Side Stream Phosphorus Removal/Recovery–Breaking Loop of Phosphorus in EBPR Plants

In a perfect EBPR system, phosphorus in the raw sewage is effectively shifted from the liquid train to the solid train by the Polyphosphate Accumulating Organisms (PAO), resulting in a relatively low level of phosphorus in the plant effluent. A large portion of phosphorus trapped in the solids is released to the liquid phase as ortho-phosphate during anaerobic digestion and finds its way back to the main reactor through recycle of dewatering supernatant. The recycle stream could contribute to more than 50% of phosphorus loads to the main bioreactors. Breaking this “loop” of phosphorus recycle through side-stream phosphorus removal/recovery is critical to reduce the overall phosphorus load to the main reactor and to improve EPBR stability. The primary goal of this study was to evaluate two typical side stream phosphorus removal technologies: (1) trivalent metal complexion/precipitation and (2) struvite formation and recovery. Evaluation criteria applied include biosolids mass production, biosolids phosphorus content, phosphorus load from the side stream recycle, and payback period estimation. It is concluded that a system with struvite recovery is the better option when all criteria are considered.