The Effect of the Antecedent Dry Conditions on Nitrogen Removal for a Modified Bioretention System

Eutrophication is defined as the ‘over enrichment’ of a water body from nutrients, resulting in uncontrolled growth of primary producers, leading to periods of oxygen depletion from decomposition of the algal organic matter. According to the 2010 Water Infrastructure Needs and Investment (a U.S. Congressional Report), 40% of U.S. water bodies are contaminated with pollutants, including nutrients. Non-point sources of nutrient pollution are a major cause of this reduction in water quality. One way to decrease eutrophication is to manage nutrients found in stormwater runoff, before they reach a receiving water body. Bioretention cells containing an internal water storage zone (IWSZ) have been shown to remove higher amounts of nitrogen than conventional cells (without an IWSZ). The IWSZ contains an organic carbon substrate, usually derived from wood chips submerged in water, which supports the biochemical process of denitrification. Characteristics of wood chips that affect nitrogen removal include carbon content (%), leaching of dissolved organic carbon (DOC), and wood chip size and type. However, there is limited information on how the intermittent hydraulic loading that is associated with these field systems impacts their performance. Accordingly, the overall goal of this research is to improve understanding of the effect that the antecedent dry conditions (ADC) have on the performance of a field scale bioretention cell modified to contain an IWSZ. The nine different types of wood chips used in laboratory and field studies identified in the literature were categorized as hardwood and softwood. Literature showed that total organic carbon (TOC) leached from softwood chips is almost double the TOC measured from the hardwood chips, 138.3 and 70.3 mg/L, respectively. The average observed nitrogen removal for softwood chips was found to be greater than the removal for the average of the hardwood chips (75.2% and 63.0%, respectively). Literature also suggests that larger wood chip size may limit the availability of the carbon for the denitrifying organisms and provides less surface area for the biofilm growth. A field study conducted for this research compared the performance of a modified bioretention system designed to enhance denitrification, addition of an IWSZ, with a conventional system that does not contain an IWSZ. Fourteen storm events were completed from January 2016 to July 2016 by replicating storm events previously completed in the laboratory using hydraulic loading rates (HLR) of 6.9 cm/h, 13.9 cm/h, and 4.1 cm/h. The goal was to have results from storm events with ADCs of two, four, and eight days, with the varying durations of hydraulic loading of two, four, and six hours. Synthetic stormwater, simulating nitrogen levels common in urban runoff, was used as the system’s influent to assist in running a controlled experiment. The resultant ADCs ranged from 0 to 33 days, with the average ADC being 9 days. The fourteen sets of influent samples were averaged to obtain mean influent concentrations for the synthetic stormwater. These values were used when calculating the percent nitrogen removal for the four measured nitrogen species (NOx – N, NH4+– N, organic N, and TN). The field storm events were separated into three groups based on HLR and duration to eliminate the affects of both variables on nitrogen removal for these results, since the focus is the ADC. For the low HLR (4.1 cm/hr), there were four storm events (ADCs of 4 to 33 days), as the ADC increased, greater percentages of ammonium – nitrogen, organic nitrogen, and total nitrogen were removed. For nitrate/nitrite – nitrogen, the percent removal was rather consistent for all four storm events, not significantly increasing or decreasing with changes in the ADC. There were five storm events (ADCs of 0 to 28 days) tested with the median HLR (6.9 cm/hr), nitrogen removal for all four species increased as the ADC increased. The increase was significant (p0.05) for nitrate/nitrite – nitrogen. The third group also contained five storm events (ADCs from 0 to 11 days) that were tested with the highest HLR (13.9 cm/hr). Ammonium – nitrogen, nitrate/nitrite – nitrogen, and total nitrogen all increased with the ADC, and organic nitrogen removal decreased with the increasing ADC. As a result, this research concluded that the difference in HLR affects the nitrogen removal efficiency, but overall increasing the ADC increased nitrogen removal for NOx – N, NH4+ - N, organic N, and TN