Nitrification and total nitrogen removal in a super-oxygenated wetland

A novel surface flow wetland demonstrates high-rate nitrification by recirculating water supersaturated with oxygen. Design of the wetland was based on two hypotheses: (1) Oxygen supersaturation of a recirculating sidestream would support nitrification in a surface flow wetland at rates comparable to aerated subsurface flow wetlands, and; (2) Nitrification would be maintained in cold water conditions (0.5-5.0 °C) with minimal bioaugmentation using commercially available liquid suspensions of nitrifying bacteria. The wetland has an area of 23,000 m², of which the final 21,000 m² is oxygenated. Mean flow was 863 m³ d^-1. A downflow oxygen contactor (Speece cone) maintains dissolved oxygen at or over saturation in the wetland. Mean inflow NH₃-N was 8.0 mg L^-1, mean outflow 0.6 mg L^-1. In winter, nitrification began to degrade slowly at a water temperature of 0.5 °C to about half the summer rate but was restored by bioaugmentation with nitrifying bacteria in one month at water temperatures less than 3.3 °C. Determination of nitrification rates by a first-order, tanks-in-series model (P-k-C*) was hindered by the method detection limit (0.2 mg L^-1 NH₃-N) of analytical methods and the inherent limitations of first order models for complete nitrification. The median monthly nitrification areal rate coefficient from April 2016 through June 2017 was at least 216 m y^-1, which is 15 times greater than median passive wetland literature rates. High nitrification rates allow for at least a 90% reduction of required treatment area for nitrification at approximately one third the capital cost of aerated subsurface flow wetlands. Denitrification rates were well within observed literature values. The peak monthly mean denitrification rate, observed in June 2017, was 76 m y^-1, but winter denitrification rates were close to zero.