"Water Disinfection with UV Radiation: A Kinetic Study"

MARISOL LABAS; CARLOS A. MARTIN; ALBERTO E. CASSANO

Barcelona (España)

Congreso; SPEA 3. 3rd European Meeting on solar Chemistry and Photocatalysis: Environmental Applications; 2004

Microbiological contamination of water has always been a problem of general concern. Among the proposed ways to deal with this type of pollution without the known difficulties associated with the use of chlorine, UV radiation is an established technology.This work presents a detailed kinetic study of the rate of removal of bacteria employing UV radiation (253.7 nm) under the following conditions: (i) the indicator microorganism is Escherichia coli, (ii) temperature is 20¢X C, (iii) four different irradiation levels are employed (iv) the initial concentration of bacteria varies from 105 to 108 CFU cm-3, (v) two different culture medium are used and (vi) the concentration of the culture medium varies between 10-3 to 10-6 g cm-3. The three last variations were introduced to simulate typical effluents from food industries and clean waters for domestic use.The study was carried out in a well-stirred reactor of cylindrical shape irradiated from both flat sides with two tubular lamps placed in the axial position of two parabolic reflectors. The reactor was operated in a recycle (VR= 70 cm3). The reactor was operated in a recycle, including a well-stirred tank (VT = VR + VTk) where sampling and temperature control was made. The concentration evolution was analyzed employing the plate count method with Petrifilm„§ specific plates. The Incident Radiation at the reactor windows was measured with ferrioxalate actinometry.The kinetic model is a modification of that originally proposed by Severin1 with a Series Events mechanism in the following aspects: (i) it employs the true value of the radiation absorbed by the microorganisms which may change significantly in the reaction space, (ii) it does not assume that the disinfection rate is of first order with respect to the absorbed radiation, (iii) it includes the growing rate of the bacteria during the treatment time and (iv) the reaction space has a non-uniform distribution of radiation due to the characteristics of the bacteria¡¦s growing medium. Thus, a complete mathematical model has been derived. It is considered that every level of bacteria damage along the disinfection process represents a different species. The kinetic constants of the model were obtained using a multiparameter optimization program. They are: n = 2 (damage threshold); k=1.3142 x 102 s-1 (cm3 s/ Einstein)m (inactivation constant), m = 0.205 (the reaction order with respect to the absorbed radiation by the bacteria) and  k* = 1.50 x 102 (CFU g-1 s-1) (the growing constant of the bacteria).