Calcite Precipitate in Biosolids Handling Processes: A Case Study

The Region of Waterloo’s (The Region) Waterloo Wastewater Treatment Plant (WWTP) located in Waterloo, Ontario, Canada has historically experienced nuisance chemical precipitate throughout the biosolids handling system. Work in 2012 identified vivianite as one of the minerals precipitating in the primary anaerobic digester; however, the identity of the precipitate found in the centrifuge equipment remained unknown.The deposits have caused equipment operational issues and have raised concern over long-term operation of the system. The scaling requires periodic maintenance to maintain the equipment’s level of service, which is time and operator intensive. Plant staff currently utilize a descaling agent to remove precipitate, which requires equipment downtime.This project was performed to:Identify the major constituents of the chemical deposit/precipitate;Quantify the amount of the deposit/precipitate;Determine the location and cause of deposit/precipitate; andDevelop conceptual solutions for mitigation.Work completed included a review of historical water quality, special water quality sampling performed at key locations, and X-Ray Diffraction (XRD) analysis of precipitate collected from within the centrifuge equipment. The historical and special water quality sampling results were utilized to perform mass balances around biosolids processing equipment to quantify the type and amount of precipitate, which is necessary in order to identify potential mitigation strategies. The XRD analysis was conducted in July 2017 and special sampling was conducted in August 2017.Results from the analyses indicated that both vivianite and calcite (CaCO3) are present in the primary digester. Calcite was the primary precipitate found in the dewatering lines/equipment. Calcite is not typically documented as a primary nuisance precipitate for municipal WWTP and presents a unique challenge that must be navigated. Eight potential mitigation strategies for vivianite and calcite were identified. This work and manuscript describes the various elements of the analysis conducted to identify the precipitate and potential solutions.