Comparison of municipal solid waste treatment technologies from a life cycle perspective in China.

China has endured the increasing generation of municipal solid waste; hence, environmental analysis of current waste management systems is of crucial importance. This article presents a comprehensive life cycle assessment of three waste treatment technologies practiced in Hangzhou, China: landfill with and without energy recovery, and incineration with waste-to-energy. Adopting region-specific data, the study covers various environmental impacts, such as global warming, acidification, nutrient enrichment, photochemical ozone formation, human toxicity and ecotoxicity. The results show that energy recovery poses a positive effect in environmental savings. Environmental impacts decrease significantly in landfill with the utilization of biogas owing to combined effects by emission reduction and electricity generation. Incineration is preferable to landfill, but toxicity-related impacts also need to be improved. Furthermore, sensitivity analysis shows that the benefit of carbon sequestration will noticeably decrease global warming potential of both landfill scenarios. Gas collection efficiency is also a key parameter influencing the performance of landfill. Based on the results, improvement methods are proposed. Energy recovery is recommended both in landfill and incineration. For landfill, gas collection systems should be upgraded effectively; for incineration, great efforts should be made to reduce heavy metals and dioxin emissions.

Life cycle and economic assessment of source-separated MSW collection with regard to greenhouse gas emissions: a case study in China.

In China, the continuously increasing amount of municipal solid waste (MSW) has resulted in an urgent need for changing the current municipal solid waste management (MSWM) system based on mixed collection. A pilot program focusing on source-separated MSW collection was thus launched (2010) in Hangzhou, China, to lessen the related environmental loads. And greenhouse gas (GHG) emissions (Kyoto Protocol) are singled out in particular. This paper uses life cycle assessment modeling to evaluate the potential environmental improvement with regard to GHG emissions. The pre-existing MSWM system is assessed as baseline, while the source separation scenario is compared internally. Results show that 23 % GHG emissions can be decreased by source-separated collection compared with the base scenario. In addition, the use of composting and anaerobic digestion (AD) is suggested for further optimizing the management of food waste. 260.79, 82.21, and -86.21 thousand tonnes of GHG emissions are emitted from food waste landfill, composting, and AD, respectively, proving the emission reduction potential brought by advanced food waste treatment technologies. Realizing the fact, a modified MSWM system is proposed by taking AD as food waste substitution option, with additional 44 % GHG emissions saved than current source separation scenario. Moreover, a preliminary economic assessment is implemented. It is demonstrated that both source separation scenarios have a good cost reduction potential than mixed collection, with the proposed new system the most cost-effective one.

Co-destruction of organic pollutants in municipal solid waste leachate and dioxins in fly ash under supercritical water using H2O2 as oxidant.

Supercritical water oxidation (SCWO), with hydrogen peroxide as oxidant, is applied to the co-disposal of two distinct waste streams: municipal solid waste leachate and incineration fly ash. The chemical oxygen demand (COD) removal efficiency increases rapidly with rising temperature and excess oxygen. Rising residence time from 1 to 2 min has surprisingly little effect. The addition of fly ash accelerates COD conversion markedly and also polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs, dioxins) in the original fly ash are efficiently destroyed. High-chlorinated PCDD/Fs are more likely to be destroyed than low-chlorinated PCDD/Fs, at all experimental conditions. In addition, PCDDs are much more reactive than PCDFs, since the PCDDs/PCDFs ratio declines from 0.17 to 0.12 as excess oxygen rises from 0% to 300%.

Supercritical water oxidation of tannery sludge: stabilization of chromium and destruction of organics.

The supercritical water oxidation (SCWO) of industrial tannery sludge was investigated to understand the simultaneous destruction of organic pollutants and recovery of high content chromium. Experiments were performed in a batch reactor at temperatures of 350-500 °C, reaction time of 150-300 s and different oxygen ratios, to exhibit the effect of operation conditions. Results showed that removal efficiency of chemical oxygen demand (COD) increased with higher temperature, larger oxidant amount and reaction time; a maximum value of 96% was obtained. Meanwhile, destruction yield was much higher under supercritical conditions than that in subcritical water. In addition, removal efficiency of Cr from sludge reached more than 98% under all conditions; higher temperature played a positive role. Further, leaching toxicity tests of heavy metals in solid products were conducted based on toxicity characteristic leaching procedure. All heavy metals except nickel showed a greatly reduced leaching toxicity through their stabilization. The chromium oxide recovered in ash was amorphous below 550 °C, so that the structure of Cr could not be identified by X-ray diffraction pattern. Special attention should be paid on nickel as its leaching toxicity increased due to the corrosion of reactor surface under severe reaction conditions.