Environmental tradeoffs of denitrifying woodchip bioreactors

bioreactors;hypoxia;nitrate;greenhouse gases;nitrous oxide;methane;carbon dioxide

Herbstritt, Stephanie ; Cooke ; Richard A.

关键词: bioreactors;    hypoxia;    nitrate;    greenhouse gases;    nitrous oxide;    methane;    carbon dioxide;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/49622/Stephanie_Herbstritt.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

The development of row cropped agriculture on hydrologically altered landscapes in theMidwestern United States has led to environmental concerns. Tile drainage in this region acts as a conduit carrying sediment, nutrients, and other environmental pollutants to surface waters. Propelled by these concerns and a desire to meet water quality standards, innovative practices like woodchip tile bioreactors, constructed wetlands, drainage water management systems, and riparian buffer strips for treating agricultural drainage water are continually being developed and implemented across the Midwestern United States. Like drainage, bioreactors may have the potential to produce additional environmental and health problems. The production of greenhouse gases like carbon dioxide, nitrous oxide and methane are one concern. Unintended increases in these gases in bioreactors would effectively trade one problem for another with equally deleterious effects on the environment. This study is an examination of the ability of two bioreactors in Central Illinois in 2013, BR1 and BR2, to reduce nitrate-nitrogen (N) loads to surface waters. It is also an exploration of the potential unintended production of nitrous oxide, methane and carbon dioxide fluxes. The average load reduction for BR1 was 18.5% with average minimum and maximum daily load reductions of 2.1% and 69.8% respectively. The average removal rate was 13 gNO3-Nm-3 day-1. BR1 removed 268 kgNm-2 and emitted 3.2 kg N2O-Nm-2. Therefore, N2O-N represents 1.2% of the total N removed. Average N2O-N flux during the study period was 1.0mgN2O-Nm-2 hr-1. In 2013, 28.7 kgN2O-N was emitted from BR1. Average CH4-C flux from BR1 was 0.02mgCH4-Cm-2 hr-1. In 2013, 15.4 kgCH4-C was emitted from BR1.Average CO2-C flux from BR1 was 2.9 gCO2-Cm-2 hr-1. In 2013, 2256.4 kgCO2-C was emitted from BR1. This represents 7.7% of total C lost from woodchips. At this rate, the carbon in the bioreactor would be depleted in 13 years. During 2013, BR2 emitted 1.0 kgN2O-Nm-2. Average N2O-N flux during the study period was 0.5mgN2O-Nm-2 hr-1. In 2013, 9.3 kgN2O-N was emitted from BR2. Average CH4-C flux was 0.02 mgCH4-Cm-2 hr-1. In 2013, 7.0 kgCH4-C was emitted from BR2. Average CO2-C flux from BR2 was 3.1 gCO2-Cm-2 hr-1. In 2013, 2578.2 kgCO2-C was emitted from BR2. This represents 8.8% of total C lost from the woodchips. At this rate, the carbon in the bioreactor would be depleted in 11 years. Gas fluxes were higher during warmer months when nitrate removal was highest. Methane fluxes were in general negligible.Phosphorus concentrations and loads leaving the bioreactors were greater than those entering them. Phosphorus influent concentrations at BR1 averaged 0.02mgL-1 whereas effluent concentration averaged 0.3mg L-1. Phosphorus loading was 0.2 g P04-Pm-3 day-1 at BR1. Phosphorus influent concentrations at BR2 averaged 0.02 mgL-1 whereas effluent concentration averaged 0.1mgL-1. Data from BR1 was used to update the interactive module for bioreactor design and performance evaluation found on the Illinois Drainage Guide. Routines were added to use inflow and outflow data to derive hydraulic conductivity, and to use inflow data and stop log settings to estimate bypass flow. Hydraulic conductivity was found to lag flow by 6 days suggesting biofilm growth during low flow events and the flushing of biofilms from control structures during peak flow events. Bypass flow represented 47% of the flow at BR1 during the study period. The routine accurately predicted 49% bypass given measured flow data and stop log settings. The visual basic interactive module was found to be a valid tool for predicting bioreactor performance.Therefore, the routine was used to determine board settings for BR1 for future study periods that would optimize bioreactor performance. The results of this study demonstrate that bioreactors are an effective means to reducing nitrate-N loads from agricultural fields while producing minimal unintended consequences that would have a deleterious effect on the environment. Under flooded conditions, bioreactor performance is not reduced, however adverse effects like methane and hydrogen sulfide gas production and biofilm formation occurred. More long term field studies examining potential adverse effects need to be performed.