Effect of water conservation and recycling on the potential for, and environmental impact of, phosphorus recovery from municipal wastewater in Australia

Abstract

Per capita water consumption declined by 25-36% while recycled water use increased by 10% in Australian capital cities from 2000 to 2009. Assuming constant per capita phosphorus loads, these changes present an opportunity for phosphorus recovery as struvite from RO brine generated at water recycling plants. A multi-parametric model for a 50,000 equivalent population wastewater recycling plant found specific power for struvite recovery from RO brine reduced from 590 to 170 kWh/kgP as per capita water consumption declined from 300 to 50 L/c/d. Phosphonate antiscalants used in RO systems at a typical concentration of 30 mg/L reduced the phosphorus removal efficiency from 29% to 20% for phosphorus recovery from synthetic solutions and altered the rhombic struvite precipitate in 1L jar tests. However, the effects were less pronounced in a 5L batch reactor at higher struvite saturation (SI 1.8), indicating the technical feasibility of struvite recovery from RO brine. An alternative magnesium source, seawater, reduced the phosphorus removal efficiency from 29% for the control to 14%, while the use of bittern increased the removal to 37% in 1L jar tests. The use of seawater also increased the calcium content of the precipitate from <0.1% for the control and bittern to 6.6% and altered the rhombic structure in 1L jar tests, though the effects were less pronounced in a 5L batch reactor. While technically feasible, analysis of wastewater catchments with declining water consumption over the 2000 to 2012 period found per capita phosphorus loads declined in 11 of 12 catchments, indicating that on an energy basis, phosphorus recovery from the RO brine is unlikely to compare with production from mined phosphate rock. A life cycle analysis comparing the environmental benefits and burdens of introducing phosphorus recovery into existing infrastructure found struvite recovery from solids dewatering streams was preferred over recovery from RO brine and over direct land application of biosolids at centralised plants. Urine separation reduced the global warming and ozone depletion potentials of decentralised systems, although the soil salinization and terrestrial ecotoxicity potentials increased. While avoided fertilizer provided benefits, it didn’t necessarily outweigh the resources required to achieve phosphorus recovery.