Quantifying the hydraulic performance of treatment wetlands using the moment index

Abstract

A new hydraulic index was derived according to residence time distribution theory. The approach quantifies hydraulic inefficiencies according to the juxtaposition of the hold back parameter relative to the residence time distribution. The index was evaluated for its ability to detect variation, for conformity with qualitative assessments, and for correlation to effluent pollutant fractions in order to assess its suitability as a predictor of treatment. The moment index overcomes many of the weaknesses inherent in existing indices. The index can be computed from a dataset considering just one volume exchange so arbitrary truncation of data due to the finite nature of data collection has no impact on the moment index. The moment index appears to be more sensitive than existing indices in detecting attenuation of a residence time distribution as well. The new index demonstrated excellent correlation to the effluent pollutant fraction predicted by a first-order reduction implying the index could be the good predictor of treatment. In addition to correlation with treatment, the moment index matched qualitative assessment precisely for eight specific cases considered. The moment index could substantially aid in the design and management of treatment wetlands for balancing cost and efficacy by resolving some of the uncertainty associated with residence time. The index could be used to help identify the optimal wetland configuration for maximizing residence time. Not only would it be useful in quantifying the effects of vegetation, bathymetry, and wetland shape on residence time; it could have utility in supplying the bounds for pollutant reduction.