Inactivation of faecal indicator microorganisms in waste stabilisation ponds: Interactions of environmental factors with sunlight

Sunlight exposure is considered to be the most important cause of "natural" disinfection in waste stabilisation ponds (WSPs). We examined the influenc e of dissolved oxygen (DO), pH, and particulate and dissolved constituents in WSP effluent, on sunlight inactivation of faecal micro-organisms, using small reactors operated under controlled physico-chemical conditions. Inact ivation of both enterococci and F-DNA phages increased strongly as DO was i ncreased, and also depended on light-absorbing pondwater constituents, but pH was not influential over the range investigated (7.5 to 10). Inactivatio n of E. coli increased strongly when pH increased above 8.5, as well as bei ng strongly dependent on DO. Inactivation of F-DNA phage was independent of the factors investigated. These results are consistent with the F-DNA phag es being inactivated as a result of direct DNA damage by UVB in sunlight, w hereas the other three microbiological indicators are inactivated as a resu lt of photo-oxidative damage, although the target of damage is apparently d ifferent. Our findings of diverse influences of physico-chemical conditions suggest difficulties in interpreting data for a single micro-organism to i ndicate WSP effluent quality. However, sunlight remains the factor of over- riding importance, and disinfection in WSPs may be enhanced by increasing s unlight exposure. (C) 1999 Elsevier Science Ltd. All rights reserved.