World Aquaculture December 2018

WWW.WA S.ORG • WORLD AQUACULTURE • DECEMBER 2018 61 meq/L. Alkalinity adjustments consisted of daily additions of 5-30 mL/reactor of 0.5 N NaHCO 3 . Daily measurements consisted of pH, DO, temperature, alkalinity, nitrite and conductivity. Daily ammonia concentration determinations were conducted using Hach kits, and twice per week, ammonia concentrations were analyzed using the Standard Methods ammonia distillation procedure. Reactor working volume was returned to 8 L with additions of distilled water after every sample extraction. AlkalinityBalances inLaboratoryReactors Reactors 1 and 2 were operated to provide an alkalinity balance indicative of in-situ ammonia excretion, as in the case of an aquaculture systemwhere the influx of nitrogen into the system is derived from feed protein-nitrogen. This case was simulated with the addition of NH 4 OH to reactors. Alternatively, reactors 3 and 4 with additions of NH 4 Cl were operated to provide an alkalinity balance indicative of a typical wastewater treatment nitrification reactor where the alkalinity “cost” of treating ammonia nitrogen is considered in the inflow of the waste stream. As expected, the alkalinity change after addition of required NaHCO 3 to maintain a stable alkalinity and addition of ammonia resulted in twice as much alkalinity destruction in the NH 4 Cl reactors compared to the NH 4 OH reactors. The relationship between alkalinity destruction and ammonia oxidation is seen in the cumulative reactor alkalinity demand and in the cumulative alkalinity destroyed as a function of cumulative nitrogen added to each reactor (Fig. 5). In reactors 1 and 2, NH 4 OH additions suggested a net alkalinity requirement of 0.96 meq alkalinity/meq nitrogen oxidized as opposed to NH 4 Cl addition, which required an alkalinity addition of 1.99 meq alkalinity/meq nitrogen oxidized. Nitrification and Denitrification Stoichiometry: Discussion It is the hypothesis of this article that applying equations (1) and (2) to predict alkalinity consumption in RAS will lead to incorrect conclusions because these equations do not account for base generation resulting from protein utilization, catabolism and in-situ ammonia excretion. When aquatic organisms are fed protein nitrogen, the end result is excretion of ammonia to the culture water: Organic-N ➝ Urea ➝ NH 3 (3) This ammonia release in turn generates a temporary base and alkalinity from the reaction with water: NH 3 + H 2 O ➝ NH 4 + + OH - (4) At the pH of a typical aquaculture system, the hydrolysis of excreted ammonia rapidly generates bicarbonate alkalinity: NH 3 + H 2 O + CO 2 ➝ NH 4 + + HCO 3 - (5) Combing equation (5) with equations (1) and (2) suggest that nitrifying aquaculture systems would destroy only one equivalent of alkalinity per mole of nitrogen oxidized and systems with complete denitrification will show no decrease in system alkalinity. A complete alkalinity balance beginning with ammonia excretion derived from feed protein and ending with nitrogen gas is proposed: ( C O N T I N U E D O N P A G E 6 2 ) FIGURE 4. Batch nitrification reactors (5-gal buckets) with aeration and CO 2 injection manifold. FIGURE 5. Cumulative alkalinity destroyed (meq/L) as a function of cumulative NH 4 OH or NH 4 Cl (meq/L) added to batch reactors during a 14-d nitrification trial. The slope represents the equivalents of alkalinity required per equivalent of N nitrified.