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Quantifying the Hydraulic Performance of Treatment Wetlands

Wahl, Mark D.

Abstract Details

2010, Master of Science, Ohio State University, Food, Agricultural and Biological Engineering.

Constructed wetlands are becoming an increasingly common best management practice for reducing pollutants. Processes like rhizofiltration, settling of suspended particles, and degradation are all time dependent. These treatment mechanisms can be limited by hydraulic inefficiencies like short-circuiting in treatment wetlands. It is not known exactly what role such inefficiencies play in treatment, but when expected water quality gains are not realized the adoption of treatment wetlands as a best management practice can be slowed. One reason the effects on treatment are not well understood is that hydraulic inefficiencies are difficult to quantify. The aim of this work was to develop a universally applicable hydraulic index to quantify the hydraulic performance of treatment wetlands. An index demonstrating strong correlation to pollutant reduction is needed to identify the optimal wetland configuration for maximizing residence time. Such an index should be related to the various wetland parameters that influence the RTD. The index would not only be useful in quantifying the effects of vegetation, bathymetry, and wetland shape on residence time; it could then be used to supply the bounds for anticipated pollutant reduction.

Three existing hydraulic indices were evaluated for their suitability both as a measure of hydraulic performance and as a predictor of treatment. Of the three existing hydraulic indices evaluated, only one demonstrated strong correlation to the effluent pollutant fraction. However, that index could not detect variations among residence time distributions that had a common centroid implying the index could not detect attenuation of a residence time distribution. Other indices are needed to better quantify the influence that various wetland parameters have on residence time and develop predictive models for treatment.

Three new indices were proposed. The moment index was derived using residence time distribution theory. This approach quantifies hydraulic inefficiencies according to the juxtaposition of the hold back parameter relative to the nominal residence time. 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. Further, two other indices were derived using a statistical approach. Recognizing the close relationship between the residence time distribution and a probability density function, an approach typically associated with failure analysis was used to develop the two new indices. The hazard index demonstrated superior agreement with qualitative assessments implying this index could be useful for characterizing the effects on the flow regime from various wetland parameters like depth, bathymetry, and shape. The temporal hazard index demonstrated superior correlation to the effluent pollutant fraction predicted by a first order reduction implying the temporal hazard index could be the good predictor of treatment.

All three proposed indices overcame weaknesses inherent in the existing indices. The arbitrary truncation of data resulting from finite resources for data collection has no impact on any of the proposed indices. All the proposed indices had the ability to detect attenuation of residence time distributions.

Larry Brown, PhD (Advisor)
Norman Fausey, PhD (Committee Member)
Jay Martin, PhD (Committee Member)
Alfred Soboyejo, PhD (Committee Member)
103 p.

Recommended Citations

Citations

  • Wahl, M. D. (2010). Quantifying the Hydraulic Performance of Treatment Wetlands [Master's thesis, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1275418194

    APA Style (7th edition)

  • Wahl, Mark. Quantifying the Hydraulic Performance of Treatment Wetlands. 2010. Ohio State University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1275418194.

    MLA Style (8th edition)

  • Wahl, Mark. "Quantifying the Hydraulic Performance of Treatment Wetlands." Master's thesis, Ohio State University, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=osu1275418194

    Chicago Manual of Style (17th edition)