerobic digestion facilities at Plant No. 1 and concluded that the digestion volume would be adequate for future loading conditions once the new primary and secondary facilities were constructed and placed into operation. However, to maintain the 15-day hydraulic retention time for meeting EPA 40 CFR Part 503 Regulations for Class B biosolids, it would be necessary to thicken the feed solids to the digesters. Preliminary estimates indicated that feed solids solids concentrations would need to be approximately 6 percent dry solids (DS) resulting in the digesters operating at solids concentrations up to about 4 percent DS.Traditionally in the US, wastewater treatment plants operate with feed solids of 4 percent DS or less and therefore 2 to 2.5 percent DS in the digested biosolids. Digested biosolids at this concentration will tend to have approximated Newtonian fluids very similar to water, making both pumping and mixing relatively easy. However, recent trends towards mechanically and chemically thickened solids (up to 14 percent DS) have highlighted the non-Newtonian nature of the material, a Bingham plastic with a high yield stress and shear thinning properties. In turn this has led to problems with pumping and digester mixing, resulting in undersized pumps, foaming and short circuiting.The paper discusses the preliminary design engineering study carried out for Plant No. 1. Two novel techniques for testing and using site-specific rheology data to produce hydraulic and pump evaluations are presented, and the use of a scale model and computational fluid dynamics are reviewed for the assessment of digester mixing.