Phosphate release from waste stabilisation pond sludge: significance and fate of polyphosphate

摘要

Net phosphorus removal from waste stabilisation pond (WSP) systems is governed by the rate ofnphosphorus incorporation into the sludge layer and the rate of phosphorus release from thisnsludge back to the overlying wastewater. Luxury uptake of phosphorus by microalgae has beennshown to occur under WSP conditions in the laboratory; however, the significance of thisnmechanism and the fate of polyphosphate contained in the settled solids have not previouslynbeen investigated. In this work the analysis of sludge samples from three WSP showed that up ton71% of the total phosphorus in the sludge was in the form of polyphosphate. This indicates thatnpolyphosphate accumulation could potentially be an important mechanism for phosphorusnsequestration in WSP and challenges the common view that chemical precipitation is thenpredominant phosphorus removal mechanism in these systems. The release of phosphate fromnWSP sludge samples was monitored in the laboratory. The samples from two different pondnsystems had release rates in the order of 4.3 mgP/gTSS.d. However, the third sample which wasncollected during an algal bloom had a release rate of 12.4 mgP/gTSS.d. Phosphate release fromnfresh microalgal sludge grown under laboratory conditions was also studied and was shown tonhave a release rate of 160 mgP/gTSS.d. Analysis of polyphosphate during the experiments onnlaboratory grown microalgal sludge showed that polyphosphate was indeed degraded resultingnin phosphate release. Interestingly, after the initial release phase phosphorus was assimilatednby the biomass and some polyphosphate was reformed. It is likely that this is due to bacterialngrowth in the sludge.