OPTIMIZATION OF SEAWATER REVERSE OSMOSIS PRETREATMENT: PART III1 A MICROBIOLOGICAL APPROACH

Control of biofouling under different pretreatment regimes i.e., type of coagulant, coagulant aids, other chemicals and their concentrations as well as disinfection method, disinfectant concentration and pH was studied in a specially designed SWRO pilot plant at SWCC, RDC Al-Jubail. Bacterial number and bacterial aftergrowth in the feed at different pretreatment stages were monitored for a period of one year and were used as a measure to evaluate the performance of SWRO pretreatment. Coagulation and dual media filtration reduced bacterial number in the feed by 32% - 100% depending on initial chemical pretreatment of raw seawater. In most of the cases coagulation and filtration effectively removed a large portion of total bacterial mass (82%) in the feed. Bacterial aftergrowth in the feed after media filter was significantly affected by chemical treatment of raw seawater. Chlorination-dechlorination promoted aftergrowth in the feed, whereas, simultaneous treatment of raw seawater with traces of copper and chlorine generally reduced aftergrowth. Biofouling of cartridge filter resulted in a maximum of 42 times increase in bacterial count in the feed after cartridge filter. Data gathered during the period signified the relevance of monitoring of planktonic bacteria in detecting biofouling. Preliminary results of an ongoing study suggested that addition of coagulant aid polyelectrolyte could result in higher bacterial activity in the feed when the chlorine level is kept low. This effect, however, is reversed by keeping the residual chlorine around 1.0 mg/l. This effect has to be further verified by laboratory and pilot plant studies.