A newly developed slurry-type titanium dioxide (TiO2) photocatalyst has been applied to the decomposition of pesticides in water. This photocatalyst is composed of the TiO2 nanoparticles deposited on the surface of a zeolite. Kinetic studies were performed on the photocatalytic decomposition using atrazine as the model pesticide. Reaction rate constants of 0.123, 0.161, and 0.512 min−1 for the TiO2 concentrations in the TiO2/zeolite complex of 0.025, 0.050, and 0.250 g/L, respectively, were obtained using an ultraviolet light source of 1 mW/cm2 at a wavelength of 350 nm. This showed that as the concentration of the TiO2 particles increased, the reaction rate constant also increased. In the prototype apparatus study, the concentration of atrazine in the treated water was maintained at 10 μg/L. Bubbling oxygen in the photoreactor was found to facilitate the decomposition of pesticides, indicating that the diffusion of oxygen gas on the TiO2 surface is the rate-determining step in the overall reaction process. The electric energy per order (EEO) under high oxygen concentration was 0.159 kWh/m3/order, which is comparable to other treatment methods.
Environmental impact assessments for wastewater treatment plants (WWTPs) have evaluated many endpoints including emissions of greenhouse gases, discharges of nutrients and discharges of toxic substances. The primary objective of this study was the development of an integrated environmental impact assessment model for wastewater treatment processes. The assessment model was based on an impact assessment methodology used in Japan for life cycle assessments. Specifically, eutrophication was taken into account in the model along with the impacts of free ammonia, because this chemical was known to have toxic effects on aquatic ecosystems. The model developed was then applied to an actual WWTP operating under two different conditions (case 1, without nitrification; case 2, with nitrification), and the best operating conditions were evaluated based on nitrogen emissions. The results showed that the main contributor to the environmental impacts of the WWTP in case 1 was ecotoxicity from discharges of NH4-N. In case 2, the main contributor was eutrophication from discharges of total nitrogen. These results demonstrated that the overall environmental impacts of WWTPs should decrease when nitrification is employed because this will reduce the impacts associated with the ecotoxicity of NH4-N.
Bacterial regrowth in reclaimed water, specifically observed when residual chlorine concentration declines along the distribution system, causes undesirable changes in water quality and hampers its acceptability. To study the impact of chlorination on regrowth and bacterial community structure, unchlorinated tertiary treated reclaimed water was collected and dosed with chlorine such that the initial doses were 1, 3, 5 mg-Cl2/L before being stored at ambient temperature under dark condition. Chlorine measurement, cell counts and bacterial community profiling were carried out at regular intervals for 21 days. Addition of chlorine caused rapid decline in intact cell concentration and no regrowth was observed until free chlorine decayed below detection limit (0.03 mg-Cl2/L). Upon regrowth, intact cell concentrations reached the initial level except in the case of 5 mg-Cl2/L where the intact cell concentration was lower by 1-log10. The dominant species that regrew under each condition were distinct, based on their capacity to withstand chlorine. The most chlorine-tolerant groups belonged to the order Sphingomonadales and Rhizobiales, which have been previously reported to initiate biofilms. This study demonstrates that chlorination selects specific bacterial groups which have the potential to regrow in the distribution network.