Reverse osmosis (RO) and electrodialysis (ED) were examined to determine an optimum way to concentrate natural organic matter (NOM) for chemical and biological characterization of disinfection by-products (DBPs). RO is an effective way to concentrate NOM. However, it also concentrates inorganic ions which lead to an increase in the osmotic pressure, eventually rendering RO ineffective, and potentially producing membrane scaling. Salts are also harmful to mammalian cells at high concentrations, which may cause artifacts in the toxicological outcomes. Thus ED was introduced to remove the inorganic ions. However, since a portion of NOM is negatively charged, it is not retained well by ED. Due to the ion exchange properties of ED membranes, sodium sulfate and sodium chloride were added at different time points and amounts in order to optimize the retention and concentration of NOM. It was found that a high concentration of sodium chloride displaced NOM that had adsorbed to anion exchange sites on the membranes, and was able to prevent loss of NOM. However, calcium was removed slowly due to the high sodium concentration, which limited the amount of sulfate that could be added. Meanwhile, continually adding sulfate at lower concentrations (low enough to prevent calcium sulfate precipitation) resulted in an initial loss of NOM, but eventually no further loss occurred as sulfate took over the anion exchange sites. However, not all of the calcium could be removed this way, and the presence of limited sulfate makes it infeasible to achieve large concentration factors. In order to optimize this process, calcium should be removed prior to concentration.
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Optimizing a hybrid reverse osmosis/electrodialysis system for natural organic matter concentration
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