Environmental impacts assessment of wastewater treatment and sludge disposal systems under two sewage discharge standards: a case study in Kunshan, China

Abstract

To improve water quality in cities, the Chinese government has raised the discharge standard in many areas from class 1B to the more stringent 1A. Therefore, sewage treatment plants must ramp up their advanced treatment. Sludge disposal system is an extension of the sewage treatment system and has a significant impact on the effectiveness of sewage treatment. The environmental impacts of two sewage treatment plants and a sludge incinerator plant in Kunshan, China were evaluated using the life cycle assessment method, and the results of the two standards were compared under four scenarios. Our results show that improving sewage discharge standards can reduce eutrophication potential of the two systems by 4% and 14%, respectively, but the impacts on fossil energy depletion, global warming potential, human toxicity, freshwater ecological toxicity, and acidification potential are increased by 40% to more than 100 times. Further analysis reveals that it is necessary to decrease the moisture content of the dewatered sludge from 80% to 60%, because it has a significant impact on fossil energy depletion. In addition, physical methods should be prioritized over chemical agents for the advanced treatment and clean energy should be used in order to minimize trade-offs with other environmental impacts. The efficiency of energy recovery in the sludge disposal system is critical to the total environmental impact of the entire system which offers opportunities for improvements.