Screw Press - An Alternative to Conventional Dewatering Technologies

An alternative to traditional biosolids dewatering technologies, including centrifuges and belt filter presses, is the screw press technology, which has been introduced to the US market over the past five years with an increasing number of existing and planned installations.Screw press dewatering is a low shear, low energy dewatering technology, which has the advantage over belt filter presses of consistent over time performance. It also provides an alternative for generating Class A sludge through steam injection and lime addition.This paper will review design and performance information for screw presses, including results from a screw press facilities survey and visit of installations, along with insights obtained from our design experience of several full-scale facilities.In addition, a case study for the City of St Petersburg in Florida will be used to demonstrate the use of pilot testing to establish the expected performance of screw presses, compared to belt filter presses and centrifuges.The existing City BFPs generated cake concentration in the range of 12% to 14%, with polymer dose varying in the range of 10 to 15 pounds per dry ton (lb/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 lb/DT. Centrifuges pilot testing resulted in cake solids concentrations from 19.8% to 22.3% using polymer dose of 25.7 to 29.2 lb/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 lbs 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, as seen in Figure 1 and Figure 2. 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 score. The final decision on the selection of the dewatering technology was made in favor of screw presses based on assigning top priority to operational simplicity.