Using a Screw Press to Produce Class A Biosolids at Small Wastewater Treatment Plant

The City of Forks, Washington and FKC Co., Ltd recently teamed up to produce Class A Biosolids using new screw press technology. The screw press simultaneously dewaters and pasteurizes biosolids at the city's wastewater treatment plant (WWTP). This new technology is very effective in dewatering sludges that are traditionally difficult to dewater. The relatively low equipment and operational costs makes the screw press technology effective and affordable for large and small municipal plants.Forks, is a small city on the Olympic Peninsula of Washington State. The plant's average flow is 135,000 gallons per day and it generates approximately 30 dry tons of biosolids per year. The WWTP is an activated sludge extended aeration and denitrifying nondischarging plant. Forks chose to use the screw press technology because of minimal operator requirements, and relative low capital and operational costs.The patent-pending Class A process begins by adding lime to batches of low consistency waste activated sludge (WAS) until a pH of 12.0 is reached. Once the sludge has been limed it is pasteurized during dewatering using a steam-heated screw press. The liming and pasteurization of the sludge meets the requirements outlined in the Part 503 Biosolids Rules.The system designed for the City of Forks is completely automated through the use of an integrated programmable logic controller (PLC) program. The program safely shuts down the equipment if Class A Biosolids are not being produced during any part of the process. This safety feature allows the process to reliably run un-staffed overnight.The screw press installed at the City of Forks was the first unit dedicated to producing Class A Biosolids using this process. The end product consists of 28% average total solids and is land applied at the local airport and City parks. The City is also accepting sludge from other WWTPs to help offset the initial capital expenditures. Future plans also include accepting sludge from septic pumping companies.