Effect of Saturated Zone on Nitrogen Removal Processes in Stormwater Bioretention Systems

Abstract

Abstract:The introduction of a saturated zone (SZ) has been recommended to address the issue of nitrogen removal fluctuation in the bioretention system, which is one of the most versatile low-impact development facilities for urban stormwater management. Nine experimental columns were used to characterize the nitrogen concentration variations over the outflow during wetting periods and in SZ during the antecedent drying periods (ADPs), as well as compare removal efficiencies of various nitrogen species in systems with different SZ depths under alternate drying and wetting conditions. Results indicated that NO 3 − -N concentrations in the outflow showed quasi-logistic curve-shaped variations over time: being low (<0.5 mg/L) in the early process, sharply increasing thereafter, and finally flattening around 3.0 mg/L with NO 3 − leaching; NH 4 + -N and organic nitrogen (ON) concentrations were consistently low around 0.5 mg/L and 1.8 mg/L, respectively during the wetting periods. NH 4 + removal efficiency in bioretention systems was consistently high around 80%, not varying with the increasing SZ depth; ON removal efficiency had a slight rise from 57% to 84% and NO 3 − removal efficiency was significantly enhanced from −23% to 62% with the SZ depth increasing from 0 to 600 mm. Deeper SZ could store more runoff and promote more denitrification of NO 3 − and mineralization of ON during the ADPs, providing more "old" water with low NO 3 − and ON concentrations for water exchange with "new" inflow of higher NO 3 − and ON concentrations during the wetting periods. The total nitrogen (TN) removal, a combined result of the instantaneous removal through adsorption and retention in the upper soil layer during the wetting periods and the gradual removal via denitrification and mineralization in SZ during the ADPs, was also improved by increasing the SZ depth; TN removal efficiency was elevated from 35% to 73% when the SZ depth increased from zero to 600 mm.

Design and Performance Characterization of Roadside Bioretention Systems

“…The artificial filler layer was uniformly mixed material of coarse sand, original soil, and wood chips in a ratio of 1.5:1:0.5. The original soil was undisturbed natural soil and the wood chips were added to the artificial filler layer mainly because they served as organic matter [12]. The second bioretention unit (unit B) had a 400 mm substrate with a 300 mm submerged zone, meanwhile, a vertical PVC tube of 300 mm height was laid at the bottom of the perforated pipe.…”

“…A new urban water resource management strategy called the Sponge City Program (SCP) was proposed by China in 2013, which totally deviated from the traditional rapid-drainage techniques [12]. The Sponge City concept is an effective urban water management plan that aims to absorb rainwater for reuse in a proper scientific manner [13].…”

Importance of the Submerged Zone during Dry Periods to Nitrogen Removal in a Bioretention System

“…In this study, SZ depth was fixed at 300 mm and V1 was relatively stable (10.6 ± 2.5 L). Wang et al [22] carried out a comparative experiment of bioretention systems with different SZ depths. In terms of the experimental data [22], the relationship between the depth of SZ and V1 was showed in shown in Figure 9.…”

“…For the bioretention system installed with a SZ, Lynn et al [26] and Berger et al [27] found that the NO3 --N removal rate of the bioretention system with a SZ increased with the increase of ADP. Subramaniam, et al [28] and Wang et al [22] both found that the effluent NO3 --N concentration gradually rose within ~30 min and tended to be stable afterwards, suggesting the removal of NO3 --N (denitrification) mainly occurred in the dry phase. However, Cho et al [29] found that the NO3 --N concentration decreased significantly with the increase of ADP when ADP was shorter than 10 days, while the NO3 --N leached out when ADP was 20 days.…”

“…Furthermore, the NO3 --N removal performance of a bioretention system could be affected by the depth of SZ. The results of comparative experiments carried out by Wang et al [22] indicated that with the depth of SZ increasing from 0 to 600 mm, the removal rate of NO3 --N increased from -23% (leaching) to 62%. However, some studies have found that the NO3 --N removal rate of biological retention system would decrease when the depth of SZ was greater than a certain optimal height [23].…”