Rapid population growth and urbanization are two main drivers for the over-abstraction of conventional freshwater resources in various parts of the world, which leads to the situation of water scarcity (per capita availability <1000 m³ /year). Predictions based on the World Bank projected population data and the FAO AQUASTAT database for freshwater availability show that by 2050, 2 billion people living in 44 countries will likely suffer from water scarcity, of which 95% may live in developing countries. Among these, the countries that will likely be most strongly hit by water scarcity by 2050 are Uganda, Burundi, Nigeria, Somalia, Malawi, Eritrea, Ethiopia, Haiti, Tanzania, Niger, Zimbabwe, Afghanistan, Sudan, and Pakistan. Currently, these countries have not yet established desalination to meet their freshwater demand. However, the current global trend shows that membrane-based desalination technology is finding new outlets for supplying water to meet growing water demand in most of the water-scarce countries. These 14 water-scarce countries will demand an additional desalination capacity of 54 Mm³ /day by 2050 in order to meet the standard of current municipal water demand and to compensate for the withdrawal of renewable resources. Case studies from India, China, and South Africa have highlighted that other countries may apply the strategy of using desalinated water for industrial users. Moreover, challenges to the widespread adoption of desalination exist such as expense, significant energy use, the need for specialized staff training, the large carbon footprint of facilities, environmental issues such as greenhouse gas emission (GHGs), chemical discharge, and operational problems such as membrane fouling.

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Many desalination plants still struggle to control biological fouling in seawater reverse osmosis (SWRO) systems as there are no standard methods to monitor this type of fouling. Strategies to control biofouling in SWRO systems have been proposed such as antifouling coating and lowering biofouling potential in SWRO feedwater through pretreatment processes. Measuring biofouling potential in the pretreatment and SWRO feedwater has gained increased interest due to its direct link to biofouling. Moreover, this approach can be used as an early warning system allowing for taking corrective actions in the pretreatment processes to meet the required SWRO feedwater quality. This article presents the biofouling potential methods/tools developed for seawater, their applications to monitor and assess raw seawater, SWRO pretreatment and SWRO feedwater, and how these methods are employed to control SWRO biofouling membrane systems. The reported removal efficiency of biofouling potential during SWRO pretreatment processes was found to be low to moderate. Threshold values for biofouling limitation were then proposed based on several lab and plant studies. Research on biofouling potential has provided insight into SWRO pretreatment performance optimisation and biofouling control. Future research is anticipated to determine better pretreatment processes and to identify robust threshold values for mitigating biofouling in SWRO membranes.

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In this study, the removal of particulate, organic and biological fouling potential was investigated in the two-stage dual media filtration (DMF) pretreatment of a full-scale seawater reverse osmosis (SWRO) desalination plant. Moreover, the removal of fouling potential in two-stage DMF (DMF pretreatment) was compared with the removal in two-stage DMF installed after dissolved air floatation (DAF) (DAF-DMF pretreatment). For this purpose, the silt density index (SDI), modified fouling index (MFI), bacterial growth potential (BGP), organic fractions and microbial adenosine triphosphate (ATP) were monitored in the pretreatment processes of two full-scale SWRO plants. Particulate fouling potential was well controlled through the two stages of DMF with significant removal of SDI15 (>80%), MFI0.45 (94%) and microbial ATP (>95%). However, lower removal of biological/organic fouling potential (24–41%) was observed due to frequent chlorination (weekly) of the pretreatment, resulting in low biological activity in the DMFs. Therefore, neutralizing chlorine before media filtration is advised, rather than after, as is the current practice in many full-scale SWRO plants. Comparing overall removal in the DAF-DMF pretreatment to that of the DMF pretreatment showed that DAF improved the removal of biological/organic fouling potential, in which the removal of BGP and biopolymers increased by 40% and 16%, respectively. Overall, monitoring ATP and BGP during the pretreatment processes, particularly in DMF, would be beneficial to enhance biological degradation and lower biofouling potential in SWRO feed water.

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Several potential growth methods have been developed to monitor biological/organic fouling potential in seawater reverse osmosis (SWRO), but to date the correlation between these methods and biofouling of SWRO has not been demonstrated. In this research, the relation between a new adenosine triphosphate (ATP)-based bacterial growth potential (BGP) test of SWRO feed water and SWRO membrane performance is investigated. For this purpose, the pre-treatment of a full-scale SWRO plant including dissolved air flotation (DAF) and two stage dual media filtration (DMF) was monitored for 5 months using BGP, orthophosphate, organic fractions by liquid chromatography coupled with organic carbon detection (LC-OCD), silt density index (SDI), and modified fouling index (MFI). Results showed that particulate fouling potential was well controlled through the SWRO pre-treatment as the measured SDI and MFI in the SWRO feed water were below the recommended values. DAF in combination with coagulation (1–5 mg-Fe³⁺/L) consistently achieved 70% removal of orthophosphate, 50% removal of BGP, 25% removal of biopolymers, and 10% removal of humic substances. Higher BGP (100–950 µg-C/L) in the SWRO feed water corresponded to a higher normalized pressure drop in the SWRO, suggesting the applicability of using BGP as a biofouling indicator in SWRO systems. However, to validate this conclusion, more SWRO plants with different pre-treatment systems need to be monitored for longer periods of time.

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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. @en

Various bacterial growth potential (BGP) methods have been developed recently to monitor biofouling in seawater reverse osmosis (SWRO) systems such as assimilable organic carbon and bacterial regrowth potential. However, the relationship between these methods and biofouling in SWRO desalination plants has not yet been demonstrated. In this research, an attempt is made to investigate if a correlation exists between BGP of SWRO feed water and the chemical cleaning frequency in SWRO plants using an ATP-based BGP method employing an indigenous microbial consortium. Using ATP-based BGP method at 5 different seawater locations showed low variations of bacterial yield. The BGP method was applied to assess the pretreatment performance of three full-scale SWRO plants with different pretreatment processes. Dual media filtration (DMF) showed the highest BGP removal (>50%) in two SWRO plants. Removal of BGP and hydrophilic organic carbon in dissolved air floatation combined with ultrafiltration was similar to the removal achieved with DMF in combination with inline coagulation. For the three SWRO plants investigated, a higher BGP in SWRO feed water corresponded to a higher chemical cleaning frequency. However, more data is required to confirm if a real correlation exists between BGP and biofouling in SWRO plants.@en

The use of adenosine triphosphate (ATP) to monitor bacterial growth potential of seawater is currently not possible as ATP cannot be accurately measured at low concentration in seawater using commercially available luciferase-based ATP detection. The limitation is due to interference of salt with the luciferin–luciferase reaction, which inhibits light production. This research demonstrates that new reagents developed for (i) ATP extraction from microbial cells and (ii) ATP detection in seawater are able to reliably detect Microbial ATP as low as 0.3 ng L^–1 in seawater. The luminescence signal of the new detection reagent is significantly higher (>20 times) than the luminescence signal of the freshwater reagent, when applied in seawater. ATP can now be used to monitor bacterial growth potential (BGP) through pre-treatment trains of seawater reverse osmosis (SWRO) plants. The level of detection of the new BGP test is significantly lower than the estimated threshold value required to prevent biofouling in SWRO systems. The new reagents have been used to monitor Microbial ATP in coastal North Sea water. Moreover, Microbial ATP has been applied to monitor the bacterial growth potential (using indigenous bacteria) through the pre-treatment train of an SWRO desalination plant. A significant reduction (>55%) of the bacterial growth potential was found through the dual media filtration with 4.5 mg-Fe(III) L^–1 coagulant. Overall, the new reagents can detect low Microbial ATP concentrations in seawater and can be used to monitor bacterial growth potential in seawater desalination plants.

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