Challenges in the potable water industry due to changes in source water quality: case study of Midvaal Water Company, South Africa

Abstract

Midvaal Water Company treats hypertrophic water abstracted from the Vaal River to supply bulk wholesome potable water to their consumers in compliance with the South African National Standard (SANS) 241:2015 for drinking water. The facility incorporates conventional and advanced treatment processes. The aims of the study were to identify how the water treatment processes of the plant have changed over time in response to the varying water quality of the Vaal River, and to consider both current and future obstacles as well as possible solutions regarding water quality and treatment. Oxidation steps such as pre-chlorination, potassium permanganate addition, pre-ozonation and intermediate ozonation have either been applied in the past or are still operational. The dissolved air flotation plant accounts for almost 70% of total chlorophyll removal and the significance of this process was confirmed during a brief maintenance shutdown during 2015. Total chlorophyll concentrations of the source water have increased extensively since 1984, while turbidity levels have remained fairly constant but with spikes at times. The facility suffers from severe taste and odour episodes during warm periods due to the presence of methylisoborneol (MIB), released by Cyanophyceae, in the Vaal River. Concentrations of > 300 ng/L MIB have been recorded, whereas the odour threshold concentration for MIB ranges from 4 ng/L to 20 ng/L. The additional application of activated carbon to alleviate taste and odour problems has to be weighed against the cost implications for consumers, the correct type to be purchased for the organic molecules to be adsorbed, the interference of natural organic matter, and the formation of additional sludge mass, as well as the intensity and duration of taste and odour events. Midvaal remains a bulk potable water supplier and therefore has to consider the socio-economic status of their consumers where water pricing is concerned. The study ultimately emphasized the intrinsic value of protecting water resources.

Keywords: oxidation processes, dissolved air flotation, chlorophyll a, taste and odour