Identification and quantification of natural organic matter in the Windhoek operating company water treatment plant in Windhoek, Namibia
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Date
2020
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Publisher
University of Namibia
Abstract
Natural organic matter (NOM) is a generic term for a mixture of organic, slightly water-soluble components found in soil, sediment and natural waters, which represents severe challenges for the process control in water treatment plants as well as water distribution systems. The effective removal of NOM during drinking water treatment requires a good understanding of its character. The current study aimed to identify and quantify NOM present in the Windhoek Goreangab Operating Company (WINGOC) water treatment plant and to assess the efficiency of the plant in removing or reducing these organic matters. Water samples were collected at different points in the treatment plant starting with the raw ponds until the final treatment point once a week over a period of twelve months. Temperature, pH, organic content, nitrates, UV absorbance at 254 nm (UV254) and specific UV absorbance (SUVA) measurements were performed on the day of sampling. The molecular weight distribution of the organic fractions was determined using high pressure size exclusion chromatography (HPSEC) with fluorescence and UV detection methods. Raw water (ponds) had high dissolved organic matter, however this was greatly reduced after the dissolved air flotation, biological and granular activated carbon treatment processes. Dissolved organic carbon (DOC) removal efficiencies of up to 98% were achieved in some cases, which indicates that the treatment process employed at WINGOC treatment plant is effective in removing organic matter from the water. UV absorbance at 254 nm was found to be directly correlated to the organic content, decreasing gradually as the water is passing through different treatment stages. The reduction in UV absorbance could be ascribed to the loss of aromaticity due to depolymerisation of high molecular weight (HMW) organic matter, as is normally observed in samples of natural waters containing high levels of humic substances. Water samples had specific UV absorbance values ranging between 2.88 and 4.80 Lmg-1m-1 before ozonation, while these values were less than 2.38 Lmg-1m-1 after ozonation. From these values, it could be concluded that the NOM fractions at the WINGOC treatment plant contained complex mixtures of humic- and non-humic substances with varying sizes and degrees of hydrophobicity/aromaticity as can be deduced from the high UV absorbance and SUVA data obtained for the treatment stages before ozonation, and these were converted to hydrophilic, low molecular weight organics with low UV absorbance and SUVA values after the ozonation process. The UV and SUVA results were well supported by the molecular weight distribution data obtained in size exclusion chromatography. No clear trends could be deduced with regard to seasonal changes.
The findings of this study provided an improved understanding on the character and fate of NOM during different water treatment processes. However, since the results were obtained for the bulk water samples, future studies should involve fractionation of the NOM to enable identification of individual compounds. Size exclusion chromatography (SEC) in combination with mass spectrometry (MS), nuclear magnetic resonance mass spectrometry (NMR) and/or Fourier transform infrared spectroscopy (FTIR) would be helpful in the determination of the chemical composition of the NOM.
Description
A mini thesis submitted in partial fulfilment of the requirements for the Degree of Master of Science in Chemistry
Keywords
Organic matter, Water treatment