Identifying the components of membrane antiscalants that cause biofouling could help make seawater desalination a more sustainable source of fresh water, following research by King Abdullah University of Science and Technology (KAUST).
“Safe drinking water is a human right,” said environmental scientist Graciela Gonzalez-Gil. “Yet roughly 800 million people have no access.”
The United Nations estimates that demand for freshwater could exceed the natural water cycle supply by as much as 40 per cent by 2030.
“Seawater desalination — particularly by reverse osmosis (SWRO), which involves pressurizing seawater through a membrane at high pressure to remove salt and impurities — has become a widely adopted low-cost source of drinking water in arid coastal countries,” said Gonzalez-Gil’s colleague and KAUST alumni Ratul Das, who now works as Head of Desalination R&D for energy company ACWA Power. It has 16 water seawater desalination plants across four countries.
However, SWRO is energy-intensive, and the used membranes create a lot of waste. Seawater is typically pretreated with antiscalants to prevent salt scaling on the membranes. “The low cost of these chemicals compared to other methods helps keep water prices low, hence their popularity,” said Das. But many of them trigger fouling by promoting microbial growth.
“Desalination operators are not fully informed about why and to what extent antiscalants cause biofouling,” said Gonzalez-Gil. “Measuring the bacterial growth caused by different antiscalants and linking this to their chemical composition can help these operators select products with minimal biofouling.”
How to help desalination membranes to protect from biofouling
Gonzalez-Gil’s team prepared vials of natural seawater with a small starting concentration of indigenous bacteria. Adding one of eight common antiscalants to separate vials, they measured daily bacterial growth and compared this to bacterial growth in seawater without antiscalants.
“We measured the carbon, phosphorous and nitrogen content of each antiscalant and used nuclear magnetic resonance to get a more detailed chemical fingerprint,” said Gonzalez-Gil.
The team found that some antiscalants contained other compounds besides the active ingredients. One particular contaminant – orthophosphate – promoted bacterial growth. “Surprisingly, not all phosphonate-based antiscalants were contaminated with orthophosphates,” said Gonzalez-Gil. “It includes antiscalants such as HEDP (1-hydroxy ethylidene-(1,1-diphosphonic acid), which was also the only antiscalant that didn’t promote bacterial growth.”
The team’s chemical fingerprinting technique could help manufacturers tailor antiscalants to contain fewer bacteria-boosting compounds. “Reducing biofouling will reduce the energy required for SWRO,” said Das. “It will lower the costs of desalination and, by reducing greenhouse emissions, will help to protect the planet.”
Reverse osmosis membranes are replaced every three to five years, despite a potential lifespan of 10 to 15 years. “Minimizing biofouling will extend their useful life and reduce the membrane waste deposited to landfill,” adds Gonzales-Gil.
Das hopes to develop a simple, low-tech test for use at desalination plants worldwide. “We want to eliminate ‘black boxes’ in the desalination industry and drive greener initiatives that impact Saudi Arabia and internationally,” he adds.
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