Energy based ecological assessment of constructed wetland for municipal wastewater treatment: Methodology and application to the Beijing Wetland

This paper improves the traditional emergy method, and presents an ecological assessment for a constructed wetland that is designed for municipal wastewater treatment. The improvement is made by redefining the ecological value of wastewater, using renovated trasformities, and considering the ecological discounting effect. The modified method is applied to assess the Beijing Wetland, a typical constructed wetland in northern China. A parallel monetary based assessment is also carried out to investigate the difference between ecological and economic perspectives. Results show that the Beijing Wetland maintains a positive ecological profit while the discount rate is below 15.58%, and economic deficit will not occur until the discount rate reaches an unusually high level of 41.21%. In the scenario of a yearly discount rate of 5%, the emergy cost to treat one cubic meter of wastewater by the Beijing Wetland is calculated to be 4.53E+10 sej, which is equivalent to 0.01 em$, and is much lower than that by the traditional treatment technology. Moreover, both ecological and economic assessments suggest the application of constructed wetland is profitable and thus should be promoted on a larger scale for municipal use of wastewater treatment if the discount rate does not exceed 15%.