Disinfection of municipal wastewater by sensitized photooxidation

Abstract

To optimize the disinfection of treated municipal wastewater by sensitized photo-oxidation as a new alternative disinfection method, secondary unchlorinated activated-sludge-treated effluents from a sewage treatment plant were treated by semiconductor-sensitized photo-oxidation. Experiments of disinfection were performed in a photo-reactor system, which is a cylindrical borosilicon glass reactor with an effective volume of 1.1 litre. A NEC black light lamp (T10 20W) is placed at the centre of the reactor as an artificial light source to provide near UV irradiation with a maximum intensity at 350 nm. Hydraulic retention time (HRT) was used up to 60 minutes in this study. The effluents, after adding titanium dioxide as sensitizer, were exposed to the illumination of near UV light for various irradiation time and incident light intensities. Number of bacteria and coliforms were significantly declined from 35,000/100 ml to 59/100 ml after 60 minutes treatment, with existing of titanium dioxide, light irradiation and aeration. After the photo-oxidation process, the titanium dioxide in the treated effluent was separated by precipitation. The influence of light energy from 5.2 to 45 W/mᒾ, absorbed by the sensitizer in the visible range, on micro-organisms in secondary effluents was determined by testing for Escherichia. coli (E. coli) and total coliforms. An optimum titanium dioxide concentration found in the test is about 1 g/L. While the destruction rate of E. coli was affected by pH, dissolved oxygen (DO) and temperature (T), it was found that number of bacteria destroyed during disinfection mainly depends on incident light intensity (I₀) and irradiation time (t), which follows an exponential relationship. A kinetic model has been developed using the product of I₀ and t defined as a Dose Number (D) for this disinfection reaction.