Assessment of Woodchip Bioreactor Characteristics and Their Influences on Joint Nitrate and Pesticide Removal

We constructed a bench-scale continuous-flow (8 L total volume, 4.3 L/day) woodchip bioreactor and operated the reactor under field-like conditions to evaluate joint pesticide and nitrate removal. The continuous-flow reactor achieved 83.5 ± 8% diuron removal and 61.6 ± 11.9% imidacloprid removal with a 24 h hydraulic retention time (HRT). We designed a sequencing-batch reactor configuration (8 L total volume) to evaluate the impact of an aerobic phase on denitrification and pesticide removal performance. The sequencing-batch reactor achieved 89.2 ± 8.8% nitrate removal with a hydraulic retention time (HRT) of 12 h, while the continuous-flow design achieved 55.6 ± 9.1% nitrate removal with a 12 h HRT. There was no significant difference between pesticide removal between sequencing-batch and continuous-flow reactor types (p = 0.655 and p = 0.316 for diuron and imidacloprid removal, respectively). Kinetic batch tests revealed sorption, not microbial degradation, as the main mechanism of removal for both diuron and imidacloprid under denitrifying conditions. Imidacloprid removal ranged from 440.4 to 532.0 ng/g-woodchip (dry mass) and diuron removal between 468.6 and 553.8 ng/g-woodchip (dry mass) over 24 h. The bench-scale evaluation of pesticide behavior in woodchip bioreactors highlights the need to improve microbial degradation in such best management practices for pesticide removal.