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

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 71% 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 $3737 for water quality work and $4666 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.