An emergy-based treatment sustainability index for evaluating waste treatment systems

The treatment of wastes can be accomplished using various combinations of natural processes and human invented technologies that operate by using a variety of energy sources and natural resources. In many waste treatment systems, the waste itself is used as a main energy source to drive treatment (e.g., activated-sludge in advanced wastewater treatment). Understanding the sustainability of waste treatment systems thus requires the capability for quantitatively comparing the energy and resource inputs with the waste-derived energy inputs and effluent quality. This study applied concepts of emergy accounting to develop a treatment sustainability index (TSI). The theoretical basis of the TSI is explained and then applied to two wastewater treatment systems—an operating passive treatment system and a hypothetical active treatment system for treating mine drainage in NE Oklahoma. The TSI accounts for renewable and purchased emergy inputs, the input of emergy from the wastewater itself, and the amount of work required by the receiving environment to further treat the effluent. Unlike other emergy-based indices, the TSI explicitly accounts for the energy provided by wastes, the waste treatment efficiency and the downstream effect on the releasing pollutants into receiving environments. Since the emergy associated with the waste used during treatment was orders of magnitude larger than renewable, non-renewable or purchased inputs, the TSI clearly captured the importance of accounting for waste emergy, and was able to quantify their sustainability. The TSI offers a new way to assess the sustainability of all types of treatment systems.