Assessing bacterial growth potential in seawater reverse osmosis pretreatment: Method development and applications

Seawater desalination is increasingly used as a means to augment freshwater supplies in regions of the world with high water stress, and reverse osmosis is increasingly the technology of choice because of the lower energy consumption compared to thermal systems. However, seawater reverse osmosis (SWRO) systems suffer from various types of fouling, which can increase energy consumption and the use of chemicals during SWRO operation. In practice, pre-treatment systems are put in place to reduce the particulate and biological fouling potential of SWRO feed water. However, simple, reliable and accurate methods to assess the extent to which biological fouling potential is reduced during pre-treatment are not available for seawater. This research developed a new method to measure bacterial growth potential (BGP) using the native bacterial consortium in seawater. New reagents to extract and detect ATP in microbial cells were tested and optimized for seawater. The new lysis and detection reagents overcame the salt interference in seawater and allowed the detection of low concentrations of total ATP, free ATP and microbial ATP in seawater. Incorporating a filtration step increased the sensitivity of the method six fold, enabling ATP detection of ultra-low levels of microbial ATP in seawater (0.06 ng-ATP/L). The newly developed ATP-based BGP method was applied to monitor and assess the pre-treatment of five full-scale seawater desalination plants around the world, which included dual media filtration, dissolved air flotation and ultrafiltration. The highest removal of microbial ATP and BGP was achieved with dual media filtration in combination with inline coagulation. A correlation was observed between BGP measured in SWRO feed water and the pressure drop increase in SWRO systems, suggesting the applicability of using the ATP-based BGP method as a biofouling indicator in SWRO. Furthermore, a safe level of BGP (<70 µg/L) is tentatively proposed for SWRO feed water in order to ensure a chemical cleaning frequency of once/year or lower. However, to validate these conclusion, more SWRO plants with different pre-treatment systems need to be monitored. In the future, on-line monitoring of ATP and BGP in SWRO feed water may further reduce the consumption of chemicals and energy and improve the overall sustainability of seawater desalination by reverse osmosis.