Life-cycle Environmental And Economic Assessment Of Sewage Sludge Treatment In China

A cost-combined life-cycle assessment was conducted to estimate the environmental and economic burdens of 12 sewage sludge-treatment scenarios in China. Results showed that for all scenarios, the impact observed from human toxicity and marine ecotoxicity served an important function. The impacts observed from freshwater eutrophication, terrestrial ecotoxicity, freshwater ecotoxicity, and climate change had small contributions, whereas those observed from the other categories could be neglected. Anaerobic digestion contributed the most to climate change, whereas gravity thickening, drying and dewatering served insignificant functions in the overall environmental burden. Anaerobic digestion was a suitable alternative to reduce both environmental and economic burdens because this approach reduced dry mass volume and applied energy recovery.Landfill and incineration technologies had the highest and lowest environmental burdens, respectively. Direct mercury and lead emissions generated from landfill process significantly contributed to human toxicity, whereas direct vanadium emissions generated from incineration process served an important function in marine ecotoxicity. However, energy recovery from the landfill and incineration stages was important to reduce both environmental and economic burdens. A linear correlation among electricity recovery capacity, midpoint score for human toxicity, and landfill and incineration costs was observed. The potential effect on human toxicity and total cost for landfill and incineration decreased with increasing electricity recovery capacity. The linear relation showed that a 100 kWh/T-DS increase in electricity recovery decreased the total cost and human toxicity potential score by approximately USD 8.2 and 23.5 kg 1,4-DB eq in both the landfill and incineration stages, respectively.By assessing the entire life cycle of all scenarios with and without anaerobic digestion, the costs of raw materials and electricity were found to serve a significant function. Besides, scenarios with anaerobic digestion had much lower life cycle costing than those without anaerobic digestion. This finding was mainly attributed to the fact that during anaerobic digestion stage, the degradation rate of organics is high (approximately 50%). Thus, the volume of sewage sludge is significantly reduced. Electricity recovery significantly contributed to the scenarios with anaerobic digestion. The sensitivity analysis was also conducted. The Impact 2002+ method was used for comparisons with the ReCiPe method, results showed that using the ReCiPe method to assess the environmental impacts was reliable as far as the current study was concerned. The sensitivity analysis of dominant contributors showed that incineration and landfill had the highest and lowest environmental benefits, which is consistent with the LCIA results. The heavy metal emissions in the agricultural use stage should meet the environmental capacity requirement.This study indicated that a sewage sludge-treatment scenario with anaerobic digestion, dewatering, and incineration technologies was the most environmentally and economically suitable method to treat sewage sludge because of energy recovery. All new sewage treatment plants should be constructed to operate according to this method, and existing plants should be retrofitted.