Use of vegetated drainage ditches and low-grade weirs for aquaculture effluent mitigation: I. Nutrients

- Effluent was passed through a vegetated ditch fit with low-grade weirs.
- Loads of all analytes tested were decreased by the system.
- Concentrations of TP, NO2−-N, and NO3−-N were decreased, while SRP increased.
- Comparable nutrient reductions as constructed wetlands and settling basins.
- Achieved decreases with a 2 h hydraulic residence time.

Implementation of water management practices could be a strategy for reducing mass discharge from aquaculture production. Of greatest concern is the delivery via discharge of excess loads of nitrogen and phosphorus to downstream systems and their effects on ecological impairments. This study assessed effects of consecutive low-grade weirs on chemical retention of aquaculture pond effluent in a single, vegetated drainage ditch. Two control (without weirs) and nine treatment (with weirs) discharges were conducted September to October 2012 at the Mississippi State University South Farm Aquaculture Facility. From discharge to drainage ditch outlet, control discharges had a 154% increase in soluble reactive phosphorus (SRP) load, whereas total phosphorus (TP), total ammonia nitrogen (TAN), and nitrate (NO3−-N) loads decreased (47%, 43%, and 63%, respectively). Additionally, concentrations of TP (F = 4.59, P = 0.02) and TAN (F = 6.70, P < 0.001) in control discharges were significantly decreased. In treatment discharges, nutrient mass loads decreased across all analytes (80% SRP, 86% TP, 89% TAN, 89% NO3−-N). Treatment discharges' concentrations of TP, nitrite, and NO3−-N (F = 4.95, P = 0.01; F = 2.91, P = 0.06; F = 5.18, P = 0.01, respectively), were significantly reduced while SRP concentrations increased significantly (F = 12.60, P < 0.001). Changes in concentration were further analyzed with additional fixed effects of distance to weir 1, number of discharges prior, initial effluent concentration, and flow rate; initial effluent concentrations were shown to have a significant negative effect on TP (F = 4.28, P < 0.001) and NO3−-N (F = 2.41, P = 0.02) concentration decrease. Results of this study indicate utilizing a ubiquitous landscape feature such as a vegetated drainage ditch with installed low-grade weirs has the potential to reduce nutrient mass loads moving downstream and can be added as a promising effluent mitigation strategy for aquaculture.