Wastewater Disinfection by UV Irradiation: Short and Long-Term Efficiency

Experimental trials were carried out at pilot scale using a UV system, fed on nitrified and filtered effluent. Irradiation doses up to 80 mJ/cm² were tested to evaluate UV efficiency with respect to initial wastewater characteristics and to the Italian regulations for discharge and reuse. Regrowth tests were performed to quantify dark repair and photoreactivation 6 h after the collection of disinfected samples. Coliform bacteria (total and fecal coliforms, Escherichia coli) and total heterotrophic bacteria (THB) were monitored by traditional plate count methods. THB were also enumerated by cytometry, evaluating bacteria viability on the basis of their cell membrane integrity. For UV doses up to 20-30 mJ/cm², a linear correlation between UV applied dose and microbial inactivation was observed, while for the whole set of data the best fitting model was the Oppenheimer relationship. The tailing-off trend of inactivation with increasing UV doses was considered as not due to total suspended solids (TSS) concentration (maximum measured value: 13.2 mg/L, average value: 2.5 mg/L), but to low initial bacterial counts. UV doses in compliance with Italian microbiological standards for discharge and reuse (reference microorganism: E. coli) were defined by a statistical approach, supported by Monte Carlo simulation. The standard for discharge into surface water (E. coli: 5,000 CFU/100 mL) was easily met even at low doses (10 mJ/cm²). Higher doses (30 mJ/cm²) were needed for the more stringent limit for reuse (E. coli: 10 CFU/100 mL in 80% of the samples). No dark repair was observed for any bacterial indicator. Photoreactivation was appreciable for coliform group bacteria at UV doses below 40 mJ/cm² and decreased with increasing doses; photoreactivation was also observed for THB but to a lesser extent. © Mary Ann Liebert, Inc. 2008.