A new biological phosphorus removal process in association with sulfur cycle

Hong Kong has practiced seawater toilet flushing since 1958, saving 750,000m³ freshwater every day. A high sulfate-to-COD ratio (>1.25mgSO₄/mgCOD) in the saline sewage resulting from this practice has enabled us to develop the Sulfate reduction Autotrophic denitrification and Nitrification Integrated (SANI^®) process with minimal sludge production. This study seeks to expand the SANI process into an enhanced biological phosphorus removal (EBPR) process. A sulfur cycle associated EBPR was explored in an alternating anaerobic/oxygen-limited aerobic sequencing batch reactor with acetate fed as sole electron donor and sulfate as sulfur source at a total organic carbon to sulfur ratio of 1.1-3.1 (mg C/mg S). Phosphate uptake and polyphosphate formation was observed in this reactor that sustained high phosphate removal (20mgP/L removed with 320mg COD/L). This new EBPR process was supported by six observations: 1) anaerobic phosphate release associated with acetate uptake, poly-phosphate hydrolysis, poly-hydroxyalkanoate (PHA) (and poly-S^2-/S⁰) formation and an "aerobic" phosphate uptake associated with PHA (and poly-S^2-/S⁰) degradation, and polyphosphate formation; 2) a high P/VSS ratio (>0.16mgP/mg VSS) and an associated low VSS/TSS ratio (0.75) characteristic of conventional PAOs; 3) a lack of P-release and P-uptake with formaldehyde inactivation and autoclaved sterilized biomass; 4) an absence of chemical precipitated P crystals as determined by XRD analysis; 5) a sludge P of more than 90% polyphosphate as determined by sequential P extraction; and 6) microscopically, observed PHA, poly-P and S globules in the biomass.