High Solids Centrifuges Vs. Plate and Frame Filter Presses: Lessons Learned After 5 Years of Operation at Fairfax County's Noman M. Cole, Jr. Pollution Control Plant

In 2004, Fairfax County began replacement of its existing plate-and-frame sludge dewatering system at the 67-MGD Noman M. Cole, Jr. Pollution Control Plant (NCPCP). The facility's new high solids centrifuge dewatering system was placed into service in late 2005. In the five years since commissioning, the new system has had a significant impact on plant operations. Furthermore, plant operations and maintenance staff have identified a number of lessons learned during that time which are of value to other facilities undergoing similar complex system upgrade projects. This paper/presentation documents the impacts this project has had on the plant and its staff, and the lessons learned, presented from the plant staff's point of view.The goal of this project was to replace the existing dewatering system with a system capable of producing an autogenous burning cake, eliminating the need for supplemental fuel in the incinerator hearths. The new system was further designed with consistent, stable incineration operation as its primary focus. The operation of upstream processes and systems (including the new dewatering system) is geared toward achieving this goal.Through startup, commissioning, and operation of the system, the plant has streamlined solids handling operations; these continual improvements have helped optimize plant staffing requirements and system performance to provide the greatest benefit to Fairfax County.This paper assesses whether the new centrifuge system met the County's project goals and generated additional cost savings resulting from the modernization of the existing system. This analysis is based on a review of filter press and centrifuge system performance (e.g., dewatered sludge solids concentration, polymer usage, lime usage, natural gas usage, and electrical usage), operating/maintenance impacts, and resulting financial impacts.This paper further provides a number of lessons learned compiled by plant staff during the transition from filter presses to centrifuges and in the past five years of operating the new system. These lessons learned are derived from actual impacts on operating/maintenance staffing levels and training/skills requirements identified since the new dewatering system was placed into service.The results of this study indicate that the new centrifuge system has consistently met its primary goal of consistently producing an autogenous cake. In nearly five years of operation, dewatered cake concentrations have averaged greater than 28%. The consistent dewatering performance of the centrifuge system has also enabled the plant to achieve its secondary goal of improving the stability of incinerator operations, yielding significant reductions in operating/maintenance costs for Fairfax County. Total calculated operating cost savings for the new centrifuge system, as compared to the filter press system, are approximately 1,557,000 per year.