Assessment of the extent of bacterial growth in reverse osmosis system for improving drinking water quality

Abstract

This study was carried out to assess reverse osmosis (RO) treatment efficacy of drinking water in terms of biological stability in the distribution system. Two flat-sheet RO membranes were used in this study. Experiments were designed to investigate the growth of biofilm and bulk phase bacteria for the RO-treated water flowing through a model distribution system under controlled conditions without disinfectants. RO membranes improved the water quality of drinking water in terms of inorganic, organic and bacterial contents. Organic matter including the fraction available for microbes was efficiently removed by the RO membranes tested. More than 99% of bacterial cells in the tap water was retained by the RO membranes, leaving <50 cells/mL in the permeate water. In spite of the low nutrient contents and few cells in the RO permeates, monitoring of the model distribution systems receiving the RO permeates showed that remarkable biofilm accumulation and bulk cell growth occurred in the RO permeate water. In quasi-steady state, the total cell numbers in the biofilm and bulk water were of order 103 cells/cm2 and 103 cells/mL, respectively, which were about 2 orders of magnitude lower than those grown in the tap water produced from conventional water treatment. The culturable heterotrophic bacteria constituted a significant part of the total cells (20.7–32.1% in biofilms and 21.3–46.3% in bulk waters). Biofilm maximum density and production rate were of the order 104 cells/cm2 and 102 cells/cm2/day, respectively. The specific cell growth rate of bacteria in the biofilms was found to be much lower than those in the bulk waters (0.04–0.05 day−1 versus 0.28–0.36 day−1). The overall specific cell growth rate which indicates the growth potential in the whole system was calculated as 0.07–0.08 day−1, representing a doubling time of 9.1–10.1 days. These observations can be indicative of possibilities for bacterial growth in the RO permeate water with easily assimilable organic carbon concentrations below values proposed for biostability. RO permeate water does not appear to be biologically stable water. Therefore, efforts to minimize bacterial growth in the RO permeate water and in the distribution system must consider post-disinfection.