Removal of assimilable organic carbon and biodegradable dissolved organic carbon by reverse osmosis and nanofiltration membranes

The main objective of this study was to evaluate the effectiveness of reverse osmosis (RO) and nanofiltration (NF), under various solution chemistries, on bacterial regrowth potential as quantified by assimilable organic carbon (AOC) and biodegradable dissolved organic carbon (BDOC). The bench-scale experiments, using tap groundwater spiked with acetate as organic carbon, revealed that AOC removals by RO/NF membranes were strongly dependent on charge repulsion. AOC removals were greater at conditions of low ionic strength and low hardness, and were slightly higher at high pH values. BDOC removals by NF membrane also increased with decreasing hardness and ionic strength, and increasing pH. However, the RO membrane showed less dependence on feed solution chemistry for BDOC removal, suggesting that BDOC removal was determined by the combined effect of both size exclusion and charge repulsion. The bench-scale observations were compared to a full-scale drinking water treatment plant that used nanofiltration as a primary treatment process. From full-scale operation, it was observed that nanofiltration was a very effective means to reduce BDOC, but conversely, did not reject the bulk of raw water AOC. The high BDOC rejection by NF membranes at full scale can be explained by size exclusion, since a significant fraction of BDOC in raw surficial ground water consists of compounds, such as humic and fulvic acids, which are larger than the pores of NF membranes. The insignificant AOC rejection observed in the full-scale system was probably due to the low pH, high hardness, and high ionic strength (TDS) of the raw groundwater combined with acid addition during pretreatment. These solution environments repress the electrostatic interaction between charged organic compounds and membranes, allowing passage of small molecular weight compounds and thus reducing AOC rejection. (C) 2000 Elsevier Science B.V. All rights reserved.