Effect of dechlorination point location and residual chlorine on biofouling in a seawater reverse osmosis plant

Abstract

Experiments were carried out to test the effect of dechlorination dosing point location and the concentration of residual chlorine on bacterial growth and biofouling in a seawater reverse osmosis (SWRO) plant. The plant is located in Al-Birk on the Red Sea coast, southern Saudi Arabia. The plant was routinely operated with injection of chlorine as a biocide in the intake chamber in the open sea and with the injection of sodium metabisulfite (SBS) to remove chlorine after the dual media filter (DMF). During the experiments, the SBS dosing point was shifted to two locations after the micron cartridge filter (MCF): first, to a point 13 m ahead of the high pressure pump (HPP) and second, to a point just ahead (<1 m) of the HPP. Due to fluctuations in residual chlorine resulting from certain operational and physical circumstances, it was possible to assess biofouling potential when free chlorine is regarded as high (≥0.5 mg/l) or low (<0.5 mg/l) within a maximum of 1 mg/l. Bacterial generation (doubling) time was used to evaluate biofouling. Generation time was higher (lower multiplication capacity) when the SBS dosing point was before the MCF (after DMF). It decreased significantly, reflecting higher multiplication capacity and higher biofouling potential, when the SBS dosing point was moved to after the MCF. Generation times in high-pressure RO feed water were similar when the SBS dosing point was moved to two locations after the MCF. This indicated minimal contribution of the low-pressure pipe between the MCF and the HPP to biofouling. In general, biofouling increased as the SBS dosing point was moved forward along the pretreatment line, closer to the RO membranes. Generation times were similar when residual chlorine was less or more than 0.5 mg/l. Bacteria were capable of biofilm formation in the chlorinated section of the plant.