Carbon and Struvite Recovery from Centrate at a Biological Nutrient Removal Plant

A one-tenth pilot-scale demonstration study of volatile fatty acid (VFA) and soluble phosphorus recovery from an Autothermal Thermophilic Aerobic Digester (ATAD) was carried out at the City of Salmon Arm Water Pollution Control Centre in British Columbia, Canada. VFA recovery returns supplemental organic carbon to the enhanced biological nutrient removal (EBNR) treatment process and simultaneous phosphorus recovery avoids return of phosphorus to the process. The pilot demonstrated that the recovery of VFA and phosphorus was practical and beneficial.Soluble phosphorus was recovered using a University of British Columbia, (Ostara Nutrient Recovery Technologies) commercialized struvite crystallizer. The crystallizer effluent, which was high in volatile fatty acid (VFA) concentration, was returned to the biological nutrient removal process. Bench-scale batch tests were routinely carried out to observe the impact of the returned VFA on phosphorus removal.The ATAD process operates at high temperatures (40°C to 70°C) and at the Salmon Arm EBNR treatment plant, the ATAD process solubilizes large amounts of phosphates and ammonia, which results in high nutrient concentrations in the centrate.The study was successful in showing that:Supplemental VFA for the EBNR process could consistently be obtained from a mix of waste biological (phosphorus accumulating organism) solids and primary solids,Phosphate could be removed from VFA laden sidestream liquors prior to return to the EBNR process, andHigh quality fertilizer struvite pellets (avg. 2 mm diameter) could be consistently obtained by removing approximately 80% of the phosphorus and (although unexpectedly high), 35% of the ammonium-nitrogen from the centrate.