Enhanced Denitrification Bioreactors Hold Promise for Mid‐Atlantic Ditch Drainage

Abstract

Core Ideas Bioreactors can be designed to remove nitrate from drainage ditches. Designing bioreactors for ditch drainage requires site‐specific flexibility. All mid‐Atlantic ditch bioreactors tested removed nitrate from drainage water. Practical concerns will require adjustments to design and installation. There is strong interest in adapting denitrifying bioreactors to mid‐Atlantic drainage systems to help address Chesapeake Bay water quality goals. Three ditch drainage‐oriented bioreactors were constructed in 2015 in Maryland to evaluate site‐specific design and installation concerns and nitrate (NO3–N) removal. All three bioreactor types removed NO3–N, as measured by load and/or concentration reduction, showing promise for denitrifying bioreactors in the mid‐Atlantic's low gradient Coastal Plain landscape. The ditch diversion bioreactor (25% NO3–N load reduction; 0.97 g NO3–N removed m−3 d−1) and the sawdust denitrification wall adjacent to a ditch (>90% NO3–N concentration reduction; 1.9–2.9 g NO3–N removed m−3 d−1) had removal rates within range of the literature. The in‐ditch bioreactor averaged 65% NO3–N concentration reduction, but sedimentation is expected to be one of the biggest challenges. A robust water balance is critical for future assessment of bioreactors’ contribution to water quality improvement in low gradient mid‐Atlantic landscapes.

Effectiveness of Denitrifying Bioreactors on Water Pollutant Reduction from Agricultural Areas

“…Three design modifications have been trialed: ditch-diversion bioreactors designed according to the NRCS Conservation Practice Standard, in-ditch bioreactors, and sawdust denitrification walls. The two ditch-diversion bioreactors have achieved 2% and 25% N load reduction efficiencies (Rosen, unpublished;Christianson et al, 2017). Load reduction efficiencies could not be calculated for the in-ditch bioreactors or sawdust denitrification walls due to difficulties in accurately assessing flow volumes; however, both designs achieved N concentration reductions >65%.…”

“…For example, sawdust denitrifying walls may require hydrogeological investigations to determine N loading and treatment effectiveness. Christianson et al (2017) suggested that it may be more practical to create groundwater flow rate estimates for regions within the Chesapeake Bay watershed, which could be used for denitrifying wall N loading estimates. Overall, more long-term studies, which face many challenges including funding cycles and limitations, staff turnover, and the need for long-term commitments by field site partners (e.g., landowners), are needed to advance bioreactor technologies.…”

“…Consideration must be given to minimizing flow restriction with in-ditch bioreactors, as flow conveyance is the primary responsibility of a ditch network. These designs have included wooden berms, gravel, and/or wire or plastic mesh to create woodchip bags or mattresses (Chase et al, 2019;Christianson et al, 2017;Dhaese et al, 2019;Pfannerstill et al, 2016;Robertson and Merkley, 2009). Design of these in-ditch systems has borrowed from the NRCS Conservation Practice Standard, but these systems have unique design, construction, and maintenance concerns (e.g., bypass flow goes over the top of the bioreactor, sedimentation is a significant issue).…”

The Possible Roles of Escherichia coli in the Nitrogen Cycle

“…With increased reliance on amended nitrogen either as ammonia-N, nitrate-N, or urea-N the environmental impacts due to ground and surface runoff are well documented (Wang et al 2012;Zhou et al 2013). Many approaches to remediate the problem have been attempted and are based on providing carbon as a source for bacterial growth (Christianson et al 2017). On the Delmarva Peninsula, poultry litter has been the primary fertilizer for corn and other crops (Howarth et al 2002;Glibert et al 2006), and contains Urea-N at 3% by weight on the average (Bolan et al 2010).…”

Mulch‐Derived Organic Carbon Stimulates High Denitrification Fluxes from Agricultural Ditch Sediments

“…As research continues to focus on improving edge of field nutrient pollution reduction techniques, methods have evolved. However, techniques still face substantial barriers to implementation including cost effectiveness and maintenance requirements (Christianson et al, 2013, 2017). Our results build on previous applications of bioreactors by demonstrating that a minimally engineered application of an overlying hardwood mulch and gypsum layer at the sediment–water interface can remove substantial proportions of contributed N loads via denitrification (65–69%).…”