Optimizing hydraulic retention times in denitrifying woodchip bioreactors treating recirculating aquaculture system wastewater

Authors: LEPINE, CHRISTINE - Freshwater Institute; CHRISTIANSON, LAURA - Freshwater Institute; SHARRER, KATA - Freshwater Institute; SUMMERFELT, STEVEN - Freshwater Institute

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/4/2015
Publication Date: 4/25/2016
Citation: Lepine, C., Christianson, L.E., Sharrer, K.L., Summerfelt, S.T. 2016. Optimizing hydraulic retention times in denitrifying woodchip bioreactors treating recirculating aquaculture system wastewater. Journal of Environmental Quality. 45:813-821. doi: 10.2134/jeq2015.05.0242.

Interpretive Summary: Nitrogen pollution from agricultural sources is a causal agent in eutrophication, hypoxia (dead zones), and habitat degradation, resulting in a loss of biodiversity in coastal waters worldwide. Woodchip bioreactors are a farm- or field-scale technology designed to remove nitrate nitrogen from point and nonpoint sources of pollution. The primary objective of this work was to evaluate woodchip denitrifying bioreactor N-removal performance under varying hydraulic retention times (i.e., the bioreactor pore volume divided by flow rate) given influent water chemistries associated with aquaculture wastewater. Findings suggest that applying a hydraulic retention time of approximately 24 h would potentially balance the optimum nitrogen removal rate and efficiency at 18 g/(m3d) and 65%, respectively. Designs using longer hydraulic retention times may maximize nitrogen removal efficiency (as high as 39 g/(m3d)), but may also result in sulfate reduction/sulfide production under highly reduced, nitrogen-limited conditions. The woodchip bioreactors also removed over 90% of the suspended solids and some phosphorus. Research findings will help calibrate woodchip bioreactor design models specifically for recirculated aquaculture wastewater. In addition, woodchip bioreactor denitrification treatment of the relatively organic-rich aquaculture wastewater is a new and useful application of this simple water treatment technology to protect against water pollution.

Technical Abstract: The performance of wood-based denitrifying bioreactors to treat high-nitrate wastewaters from aquaculture systems has not previously been demonstrated. Four pilot-scale woodchip bioreactors (approximately 1:10 scale) were constructed and operated for 268 d to determine the optimal range of design hydraulic retention times (HRTs) for nitrate removal. The bioreactors were operated under HRTs ranging from 6.6 to 55 h with influent nitrate concentrations generally between 20 and 80 mg/L NO3-N. These combinations resulted in N removal rates >39 g/(m3d), which is greater than previously reported. These high removal rates were due in large part to the relatively high chemical oxygen demand and warm temperature (~19°C) of the wastewater. An optimized design HRT may not be the same based on metrics of N removal rate versus N removal efficiency; longer HRTs demonstrated higher removal efficiencies, and shorter HRTs had higher removal rates. When nitrate influent concentrations were approximately 75 mg/L NO3 -N (n = 6 sample events), the shortest HRT (12 h) had the lowest removal efficiency (45%) but a significantly greater removal rate than the two longest HRTs (42 and 55 h), which were N limited. Sulfate reduction was also observed under highly reduced conditions and was exacerbated under prolonged N-limited environments. Balancing the removal rate and removal efficiency for this water chemistry with a design HRT of approximately 24 h would result in a 65% removal efficiency and N-removal rates of at least 18 g/(m3d).