Alternative Water Disinfection Processes: UV inactivation and Photorepair of Escherichia coli in photochemical reactors

A.R. TROMBERT, H. IRAZOQUI, F. ZALAZAR, Y C. MARTÍN

Islas canarias, España

Congreso; 4th European Meeting on Solar Chemistry and Photocatalysis: Environmental Applications SPEA4; 2006

Although chemical disinfection with chlorine has successfully protected public health against waterborne disease since the early 1900s, the reaction between chlorine and organic compounds can result in the production of DBPs, some of which have been shown to have mutagenic properties. In pursuit of alternatives for drinking water treatments there has been an increased use of ultraviolet (UV) radiation, alone as well as combined with H2O2 and O3 (UV 253.7 nm/H2O2/O3) or TiO2(UV < 380 nm /TiO2). The main advantages of these Advanced Oxidation Processes are the absence of secondary effects on both  taste and odour (usually found using chemical disinfectants) and the effective inactivation of waterborne pathogens resistant to traditional chemical disinfection (i.e. Crysptosporidium and Giardia).            There are several differences among the effects of the inactivation processes. The lethal effect of the germicidal UV light is mainly due to the formation of cis-syn cyclobutane pyrimidine dimers (Pyr<>Pyr) between two adjacent thymine bases in DNA. These lesions produce cell injury, inhibition of the normal replication and loss of viability. On the other hand, in the photocatalytic inactivation mechanism, the oxidative atacck by hydroxyl radicals founds three sequential targets: 1) the cell wall, 2) the cell membrane and 3) the cytoplasmatic content. Another main difference lies on the fact that microorganisms posses DNA repair systems (photoreactivation, excision repair, recombination repair and SOS repair) which permit them recovery from sub-lethal UV induced damage. Some of these repair mechanisms would be promoted by UV and visible radiation, such as those included in  the solar spectrum.            Some investigations on the  UV disinfection process focused on the development of a plausible kinetic model to be used as a tool for reactor design. However, the detection of ?anomalous behavior? in the plot of bacterial concentration as a function of irradiation time, such as tailing and plateau needs to be clarified because this phenomenon would be associated with the capability of bacteria to reactivate and regrowth.            This study investigate the response of an Escherichia coli ATCC 25299 strain in a lab scale annular batch photoreactor, provided with a 15 W low-pressure mercury lamp (253.7 nm, UV-C). At a certain time this germicidal lamp was interchanged with a low-pressure mercury vapour discharge lamp whose inner envelope is coated with a fluorescent powder, transmitting UV-A radiation and only minimum visible radiation.            Besides the standard measurements typically used in this kind of experimental research (bacteria plate count, medium optical characterization, radiation field modeling), in this work molecular biology techniques were applied to improve the interpretation of experimental data. The samples (suspensions taken from the reactor as well as colonies developed on LB agar after incubation at 37°C for 24 hs) were studied with the Rep (Repetitive extragenic palindromic) ? PCR (polymerase chain reaction) technique.