Water samples from four different sources of varying physico-chemical and microbial quality with their naturally occurring microorganisms were exposed to sunlight in polyethylene terephthalate bottles under similar conditions. Up to 3-log10 reduction of total coliforms (TC) was observed during a 6-h exposure period under weak/moderate radiation conditions (<600 W/m2). Complete inactivation of TC was not achieved in 6 h of exposure for waters with larger initial TC such as river water (1 × 103 most probable number [MPN]/100 mL) and treated municipal wastewater (2 × 105 MPN/100 mL) under these conditions. Heterotrophic bacteria showed lower inactivation rates than did TC. The inactivation rate for spiked Escherichia coli was faster than for naturally occurring coliforms. Further tests with compound parabolic collectors showed that complete inactivation of naturally occurring TC could be achieved within 6 h of exposure for all the natural waters tested. The results of the study thus indicate the need to use naturally occurring organisms in testing the effectiveness of solar disinfection, and the importance of source quality on the inactivation rates of microorganisms.

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