Influence of saturation on denitrification in a two-stage, vertical flow treatment wetland at Bridger Bowl ski area
Woodhouse, Shayla Lee
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A full-scale two-stage vertical flow treatment wetland (VF TW) pilot system was installed at the Bridger Bowl Ski Area in 2013 to test its capability as a secondary wastewater treatment system. Water quality was monitored throughout the 2015-2016 and 2016-2017 ski seasons with the primary objective to optimize the system for total nitrogen removal. Eight different combinations (schemes) of daily hydraulic and nutrient loading, dose frequency, effluent recycle, and depth of saturation of the first stage were tested. Average system removal was 93% and 95% for chemical oxygen demand (COD) and 70% and 75% for total nitrogen (TN) in 2016 and 2017, respectively, despite elevated influent concentrations of 930 mg x L -1 COD and 195 mg x L -1 TN. In addition, the system converted virtually all influent TN to nitrate. Both 1:1 and 2:1 (recycle to influent) ratios were effective as were saturation depths of 53 and 71 cm. At the 2:1 recycle ratio, the higher saturation level was superior at light hydraulic and constituent mass loadings while the lower saturation level was superior at higher loadings, but the differences were small. Overall, TN removal and nitrogen species transformations were linearly related to the mass load applied (surface area basis) over the range evaluated. In addition, the maximum removal capacity of the system was not exceeded during any scheme, thus the inherent removal capacity of the system is greater than evaluated. Nevertheless, the removal efficiencies of the VF TW from the past two seasons has shown that this technology can successfully perform secondary wastewater treatment in cold climates with efficient nitrogen removal and can exceed the regulatory requirements under which Bridger Bowl operates.