Nitrogen Removal in Constructed Wetlands: Enhancement of Nitrate Mass Transfer in the Denitrification ZoneEPA Grant Number: U915482
Title: Nitrogen Removal in Constructed Wetlands: Enhancement of Nitrate Mass Transfer in the Denitrification Zone
Investigators: Fleming-Singer, Maia S.
Institution: University of California - Berkeley
EPA Project Officer: Lee, Sonja
Project Period: January 1, 1998 through January 1, 2001
Project Amount: $26,408
RFA: STAR Graduate Fellowships (1998) RFA Text | Recipients Lists
Research Category: Fellowship - Environmental Engineering , Engineering and Environmental Chemistry , Academic Fellowships
The objective of this research project is to explore three hypotheses regarding control and enhancement of denitrification rates in constructed wetlands designed for nitrogen removal. The central hypothesis is the following: (1) denitrification in constructed wetlands follows first-order kinetics with respect to nitrate; (2) denitrification rates can be enhanced by increasing mass transfer of nitrate from the bulk water to the denitrification zone; and (3) promotion of a denitrifying episediment zone will enhance denitrification rates above those found in purely sediment-based denitrification zones.
Laboratory bench-scale wetland microcosms and field-scale mesocosms will be used to measure denitrification rates and nitrate mass transfer under four experimental conditions. Two of the experimental conditions will focus on a sediment-based denitrification zone. In these experiments, a clearly defined sediment-water interface will be promoted to test the extent of nitrate mass transfer from the overlying water into the sediments for denitrification. The efficacy of an episediment denitrification zone will be tested under the two remaining experimental conditions, using a constructed thatch of cattail leaves and stems positioned at the sediment-water interface. The laboratory microcosms will be flow-through, temperature- controlled (20 EC), anoxic (N2 sparged), and will include sediments, water column, and a headspace. Nitrate levels in the microcosm influent will be manipulated within the 1-100 mg/L range, based on realistic-constructed wetland concentrations. Steady-state effluent nitrate concentrations will be measured using ion chromatography, and denitrification rates will be calculated by nitrate mass balance. To determine the location and extent of the denitrification zone, water column and sediment vertical profiles of nitrate and chloride (a conservative species) also will be measured at steady state using a thin-film polyacrylamide gel sampler. Multiway Analysis of Variance (ANOVA) will be used to evaluate statistical differences between treatments, while linear regression and/or curve fitting will be used to determine the appropriate relationship between denitrification rate and nitrate concentration within treatments. Assessment of surface area and microbial populations will be conducted in all treatments.
I expect to find that episediment denitrification is greater than denitrification in a purely sediment-based wetland. First-order kinetics should be apparent in the sediment-based treatments because of a diffusional mass transfer limitation of nitrate. However, denitrification in episediments may follow zero-order or mixed-order kinetics because of the lack of a diffusional constraint.