Metal leaching and toxicity of denitrifying woodchip bioreactor outflow - Potential reuse application

Authors: LEPINE, CHRISTINE - Freshwater Institute; CHRISTIANSON, LAURA - University Of Illinois; SOUCEK, DAVID - University Of Illinois; MCISAAC, GREGORY - University Of Illinois; SUMMERFELT, STEVEN - Superior Fresh

Submitted to: Journal of Aquaculture Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/14/2020
Publication Date: 10/14/2020
Citation: Lepine, C.A., Christianson, L., Soucek, D., Mcisaac, G., Summerfelt, S. 2020. Metal leaching and toxicity of denitrifying woodchip bioreactor outflow - Potential reuse application. Journal of Aquaculture Engineering. 93:102129. https://doi.org/10.1016/j.aquaeng.2020.102129.
DOI: https://doi.org/10.1016/j.aquaeng.2020.102129

Interpretive Summary: Recent cost-effective technologies such as denitrifying woodchip bioreactors are useful nitrogen-remediation solutions. However, this technology may inadvertently release harmful woodchip-bound compounds through pollution swapping, where the reduction of one pollutant subsequently increases another. Pollution swapping could prevent bioreactor outflows from reuse in other applications, such as aquaculture. Outflowing water from six experimental bioreactors was captured and analyzed for metals content and its aquatic toxicity to a common invertebrate indicator species, Ceriodaphnia dubia, under four real-world conditions. Woodchips initially leached some metals at concentrations of concern, but aquatic toxicity was limited. Woodchip outflows remained low in metals and non-toxic under most operating conditions demonstrating its reuse potential.

Technical Abstract: Denitrifying woodchip bioreactors, a nutrient mitigation technology, are designed to remediate nitrate-rich water, but this treatment process may also inadvertently leach wood-bound organic and inorganic elements or compounds. Dissolved nutrients and toxic metals in bioreactor leachate could impact downstream environments and prevent the reuse of bioreactor outflows in applications such as aquaculture. In the present study, a 206-d column experiment with two hardwood species (white ash, Fraxinus Americana; Norway maple, Acer platanoides) included operation under a range of conditions (bioreactor startup, steady-state, reducing conditions, and drying and rewetting cycles) to evaluate risks related to potential reuse of bioreactor outflow in an aquaculture system. Column outflows were evaluated for a spectrum of potentially harmful dissolved nutrients and metals and also for acute and chronic toxicity to a sensitive invertebrate test organism, the cladoceran Ceriodaphnia dubia. Woodchips initially leached copper (Cu) and zinc (Zn) at concentrations of either concern for salmonid production (6-70 µg/L as Cu and 5-269 µg/L as Zn, ranges based on water hardness) or exceeding recommended aquatic life water criteria (15.57 and 10.69 µg/L as Cu and 204.42 and 140.79 µg/L as Zn, calculated using pH 7.5, dissolved organic carbon of 1 mg/L, and hardness of 191 and 123 mg/L as CaCO3 for ash and maple, respectively). Metal leaching generally resolved by 21 cumulative pore volumes, where pore volume represents the total water volume within the woodchip packed column, regardless of operating conditions. Elevated concentrations of tannins and lignin and total ammonia nitrogen (range 1.5-8.4 mg/L tannic acid and 0.10-9.45 mg TAN/L) were detected in the outflow throughout the study and were influenced by operational phase. Acute toxicity was not detected through the first ten pore volumes except at three cumulative pore volumes in the ash outflow. During drying and rewetting cycles, chronic toxicity was detected in maple outflows, though the measured toxicity was not correlated to measured water chemistries, indicating toxicity may have occurred as an additive effect of combined toxicants. Significant differences in outflow water quality between the ash and maple wood species were not consistent enough for one species to be recommended over the other, regardless of the test phase. Results indicated woodchip bioreactor outflows may be applicable for aquacultural reuse provided operators utilize appropriate pre-treatments such as woodchip flushing, aeration, ozonation, and UV treatments.