8 ENVIRONMENTAL CONNECTION / THIRD QUARTER 2025 / IECA.ORG Rich McLaughlin, Ph.D. Professor Emeritus North Carolina State University, Raleigh, NC Wetlands to Remediate Agricultural Drainage Reducing Nutrient Discharges to Minimize Impacts to Waterways Agricultural production often results in discharges of water heavily laden with nutrients, which results in negative impacts on receiving waters including algal blooms and eutrophication. Several types of constructed wetlands were tested recently to determine their potential for treating these waters to remove nutrients before discharge. Modeling Constructed Wetlands Researchers in one study tested different substrates in model constructed wetlands to determine which would best remove nitrogen (N), phosphorus (P) and chemical oxygen demand (COD) from simulated farm runoff.1 The model systems were contained in 16 cm diameter by 45 cm tall (6.3 x 17.7 inch) plastic pipes. The control treatment had 20 cm (7.9 inches) of 3 to 5 mm (0.12 to 0.2 inch) gravel, while two other treatments included 10 cm (3.9 inch) gravel plus 10 cm (3.9 inch) iron-carbon composites (FeC) or 10 cm (3.9 inch) iron-carbon composites mixed with ground walnut shells (WFeC). In addition, another treatment was the same as WFeC but included inoculation with a denitrifying phosphate-accumulating bacteria (MWFeC). All of the columns were inoculated with activated sewage sludge, planted with four Iris tectorum plants, and allowed to equilibrate for a month before dosing. There were three dosing periods of 40 days each, with simulated runoff being added daily to the top while removing an equivalent volume from the bottom. The pollutant concentrations during the first period were 6.0 mg NO3 -N L-1, 3.0 mg NH4+-N L-1, 0.4 mg total phosphorus L-1 (TP) and 15.0 mg COD L-1 in the form of sucrose and fulvic acids. In the second phase, the concentrations were doubled, and in the third phase, the concentrations were doubled again. The removal of NO3 increased substantially (>2X) in all treatments relative to the gravel control, with the WFeC and MWFeC treatments achieving up to 88% and 94% removal, respectively. Increasing the NO3 concentration decreased removal rates, but the MWFeC treatment still removed 77%. Removal of NH4 hovered around 60%–70% regardless of the substrate or the concentration. Total N removal followed the NO3 pattern for the treatment effects and peaked at around 75% for the MWFeC treatment at the highest concentration. Removal of TP was in the 30%–40% range for the gravel alone and about twice that for the other substrates. Similar to the nutrients evaluated, COD removal was increased with the FeC substrate and was the greatest in the MWFeC treatment at up to 90%. Iron concentrations remained low (<0.2 mg L-1) in the effluent. The authors also measured several greenhouse gases (CO2, CH4, N2O) to determine the potential for the different substrates to negatively impact global warming initiatives. The results were much more mixed than those of the column water. At the lower nutrient concentrations, generally fewer greenhouse gas emissions came from the FeC columns compared to the gravel alone. However, at the highest nutrient concentration, emissions were about 2X when the FeC material was used in the columns. Removing Nutrients with Cattail In a second study, aquaculture waste water (WW) was collected from a trout production facility and added to 2.5 m (8.2 feet) diameter stock tanks consisting of 15 cattail plants planted into 20 cm (7.9 inch) topsoil and flooded to a 30 cm (11.8 inch) depth.2 The system was allowed to establish itself for 40 days before adding the WW. Then the WW was added weekly at five doses over five weeks. The total N and P loading over RESEARCH BRIEFS
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