ULTRAVIOLET DISINFECTION OF WASTEWATER: PHOTOREACTIVATION AND PROTECTION OF WATER-BORNE BACTERIA BY SUSPENDED MATERIAL AS FACTORS LIMITING TREATMENT EFFICIENCY (BIOASSAY, INDICATOR ORGANISM, DARK REPAIR)

Ultraviolet disinfection followed by sedimentation was investigated as a secondary disinfection technology. Field studies were conducted to evaluate the feasibility of removing water-borne bacteria and suspended solids from wastewater lagoon effluents by UV/sedimentation. A field scale UV/sedimentation research facility (capacity of approximately 275 m('3)/day) consisting of a teflon-tube type UV disinfection unit followed by a rectangular sedimentation basin was operated for 4 months in Logan, Utah, using a lagoon effluent. The two major factors influencing disinfection efficiency were protection from UV radiation by suspended material in the wastewater lagoons and reactivation of UV-irradiated bacteria within the basin.; The identification of the bacterial population surviving UV radiation and growing on KF streptococcal agar showed the presence of the family of Micrococcaceae (Micrococcus luteus, Staphylococcus saprophyticus), and Bacillaceae in addition to fecal streptococci. Fecal streptococci, commonly used as indicator organisms were not adequate for determination of UV disinfection efficiency, having little resistance to UV light. Micrococcus luteus was isolated from wastewater samples after UV light irradiation and was chosen as an indicator organism due to its very high resistance to UV light (50,000 (mu)W-sec/cm('2) resulted in 5.3 log reduction).; During laboratory studies, a dose-response curve for M. luteus was determined using a collimated beam apparatus for UV irradiation. The bioassay results served for further evaluation of the ultraviolet dose rate emitted by the UV reactor used in the field studies.; Studies on bacterial recovery after radiation were conducted using Micrococcus luteus ATCC 381. UV-irradiated bacteria in replicate samples were alternatively exposed to sunlight wavelengths of 300-700 nm, or kept in the dark for 2 hours. Dose response curves showed that dark repair resulted in an average of 0.25 log increase of survival, while photoreactivation increased survival an average of 0.4 log units with the maximum of 1.0 log.; Particle protection studies were also performed using wastewaters containing different size particles. A 0.3 log increase in bacterial survival was observed in raw wastewater compared to that in the filtered wastewater.; The impact of radiation protection and recovery on the overall disinfection efficiency are discussed in relation to the design of disinfection systems for providing disinfected wastewater effluents.