SFGP 2007 - The Effects of Sludge Digester Biogas Recovery on WWTP Ecological Impacts and Exergetic Balance

摘要

Exergetic Life Cycle Assessment (ELCA) is applied to a wastewater treatmentnplant (WWTP) by activated sludge in order to demonstrate the environmental andnexergetic effect of biogas recovery within the whole system boundaries. Twonmethods of recovery are compared: a simple burner (Burner scenario) and a combinednheat and power engine (CHP scenario). The results showed that biogasnrecovery reduces the depletion of natural resources and decreases ecological impacts.nEutrophication, toxicity and ecotoxicity impacts are not affected by biogasnrecovery in the Burner scenario and the change is not significant in the CHP scenario.nIrreversibility is added to the system by biogas recovery. This is becausenof the nature of the burnt mixture, which is different from conventional fossil fuelnand because of the addition of units such as CHP and heat exchangers. The CHPnfed with the biogas cannot fulfill the total demand in thermal energy for heatingnand drying in comparison to the burner. Natural gas should be burnt additionallynand a heat exchanger introduced. The irreversibility of the CHP scenario is largernthan the Burner’s one, but the former allows reducing electricity consumption byn22.7%. Moreover, exhaust gas has a positive effect from the exergy point of view.nIt contains an excess potential work which could be recovered in another function.nBased on ozone layer depletion, photochemical oxidation, acidification andnmarine aquatic ecotoxicity impacts, biogas recovery with a CHP scenario is betternthan with a Burner scenario. But based on abiotic resources depletion, globalnwarming potential, irreversibility and excess potential work the second strategy,nwith a Burner scenario, is the best one. In process optimization, the focus shouldnnot be only on potential impacts reduction. The decrease of the irreversibilitynproduced by new functions introduced in the system should also be taken into account.nThe other subsystems within the system boundaries should be looked atnespecially those related to electricity production. The ELCA is a useful tool for system ecodesign and for finding the best way to promote renewable fuels versusnfossil ones.