Researchers remove phosphorus from sewage waste

Researchers from the University of Wollongong (UOW) have led the development of a system which captures phosphorus from sewage waste and delivers clean water. Phosphorus is in high demand for use as an agricultural fertiliser but, if released into natural waterways, phosphates can cause harmful algal blooms and low-oxygen conditions that can threaten to kill fish.

Conventional methods for extracting phosphorus from wastewater use chemicals to precipitate a mineral called struvite from the water, making it a time-consuming and costly process. To bring down the costs of struvite recovery, Associate Professor Long Nghiem of UOW’s School of Civil, Mining and Environmental Engineering and his colleagues decided to modify their system for harvesting freshwater from sewage waste using forward osmosis (FO) and membrane distillation (MD).

The prototype system has two parts that work in tandem, one for harvesting struvite and the other for recovering freshwater. The first part is a two-sided cell divided by a semi-permeable membrane, with concentrated wastewater on one side and a highly concentrated salt solution on the other. Osmosis drives water across the membrane from the more dilute wastewater side to the saltier ‘draw’ solution, increasing the concentration of phosphorus and ammonium in the wastewater.

“FO concentrates orthophosphate and ammonium for subsequent phosphorus recovery in the form of struvite … while MD is used to recover the draw solution and extract clean water from the digested sludge centrate,” the researchers explained.

The team have thus reduced the amount of chemicals required to mine struvite from wastewater. Their next mission is to address the fouling of the system’s semi-permeable membrane, which must be frequently flushed with deionised water to maintain maximum flow rates. After that, they hope to test the system on a pilot scale within the next few years.

“We need to develop technology to convert every wastewater treatment plant into a phosphorus quarry,” Professor Nghiem said. “This is the only way to ensure that future generations will never run out of phosphorus.”

The system already has “an excellent capacity for the removal of trace organic contaminants (TrOCs)”, according to the researchers, “with removal rates ranging from 91 to 98%”. They suggest “granular activated carbon adsorption or ultraviolet oxidation could be used to prevent contaminant accumulation in the draw solution, resulting in near-complete rejection (>99.5%) of TrOCs”.

The research has been published in Environmental Science & Technology and Environmental Science & Technology Letters.