Nanofiltration and reverse osmosis biostability relative to alternative methods of water treatment

Biostability of finished waters was assessed statistically using assimilable organic carbon, biodegradable dissolved organic carbon and heterotrophic plate counts. Biofilm growth on unlined cast iron, galvanized steel, lined cast iron and polyvinylchloride pipe material was assessed visually and statistically using potential exoproteolytic activity, which is a measure of biofilm cell density. Seven different treatment processes were used to simulate full scale treatment and distribution of ground, surface and saline sources to pilot distribution systems made from unlined cast iron, galvanized, lined cast iron and polyvinylchloride pipes taken from actual distribution systems. Bulk water biostability parameters as measured by assimilable organic carbon, biodegradable dissolved organic carbon and heterotrophic plate counts were lower in reverse osmosis finished water and higher in conventionally treated groundwater. Average finished water assimilable organic carbon indicated reverse osmosis and nanofiltration membrane processes reduced assimilable organic carbon relative to finished groundwater produced by conventional treatment or softening, and finished surface water produced by enhanced coagulation. This relationship was not observed clearly for biodegradable dissolved organic carbon or heterotrophic plate counts. Biofilm growth on coupons cut from the pipes used to build the pilot distribution systems typically decreased as the level of treatment increased with the exception of reverse osmosis finished water, which produced very high biofilm growth. However, the assessment of biostability indicated biostability generally increased as the level of treatment increased, and the general order of biostability of process finished waters was: membrane > precipitative > conventional; and the order of biofilm growth with respect to pipe material was unlined cast iron > galvanized > lined cast iron > polyvinylchloride. Hence, improved distribution system biological water quality, as measured by lower assimilable organic carbon, biodegradable dissolved organic carbon, heterotrophic plate counts and biofilm growth, was directly dependent on nonpurgeable organic carbon and improved as finished water nonpurgeable organic carbon decreased.