Phosphorus removal and recovery from domestic wastewater in a membrane bioreactor with external recycle anaerobic process of sludge

The removal of phosphorus from domestic wastewater in a membrane bioreactor with external recycle anaerobic process of sludge was investigated. More than 85% of phosphorus removal efficiency could be achieved steadily with external recycle of sludge. In the anaerobic reactor, the sufficiency of phosphorus released from the phosphorus-enriched sludge was an important factor for phosphorus removal from the system. Sodium acetate as the external carbon source was much better than the real domestic wastewater for the release of phosphorus during the anaerobic process. And the release quantities of phosphorus increased with improving the dose of sodium acetate in some extent. The experimental data showed that one millimole phosphorus releasing required about 1.048mmol sodium acetate. Moreover, the recovery of phosphorus in the supernatant was also studied by batch tests. Under the conditions of [Mg²⁺]:[NH₃-N]:[P] =2:2:1 and the initial phosphorus concentration of 49.0mg/L, the optimal pH value was about 9.0 considering the struvite as the dominant product. Although increasing pH value could improve the phosphorus recovery efficiency, the precipitate transformed from struvite to magnesium phosphate as the pH value above 9.0. During struvite precipitation process, increasing the concentration of NH₃-N to improve struvite production was more suitable than increasing the dose of [Mg²⁺]. In addition, calcium ions in the domestic wastewater had competition effect on struvite precipitation.