World Aquaculture December 2018

62 DECEMBER 2018 • WORLD AQUACULTURE • WWW.WA S.ORG Alkalinity (bicarbonate) generation from ammonia hydrolysis: NH 3 + H 2 O + CO 2 ➝ NH 4 + + HCO 3 - + 1 meq alkalinity (6) Alkalinity destruction from acid production from nitrification: NH 4 + + 2 O 2 ➝ NO 3 - + H 2 O + 2 H + - 2 meq alkalinity (7) Alkalinity (bicarbonate) generation from denitrification: NO 3 - + organic-C ➝ ½ N 2 + HCO 3 - + 1 meq alkalinity (8) Net alkalinity change: ± 0 meq alkalinity Alkalinity and nitrogen mass balances of operational RAS suggests ratios of overall alkalinity destruction to am- monia removal of 0.82:1 in the Kent SeaTech systems and 0.95:1 in the Freshwater Insti- tute systems. Furthermore, an alkalinity destruction to am- monia removal ratio of 0.96:1 was observed in duplicate laboratory-scale nitrification batch reactors provided with NH 4 OH as a nitrogen source as compared to 1.99:1 in duplicate reactors provided with NH 4 Cl as a nitrogen source. Similar mass balances of shrimp culture systems after sustained operation with complete nitrification and denitrification suggests that no alkalinity supplementation is required. These results support the hypothesis that in-situ ammonia excre- tion within RAS provides an additional equivalent of alkalinity per equivalent of ammonia nitrogen added to the system. This additional equivalent of alkalinity will supply 50 percent of the alkalinity de- stroyed by complete nitrification of added nitrogen. Furthermore, because complete denitrification of added nitrogen supplies an addi- tional equivalent of alkalinity per equivalent of nitrogen removed, no alkalinity supplementation is required in such systems. Recirculating aquaculture systems with coupled nitrification- denitrification will not require alkalinity addition, which may significantly reduce operational costs, and furthermore, will not result in discharge of nitrate to the environment, a nutrient with potential to cause eutrophication of receiving waters. The apparent discrepancy between observations of alkalinity destruction in wastewater treatment, in contrast with aquaculture water treatment, results from alkalinity generation arising from the hydrolysis of ammonia excreted by fish or shrimp in contrast to previously accounted for ammonia and alkalinity in wastewater (Fig. 6). Summary Alkalinity requirements proposed by authors detailing stoichiometric alkalinity destruction in nitrifying wastewater treatment plants is not wrong, rather, conventional wastewater treatment systems represent incomplete or abbreviated ecosystems compared to aquaculture systems. Alkalinity contribution from ammonia in the influent to a wastewater plant is accounted for prior to the nitrogen treatment process. Nitrifying aquaculture systems require approximately 250 g NaHCO 3 /kg feed to stabilize alkalinity, not 500 g/kg as suggested by conventional wastewater treatment theory and observation. Fully integrated nitrification-denitrification aquaculture systems do not require any alkalinity addition, thereby enabling zero-discharge operation. Past work indicates that internal denitrification systems can be inexpensively provided to RAS facilities (Brune et al. 2004), taking full advantage of internal alkalinity regeneration and eliminating the cost of lime or water additions for alkalinity control. This reduction can represent a 5 to 10 percent (of feed) cost savings in RAS, depending of the relative costs of fish feed and lime or bicarbonate. Notes David E. Brune, Professor of Bioprocess and Bioenergy Engineering, University of Missouri, Columbia, MO, USA. This study was funded by the Clemson University Newman Endowed Chair Fund. The author is indebted to Mike Massingill at Kent SeaTech, and Steve Summerfelt at the Freshwater Institute for supplying RAS water quality data. References Brune, D.E., K. Kendall and A.G. Eversole. 2004. Autotrophic intensification of pond aquaculture: shrimp production in a partitioned aquaculture system. Proceedings of the Fifth International Conference on Recirculating Aquaculture. Brune, D.E., G. Schwartz, A.G. Eversole, J.A. Collier and T.E. Schwedler. 2003. Intensification of pond aquaculture and high rate photosynthetic systems. Aquacultural Engineering 28:65–86. Brune, D.E. 2007. Alkalinity destruction in laboratory nitrifying reactors. Clemson University, unpublished data. Ebeling, J.M., M.B. Timmons and J.J. Bisogni. 2006. Engineering analysis of the stoichoiometry of photoautotrophic, autotrophic and heterotrophic removal of ammonia nitrogen in aquaculture systems. Aquaculture 257:346-358. Massingill, M. 2007. Alkalinity in recirculating bass culture. Unpublished operational data. Rittmann, B.E. and P.L. McCarty. 2001. Environmental Biotechnology: Principles and Applications, McGraw-Hill. Summerfelt, S.T. 2006. Design and management of conventional fluidized-sand biofilters. Aquacultural Engineering 34:275-302. Summerfelt, S.T. 2007. Alkalinity balances in recirculating salmonid culture. Unpublished operational data. Timmons, M.B., T. Guerdat and B.J. Vinci. 2018. Recirculating Aquaculture, 4th edition. Ithaca Publishing Company, Ithaca, NY, 779 p. FIGURE 6. Difference between alkalinity balances in wastewater treatment systems and aquaculture water treatment systems.