Effect of different C/N ratios and hydraulic retention times on denitrification in saline,recirculating aquaculture system effluents |
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| Institution: | 1. Department of Aquaculture, University of Dubrovnik, ?ira Cari?a 4, Dubrovnik 20000, Croatia;2. Department of Marine Biotechnology, Institute of Marine and Environmental Technology, University of Maryland Baltimore County, 701 E. Pratt St., Baltimore, MD 21202, USA;3. Department of Biological Sciences, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA;4. Institute of Oceanography and Fisheries, PO Box 500, ?etali?te Ivana Me?trovi?a 63, Split 21000, Croatia;5. Department of Fisheries, Beekeeping, Game Management and Special Zoology, University of Zagreb Faculty of Agriculture, Sveto?imunska cesta 25, Zagreb 10000, Croatia;6. Sustainable Aquaculture Systems Inc., 715 Pittstown Road, Frenchtown, NJ 08825, USA |
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| Abstract: | Data on operation and performance of cost-effective solutions for end-of-pipe removal of nitrate from land-based saltwater recirculating aquaculture systems (RAS) are scarce but increasingly requested by the aquaculture industry. This study investigated the performance of a (semi)commercial-scale fixed-bed denitrification unit using single sludge for treating effluent from a commercial, saltwater RAS used for production of Atlantic salmon (Salmo salar). A fixed-bed denitrification reactor was fed continuously with 3-days hydrolyzed sludge from the commercial RAS, and was operated at different hydraulic retention times (HRTs; 1.82, 3.64, 5.46, or 7.28 h) or influent C/N ratios (3, 5, 7, or 10). Twenty-four h pooled samples were collected from the inflowing RAS water and the hydrolyzed sludge as well as from the denitrification reactor outlet, and samples were analyzed for nutrients and organic matter content.Nitrate removal rates increased consistently with decreasing HRT (from 64.3 ± 5.2–162.7 ± 22.0 g NO3-N/m3/d within the HRTs tested) at non-limiting C/N ratios, while nitrate removal efficiencies decreased (from 99.6 ± 0.3–58.2 ± 8.9 %). With increasing influent C/N ratios at constant HRT (3.64 h), nitrate removal rates increased until the removal efficiency was close to 100 % and nitrate concentration in the denitrification reactor became rate-limiting. A maximum nitrate removal rate of 162.7 ± 2.0 g NO3-N/m3/d was achieved at a HRT of 1.82 h and an influent C/N of 6.6 ± 0.5, while the most efficient use of hydrolyzed sludge (0.19 ± 0.02 g NO3-N removed/g sCOD supplied) was obtained with a HRT of 3.64 h and a C/N ratio of 2.9. Removal rates of organic matter significantly and consistently increased with decreasing HRT and increasing C/N ratio. In addition, reducing HRT and increasing C/N ratios significantly improved removal of total phosphorus (TP) and PO4-P.In conclusion, optimal management of the operating parameters (HRT and C/N ratio) in a single-sludge denitrification process can significantly reduce the discharge of nitrogen, organic matter, and phosphorous from land-based saltwater RAS and thus contribute to increased sustainability. |
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| Keywords: | Nitrate Nitrogen Sludge Organic waste End-of-pipe |
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