A GIS-Based Estimation of Comparative Occurrence and Fate of Waste-Related Protozoan Pathogens in a Peri-Urban Canal Network in a Tropical Environment

摘要

The role of extensive Thai canal network (16-21 km in length) is scrutinized and evaluated in terms of wastewater treatment, pathogen removal and pollution mitigation. It is a pioneering effort for a tropical country. Three different canals, namely: (ⅰ) one receiving municipal wastewater, (ⅱ) one situated in agricultural area, and (ⅲ) one receiving nearly almost only industrial wastewater were investigated. Hydraulic regimes and pollution loads were studied using conventional environmental techniques, as well as geographic information systems (GIS). The study demonstrated that during rainy season the canals are free-flowing water bodies that can be compared to rivers, while in dry season they behave as ponds. The field concentrations of the protozoan pathogens {Cryptosporidium and Giardia) were monitored by realtime PCR for more than 12 months. Despite the highly polluted status of the canals, predominantly caused by point pollution sources (0.04 kg BOD/m~3/day), a 70% BOD5 and 4 log_(10) pathogen removal have been achieved. The main removal factor was sedimentation and, to an extent, solar irradiation. Cryptosporidium removal appeared to be accomplished by other mechanisms (potentially, predation by protozoa and rotifers) since no oocysts were detected in the sediments. Genotyping studies revealed the presence of both human and animal genotypes of the protozoan pathogens. Quantitative microbial risk analysis and Monte Carlo risk simulations showed that single exposures to the canal waters present risks of infection greater than both the yearly accepted risk by the World Health Organization and the yearly reported risk in Thailand.