Nutrient mass balance and denitrification rates in a constructed wetland used to process milkhouse waste

Date of Completion

January 1997


Biology, Ecology|Environmental Sciences|Engineering, Environmental|Biology, Limnology




Nonpoint source pollution is the major cause of impairment of surface waters in the United States. Agricultural activities, such as wastewater from dairy milking parlors, are a dominant source of nonpoint source pollution. Constructed wetlands are gaining increased attention for treatment of nonpoint sources. Although constructed wetlands have been utilized for wastewater treatment in warm climates, their performance in cold climates has been questioned. A surface-flow wetland, designed to treat 2.65 m$\sp3$/d of milkhouse wastewater, was constructed on the University of Connecticut's Storrs campus in 1994. The purpose of the project was to determine the efficiency of the system to remove nutrients, five-day biochemical oxygen demand (BOD$\sb5),$ and total suspend solids (TSS), as well as to reduce fecal coliform bacteria count (FC). This dissertation reports on the nitrogen mass balance, denitrification rates and treatment efficiencies of the wetland from August 1994 to December 1996.^ The wetland consisted of three treatment cells 40 meters by 3.5 meters, providing approximately 420 m$\sp2$ of surface area. Flow was monitored hourly at both the inlet and the outlet. A composite sample was collected weekly at the outlet and inlet. Denitrification rates were measured seasonally using the acetylene block method.^ The wetland was designed to process an estimated BOD$\sb5$ loading rate of 7.3 g/m$\sp2$/d, which was less than half of the average actual loading rate. The overall percent mass retention was 94%, 85%, 68%, 60% and 53% for TSS, BOD$\sb5,$ total phosphorus, nitrate/nitrite and total Kjeldahl nitrogen, respectively. Although the wetland became a source of ammonia nitrogen (NH$\sb3$-N) following plant die back in fall 1994, NH$\sb3$-N outflow concentrations have gradually declined. Mass retention was significantly greater (p $<$ 0.05) during the summer than during the winter for all variables except FC. Denitrification rates measured using the acetylene block method have shown denitrification to be a minor removal mechanism ($<$1%) for nitrogen in this wetland. The mass balance indicated that settling and increased storage was the largest removal mechanisms in this wetland. The indications are that the treatment of wastewater in this wetland does not meet design criteria, most likely due to BOD$\sb5$ overloading. ^