Abstract
A seawater reverse osmosis (SWRO) plant in the Arabian Gulf region showed serious membrane biofouling that appeared to be due to the presence of planktonic particulate bacterial aggregates that can be termed as "protobiofilms", which possessed the same characteristics of surface-attached biofilms. This SWRO plant utilized dissolved air flotation and ultrafiltration for pretreatment, high permeable SWRO membranes for desalination and incorporated rotary energy recovery devices (RERD). During the first year of operation, a slow steady increase in TMP and channel Delta P was observed that appeared due to membrane biofouling from either colloidal small bacterial aggregates or individual bacterial cells. After the first year of operation, the TMP and channel Delta P increased at an accelerated rate that appeared due presence of planktonic particulate bacterial aggregates. An internal membrane hybridization (IMH) program was initiated after the second year of operation by replacing the two highly fouled high permeable membranes in the feed-end of the pressure vessel with new low permeable membranes to lower the flux and concentration polarization (CP) of the feed-end membranes and to lower the membrane fouling potential. Biofouling of these new low permeable membranes in the IMH program resulted in an immediate and an accelerated increase in both TMP and channel Delta P, which was attributed to the presence of significant concentration of planktonic particulate bacterial aggregates that appeared due to dispersal of a biofilm from the RERD. An important factor that contributed to the dispersal of bacterial aggregates into the RO feed water was the flow velocity through the RERD. Decreasing the flow velocity into and cleaning of the RERD resulted in a significant decrease in the rate of RERD biofilm dispersal and biofilm formation on the SWRO modules.