(177g) Using Membranes to Recover Phosphate in High Nutrient Density Forms

Depletion of world phosphate (PO₄^3- ) supplies is driving research into options to recover and recycle this essential, non-renewable resource. PO₄^3- recovery at centralised wastewater treatment plants is seen as economically unviable because of low PO₄^3- levels compared with decentralised systems based on source separation. However, the recent growth in membrane based water recycling projects, where reverse osmosis is used to produce high quality water has resulted in the production of liquid waste streams with high concentrations of PO₄^3-. By using data from centralised water recycling plants in California, Queensland and Victoria, we have investigated PO₄^3- recovery via struvite (magnesium ammonium phosphate) from membrane concentrate. The process uses nanofiltration membranes to concentrate COD and achieve stoiciometrically optimum ratios of N and P to improve struvite reactor performance. Screening studies of six nanofiltration membranes showed that while high organic removal membranes exhibited promising trivalent rejection, fouling of the membranes limited recovery. However, membranes with low organics removal have poor trivalent rejection and are therefore not suitable for concentrating PO₄^3-. However, we have identified a nanofiltration membrane that provides 91% organics removal and 75% rejection of trivalent ions, which allows for stable operation and reasonable PO₄^3- recovery. By incorporating an anaerobic reactor in the flow diagram we have been able to investigate the possibility of cogeneration of electricity using methane. The net result is an improvement in the kWh/kg P recovered that compares favourably with source separated decentralised facilities under some circumstances.