Incidence of Off-Flavors in South African Surface Waters

Incidents of odor in selected South African subtropical water bodies, the majority of which are sources of potable water, are presented. The presumed origin, prevalence and frequency of earthy/muddy off-flavors are described. Investigation into odor cases included the following: sensory and instrumental (gas chromatography/mass spectrometry) analyses of odorous water samples, enumeration and identification of cyanobacteria, algae and actinomycetes present in the samples and attempts to establish cultures of responsible microorganisms for confirmation of odor production. The possibility of concomitant production of toxins by odor-forming cyanobacteria is also discussed. Incidence of off-flavors of 30% was established. All affected reservoirs were dominated by cyanobacteria, mainly Microcystis aeruginosa. Odorous substances detected in water were geosmin and dimethyl trisulphide. Efforts to isolate odor-producing organisms in culture were unsuccessful. From individual case histories studied it is clear that odor incidents in man-made lakes present a serious problem in South Africa. The warmer climatic conditions and seasonally fluctuating water level create additional factors for odor formation, which are different from those experienced in more temperate regions.