Treatability of South African surface waters by activated carbon

Abstract

Natural organic matter (NOM) in water resources for drinking purposes can be removed by different methods, including activated carbon adsorption. Due to the variability of NOM in natural waters, both in terms of its nature and its concentration, a study was undertaken to investigate NOM removal for a wide range of South African surface waters, sampled at different periods, by the use of granular activated carbon (GAC). NOM removal was assessed by measuring the ultraviolet (UV) absorbance at 3 wavelengths, namely, 254 nm (UV 254 ), 272 nm (UV 272 ) and 300 nm (UV 300 ). A comparison of data between the three wavelengths showed that any of the three wavelengths can be used to assess NOM removal by GAC, which is well described by the Freundlich equilibrium equation. A treatment target of 40% removal of initial UV 254 absorbance was considered. It was observed that, although the GAC dosage was generally a function of the initial UV 254 absorbance, differences existed between waters. This suggests that GAC usage rate is not only a function of the initial UV absorbance but also of the NOM composition, indicating a need for improved NOM characterisation. Comparison between the UV absorbance and dissolved organic carbon (DOC) data suggested that for some waters UV 254 absorbance can be used as a rapid substitute for DOC. Finally, the high GAC dosage rates required for the target criterion revealed that the process is inadequate for use at the initial stage of raw water treatment; GAC adsorption should be used at later stages of drinking water treatment.

Review: Natural organic matter in aquatic systems – a South African perspective

“…More than 600 DBP compounds have been identified and their formation in drinking water is based on various factors such as pH, temperature, contact time, dose, inorganic compounds, the type of NOM present in water, and the treatment processes being used (Gopal et al, 2007;Kim and Yu, 2007). Previous studies have shown that the highest contributor of DBP precursors is the hydrophilic fraction of NOM; however, even the hydrophobic fraction contributes to the disinfection by-product formation (DBPF) if it is not effectively removed after coagulation Li et al, 2014). In contrast, other studies shows that humic substances are the main contributor of DBPF (Kim et al, 2006).…”

“…Moreover, it is less effective compared to chlorine due to the fact that it requires longer contact time and produces volatile by-products that are responsible for bad taste and malodour in water (Gopal et al, 2007). In addition, the use of iodine and bromine can result in the formation of more hazardous DBPs compared to the chlorinated precursors because they are highly reactive to the hydrophilic fraction of NOM Li et al, 2014).…”

Characterization of natural organic matter in South African drinking water treatment plants: Towards integrating ceramic membrane filtration

“…However, these traditional unit processes do not efficiently remove natural organic matter (NOM) (around 35% at conventional pH) (Tshindane et al, 2019). Detrimental to water treatment and distribution, NOM is the major contributor to the fouling of membranes, is a precursor to the formation of disinfection by-products (DBPs), impacts on the organoleptic properties, accelerates the clogging of the pores of activated carbon, and thus decreases their efficiency, and certain fractions of NOM promote biological growth in the distribution networks (Hua et al, 2015;Lobanga et al, 2013;Lyon et al, 2014;Park et al, 2016). Therefore, modifications of the existing water treatment methods or adding new unit processes are required.…”

Investigating the fate of natural organic matter at a drinking water treatment plant in South Africa using optical spectroscopy and chemometric analysis

“…A single set of results is therefore not enough to either project these to a new treatment system or predict future trends for an existing system. Detrimental to water treatment and distribution, NOM is the major contributor to the fouling of membranes and activated carbon, is a precursor to the formation of disinfection by-products (DBPs), impacts on the organoleptic properties of water, and certain fractions of NOM promote biological growth in distribution networks (Lobanga et al, 2013;Lyon et al, 2014;Hua et al, 2015;Park et al, 2016). Therefore, closely monitoring the transformation of NOM along the treatment train is essential for the management of NOM-related problems.…”