Emergy analysis of municipal wastewater treatment and generation of electricity by digestion of sewage sludge

This study examines and evaluates, by using emergy analysis, the use of env ironmental resources for wastewater treatment in a Swedish town. Emergy ana lysis was applied, while it facilitates the comparison of resource use of s ubstantially different kind. In the emergy analysis, all resources are asse ssed on the basis of the amount of direct and indirect solar energy require d in their generation. The study also includes an evaluation of the amount of emergy associated with the production of wastewater. On the basis of our analysis, we suggest that the large amount of emergy that wastewater conta ins are in proportion to the amount of resources employed for wastewater tr eatment and the extensive effects on surrounding ecosystems of discharge of untreated wastewater. The use of local renewable natural resources in Swed ish municipal wastewater treatment systems is negligible compared with the use of purchased inputs, processed largely with the support of fossil energ y. A drastic shift of this order would demand that extensive land areas sur rounding human settlements be (indirectly or directly) devoted to wastewate r treatment. These areas are not accessible today. Our analysis also indica tes that resource requirements from the economy in the production of electr icity by the digestion of sewage sludge is about two times the total resour ce use for generation of the average mix of electricity used in the town. W e, therefore, conclude that if the only reason to digest the sludge were to produce electricity, it would be more resource-efficient to purchase the e lectricity on the Swedish distribution net. Accordingly, there is no resour ce economy in producing biomass to digest just to increase the energy produ ction at the wastewater treatment plant. (C) 2001 Elsevier Science B.V, All rights reserved.