Treatment of seawater with specific chemicals prior to desalination may prevent biofouling and extend the lifespan of filter membranes. Identifying the biofouling-causing components of membrane antiscalants could aid in making seawater desalination a more sustainable source of fresh water.
“Safe drinking water is a human right,” says environmental scientist Graciela Gonzalez-Gil, “yet roughly 800 million people do not have access.” According to the United Nations, demand for fresh water may exceed natural water cycle supply by up to 40% by 2030.
“Seawater desalination—particularly by reverse osmosis (SWRO), which involves pressing 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,” says Gonzalez-Gil’s colleague and KAUST alumni Ratul Das, who now works as Head of Desalination R&D for energy company ACWA Power, which has 16 seawater desalination plants across four countries.
SWRO, on the other hand, consumes a lot of energy and generates a lot of garbage. To prevent salt scaling on the membranes, seawater is usually prepared with antiscalants. “The low cost of these chemicals compared to other methods helps keep water prices low, hence their popularity,” Das explains. However, many of them cause fouling by encouraging microbial growth.
Desalination operators are not fully informed about why and to what extent antiscalants cause biofouling
“Desalination operators are not fully informed about why and to what extent antiscalants cause biofouling,” Gonzalez-Gil explains. “Measuring the bacterial growth caused by different antiscalants and linking this to their chemical composition can help these operators select products with minimal biofouling.”
Gonzalez-Gil’s group manufactured vials of natural saltwater containing a low initial concentration of indigenous bacteria. They measured daily bacterial growth in separate vials after adding one of eight common antiscalants and compared it to bacterial growth in saltwater without antiscalant.
“We measured the carbon, phosphorous, and nitrogen content of each antiscalant, and then used nuclear magnetic resonance to get a more detailed chemical fingerprint,” Gonzalez-Gil explains.
The researchers discovered that certain antiscalants contained molecules other than the active ones. One pollutant, orthophosphate, obviously aided bacterial growth.
“Surprisingly, not all phosphanate-based antiscalants were contaminated with orthophosphates, such as HEDP (1-hydroxyethylidene-(1,1-diphosphonic acid), which was also the only antiscalant that did not promote bacterial growth,” Gonzalez-Gil explains.
The chemical fingerprinting technology developed by the scientists could help manufacturers customize antiscalants to incorporate fewer bacteria-promoting chemicals. “Reducing biofouling will reduce the amount of energy required for SWRO,” Das explains. “It will reduce desalination costs while also helping to protect the environment by lowering greenhouse emissions.”
The chemical fingerprinting technology developed by the scientists could help manufacturers customize antiscalants to incorporate fewer bacteria-promoting chemicals.
“Reducing biofouling will reduce the amount of energy required for SWRO,” Das explains. “It will reduce desalination costs while also helping to protect the environment by lowering greenhouse emissions.”
Despite a theoretical lifespan of 10 to 15 years, reverse osmosis membranes are currently replaced every three to five years. “Reducing biofouling will extend their useful life and reduce the amount of membrane waste deposited in landfills,” Gonzales-Gil adds.
Das intends to create a low-tech test that may be used at desalination plants around the world. “We want to eliminate ‘black boxes’ in the desalination industry and drive greener initiatives that have an impact for Saudi Arabia and internationally,” he adds.
Source:King Abdullah University of Science and Technology
