The response of typical South African raw waters to enhanced coagulation

Abstract

Drinking water treatment plants in South Africa rely almost entirely on surface water sources, which are often compromised due to high return flows and indirect reuse. The typical treatment plants focus on the removal of physical and microbial contaminants which include turbidity, colour, chemical compounds and microorganisms. A relatively new concern to this list is natural organic matter (NOM) which has become a major concern in potable water treatment due to its recent regulation. In this study, eight different raw water samples from the various water types found in the country were seasonally collected and treated for the removal UV absorbance at a wavelength of 254 nm (UV254) using enhanced coagulation (EC). The efficacy of EC, which can be employed as a practical technology in the removal of both turbidity and NOM, was evaluated in remaining UV254 from these raw water sources. Jar tests were conducted, with ferric chloride used as the coagulant (due to its extensive use as a coagulant in the water treatment industry in South Africa) and specific pH values (initial water pH, 7.0, 6.0, 5.5, 5.0 and 4.5) were chosen as target values guiding the six different coagulant dosages for the jar tests. The pH of the low-alkalinity (<60 mg/L CaCO3) raw waters were adjusted and raised by the addition of sodium carbonate. The response parameters of the tests were turbidity (NTU), pH and UV254. Algorithms for finding the optimum coagulant dosage for UV254 removal were developed and consistently applied to all the results. Results showed large variations in the nature of NOM across the country from specific ultraviolet absorbance values. From the UV254 values, the concentrations of NOM also varied greatly geographically than temporally. The general trend observed in the EC results suggested that the pH should always be dropped to between 4.5 and 7.0 to lower the amounts of UV254 and turbidity to reasonable levels.