Flux Capacity: Screw Press Sizing Of The Future

The most common technologies for sludge dewatering in wastewater treatment plants include belt filter presses, centrifuges, and screw presses, with the latter known for their significant advantages in low power consumption and high efficiency during the dewatering process. Various manufacturers supply screw presses to wastewater treatment plants, offering pilot testing to assess the performance, throughput capability, and feasibility of dewatering units in diverse practical and controlled environments. However, unlike centrifuges and belt filter presses, the capability to compare the throughput of screw presses throughput between manufacturers is lesser known. While the throughput equipment capacity comparisons for centrifuges and belt filter presses are well-established through G-Volumes and filter areas, respectively, a similar standardization for screw presses is not widely recognized. The Tres Rios Water Reclamation Facility, owned by The Pima County Regional Wastewater Reclamation Department, serves as the centralized sludge handling and solids treatment system for the entire county, offering a treatment capacity of 50 MGD and over 180 WTPD of solid dewatering. Currently utilizing three centrifuges for sludge dewatering, the facility faces challenges related to high polymer and power usage. In response, as part of the county's transition from achieving Class B to Class A biosolids, a new dewatering facility employing screw presses was proposed aimed to reduce energy costs and enhance the efficiency and reliability of the dewatering process, ultimately supplying future solar dryers. During the summer and fall of 2023, three screw press pilots, and one volute press pilot were conducted in collaboration with various equipment manufacturers. The primary objective of these pilots was to verify and evaluate dewatering performance and ultimate equipment selection using digested sludge from the Tres Rios WRF. A new approach was implemented when comparing pilot test results relative to full-scale equipment selection. The approach is to compare 'flux' of each machine at its best performance point. The machine's flux is the rate of liquid and/or solids flow rate per surface area of the machine. In doing this analysis, full-scale equipment is sized relative to the pilot test's best performance at a proven flux rate. An equal flux rate from pilot to full-scale equipment will help ensure similar and expected performance at full-scale operation. To compare performance of the screw press units during the pilot testing, liquid and solids flux rates were calculated for each manufacturer. The liquid flux rate results of the three screw press manufacturers are shown in Table 1. The volute press manufacturer was not included in the evaluation due to the poor pilot test results compared with the other manufacturers. The lower flux rate, indicative of a higher dewatering capability, signifies superior dewatering through an extended residence. Manufacturer 2 and 3 out-performed Manufacturer 1 by providing higher filtration area. Manufacturer 1 was not recommended for further consideration due to small full-scale equipment size options, resulting in the preferred maximum liquid flux rate of 0.31 gpm/sf for best performance. The results from the pilot testing aided in identifying the required flux rate achieve best performance, and ultimately confirms full-scale equipment sizing for a range of operating conditions. Table 2 shows that both manufacturers required five installed units dewater the design load, and a throughput derating of up to 75% is needed compared with the listed manufacturer nameplate. In contrast to using screw press volume, relying on manufacturer recommendations, or arbitrary derating of equipment as a metric for performance comparison among different manufacturers, the presentation will present findings showing that flux rate calculation during pilot units provides fair and more realistic results. Larger screw presses are noted to lose efficiency as the ratio of surface area to volume decreases with the growth in diameter and length of the screw. This claim can be better studied with a flux rate that remains constant regardless of the equipment size due to the solids retention time being equal, which this pilot study investigated. Attendees of this presentation will gain valuable insights into the comparative performance of screw presses from different manufacturers post-pilot testing. This knowledge will aid in establishing informed selection criteria for optimizing dewatering solutions in wastewater treatment plants.