Assessing the benefits of a passive treatment system for mine drainage in northeast oklahoma using emergy analysis

A ten-cell passive treatment system (PTS) in the Tar Creek Superfund Site in Ottawa County, Oklahoma treats approximately 605,000 L of net-alkaline, lead-zinc mine drainage daily using a single initial oxidation pond followed by two parallel treatment trains of aerobic surface flow wetlands, vertical flow bioreactors, re-aeration ponds and horizontal flow limestone beds, and a common final polishing cell. Re-aeration is achieved via renewable energy resources (solar and wind). Design and construction of the PTS cost $1.2 million and it has a design life of 30 years. Prior to treatment, water from boreholes flowed into a horse pasture, forming volunteer wetlands and discharged to an unnamed stream that eventually empties to Tar Creek, a tributary to the Neosho River. Emergy (spelled with an "m") analysis is a method used to quantitatively classify energy flows in systems with regard to the amount of embodied energy of a lesser quality (usually solar energy) used to form that flow. Because different forms of energy are not necessarily capable of doing the same amount of work (e.g., one joule of solar energy cannot do the same work as one joule of fossil fuel), emergy analysis is useful because it normalizes these differences for meaningful comparisons. Using emergy analysis, the emergy inputs of this PTS were compared to the amount of work required by the environment to achieve the same treatment performance with no PTS. When less work is done by the environment mitigating this mine drainage, more resources become available for other systems. In addition, the emergy costs of a modeled active treatment system (ATS) were considered. These three treatment scenarios (ATS, PTS, and No Treatment) were compared using the Treatment Sustainability Index for determining relative sustainability of treatment systems based on their emergy inputs. The TSI revealed that the PTS is 6 times more sustainable than the ATS.