Eco-efficiency of system alternatives of the urban water-energy-waste nexus

Abstract Wastewater systems in developed cities contribute significantly to public health. The related systems are energy and resource intensive and do not recover nutrients in an efficient and effective way. Separating wastewater to greywater and blackwater at the source and exploiting organic municipal solid waste as an additional feed to an adjunct biogas plant could support efforts to make use of the potentials to reduce the environmental impacts, to increase the energy efficiency of winning nutrients, and to implement an additional, locally available energy source. However, the implementation of such systems is seen as expensive. The overarching aim of the paper is to analyze the eco-efficiency of transforming the current separately organized wastewater-energy-waste systems to an integrated one. The study differs between three system alternatives. The least invasive system change assumes a separation of wastewater at the source without a complete overhaul of the current system; the most elaborated one takes the current wastewater system fully out of operation. The reference for the current system is the existing system of a German medium-sized urban neighborhood. The analysis considers the eco-efficiency of two resource-related (fossil and metal depletion) and three emissions-related (climate change, photochemical oxidant formation and terrestrial acidification) impacts. Under the conditions of the settlement investigated, a transformation to the system alternatives will generate in all cases a weak eco-efficiency, i.e. the higher costs of implementing a new system counteracts with the noteworthy environmental improvement. Of the three options, the most elaborated one sees the best performance.