Using reverse osmosis to isolate natural organic matter from large samples of fresh water

Natural organic matter (NOM) has been concentrated from five drinking water sources in Minnesota, Florida, Washington, California, and North Carolina, USA. Reverse osmosis (RO), coupled with cation exchange (CEX-H+) and/or electrodialysis (ED), was used to obtain several hundred grams of NOM from each source. The scale of the project exceeds that of most prior applications of this methodology for NOM concentration. The water concentration factor (WCF), which is the ratio of the total volume of processed sample to the final volume of concentrated sample, ranged from 200 to 408. In contrast, WCF is in the range of 20-50 in most prior studies. This paper provides technical background for the work, including the basic equations that relate the yield of NOM (the ratio of isolated organic carbon to processed organic carbon) to WCF and the RO rejection coefficients of its components. A numerical simulation of the yield of NOM and the degree of NOM fractionation that likely occurs during RO processing is presented. The inorganic chemical compositions of source waters have been used to anticipate and mitigate membrane fouling at very high WCF during the RO process. The relationship between NOM yield, the RO rejection coefficients of its components, and WCF is explored and used to interpret differences in NOM yield among the five sampling sites. Even with the large WCF values at the five sampling sites, the NOM yield ranged from 58% to 91%, within the predicted range for a pure solute with an RO rejection coefficient of 0.995-0.999.