In 2016, Sustainable Environmental Consultants (SEC) began looking to expand their agricultural compliance and engineering division portfolio with denitrifying bioreactor design services. Used to treat subsurface drainage water from agricultural fields, bioreactors are an edge-of-field practice that help to reduce nitrate loading to nearby streams. They are essentially underground wood chip “filters” designed to route subsurface drainage water through a buried trench. Natural bacteria in the bioreactor use the carbon from the wood chips as their food source and convert nitrates in the water to nitrogen gas during the respiration process, also known as denitrification. The nitrogen gas is then freely released into the atmosphere while the water is discharged with a reduced nitrate concentration.
Iowa is one of 12 states that comprises the Mississippi River Basin, which has generated a Nutrient Reduction Strategy to improve water quality and combat the hypoxia zone in the Gulf of Mexico. Iowa’s goal is to reduce nitrogen and phosphorus loading by 45%. According to the Iowa Nutrient Reduction Strategy, bioreactors are an effective land management practice, estimated to reduce 43% of nitrate loads annually to local streams. In the summer of 2019, SEC signed a contract with the Iowa Department of Agriculture and Land Stewardship (IDALS) to design bioreactors throughout central Iowa. Each bioreactor will be developed in three phases.
Phase 1 – Design & Layout
Phase 1 is the design and layout phase. During this phase, IDALS completes an initial field investigation to acquire site specific survey data and information needed to facilitate the design process. This typically includes a map of the existing subsurface drainage system, identifying the location and diameter of subsurface pipes and their outlet locations. SEC utilizes this information to design and place the bioreactor. To maintain productivity and soil quality of the field, bioreactors are generally built on the edge of the field. An initial water sample is also collected during this phase for a pre-bioreactor and post-bioreactor analysis and comparison.
Phase 2 – Landowner Operation & Management Plan
Phase 2 involves providing the landowner with an operation and management plan. The plan outlines site-specific maintenance for the bioreactor, including items such as managing the water level in the bioreactor, inspection and maintenance requirements, and monitoring the wood chips for replacement. To reach the estimated lifespan of 20 years for feeding the microbes, the wood chips require a consistent flow of water, which is controlled within the drainage structure by way of stoplogs that are moved up and down seasonally and for rain events.
Phase 3 - Construction
Phase 3 is the construction phase. This includes coordinating a pre-construction meeting with the contractor and providing subsequent oversight for critical aspects of the bioreactor. A post-installation inspection is also completed to ensure the bioreactor was constructed as designed and functioning properly.
Additional states in the Mississippi River Basin have identified bioreactors to be a best management practice (BMP) in improving water quality and helping to meet water quality targets. The SEC team looks forward to assisting with the implementation of future bioreactors and accelerating their adoption rate.