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 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 dN/dt=−k′CnN. 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, (K1 and K2). 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.