Assessment of Constructed Wetlands for Mass Removal of Nitrogen from Swine Wastewater

Disposal of wastewater generated by confmed swine production units is a problem when land is limited. Improperly sized or managed waste treatment systems can cause odor and water pollution problems. Our objective was to compare the mass removal of nitrogen from swine wastewater by the current technology that uses anaerobic lagoon treatment with alternative technologies using constructed wetlands, overland flow, and media filter. Lagoons are a common component of nutrient management systems because they can reduce nitrogen in wastewater up to 80%. However, residual ammonia-N concentrations in the lagoon effluent are very high. When land is limited, constructed wetlands can be a viable component of a system for safe disposal of lagoon effluents. Nitrogen removal efficiencies more than 80 % were obtained for wetlands planted to two mixtures of rush/bulrush and cattail/bur-reed plants with N application rates up to 15 kg/ha-day. Two critical aspects of the wetland system are plant tolerance to ammonia-N and nitrification-denitrification rates to transform ammonia into N2. Because levels of ammonia-N tolerated by plants were unknown, the lagoon effluent was diluted with fresh water. A microcosms study showed that wetland plants grew vigorously with ammonia-N concentration levels up to 240 mg/L. Since nitrification seems to be the limiting factor to treat high ammonia loads, aeration of the wastewater may enhance treatment. When lagoon waste was pretreated by overland flow, the N removal efficiency was 59 % for a N application rate of 50 kg/ha-day. Arnmonia- N in the effluent was reduced to levels that can be efficiently treated by constructed wetlands. In the media filter treatment, up to 32 % of the influent TKN was transformed into nitrate-N when wastewater was recycled four times. Either overland flow or media filter offered the opportunity to eliminate the dilution of lagoon effluents prior to wetland treatment. By sequencing nitrification and denitrification processes, advanced wastewater treatment levels could be achieved. Such a system could offer a safer alternative to anaerobic lagoons