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As the center of human activities and social civilization,city has developed at the cost of serious environmental degradation and huge eco-environmental pressure,which further lower the human living standard and impeded the sustainable development of the urban ecosystem.Current urban metabolic methods have broader views and focus more on ecological aspects ― impact of emissions and resource consumption ― but they lack a rigorous thermodynamic framework.Consequently,there is a need for modifying existing processes and developing new technologies that minimize environmental impact while providing stimulating ecological flow.This study developed an emergy-based framework for evaluating ecological objectives in traditional urban metabolic process.It started with a brief historical review of urban sustainable development and examines the controversial concept critically from a thermodynamic point of view.Then the assessment framework was proposed by reviewing and compiling existing data and studies on environmental issues available primarily at some sources,which includes resource accounting and environmental impact assessment.The direct and indirect emergy demand was assessed and quantied based on airborne/waterborne pollutants dilution patterns,and concepts of Life Cycle Impact Assessment.Such a framework is a necessary pre-requisite to perform a reliable cost-benefit evaluation of urban sustainability strategies,and provide guidance to policy decisions.

The knowledge of urban metabolism process is a major step towards the design of sustainable development schemes and environmental management.This study systematic simulated and analyzed the mechanisms of Beijing urban ecosystem from a thermodynamic point of view.This assessment model review and compile existing data and studies on environmental issues available primarily at some sources,which include resource accounting and environmental impact assessment.The direct and indirect emergy demand was assessed based on airborne and waterborne pollutants dilution patterns,and concepts of Life Cycle Impact Assessment followed the DALY and PDF methods.Results pointed out(1) the development of economy in Beijing was closely correlated with the consumption of the nonrenewable resources and exerting rising load on environment;(2) of the total emergy use by the economic system,the imported nonrenewable resources from other province contribute most with increasing use from imported nonrenewable resources;(3) the rapid growth of society fixed capital investment drave Beijing′s economic development and GDP′S growing;(4)emissions greatly reduced the sustainability of the urban metabolic system by pulling resources for damage repair and for replacement of lost natural and human-made capital.Such a knowledge is a necessary pre-requisite to perform a reliable cost-benefit evaluation of urban sustainability strategies,and provide guidance to policy decisions.

Biogas-linked agro-ecosystem can bring in economic and environmental benefits via integrated resource utilization,which provides an energy consuming pattern that has prominent advantages in relieving energy shortage and reducing environmental pollution.It is important to make clear how materials and energy convert in the agro-ecosystem,because the facts could suggest us how to improve the performance of the agro-ecosystem and get more benefits.Exergy of a system in a certain environment is defined as the amount of work that can be extracted from the system,which can be regarded as a uniform measure for different forms of energy and different states of matter.This paper presents an exergy-based life cycle accounting method for household biogas system.The proposed method is applied to a case study of Gongcheng as a "National Eco-agriculture Demonstration County" due to its demonstration status of biogas-linked agro-ecosystem in China.It has a high popularization rate of household biogas system.The typical household biogas system of Gongcheng is supposed to have three stages,i.e.,construction,operation and utilization.We calculate the exergy input-output of each subsystem and hereby analyze the exergy efficiency of the whole agro-ecosystem.The results show that,the exergy input of the system is 1.06×108kJ/a while the exergy output(mainly in the form of biogas residue) is 5.00×107kJ/a,with the exergy conversion rate being 48.82%.The exergy utilization rate is 21.60%,and biogas contributes more than biogas residue and biogas slurry.The exergy of system emission reaches 3.42×105kJ/a,which mainly comes from the biogas combustion.Major conclusions are as follows:(1) Compared with the traditional agro-ecosystem,biogas-linked agro-ecosystem has a longer utilization chain for agricultural wastes,which takes in nonrenewable resources and outputs renewable resources.Therefore,household biogas system is an effective measure in relieving energy shortage and reducing environmental pollution in rural area and realizing the sustainable development.(2) The exergy efficiency of the household biogas system is at a low level,and still needs to be promoted.Improving the process technology and optimizing the fermentation condition may be a choice to increase the exergy conversion rate.Meanwhile,technological innovation in the multi-purpose use of biogas residue and biogas slurry can increase the exergy utilization rate.(3) The exergy-based life cycle accounting method of household biogas system in this paper extends the application of exergy accounting,and fills in the gap of household biogas research field.The method may help us understand the household biogas system more clearly and directly in energy conversion and utilization mode.As a result,it is convenient to find out the weak points of the agro-ecosystem that waste more energy and seek for optimal solutions accordingly.