Nitrate removal effectiveness of fluidized sulfur-based autotrophic denitrification biofilters for recirculating aquaculture systems

Authors: CHRISTIANSON, LAURA - Freshwater Institute; LEPINE, CHRISTINE - Freshwater Institute; TSUKUDA, SCOTT - Freshwater Institute; SAITO, KEIKO - University Of Maryland; SUMMERFELT, STEVEN - Freshwater Institute

Submitted to: Journal of Aquaculture Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/17/2015
Publication Date: 7/21/2015
Citation: Christianson, L., Lepine, C., Tsukuda, S., Saito, K., Summerfelt, S. 2015. Nitrate removal effectiveness of fluidized sulfur-based autotrophic denitrification biofilters for recirculating aquaculture systems. Journal of Aquaculture Engineering. 68:10-18.

Interpretive Summary: Reducing the amount of nutrients in agricultural outflows continues to be important for environmental health and the sustainability of our agricultural systems. One such solution may come from “denitrification biofilters”, where excess nitrate in water (from aquaculture wastewater) may be converted into harmless and inert nitrogen gas. Previous work characterized the best size sulfur particles, which could theoretically be used to create such a biofilter, as grain sized. Taking this preliminary research to the next step, sulfur “grains” were fluidized (“fluidization”, like quicksand) in pilot-scale biofilters and analyzed for nitrate removal during different retention times (how long water was retained within the system). This experiment demonstrated the highest reported nitrate removal rate for any sulfur biofilter system, though the removal rate was low compared to other types of denitrification biofilters. Longer retention times resulted in higher nitrate removal rates, but created challenges for retaining sulfur grains within the fluidized system. These results are important to USDA ARS stakeholders because they provide additional information about alternative clean-water treatment technologies. Finding new low-cost and efficient ways to reduce nutrients such as nitrate will continue to be important as land-based aquaculture production increases and regulatory permitting requirements for water discharge strengthen.

Technical Abstract: There is a need to develop practical methods to reduce nitrate -nitrogen loads from recirculating aqua-culture systems to facilitate increased food protein production simultaneously with attainment of water quality goals. The most common wastewater denitrification treatment systems utilize methanol-fueled heterotrophs, but sulfur-based autotrophic denitrification may allow a shift away from potentially expensive carbon sources. The objective of this work was to assess the nitrate-reduction potential of fluidized sulfur-based biofilters for treatment of aquaculture wastewater. Three fluidized biofilters (height 3.9 m, diameter 0.31 m; operational volume 0.206 m3) were filled with sulfur particles (0.30 mm effective particle size; static bed depth approximately 0.9 m) and operated in triplicate mode (Phase I: 37 -39%expansion; 3.2 -3.3 min hydraulic retention time; 860 -888 L/(m2min) hydraulic loading rate) and independently to achieve a range of hydraulic retention times (Phase II: 42 -13% expansion; 3.2 -4.8 min hydraulic retention time). During Phase I, despite only removing 1.57 plus or minus 0.15 and 1.82 plus or minus 0.32 mg NO3 -N/Leach pass through the biofilter, removal rates were the highest reported for sulfur-based denitrification systems (0.71 plus or minus 0.07 and 0.80 plus or minus 0.15 g N removed/(L bioreactor-d)). Lower than expected sulfate production and alkalinity consumption indicated some of the nitrate removal was due to heterotrophic denitrification, and thus denitrification was mixotrophic. Microbial analysis indicated the presence of Thiobacillus denitrificans, a widely known autotrophic denitrifier, in addition to several heterotrophic denitrifiers. Phase II showed that longer retention times tended to result in more nitrate removal and sulfate production, but increasing the retention time through flow rate manipulation may create fluidization challenges for these sulfur particles.