Reducing Ammonia and Phosphorus Recycle Loads by Struvite Harvesting

The Clean Water Service's Durham AWWTP has a 0.10 mg/l total P discharge limit, which is achieved through a combination of Enhanced Biological Phosphorus Removal (EBPR) and tertiary chemical polishing. EBPR Sludge is digested anaerobicly resulting in a dewatering centrate that is rich in phosphorus and ammonia. Typical values for the centrate are 600 mg/l soluble PO4-P and 1,200 mg/l NH3-N, resulting in 25% and 30% of the phosphorus and ammonia load to the plant, respectively. Centrate storage is used to shave the diurnal peak loads and fill the diurnal low points. Clean Water Services (CWS) did not want to add metal salts to the centrate to capture the phosphorus since it would defeat the point of removing phosphorus biologically with EBPR and result in increased operating costs for chemical consumption and sludge disposal. Metal salt addition would also not reduce the ammonia recycle.Clean Water Services was looking at side stream treatment options that could reduce the nutrient recycle load to the plant. The Ostara struvite recovery process, which adds magnesium to the phosphorus and ammonia rich centrate to create struvite, (magnesium ammonium phosphate hexahydrate, commonly called struvite or MAP) was piloted over an 8 week period from April to June 2007. This could reduce both phosphorus and ammonia that is returned to the plant for treatment and makes a product that has beneficial use as a slow release fertilizer. The pilot was very successful, demonstrating process stability despite multiple power interruptions from switchgear maintenance and re-started without difficulty. Ortho-phosphate was reduced from an average of 505 mg P/l to25 mg P/l for a 95% average reduction in the recycle stream. Ammonia was reduced from an average of 1,200 mg N/l to 980 mg N/l for a19% average reduction. That translates to a plant influent ammonia reduction of 6% and an influent phosphorus reduction of 24%, which is greater than the 22% reduction resulting from the statewide phosphate detergent ban. The pilot was operated at four different hydraulic loading conditions to determine the optimal reactor throughput for a full-scale facility. Installation of a full scale facility based on the results of this demonstration is currently planned for construction in fourth quarter of 2008.