Biofouling in a seawater reverse osmosis plant on the Red Sea coast, SaudiArabia

The study investigated the environmental and pretreatment impact on biofoul ing in a seawater reverse osmosis (SWRO) plant. The effect of a pretreatmen t chemical (chlorine) and certain alterations of chemical dosing on membran e biofouling was also investigated, as well as the biofouling potential of the source water and the effect of chlorination on this biofouling potentia l. Experiments were carried out on biofouling in a SWRO plant on the Red Se a coast with a set of four pretreatment modes: (1) normal operation mode, w here coagulant is dosed immediately before the media filter and where sodiu m metabisulfite (SBS) is dosed after the media filter; (2) operation with t he coagulant dosing point shifted back to the pressure side of the seawater intake pump; (3) with the SBS dosing point shifted to after the micron car tridge filter; (4) while the plant was operating without chlorination/dechl orination. Bacterial generation time and biofilm attachment slides were use d to evaluate biofouling. Generation (doubling) times were lowest (higher m ultiplication capacity) nearest the intake, and they increased gradually al ong the pretreatment line, becoming the highest closest to the membranes an d in the brine reject. When the SBS was shifted, chlorine removal became cl oser to the membranes. Following this, generation time in the water samples taken after the dual media filter (ADMF), after the micron cartridge filte r (AMCF) and immediately before the membranes, decreased significantly, ref lecting more biofouling potential in the membranes. This correlates well wi th operational data where the SBS shift resulted in doubling the frequency of membrane cleaning. Generation times were higher when no chlorine was use d, indicating less membrane biofouling potential. Water samples from the pl ant's intake in the sea had 24-h generation time values less, but close to those of chlorinated seawater. This indicated high nutrient load and questi onable water quality of the intake. The bacterial attachment to the biofilm slides showed the general trend exhibited by the generation time of plankt onic bacteria. The biofilm formation in the brine was generally less than t he preceding sampling stations ADMF and AMCF, indicating removal of nutrien ts along the pretreatment line, However, when the SBS dosing point was shif ted closer to the membranes, the number of attached bacteria in the brine i ncreased significantly. Bacteria attached to the biofilm slides randomly an d in microcolonies with vesicles in between. Although chlorination enhances biofouling, the current experiments indicate that the questionable quality of source water is one cause of the operational problems in this plant.