Efficacy of conventional drinking water treatment processes in removing problem-causing phytoplankton and associated organic compounds

He Ewerts,Hh Du Preez,A Swanepoel

doi:10.4314/wsa.v39i5.19

Abstract

Seven phytoplankton groups were recorded in the source water supplied to South Africa’s largest conventional drinking water treatment plant (DWTP). Two phytoplankton genera, Anabaena and Ceratium were identified as the problem-causing phytoplankton due to their ability to interfere with the water treatment process and negatively impact on water quality. The objectives of this study were to identify problem-causing phytoplankton genera and investigate the efficacy of unit processes in removing phytoplankton genera and associated organic compounds. Phytoplankton and organic compound data were obtained from four different sampling localities throughout the treatment plant and statistically analysed to evaluate the removal efficiencies of unit processes. The highest percentage removal for the Cyanophyceae average seasonal concentration (> 1 000 cells/mℓ) was recorded at 98%, while the highest percentage removal for the Dinophyceae average seasonal concentration (± 9 cells/mℓ) was recorded at 100%. Microcystis and Anabaena were removed by the processes of coagulation, flocculation and sedimentation (> 95%), while Ceratium cells were removed by sand filtration (> 80%). Ineffective removal of Ceratium by coagulation, flocculation and sedimentation (and subsequent penetration to the sand filtration step) will negatively impact on filter run times when these phytoplankton genera are present in high concentrations in the source water. Total photosynthetic pigments (TPP) were removed effectively by all the different water treatment processes. Not enough statistical evidence could be displayed to suggest effective removal of geosmin in this conventional water treatment plant. With good removal of intact cyanobacteria cells during coagulation, flocculation and sedimentation, geosmin concentrations in the final water could be kept to accepted organoleptic levels of 5–10 ng/ℓ in the final water. Optimising conventional drinking water treatment processes can effectively remove problem-causing phytoplankton as well as their associated organic compounds and thereby reduce the potential risk to drinking water consumers.Keywords: coagulation, sand filtration, drinking water, Ceratium, Anabaena, Microcystis, geosmin

Highlights

Pollution and eutrophication lead to the presence of high concentrations of organic and inorganic compounds, which enhance phytoplankton blooms and concomitantly decrease water quality (Venter et al, 2003; Heisler et al, 2008 and Li et al, 2011)
Source water supplied to the conventional water treatment plant contains a phytoplankton community that consists of 7 major groups, which are sub-divided into genera
The Cyanophyceae group was identified as the dominant group, which consisted of Anabaena, Microcystis and Oscillatoria as well as the genus Ceratium, belonging to the Dinophyceae group

Summary

Introduction

Pollution and eutrophication lead to the presence of high concentrations of organic and inorganic compounds, which enhance phytoplankton (including Cyanophyceae) blooms and concomitantly decrease water quality (Venter et al, 2003; Heisler et al, 2008 and Li et al, 2011). The occurrence of these blooms in the source water for drinking water production is of critical importance to drinking water providers as phytoplankton can have both a physical impact (e.g. clogging of filters) and chemical impact (e.g. production of cyanotoxins, disinfection by-products and taste and odour compounds) on the treatment process (Du Preez et al, 2007; Merel et al, 2010). Blooms with high cell numbers can have a significant effect on the formation of disinfection byproducts after oxidation with chlorine (Rositano et al, 2001; Rodríguez et al, 2007; Zamyadi et al, 2011 and Zamyadi et al, 2012a)

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Conclusion