UV disinfection of treated wastewater in a large-scale pilot plant and inactivation of selected bacteria in a laboratory UV device

Abstract

Efficiency of UV disinfection of unfiltered and filtered secondary wastewater effluent, using a large-scale pilot system, and the inactivation of six bacterial species in a laboratory UV-device have been studied. Pilot plant studies revealed low levels of coliforms and streptococci (3 logarithmic units reduction) when a wastewater UV transmission of 45% and an average effective UV dose of 100 mW s cm −2 were used. By contrast, removal of Pseudomonas aeruginosa appeared insufficient (<1.1 logarithmic units). Laboratory investigations, using the six bacterial species as reference, showed that the kinetics of bacterial removal agreed with the model of Chick–Watson, which is formulated by the equation d N/d t=− k ′ C n N. It also appeared, that according to the UV dose applied, two types of inactivation prevailed: a high rate of inactivation, with weak UV doses, and a low rate of inactivation with relatively high doses. Each examined strain was characterised by two K coefficients, ( K 1 and K 2). Lower the K value, more the bacterial resistance to UV. Hierarchical classification of bacteria, based on the values of kinetic constants, revealed three groups of different sensitivity to UV. A resistant group, represented by the two strains of P. aeruginosa ATCC 15442 and Bacillus subtilis 6633, and a sensitive group, with the strain of Escherichia coli ATCC 11229. A medium group was represented by Enterococcus faecalis ATCC 19433, Serratia marcescens ATCC 8100 and P. aeruginosa S21. Kinetic studies revealed that the first instants of exposure (2–10 s) to a UV intensity of 5–8 mW cm −2 appeared as deciding factors in the disinfection with UV irradiation. The importance of cumulative UV dose was only secondary. Photoreactivation of micro-organisms following UV disinfection was observed with all bacterial strains studied here, except for P. aeruginosa ATCC 15442, P. aeruginosa ATCC 15442 mutant m1 and Enterococcus hirae ATCC 10541.