NITRIFICATION AND DENITRIFICATION IN A TIDAL VERTICAL FLOW WETLAND PILOT

This paper describes nitrification and denitrification in a novel tidal vertical flow wastewater pilot treatment wetland (U.S. and international patents pending). The pilot is a flood and drain, serial batch reactor system with process recycle. All wetland cells are covered with mature plants. Tertiary treatment standards (≤ 10 mg/L BOD5, ≤ 10 mg/L TSS, ≤ 10 mg/L TN) were achieved from manufactured wastewater (428 mg/L BOD, 48 mg/L TN) without use of aeration machinery. The average hydraulic residence time was approximately 24-hours. Oxygen transfer is a complex process in tidal vertical flow systems. Ammonia ions (NH4+) adsorb to media biofilms in the flood stage and then rapidly nitrify when exposed to atmospheric oxygen in the drain stage. Nitrate ions (NO3−) desorb into bulk water in the next flood stage and are used as terminal electron acceptors in bacterial respiration. Turnover of the water column during tidal exchanges also promotes atmospheric diffusion of oxygen into bulk water. Apparent simultaneous nitrification and denitrification has been observed directly and indirectly by standard methods, in situ probes, and analyses of wetland cell exhaust gases. Genetic probes (fluorescent in situ hybridization - FISH) of biofilm bacteria in this study are consistent with literature results for municipal and industrial wastewater treatment plants except for an elevated percentage of Planctomycetes. Results of this pilot study, and information available in the literature, indicate that tidal flow systems can provide advanced, biological nitrogen removal for substantially less energy than activated sludge systems, and occupy a substantially smaller treatment footprint than conventional constructed wetlands for wastewater treatment.