Reactive ditches: A simple approach to implement denitrifying wood chip bioreactors to reduce nitrate exports into aquatic ecosystems?

Abstract

Despite intensive efforts motivated by the European Water Framework Directive, many water bodies still suffer from poor water quality in Germany. Intensively drained agricultural areas are still a critical source for nitrate which is responsible for negative effects on aquatic ecosystems. Basic measures such as the fertiliser ordinance are expected to be not sufficient to completely eliminate nitrate exports via drainage tiles and ditches. Consequently, there is the demand to manage the reduction of nitrate concentrations with new additional end of pipe solutions. For this, the presented study focuses on a simply to implement reactive ditch for denitrification, which is installed into drainage ditches to reduce nitrate concentrations. An existing drainage ditch that is fed by tile drainage water was filled with wood chips to keep installation efforts as simple and cheap as possible. In situ parameters and nutrient concentrations were determined at the inflow and the outflow of the reactive ditch for 2 years.The results reveal a promising potential for wood chipbased filters to reduce nitrate concentrations by 28 % on average over all seasons. Within the filter, favourable conditions for denitrification were predominantly found without flooding of surrounding areas. Investigations revealed decreasing nitrate concentrations especially in cases of low flow rates and high temperatures. These encouraging results demonstrate the successful application of reactive ditches under German lowland conditions in general. Since tile drainages are installed for many agricultural areas in this region, there seems to be high potential for the application of this easily implementable type of bioreactor.

Effectiveness of Denitrifying Bioreactors on Water Pollutant Reduction from Agricultural Areas

“…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).…”

Sizing an open-channel woodchip bioreactor to treat nitrate from agricultural tile drainage and achieve water quality targets

“…Woodchip bioreactors have demonstrated the capacity for substantial nitrate removal from agricultural tile drain water and runoff over a variety of designs, including subsurface flow, tile drain sidewall, in-streambed, and in-channel alternatives (Blowes et al 1994;Jaynes et al 2008;Robertson and Merkley 2009;Schipper et al 2010a;Pfannerstill et al 2016). WBRs have been more widely studied in the Midwest over the past 15 years as subsurface flow systems intercepting tile drain water.…”

Soil and water management: opportunities to mitigate nutrient losses to surface waters in the Northern Great Plains

“…For example, Salo et al (2015) demonstrated that a woodchip bioreactor system with iron media as post-treatment removed nitrate and phosphate from subsurface drainage. While bioreactors are typically applied for tile drains, they can also be used in surface drainage (Pfannerstill et al 2016;Pluer et al 2016). Bioreactor technology has not been directly assessed in Northern Great Plains landscapes, but given that bioreactors tend to be most effective under warm conditions and at low-flow volumes (Pfannerstill et al 2016), they may not be ideally suited to snowmelt dominated drainage.…”

“…While bioreactors are typically applied for tile drains, they can also be used in surface drainage (Pfannerstill et al 2016;Pluer et al 2016). Bioreactor technology has not been directly assessed in Northern Great Plains landscapes, but given that bioreactors tend to be most effective under warm conditions and at low-flow volumes (Pfannerstill et al 2016), they may not be ideally suited to snowmelt dominated drainage. However, with adequate effluent storage capacity or additional chemical amendments (e.g., physico-chemical treatments such as P sorbing materials) and sufficient retention time to enable substantial nutrient removal (Bock et al 2015;Addy et al 2016;Hoover et al 2016), the relatively low cost, and small footprint (Moorman et al 2015) of bioreactors and chemical nutrient removal technologies may make them an appealing nutrient management BMP in this landscape.…”