Sulfur cycle associated enhanced biological phosphorus removal

Hong Kong has practiced seawater toilet flushing since 1958, saving 750,000 m3 freshwater every day. A high sulfate-to-COD ratio (>1.25 mg SO4/mg COD) in the saline sewage resulting from this practice has enabled us to develop the Sulfate reduction Autotrophicdenitrification and Nitrification Integrated (SANIRTM) process for biological treatment of saline sewage with minimal sludge production. To expand the SANI process into an enhanced biological phosphorus removal (EBPR) process, this study aimed to explore a new EBPR in association with a sulfur cycle and develop an EBPR SANI. An alternating anaerobic/limited-oxygen sequencing batch reactor (SBR) fed with acetate as the sole electron donor and sulfate as the sulfur source, at total organic carbon to sulfur ratios of 1.1 to 3.1 (mgC/mg S), was adopted to develop a Limited-Oxygen Sulfur cycle-associated EBPR (LOS-EBPR). Phosphate (P) release/uptake and polyphosphate formation were observed in this LOS-EBPR that sustained high phosphate removal (20 mg P/L removed with 320 mgCOD/L) and oxygen acted as electron acceptor in the P uptake reaction. A Denitrifying Sulfur cycle-associated EBPR (DS-EBPR) process was further developed in an alternating anaerobic/anoxic SBR. This DS-EBPR process not only significantly reduced the hydraulic retention time and enhanced volumetric loading of the reactor, but also performed well at a temperature of 30 °C as well as a salinity of 20% seawater. A synergistic relationship between sulfur cycle and biological phosphorus removal was envisioned in this study. The new microbial function was observed: anaerobic phosphate release associated with acetate uptake, poly-phosphate hydrolysis, poly-hydroxyalkanoate (PHA) (and poly-S2-/S0) formation and an "aerobic"/anoxic phosphate uptake associated with PHA (and poly-S2-/S0) degradation, and polyphosphate formation. No 16S rRNA gene sequences related to known PAOs, e.g. Accumulibacter and Tetrasphaera were detected. Instead, sulfate-reducing bacteria, sulfur-oxidizing bacteria and a large portion of unclassified species were identified to be the dominant groups in the reactor through a 454-pyrosequencing analysis. This new sulfurcycle-associated EBPR is a promising process for simultaneous removal of organic carbon, nitrogen and phosphorus with minimal sludge production and oxygen demand.