Effect of Sunlight on Coliphages in an Oxidation Pond

Escherichia coli B were used as host cells for isolation and multiplication of coliphages. The coliphages were isolated from a small canal which receives the effluent from an oxidation pond in Bangkok, Thailand, and were used as a virological indicator. Coliphages were assayed by the plaque forming technique (PFU method). The coliphage which was used in this study had no resistance to acid (pH3) and weak resistance to alkali (pH 10).The fate of coliphages in the oxidation pond was investigated by field measurement and laboratory experiments. Batch experiments showed the adsorption of coliphage to microbial particulates (mainly algae) occurred under aerobic conditions. The desorption of coliphage from the particulates was observed under anaerobic conditions. The adsorption-desorption process was reversible and was controlled by dissolved oxygen concentration. The same mechanism of adsorption-desorption was observed in the oxidation pond as well. During day-time, the concentration of coliphage decreased under high concentration of dissolved oxygen caused by photosynthesis. On the other hand, the concentration of coliphage increased after sunset because the dissolved oxygen concentration in the pond decreased to zero due to respiration of algae. The degree of removal of coliphage in the oxidation pond (design retention time = 20 days) was 90% (1 log). Field measurement with submerged bottles in the oxidation pond and sunlight-exposure experiment using beakers showed that coliphage could be inactivated by sunlight only near the water surface (less than 10 cm depth, the highest estimate) in the oxidation pond.