Nitrate removal in biochar amended, electrically stimulated up-flow wetlands

Abstract

Human disruption of the natural nitrogen cycle has major environmental repercussions. Increased in nitrate concentrations in surface water, due to increased fertilization of agricultural farmland and subsequent run-off, can lead to saltwater ecosystem eutrophication and hypoxia, as seen seasonally in the Gulf of Mexico. A major source of nitrate contamination in Minnesota is the result of fertilizer application increasing the nitrate concentration in snow melt run-off. As the Mississippi River eventually feeds into the Gulf of Mexico, finding efficient, cost-effective treatment technologies for agricultural run-off is essential. An emerging treatment strategy of interest is the implementation of constructed wetlands, which aim to recreate the natural chemical, microbial, and physiological processes of a natural wetland. Constructed wetlands are low maintenance and are low-cost, but nutrient removal is highly dependent on temperature and flow rate. Therefore, with increased runoff during the spring, nitrate removal efficiencies in constructed wetlands decrease. To improve the constructed wetland’s nitrate removal efficiency, electrical stimulation could be applied to enhance autotrophic denitrification through the addition of an electron donor source. Here, synthetic agricultural runoff was fed to two biochar-amended, bench-scale, up-flow constructed wetland reactors to test the nitrate removal efficiency of a control system and a system with alternating between applied and no applied current. The reactors were amended with biochar, a highly porous, electroactive substrate, to enhance electron transfer efficiency and provide ample surface area for biofilm growth. The experiments were conducted twice, and effluent nitrate concentrations were monitored using segmented flow analysis. The total carbon concentration and pH were monitored throughout experimentation. Denitrifier copy numbers were measured upon experimental completion. The control systems exhibited variability, both showing low nitrate removal in experiment one and nitrate elution in experiment two. The applied current resulted in a pH change in the effluent which deteriorated the electrode and inhibited microbial growth. Without buffer, the electrically stimulated system showed low denitrification efficiencies, with high variability in performance. It is hypothesized that biochar adsorption influenced nitrate removal because reapplying current resulted in nitrate elution from the reactor. The denitrifier gene copies of DNA extracted from all experiments were low, but their presence signified the possibility of biological denitrification. Overall, low nitrate removal rates were measured throughout experimentation, but the data confirms that hydrolysis of water was occurring, which could act as an electron donor for autotrophic denitrification in the presence of a more abundant microbial community. Also, nitrate removal was achieved in both systems without a carbon source. Optimization of current density and electrode material is necessary for a complete understanding of nitrate removal mechanics, but this data reveals the practical limitations of upscaling bioelectrochemical systems in this manner.