EVALUATION OF BELT FILTER PRESSES, CENTRIFUGES, AND SCREW PRESSES FOR DEWATERING DIGESTED WASTE ACTIVATED SLUDGE AT ST. PETERSBURG'S WATER RECLAMATION FACILITIES

The City of St. Petersburg, Florida uses belt filter presses (BFPs) to dewater anaerobically digested waste activated sludge (WAS) at its four regional water reclamation facilities. These BFPs are nearing the end of their useful life and require replacement or major overhaul in the near future. As part of a dewatering upgrade project initiated by the City, belt filter presses, screw press and centrifuge dewatering technologies were evaluated. The compatibility of the selected dewatering process with possible future Class A quality biosolids disposal and/or reuse options was also considered.To evaluate the effects of Class A treatment options on the dewatering options, the following three means of achieving Class A biosolids were considered in this study: heat drying, the FKC screw press Class A process, and conversion of the existing mesophilic digestion process to the three phase digestion process (Drury et al., 2002).The existing BFPs generated cake concentration in the range of 12% to 14%, with polymer dose varying mainly in the range of 10 to 15 pounds of active polymer per dry ton (1b/DT). Previous pilot testing of a more advanced BFP model resulted in cake solids of 18.4 to 19.9 % with polymer dose ranging from 23 to 32 active 1b/DT. Centrifuges pilot testing resulted in cake solids concentrations from 19.8% to 22.3% using polymer dose of 25.7 to 29.2 1b/DT. Screw press pilot testing resulted in cake solids concentrations ranged from 14.3% to 17% depending on polymer dose and solids loading rate, using the Ciba Zetag 7878 polymer. Due to the highly buffered nature of the digested biosolids, a large dose of lime (950 lb Ca(OH)2/DT) was required to generate class A biosolids.Using the pilot test results and the results of a survey of installed operational facilities, a qualitative decision matrix was generated and presented to the City staff in a workshop. Detailed present and lifecycle cost estimates were generated for the centrifuge and screw press technologies. The analysis indicated that centrifuges had the lowest lifecycle cost under all considered Class A and Class B biosolids conditions, with the cost difference increasing under Class A biosolids conditions. The FKC Class A process had lower capital but higher lifecycle cost compared to the 3-phase digestion option of generating Class A biosolids. A new technology evaluation decision matrix was assigned weighting factors and scores by City staff and resulted in the centrifuge technology having the most favorable cost. The final decision on the selection of the dewatering technology will be made following additional planned pilot testing of BFPs and centrifuges.