Technology and Innovation for Sustainable Development Conference (TISD2006) Faculty of Engineering, Khon Kaen University, Thailand 25–27 January 2006 Manuscript Preparation Guidelines for the TISD2006 Conference Chomsri Siriwong 1 and Kanyarat Holasut 2* 1 Division of Chemistry, Faculty of Science, Rajamangala University of Technology (Khon Kaen Campus), Khon Kaen 40000 2 Department of Chemical Engineering, Faculty of Engineering, Khon Kaen University, Khon Kaen 40002 E-mail: kanyarat@kku.ac.th * Abstract In most developing countries, contaminated water is the major cause of most water-borne diseases. Solar energy can be used effectively for drinking water treatment because inactivation of microorganisms can be done either by heating water to a disinfecting temperature or by exposing to ultraviolet (UV) solar radiation. A series of experiment was studied to characterize the solar disinfection (SODIS) process of water stored in transparent plastic bottles that were then exposed to sunlight. Thermal inactivation was found to be important only at water temperatures above 40 o C at which point a synergy between thermal inactivation and UV irradiation process was observed. The disinfection kinetic model which taking into account of the synergy or the coupling effect between thermal inactivation and UV irradiation was developed and verified by experimental results, finally the decoupling effect was quantified to be 0.5665 for the present study. Keywords: SODIS, solar disinfection, coupling effect, drinking water 1. Introduction Drinking water disinfection has been a major contributor to the reaction if water-borne deceases in people living in rural communities in the developing world. In such communities, however, there are often neither the finance nor the resource to construct and maintain the type of water disinfection processes used in more industrialized countries, which tend to be either energy or chemical intensive. Hence, an appropriate water disinfection process, which uses locally available resource and can be constructed of low cost and low maintenance, is required in such situations. Solar disinfection (SODIS) seems to meet these requirements. In this process of drinking water, the drinking water is stored in 1.5-litre plastic (Polyethylene Terepthalate; PET) bottles that are placed in direct sunlight for continuous periods of not less than 6 hours in order to inactivate microorganisms contained in the water effectively. It has been reported from previous research that the most microorganisms are killed when exposed to temperatures of over about 70 o C for a certain period of time [13] and also when exposed to ultraviolet (UV) solar radiation [10]. In a detail study of SODIS systems, Sommer et al. [8] have shown that the SODIS systems can reduce culturable bacteria 3.0 log units after 140 minutes of direct sunlight exposure. This effect has been attributed to heating and UV radiation. Furthermore, Wegelin et al. [9] have found that photochemical ageing of the bottles does not change the quality of the water stored in the bottles with regard to aldehyde, organic photoproduct, additive or phthalate concentrations. Their studies also suggest that PET bottles are excellent containers for soft drinks that might be exposed to sunlight over extended periods of time. Recently, McGuigan et al. [4], [7] have found that the combined exposure to UV radiation plus solar thermal in the SODIS process has a synergistic effect on microbial inactivation, producing greater inactivation than predicted to either one of the two agents alone. However, no attempt has been made to theorize or quantify the synergistic effect apart from reporting the effect. The application of SODIS process is simple, but the interaction between UV radiation and solar thermal is not yet clearly understood. 2. The development of kinetic model for thermal disinfection The Chick-Watson model [1] which describes the first order kinetics of disinfection process, can be expressed as: 0 Kt t N e N − = (1) which can be written in the more similar term as: 0 ln t N Kt N ⎛ ⎞ = − ⎜ ⎟ ⎝ ⎠ (2) This equation describes the linear survival curve, which can be used as the first-order approximation for all types of disinfection processes. However, in the case of thermal disinfection the death of microorganisms is caused by inactivation of some critical enzyme (or enzyme system). Therefore, the killing rate is temperature dependent. According to