Managed Denitrification with Woodchip Bioreactors: Pilot-Scale Testing to Address Aquaculture Wastewater Treatment Challenges.

Denitrification “woodchip” bioreactors are a promising technology to remove nitrate from agricultural effluents ranging from tile drainage to hydroponic waste water to land-based fish farm outflows. For woodchip bioreactor treatment to be successful and cost effective over the entire bioreactor life, several key design and operational issues need to be resolved. Firstly, hydraulic retention time (HRT) is a primary reactor design parameter, but there has been no woodchip bioreactor HRT model calibrated specifically for the high nitrate concentrations and other parameters (for example, biological oxygen demand (BOD), dissolved oxygen, temperature) associated with recirculated aquaculture effluent. Secondly, there is ostensible evidence of woodchip bioreactor clogging under high BOD conditions as expected with treatment of some aquacultural waters. Lastly, woodchip bioreactors release a tea-colored flush of organics during start-up, and anecdotal evidence suggests this enriched outflow may contain environmentally detrimental levels of phosphorus. The associated risk of pollution swapping (i.e., removing nitrate from water, only to increase phosphorus pollution) must be quantified. To address these concerns, four pilot-scale woodchip bioreactors (L x W x D: 3.8 x 0.75 x 0.61 m; approximately 1/10th scale based on surface footprint) were constructed and operated at the Freshwater Institute in Shepherdstown, WV. Each reactor was operated under a unique treatment HRT (12, 24, 36, or 48 hr HRTs) that will allow further calibration of woodchip bioreactor design models, with this HRT range providing an array of loading scenarios for bacterial clogging and start-up phosphorus flushing. Preliminary results from this work will be presented.