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

Efficiency of UV disinfection of unfiltered and filtered secondary wastewat er 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 un its reduction) when a wastewater UV transmission of 45% and an average effe ctive UV dose of 100 mW a cm(-2) were used. By contrast, removal of Pseudom onas aeruginosa appeared insufficient (<1.1 logarithmic units). Laboratory investigations, using the six bacterial species as reference, showed that t he kinetics of bacterial removal agreed with the model of Chick-Watson, whi ch is formulated by the equation dN/dt = -k'(CN)-N-n. It also appeared, tha t 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 inactivati on with relatively high doses. Each examined strain was characterised by tw o K coefficients, (K-1 and K-2). Lower the K value, more the bacterial resi stance 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 15 442 and Bacillus subtilis 6633, and a sensitive group, with the strain of E scherichia coli ATCC 11229. A medium group was represented by Enterococcus ATCC 19433, Serratia marcescens ATCC 8100 and P. aeruginosa S21. Kinetic st udies revealed that the first instants of exposure (2-10 s) to a UV intensi ty of 5-8 mW cm(-2) appeared as deciding factors in the disinfection with U V irradiation. The importance of cumulative UV dose was only secondary. Pho toreactivation of micro-organisms following UV disinfection was observed wi th all bacterial strains studied here, except for P. aeruginosa ATCC 15442, P. aeruginosa ATCC 15442 mutant mt and Enterococcus hirac ATCC 10541. (C) 2000 Elsevier Science Ltd. All rights reserved.