A Novel Process for Biological Nitrogen Removal From Dairy Wastewater Using Constructed Wetlands

Dairy farms generate liquid manure, milking washwater, and contaminated runoff, which are often characterized as ammonia-rich. Dairy wastewater, left untreated, poses environmental effects and health concerns when discharged to the environment. Traditional nitrogen removal systems require an energy intensive two-step process: nitrification-denitrification. In our integrative constructed wetland treatment system, total nitrogen (TN) removal is achieved by a nitritation-anammox process. Partial nitrification (nitritation) is responsible for producing nitrite, a reactant for the anammox process. Autotrophic anammox bacteria provide a short-cut pathway for TN removal while requiring less energy, producing less waste, and minimizing greenhouse gas emissions. In the present study, four subsurface flow constructed wetlands have been operated in series with three free water surface constructed wetlands since January 2009. All of the constructed wetlands were seeded with anammox bacteria. Two subsurface flow wetlands were packed with marble chips and the other two with pea pebbles. The free water surface wetlands contain silt loam as rooting substrate. Aquatic plants present in the free water surface wetlands include cattails, bulrushes, reeds, and pondweed. The system was operated in batch mode with synthetic dairy wastewater (targeted ammonia-N concentration at 90 mg/L) from January to April 2009. Each wetland had a hydraulic retention time of 7 days and a surface area of 0.21 square meters. The marble chip subsurface flow wetlands (BM) were able to achieve a TN mass removal rate of 87 mg/d. The subsurface flow wetlands using pea pebbles (BP) had TN removal rate less than 38 mg/d and showed a decreasing trend, likely due to the absence of nitritation at acidic conditions. In series with a free water surface wetland (CW1), the marble chip series (BM-CW1) achieved a TN removal rate of 347 mg/d. In series with another free water surface wetland (CW2), the pea pebble series (BP-CW2) achieved a TN removal rate of 229 mg/d. The BM-CW1 series achieved up to 100% ammonia removal efficency and 69% TN removal efficiency. An additional free water surface polishing wetland (CWp), which was in series with BM-CW1 and BP-CW2 series, was able to further remove TN up to 179 mg/d. Since May 4, 2009, the system has been tested to treat more ammonia-rich dairy wastewater (ammonia-N concentration at 200 mg/L). Increasing regulation for dairy farms to manage waste streams will require that farmers implement wastewater management plans. Constructed wetlands provide a low-cost, low-energy on-site treatment alternative for effective biological nitrogen removal.