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Terra Cubes Design Final Technical Report: Design of a Modular Bioretention System for Nitrate, Phosphate and Herbicide Treatment Public Deposited

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  • Plant nurseries comprise the largest agricultural sector in Oregon at nearly $1 billion per year. Even at this scale, individual nurseries and greenhouses do not have clear water quality standards set by the EPA. Watershed-level total maximum daily loads (TMDLs) have been set for excess nutrients and herbicides due to their harmful impacts on waterways. Nitrates and phosphates in runoff can lead to harmful algae blooms and subsequent hypoxic “dead zones” through eutrophication. Herbicides such as glyphosate (Roundup) are toxic to aquatic life and dangerous enough to human health to have maximum contaminant levels (MCLs) set for drinking water. To address the challenge of untreated nursery effluent, the Terra CuBES team developed a compact, modular bioretention system using an upcycled intermediate bulk container (IBC) tote with replaceable exterior adsorption columns and a passive solar heater. Removal targets for nitrate, phosphate, diuron, and glyphosate were set between 95-99% removal, based loosely on drinking water standards. The Terra Cube was designed with nitrate removal as the limiting factor for the initial design flow of 150 L/day, based on a comprehensive literature review of similar systems. The Cube depends on biological denitrification for nitrate removal, using corn and squash as labile carbon sources. Phosphate removal was designed to rely on uptake from a variety of wetland sedges and rushes as well as activated carbon as a polishing step. The adsorption columns were sized for removal of herbicides for a six-month period, with adsorptive materials including biochar, chitosan (upcycled and processed shrimp shells), and activated carbon. After the six-month cycle, the columns would need to be leached with an acid rinse to remove adsorbed materials and free binding sites. The system was tested for two months at four flow rates from 120 L/day to 500 L/day with contaminant concentrations commonly released from plant nurseries and greenhouses. The Cube was evaluated against a bioretention system called the “Grattix”, which is comprised of gravel, soil, and wetland plants. The Cube significantly outperformed the Grattix and exceeded removal targets for nitrates and glyphosate. Diuron could not be tested due to budget constraints and reagent availability, and both systems leached additional phosphates throughout the testing phase. Nitrate removal exceeded expectations twofold compared to literature values, achieving 99.8% removal at 150 L/day and 97.5% removal at 500 L/day. Glyphosate concentrations at the final outlet showed 99.3% removal at 150 L/day and 99.2% removal at 500 L/day. Phosphate leaching occurred in both systems, likely due to existing phosphate present in the soil and adsorption media and cold temperatures that did not promote plant uptake. If the Terra Cube was scaled up to production, it is estimated that it would have a net present cost of $2640 after accounting for construction, maintenance, disposal, and labor. It would treat 183 cubic meters annually at a flow rate of 500 liters per day. It would cost $2.54 per cubic meter of effluent treated. Each cube is estimated to treat runoff form a greenhouse a tenth of an acre large. The Terra Cube is a promising technology for commercial greenhouses, but the current model may be too expensive to use on an agricultural scale. Deploying the system inside greenhouses may eliminate the need for a solar heater and promote denitrification. Chitosan did not yield effective relative removal rates, and its potential for clogging discourages its use. The success of nitrate and glyphosate removal indicates that further testing at higher flows is necessary to evaluate the upper ends of performance and the product lifespan.
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