Denitrification in free water surface wetlands receiving carbon supplements

Abstract

Wetlands may be used as fixed film reactors for removal of NO3-N in wastewater. Two 1 hectare free water surface wetlands were constructed for denitrification of nitrified effluent from intermittent sand filters in Connell, Washington. The wetlands were designed to remove NO3-N from wastewater prior to land application. The design flow as 5300 m3/d (1.4 mgd). Primary effluent from a potato processing facility was used as a carbon supplement for detnitrification. Addition of primary effluent (COD = 2800 mg/L) resulted in a COD:NO3-N mass load ratio that ranged from 10 to 25. The total hydraulic retention time in the two wetlands varied from 1-2 days in the summer and winter. The NO3-N load ranged from 10 to 110 kg/ha d. The NO3-N mass removal rate ranged from 50 to 99% of the influent load. During the five months of data presented, January to May 1998, average monthly effluent NO3-N was 1.6 mg/L; monthly average influent NO3-N was 20.5 mg/L. An average of >85% of the NO3-N influent load was removed. The NO3-N removal rate coefficient (K20 = 358 m/yr) was higher than that measured in wetlands without carbon supplements and was independent of temperature above 12 degrees C.