Enhanced Denitrification in Bioretention Using Woodchips as an Organic Carbon Source

Abstract

Bioretention systems fundamentally lack the ability to effectively reduce total nitrogen concentrations from urban stormwater. Column tests were conducted to simulate a bioretention anoxic zone and evaluate the effects of stormwater retention time, limestone addition, and woodchip species, size, and mass percentage on the bioretention denitrification process. Denitrification of an artificial stormwater was demonstrated and appeared to follow first-order kinetics. A 0.8-day average retention time showed the highest nitrate removal percentage of 82.4 AE 0.4%. Willow Oak and Red Maple woodchips resulted in the highest total nitrogen removal efficiencies at 61.9 AE 0.8% and 61.8%, respectively; Wild Cherry, Virginia Pine, and American Beech were less effective. Smaller woodchips and higher woodchip mass percentage corresponded to greater nitrate removal efficiencies, but also higher organic nitrogen leaching. Media containing 4.5% 5-mm Willow Oak woodchips by mass represented optimum conditions. Adding limestone to raise the pH did not increase denitrification rates. Installing a first flush bioretention denitrification zone may represent a way to incorporate the longer retention times needed for denitrification.

Performance of Denitrifying Stormwater Biofilters Under Intermittent Conditions

“…This indicates that different processes control TKN and DOC release from the wood chips or biofilm. Similar results were observed by Schmidt and Clark (2012) who observed no correlation between TKN and DOC in a denitrification wall for wastewater treatment; however, Peterson et al, (2015) observed similar trends between TKN and DOC in a denitrifying biofilter that was evaluated under alternating saturated and unsaturated conditions.…”

“…3a). Based on these data alone, an increase in detention time will increase NO 3 -removal as was previously observed by others (Kim et al, 2003;Smith, 2008;Lucas and Greenway 2011;Peterson et al, 2015). However, mixing of the influent and pore water may also affect effluent NO 3 -concentrations (Kim et al, 2003) (Fig.…”

“…However, some export of TKN was observed, possibly due to leaching of organic nitrogen from the wood chips (Schmidt and Clark, 2012;Peterson et al, 2015). The overall PO 4 3 -results provide evidence that denitrifying biofilters are capable of retaining phosphorus.…”

“…Factors that have been reported to affect NO 3 -removal in denitrifying biofilters include: influent NO 3 -concentration, oxidation-reduction potential, temperature, detention time, and electron donor bioavailability (Kim et al, 2003;Smith, 2008;Schmidt and Clark, 2013;Peterson et al, 2015). Other factors that may be important include biofilm acclimation, biofilter depth, electron donor hydrolysis rate, transport of influent dissolved organic carbon (DOC), and antecedent dry conditions (ADCs) (Kim et al, 2003;Chun et al, 2009;Chu and Wang, 2013).…”

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

“…However, denitrification or nitrification is not likely to occur significantly within the short-time (around 1 h) wetting period [36], but should mainly occur during the ADPs. The introduction of an SZ with stored runoff could pose little effect on nitrification (mostly aerobic process) but significantly affect denitrification by facilitating the growth of anaerobic denitrifiers to promote the removal of NO 3 − during the ADPs [20,34,[36][37][38]. This can explain the growing NO 3 − removal efficiencies with the increasing SZ depths (Figure 6), which can be strongly supported by the significant difference of ANOVA analysis (p < 0.05 for NO 3 − ).…”