Influence of dose volume on nitrogen removal in a two stage vertical flow treatment wetland: Bridger Bowl ski area case study

Abstract

Treating wastewater in remote locations does not require compromising the effluent quality discharged to the environment. A two-stage vertical flow treatment wetland (VF TW) with recycle meets this objective by removing high inputs of chemical oxygen demand (COD) and nitrogen (N) while requiring minimal maintenance and operator oversight. A 95.2 m 2 pilot scale VF TW at Bridger Bowl Ski Area, near Bozeman, MT, effectively treats the high strength domestic wastewater produced onsite. The partially saturated first stage of the VF TW removes influent COD and an unsaturated second stage nitrifies influent ammonium. Recycling second stage effluent to the first stage allows for nitrate removal by denitrification in the saturated zone of the first stage. Previous research indicated the system experiences near complete nitrification in the second stage and that total nitrogen removal is limited by denitrification in the first stage, potentially due to low organic carbon (COD) availability in the saturated zone. Therefore, the goal of the current study was to increase the COD:N ratio of the water entering the first-stage saturated zone by increasing the dose depth of influent (septic) water, high in COD, thereby reducing COD removal in the unsaturated layer. To evaluate denitrification performance a simplified stoichiometric process model accounted for both nitrate created and COD removed in the first stage unsaturated zone. During the 21-22 season, approximately 7 cm/day of septic water was applied to the first stage in either 1.2 or 2.5 cm doses. The larger doses showed enhanced nitrate removal efficiency in the saturated zone; however, a changing influent water quality may have supplemented efficiency improvement. During the 22-23 season, 12 cm/day of septic water was applied to the first stage in either 1 or 4 cm doses. During this experiment, influent water quality was the same, and the larger dose depths did not show enhanced nitrate removal. However, decreasing the septic dose depth increased first stage nitrification from 20 to 48% and COD removal from 77 to 82%. Throughout both experiments, system COD removal was > 95% (influent COD > 750 mg/L) and system ammonia removal was > 98% (influent NH 4 >160 mg/L).