SUNLIGHT INACTIVATION OF ENTEROCOCCI AND FECAL-COLIFORMS IN SEWAGE EFFLUENT DILUTED IN SEAWATER

Inactivation (loss of culturability) by sunlight of enterococci and fe cal coliforms within sewage effluent diluted in seawater was investiga ted in held experiments. In most experiments, 500-ml flasks of pure si lica were used to confine activated sludge effluent diluted to 2% (vol /vol) in seawater. Inactivation of bacteria in these basks (diameter, 0.1 m) was faster than in either open chambers (depth, 0.25 m) or patc hes of dyed effluent (depth of order, 1 m), probably because of the lo nger light paths in the latter two types of experiment, which caused g reater attenuation of sunlight. Inactivation of 90% of enterococci gen erally required 2.3 times the insolation required for 90% inactivation of fecal coliforms, because of both the presence of larger initial sh oulders on survival curves and a lower final inactivation rate. Two pa rameters are required to model inactivation of enterococci, a shoulder constant as well as a rate coefficient. The depth dependence of inact ivation rate for both fecal indicators matched the attenuation profile of UV-A radiation at about 360 nm. Inactivation by UV-B radiation (29 0 to 320 nm), which penetrates much less into seawater, is of minor im portance compared with the UV-A and visible radiation in sunlight, con trary to expectations in consideration of published action spectra for bacterial inactivation.