Denitrification in Wood Chip Bioreactors at Different Water Flows

Abstract

Subsurface drainage in agricultural watersheds exports a large quantity of nitrate-nitrogen (NO(3)-N) and concentrations frequently exceed 10 mg L(-1). A laboratory column study was conducted to investigate the ability of a wood chip bioreactor to promote denitrification under mean water flow rates of 2.9, 6.6, 8.7 and 13.6 cm d(-1) which are representative of flows entering subsurface drainage tiles. Columns were packed with wood chips and inoculated with a small amount of oxidized till and incubated at 10 degrees C. Silicone sampling cells at the effluent ports were used for N(2)O sampling. (15)Nitrate was added to dosing water at 50 mg L(-1) and effluent was collected and analyzed for NO(3)-N, NH(4)-N, and dissolved organic carbon. Mean NO(3)-N concentrations in the effluent were 0.0, 18.5, 24.2, and 35.3 mg L(-1) for the flow rates 2.9, 6.6, 8.7, and 13.6 cm d(-1), respectively, which correspond to 100, 64, 52, and 30% efficiency of removal. The NO(3)-N removal rates per gram of wood increased with increasing flow rates. Denitrification was found to be the dominant NO(3)-N removal mechanism as immobilization of (15)NO(3)-N was negligible compared with the quantity of (15)NO(3)-N removed. Nitrous oxide production from the columns ranged from 0.003 to 0.028% of the N denitrified, indicating that complete denitrification generally occurred. Based on these observations, wood chip bioreactors may be successful at removing significant quantities of NO(3)-N, and reducing NO(3)-N concentration from water moving to subsurface drainage at flow rates observed in central Iowa subsoil.

Internal hydraulics of an agricultural drainage denitrification bioreactor

“…8). Others have similarly noted increased N removal at higher retention times in both lab and field studies (Chun et al, 2009;Greenan et al, 2009;Woli et al, 2010;Christianson et al, 2012). Here, to meet a 45% reduction of N as recommended by the U.S. Environmental Protection Agency Science Advisory Board (USEPA, 2007), the original data would require a theoretical retention time of 27 h, and the modified models would require 15 and 8.5 h for the May 30 tracer (outlet only sampled) and May 17 tracer (all wells sampled) model corrections, respectively.…”

Temperature and Substrate Control Woodchip Bioreactor Performance in Reducing Tile Nitrate Loads in East-Central Illinois

“…A wide range of nitrate removal effectiveness (up to 98% nitrate reductions) in laboratory and field studies has been reported in the literature (Greenan et al, 2009;Chun et al, 2009Chun et al, , 2010Moorman et al, 2010;Verma et al, 2010;Woli et al, 2010;Christianson et al, 2012b;Bell et al, 2015). Most have been designed as plasticlined trenches, filled with mixed-species wood chips, that receive tile flow from fields ranging from 1 to 20 ha in size (e.g., Woli et al, 2010;Christianson et al, 2012b).…”

Efficiency and detrimental side effects of denitrifying bioreactors for nitrate reduction in drainage water

“…As nitrate (NO 3 -N) is highly soluble and mobile, excess NO 3 -N may be released to groundwater and surface waters, reducing water quality there (Ciarlo et al 2007;Kröger et al 2007;Gordon et al 2008;Greenan et al 2009;Seitzinger et al 2010;Chintala et al 2013). Denitrifying bioreactors within subsurface water pathways have proven to be effective tools for the mitigation of NO 3 -N loads in impacted groundwater or drainage water (Schipper et al 2010a;Christianson et al 2012b).…”

“…Denitrifying bioreactors facilitate denitrification by supplying these bacteria with an additional carbon source in an oxygen-poor and nitrate-enriched environment (Schipper et al 2010a;Christianson et al 2012b). The efficiency of the NO 3 -N removal in bioreactors depends on the retention time (Chun et al 2009;Greenan et al 2009), the amount and quality of the organic carbon source (Robertson et al 2000;Schipper et al 2010a), the NO 3 -N concentration in the source water (Schipper et al 2005;Chun et al 2009), the temperature (Robertson et al 2000), and the pH (Rivett et al 2008). Infield subsurface bioreactors can reduce NO 3 -N concentrations by up to 100 % of the original loads and often maintain their removal efficiency over several years (Robertson et al 2000;Schipper et al 2010a;Woli et al 2010).…”

“…Large amounts of DOC in the effluent of bioreactors may enhance oxygen consumption in the recipient stream and reduce the saprobic state (Teufl et al 2013). DOC release from bioreactors is highly variable and ranges from 0 to 45 mg L −1 in the literature (Robertson et al 2000;Healy et al 2006;Greenan et al 2009), with peak concentrations of several 100 mg L −1 during the start phase (Greenan et al 2009;Schipper et al 2010b). Alternating desiccation and flushing of denitrifying bioreactors could, thus, result in high DOC releases.…”