Design of wood chip denitrification bioreactors treating nitrate-N from agricultural drainage

In this field study, four bioreactors filled with wood chips were tested to reduce nitrate concentration in tile drainage water from agricultural fields in a cool maritime climate. There were 12 experimental forage plots arranged in two rows of six. Six plot tile lines enter on one side of the collection hut and six on the other. The plots were spaced 32 m apart and separated by buffer lines located 10 m from the plot lines. All tiles were 0.1 m diameter and had a slope of 2%. Based on plot discharge curves, drainage coefficients were calculated to be 0.21 – 0.25. Maximum discharge per plot was recorded as approximately 2L s-1. The soil has a gravelly texture, so most flow events occur within 36 hours of a rainfall event. During the drier parts of the season, precipitation exceeding 10 mm will initiate the flow. Each plot is 22 m x 60 m for a total tile drainage area of 1.6 ha. All four bioreactors measured 24 x 3 x 1 m (l x w x d). Individual drainage plot tile lines were routed into a drainage hut, where they can be piped into 1 of 4 pump pits. Each pump pit was fitted with a submersible water pump, which pumps the collected drainage water to 1 of 4 dedicated bioreactors. Each pump has an outflow capacity of 1.5L s-1, which is equal to the treatment capacity of the constructed bioreactors having an 8 h retention time. Eight flow control structures (Agri-Drain) were installed on both ends of the bioreactor. The experimental layout of the forage treatments in the field plots was an incomplete Latin square design with three blocks. The forage treatments consisted of two typical regional cropping systems x two methods of dairy slurry manure application; conventional broadcasting and low trajectory broadcasting. The three reps are combined into one bioreactor for each treatment. Preliminary results indicate that bioreactors remove significant amounts of nitrate from drainage water under eastern Newfoundland and Labrador conditions.