The occurrence and removal of algae (including cyanobacteria) and their related organic compounds from source water in Vaalkop Dam with conventional and advanced drinking water treatment processes

A Swanepoel,Hh Du Preez,N Cloete

doi:10.4314/wsa.v43i1.10

A Swanepoel, Hh Du Preez + Show 1 more

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https://doi.org/10.4314/wsa.v43i1.10

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Journal: Water SA	Publication Date: Feb 1, 2017
Citations: 16	License type: cc-by

Affiliation: Mintek, University of Johannesburg

Abstract

Cyanobacterial bloom formation in freshwaters, such as rivers, lakes and dams, is known to occur throughout the world. The Vaalkop Dam, which serves as source to the Vaalkop drinking water treatment works (DWTW), is no exception. Blooms of cyanobacteria occur annually in Vaalkop Dam as well as in dams from which Vaalkop is replenished during low-rainfall periods. These blooms during the summer months are associated with the production of cyanotoxins and taste and odour compounds such as geosmin and MIB. The Vaalkop DWTW uses a combination of conventional and advanced water treatment processes to deal with the cyanobacteria and their related organic compounds in the source water. The overall objectives of this study were to: (i) investigate the occurrence of algae (including cyanobacteria) and cyanotoxins in the Vaalkop Dam; (ii) establish which environmental variables are responsible for the development and onset of algae, and (iii) determine whether the Vaalkop DWTW is able to eliminate the influence that algae (including cyanobacteria) may have on the drinking water. Multivariate statistical analyses revealed seasonal variation in algal assemblages in the raw water. The risk of cyanobacteria bloom formation proved to be especially high during the summer months as the nutrients needed for cyanobacterial growth are available and the temperature range is optimal, causing the production of geosmin, MIB and microcystin. The presence of Ceratium hirundinella (O.F. Muller) Dujardin, in the source water appears to exacerbate the negative effect that cyanobacteria have on the drinking water treatment process. When Ceratium hirundinella is present, floc formation is inhibited, causing more of the problematic algae to penetrate into the drinking water. Even though advanced treatment options such as ozone and granular activated carbon filters are used at Vaalkop DWTW, the effects of the organic compounds produced by cyanobacteria are not entirely eliminated during the treatment process. Keywords : potable water treatment, geosmin, MIB (2-methylisoborneol), microcystin, granular activated carbon (GAC), ozone

Highlights

IntroductionHuman influences due to anthropogenic activities (which include agricultural, social and economic activities) have increased the rate of eutrophication of source waters due for drinking water treatment (Strydom and King, 2009)
Human influences due to anthropogenic activities have increased the rate of eutrophication of source waters due for drinking water treatment (Strydom and King, 2009)
Extensive growth of cyanobacteria in reservoirs or dams used as sources for potable water production can create severe problems such as ineffective coagulation, flocculation and sedimentation (Ewerts et al, 2013), clogging of sand filters (Steynberg et al, 1998), penetration of algae into the final treated water (Swanepoel, 2015), increase of organic loading in water (Ferreira and Du Preez, 2012) and the production and release of cyanotoxins as well as taste and odour compounds (Oberholster et al, 2005; Du Preez and Van Baalen, 2006; Du Preez et al, 2007; Swanepoel, 2015)

Summary

Introduction

Human influences due to anthropogenic activities (which include agricultural, social and economic activities) have increased the rate of eutrophication of source waters due for drinking water treatment (Strydom and King, 2009). Extensive growth of cyanobacteria in reservoirs or dams used as sources for potable water production can create severe problems such as ineffective coagulation, flocculation and sedimentation (Ewerts et al, 2013), clogging of sand filters (Steynberg et al, 1998), penetration of algae into the final treated water (Swanepoel, 2015), increase of organic loading in water (Ferreira and Du Preez, 2012) and the production and release of cyanotoxins as well as taste and odour compounds (Oberholster et al, 2005; Du Preez and Van Baalen, 2006; Du Preez et al, 2007; Swanepoel, 2015) These problems in the purification process may lead to undesirable cyanotoxins as well as taste and odour compounds in the drinking water. Since cyanobacteria require light for growth, they are more likely to be found in surface waters, but they may be transported into groundwater (Falconer, 2005)

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