Woodchip bioreactors as treatment for recirculating aquaculture systems’ wastewater: A cost assessment of nitrogen removal

Authors: LEPINE, CHRISTINE - Freshwater Institute; CHRISTIANSON, LAURA - University Of Illinois; DAVIDSON, JOHN - Freshwater Institute; SUMMERFELT, STEVEN - Freshwater Institute

Submitted to: Aquacultural Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/7/2018
Publication Date: 9/27/2018
Citation: Lepine, C.A., Christianson, L.E., Davidson, J.W., Summerfelt, S.T. 2018. Woodchip bioreactors as treatment for recirculating aquaculture systems’ wastewater: A cost assessment of nitrogen removal. Aquacultural Engineering. 83:85-92. doi:10.1016/j.aquaeng.2018.09.001.
DOI: https://doi.org/10.1016/j.aquaeng.2018.09.001

Interpretive Summary: Denitrification "woodchip" bioreactors are simple systems designed to naturally remove excess nitrogen from water. These systems have been used in the mid-western United States to treat agricultural runoff. More recently, the aquaculture sector has demonstrated the systems to be acceptable for treating wastewater but no study has addressed if these systems are cost-effective in their new application. This study estimated capital costs for a full-scale bioreactor designed for effluent from a land-based aquacultural facility and previous pilot-scale experimental data was used to estimate bioreactor lifetime nitrogen removal costs. Final cost efficiencies ranged from $2.83 to $13.35 per kg N removed, which is comparable to cost-estimates for bioreactors used for other applications, making bioreactors an alternative N treatment option for aquaculture farmers.

Technical Abstract: Denitrifying “woodchip” bioreactors are engineered systems, consisting of a carbon filled trench (e.g., with woodchips), designed to remediate nitrogen (N)-enriched water through naturally occurring denitrification, a process where microbes reduce nitrate into inert di-nitrogen gas during their respiration processes. Recent studies have demonstrated the feasibility of woodchip bioreactors for treating aquacultural wastewater, specifically the concentrated effluents generated from recirculating aquaculture (RAS), with the caveat that system lifespan can be reduced from clogging associated with high organic solids loading and bacterial overgrowth. Because this technology is relatively new, particularly for aquaculture applications, lifetime cost-efficiency has not been fully assessed. A cost-estimate of N removal over a one- to five-year anticipated lifespan was obtained by estimating initial capital, recurring, and operational expenditures of a full-scale bioreactor system designed for a RAS production facility, using N removal rates from previous pilot-scale aquaculture wastewater bioreactor research. Assumptions included static N removal rates of 6.06 or 11.75 kg of N removal per year for conservative and maximum sensitivities, respectively 49 and 71percent N removal efficiency, and at least one woodchip replacement over the system lifetime across a ten-year planning horizon. Initial capital expenditure totaled $47,838 or roughly $139.88 per m3 installed with woodchip replacements each $19,469. Ten-year operational expenditure total present value costs included of $3,737 for water quality work and $4,666 for lifetime maintenance. Cost per kg of N removed per year ranged from $13.35 to $2.83, dependent on woodchip replacement scenarios of one- to five-years, respectively, which demonstrated denitrification bioreactors might offer a low-cost N treatment option for aquacultural farmers.