Extracting phosphorus from wastewater

Impact: Reducing the cost and environmental effect of capturing phosphorus from wastewater

Mineral phosphorus is an increasingly scarce natural commodity valued in agriculture as a fertiliser.

A UOW team is meeting the challenge posed by the shortage of phosphorus by developing an efficient method for extracting phosphorus from wastewater, with the additional benefit of delivering clean water.

Traditional methods for capturing phosphorus from wastewater involve using chemicals to precipitate a mineral called struvite from the water - struvite contains phosphorus, ammonium, and magnesium, and is a useful slow-release fertilizer. Currently, however, extracting it is a costly process.

A team from the UOW School of Civil, Mining and Environmental Engineering, led by Professor Long Nghiem, are working to solve the cost of this process by reducing the amount of chemicals required to remove struvite from wastewater using an osmosis-distillation method.

Their prototype system involves a two-sided cell divided by a semipermeable membrane, with concentrated wastewater on one side and a highly concentrated salt ‘draw’ solution on the other. Osmosis drives water across the membrane from the more dilute wastewater side to the ‘draw’ solution, increasing the concentration of phosphorus and ammonium in the wastewater.

To enhance struvite recovery, the usual sodium chloride (NaCl) draw solution is replaced with one of magnesium chloride (MgCl2), creating a more favourable environment for struvite precipitation.

In the second part of the membrane system, where freshwater is recovered, the draw solution is heated and introduced into a cell with a microporous, hydrophobic membrane. Water vapor passes through to the cooler side of the membrane and condenses.

This distillation process removes freshwater from the MgCl2 solution, effectively renewing the draw solution for continuous operation of the two-part system. The forward osmosis and distillation process requires less than one-sixtieth of the MgCl2 and one-fifteenth of the NaOH compared with the current conventional method.

The research team is working to improve the system’s flow rates, which are impeded by fouling of the semipermeable membrane, requiring frequent flushing with deionised water. Once resolved, the team hope to have a commercially viable system that will provide a reliable and inexpensive supply of phosphorus, as well as a safe method of extracting freshwater from wastewater.