- Five unique but related studies were conducted at the Oregon State University Dairy
Wetland Treatment System (OSUDWTS), Corvallis, OR. The research site consisted of
six parallel wetland cells, which were built in 1992 and began receiving concentrated dairy
wastewater in the fall of 1993. Hydrologic, hydraulic, and water quality data were
collected at the site for three years. The five resulting studies were:
1. the prediction of evapotranspiration (ET) from wetlands;
2. the development of a hydrologic model and water budget for the OSUDWTS;
3. a preliminary investigation of the hydraulics of the OSUDWTS;
4. an overall evaluation of the treatment performance of the OSUDWTS and
applicability of current constructed wetland design methods to livestock
wastewater wetlands; and
5. the development of a conceptual model for nitrogen removal in constructed
Average ET rates for the wetland cells were found to be 1.6 times as great as the Penman-
Monteith alfalfa reference ET. Specific crop coefficients were 1.72, 2.32, and 0.57 for
bulrush, cattails, and floating grass mats. The detailed hydrology model predicted daily
water levels very accurately (R²=0.95) and showed seasonal rainfall and ET could
increase or decrease the average detention time by as much as 18%.
Tracer studies indicated that non ideal flow existed in the wetlands. Actual
detention times were found to be an average of 43% shorter than theoretical detention
times. Tank-in-series and plug flow modified by dispersion models were inadequate at
describing the observed tracer response.
Constructed wetlands were shown to be able to reduce a high percentage of most
waste constituents in concentrated livestock wastewaters. Average reductions for COD,
BOD, TS, TSS, TP, TKN, NH₃ and fecal coliforms were 45, 52, 27, 55, 42, 41, 37 and
80%, respectively. Rate constants for volumetric and areal first-order plug flow models
were found for each wastewater constituent. Overall, both models were fair at predicting
wastewater reduction at the OSUDWTS.
A conceptual model of nitrogen cycling showed denitrification to be the most
important process for nitrogen removal in constructed wetlands. However, low dissolved
oxygen in constructed wetlands limits nitrification, which in turn limits denitrification.