Enhanced hydraulic cleanability of biofilms developed under a low phosphorus concentration in reverse osmosis membrane systems

Abstract

A critical problem in seawater reverse osmosis (RO) filtration processes is biofilm accumulation, which reduces system performance and increases energy requirements. As a result, membrane systems need to be periodically cleaned by combining chemical and physical protocols. Nutrient limitation in the feed water is a strategy to control biofilm formation, lengthening stable membrane system performance. However, the cleanability of biofilms developed under various feed water nutrient conditions is not well understood. This study analyzes the removal efficiency of biofilms grown in membrane fouling simulators (MFSs) supplied with water varying in phosphorus concentrations (3 and 6 μg P·L -1 and with constant biodegradable carbon concentration) by applying hydraulic cleaning after a defined 140% increase in the feed channel pressure drop, through increasing the cross-flow velocity from 0.18 m s -1 to 0.35 m s -1 for 1 h. The two phosphorus concentrations (3 and 6 μg P·L -1 ) simulate the RO feed water without and with the addition of a phosphorus-based antiscalant, respectively, and were chosen based on measurements at a full-scale seawater RO desalination plant. Biomass quantification parameters performed after membrane autopsies such as total cell count, adenosine triphosphate, total organic carbon, and extracellular polymeric substances were used along with feed channel pressure drop measurements to evaluate biofilm removal efficiency. The outlet water during hydraulic cleaning (1 h) was collected and characterized as well. Optical coherence tomography images were taken before and after hydraulic cleaning for visualization of biofilm morphology. Biofilms grown at 3 μg P·L -1 had an enhanced hydraulic cleanability compared to biofilms grown at 6 μg P·L -1 . The higher detachment for biofilms grown at a lower phosphorus concentration was explained by more soluble polymers in the EPS, resulting in a lower biofilm cohesive and adhesive strength. This study confirms that manipulating the feed water nutrient composition can engineer a biofilm that is easier to remove, shifting research focus towards biofilm engineering and more sustainable cleaning strategies.

Periodic chemical cleaning with urea: disintegration of biofilms and reduction of key biofilm-forming bacteria from reverse osmosis membranes

“…They are natural amphiphilic polymers that aid in bacterial detachment from interfaces ( Neu and Lawrence 2010 ). Recently, it was reported that the higher presence of fulvic acids in the biofilm grown at a lower phosphorus concentration helped solubilize and detach the biofilm from the membrane and spacer ( Javier et al. 2021 ).…”

“…For efficient biofouling control, it may be suggested to (i) understand the site-specific processes influencing biofilm formation, (ii) implement preventive control strategies such as effective pretreatment to delay biofouling and (iii) examine biofouling characteristics to select the most effective membrane cleaning method. It is known that biofilm EPS composition can vary with age, nutrient availability in feed water and membrane surface properties ( Javier et al. 2021 ).…”

Phosphorus Concentration in Water Affects the Biofilm Community and the Produced Amount of Extracellular Polymeric Substances in Reverse Osmosis Membrane Systems

“…Dechlorinated tap water supplemented with nutrients (Table 1), containing an ultralow phosphorus concentration of ≤0.3 µg PO 4 -P•L −1 measured as orthophosphate according to Javier et al (2020Javier et al ( , 2021 [9,31,33], was used as the feed water for this study. The tap water was produced through seawater reverse osmosis (SWRO) desalination at the King Abdullah University of Science and Technology desalination plant (Thuwal, Jeddah, Saudi Arabia).…”

“…The 125 µg C•L −1 assimilable organic carbon concentration of glucose was used based on measurements performed at a desalination plant [42]. The dosed phosphorus concentrations for this study, 3 µg P•L −1 and 6 µg P•L −1 , were selected based on phosphorus measurements that are typically present in seawater and the RO feed water after the addition of phosphorus-based antiscalants [31,43]. The selected dosed phosphorus concentration of 0 µg P•L −1 was used as removing the phosphorus concentration from the feed water has been suggested as a biofilm control strategy [12,44].…”

“…It has been suggested that the stratification of bacterial populations in the inner biofilm layers developed a base biofilm attached to the membrane that is harder to remove [29,30]. Previously, it was demonstrated that reducing the phosphorus concentration in the water from 6 µg P•L −1 (P in RO feed water after dosage of phosphorus-based antiscalants) to 3 µg P•L −1 (P in seawater) enhanced the hydraulic cleanability of reverse osmosis membranes and feed spacers [31]. However, it is unknown which bacterial communities grow under phosphorus-limiting conditions and if the bacterial community in the biofilm that remains after membrane flushing (by increasing the crossflow velocity of reverse osmosis systems) is of a different composition.…”