ABSTRACT Batch tests show SP, SFe, ammonia, and sCOD increasing over 72 hours, driving highly variable phosphorus recycling. The Great Lakes Water Authority (GLWA) Water Resource Recovery Facility (WRRF) practices chemical and biological phosphorus removal. Investigations identified the sources of highly variable solids-handling recycle phosphorus loading, averaging 35% of plant influent loading, but ranging above 100%. Batch reactor testing demonstrated that soluble phosphorus (SP) and iron (SFe) were released when primary sludge (PSD) was held under conditions similar to those in the PSD gravity thickeners. Reduction of ferric to ferrous ions dissolved ferric hydroxide and released ferrous and associated phosphorus into solution. Organics formed through fermentation of raw wastewater solids contained in the PSD chelated the released ferrous, preventing the formation of vivianite that would have reduced SP and SFe concentrations. Released SP and SFe are recycled to the main wastewater stream plant when thickened WAS and PSD are dewatered. The proportionate recycling of phosphorus and iron explains the highly variable primary clarifier phosphorus load, while primary treatment phosphorus removal is sustained at modest mainstream ferric chloride dose, and downstream biological phosphorus removal remains sufficient to meet stringent effluent TP requirements. These results suggest potential opportunities to separately recover phosphorus and recycle iron, which should interest other WRRFs using iron-based chemical phosphorus removal without biological primary sludge stabilization.
Liu et al. (Fri,) studied this question.