"Kinetic model of bacteria disinfection employing UV-C Radiation"

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

Buenos Aires

Conferencia; International conference. Monitoring systems and novel technologies for detection /removal of pollutants in/from ecosystems; 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 (figure 1). 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.  Fig. 1. Experimental set-up     Fig. 2. Calculated vs. measured bacteria concentrations(keys: ¡¼culture medium 1, ◊ culture medium 2)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. Thus, a complete mathematical model has been derived. 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). In figure 2, it can be seen a direct comparison between the model predictions and the experimental results. It includes all experimental runs performed with four different radiation intensities and different but diluted concentrations of the culture media.One of the most important conclusions of this work is the observed reaction order with respect to the volumetric rate of radiation absorption by the bacteria. This result may have very important economical consequences.