Comparative study for the removal of natural organic matter from different raw waters by ion exchange

Abstract South African water treatment plants mainly utilise surface water sources whose quality is continuously compromised due to indirect reuse and high return flows. In general the methods used by these water treatment plants are based on the removal of turbidity, various chemical contaminants and microorganisms. Recently introduced NOM regulations added the removal of natural organic matter (NOM) as a priority. Basic anion exchange resins are one way of NOM removal, reported on this paper. Due to the variability in the composition of NOM in natural raw water, eight natural raw waters were collected throughout South Africa as representative of the different water types of the country. They were collected three times during a period of eight months. The strong and weak – base anion resins used in the study were chosen for their availability and competitive price at the local market. The removal of the NOM fractions was assessed by measuring ultraviolet absorption at a wavelength of 254 nm (UV254). Freundlich equilibrium isotherms were fitted. It was observed that the Freundlich parameters K and n are related reducing the Freundlich isotherm to a single-parameter model. The two parameters cluster for some waters regardless of season. Two performance indicators, namely 65% initial UV254 removal and absolute level of 6 /m, were adopted. It was found that if the initial UV254 is less than about 15 /m the absolute level of 6 /m is reached first while when the initial UV254 is greater than 20 /m the removal of 65% of UV254 is reached first. It was also observed that the resin usage rate is not only dependent on the initial UV254 value but also on the nature and concentration of the NOM, indicating a need for a better NOM characterization