Evaluation of photoreactivation for thermotolerant coliforms and enterococci after disinfection of wastewater by ultraviolet radiation.
Abstract Text
Disinfection of wastewater with UV radiation is an efficient mean of control of the pathogen microorganisms. The disinfection mechanism by UV results from dimerization of thymine residues located in their DNA. The absorption of UV radiations by two adjacent thymines urge them to form a dimer. This phenomenon inhibits the transcription and the replication of DNA. Microorganisms can possess multiple pathways to repair UV-induced DNA damage. Nucleotide excision repair (dark repair) and photoreactivation are two major removal pathways used to correct the alteration. Aside from dark repair, many organisms repair damage through a light-dependent process; photoreactivation. This process aims to eliminate directly the lesions caused by the thymine dimers. During photoreactivation, a photolyase uses the visible light to catalyze a scission reaction of thymine dimers and thus, reform two distinct thymine residues. Coliform and E.coli bacteria, which are used as disinfection indicators, are likely to photoreactivate. It is important that wastewater treatment plants consider this phenomenon in order to meet the discharge standards. The ministry of environment of Quebec applies a 1log factor to compensate for photoreactivation. The present project attempts to confirm the photoreactivation of thermotolerant coliforms, E.coli and enterococci from UV disinfected wastewater and the appropriateness of the factor. Disinfected effluent of Fabreville wastewater treatment plant was used. Experiments were realised during the summer of 2002. Samples were exposed to artificial daylight. To evaluate effects of light intensity and time, samples were exposed at three luminosity levels (obscurity, low, high) at 25°C or at 4°C, and aliquots were taken at 0, 2, 4 and 6h. Some samples were kept in obscurity for 1h before they were placed under fluorescents. Thermotolerant coliforms, E.coli and enterococci were enumerated by membrane filtration in accordance to the “Standard methods for the analysis of water and wastewater”. Photoreactivation did not play an important role for enterococci, at any light intensities and temperatures. Photoreactivation was observed for thermotolerant coliforms and E.coli. Counts of E.coli, increased up to 1log after a period of 6h and under high light intensity (5600lux). Temperature influenced photoreactivation which was lower by 0,5log at 4°C than at 25°C. Several authors have demonstrated that E.coli photoreactivates and that concentration in water of these bacteria can increase with a 1log factor. This study has shown that coliform bacteria and especially E.coli bacteria in UV disinfected wastewater, are capable of photoreactivation up to a ten times factor depending on environnemental conditions. This observation confirms the validity of the factor imposed by the MENV. The use of non-photoreactivating enterococci as a disinfection indicator may become an interesting alternative to coliforms.
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