Observing the Biologically Induced Phosphate Precipitation by Sludge Extracellular Polymeric Substances in Enhanced Biological Phosphorus Removal

Biologically induced phosphate precipitation is a common phenomenon in the enhanced biological phosphorus removal (EBPR) process. In this study, as observed by backscattered scanning electron microscopy coupling energy-dispersive X-ray spectroscopy, large amounts of phosphate precipitates (mainly Ca phosphate) were found to be accumulated in EBPR granular sludge. The Ca phosphate mineral inside EBPR granules was present in two forms: independent mineral clusters and aggregates along with cells packaged by sludge extracellular polymeric substances (EPS). In addition, speciation of phosphorus in EPS extracts was investigated. It was found that about 20.7 ± 2.4% of the EPS-associated phosphorus was found present in a nanosized particulate form by single particle-inductively coupled plasma-mass spectroscopy, and phosphate precipitation induced by isolated EPS was confirmed. Therefore, it was proposed that EPS might play an important role in promoting the phosphate precipitation process through adsorption or entrapment of precipitates to EPS or growth of precipitates inside EPS. Results in this study will be helpful to better understand the phosphorus removal approach through induced precipitation in granular EBPR systems.