A recirculating aquaculture system (RAS) treating tilapia production wastewaters used a two-step process combining a simple clarifier and a submerged surface flow (SSF) constructed wetlands for suspended solids removal and removal of nitrogenous compounds. This system successfully supported a commercial scale level of production (>35 kg/m3) for over 36 months of operation. The innovative SSF wetland design incorporated a high hydraulic loading rate (3.03 m/day), larger effective diameter media (380 mm), and a deeper bed depth (0.90 m) than previously suggested design guidelines. The SSF wetland flow pattern was characterized as plug flow with dispersion, but this analysis based on bed volume data indicated that media porosity was reduced from an assumed design value of 54-27% under operating conditions. The TSS, TAN, NO2-N, and NO3-N, percent mass removal for the SSF wetlands was 67.2, 46.0, 87.0, and 40.6, respectively. The TSS, TAN, NO2-N, and NO3-N mass removal for the SSF wetlands was 8.21, 0.58, 0.63, and 0.93 g/(m2 day), respectively. Optimal performance of the SSF wetlands with simultaneous removal of TAN and NO2-N, and NO3-N occurred at TAN loadings less than 6.0 g/(m2 day). Bed depth and hydraulic loading rates were major factors controlling this aerobic/anaerobic removal of nitrogen. The wetlands appeared to be oxygen limited at very high TAN loadings above 6.0 g/(m2 day). Site elevation (1189 m) and warm culture temperatures (∼25 °C) contributed, but supplemental aeration could provide better TAN removal. Summary of design parameters were presented. Mean KT values calculated for TSS, TAN, NO2-N, and NO3-N were 9.861, 0.614, 20.033, and 8.292 days-1 compared favorably to other SSF systems.
Bibliographical noteFunding Information:
This research was supported by a grant from USDA CREES program and the USDA sponsored Western Regional Aquaculture Consortium, and in part, by a grant from USDOE, Idaho Falls, ID. Further support for the project was provided by the Department of Civil, Agriculture, and Geological Engineering and the Southwest Technology Development Institute both at New Mexico State University; Las Cruces, NM. The authors would like to thank Ronald Polka for his assistance in this project.
- SSF wetlands