World Aquaculture December 2018

58 DECEMBER 2018 • WORLD AQUACULTURE • WWW.WA S.ORG assumed bacterial yields. Furthermore, in systems with complete bacterial denitrification of NO 3 - to N 2 , Rittman and McCarty (2001) suggest that base production from heterotrophic denitrification is represented as: NO 3 - + 0.625 C 2 H 4 O 5 + 0.5 H 2 O = 0.5 N 2 + 1.25 H 2 CO 3 + OH - (2) Therefore, alkalinity increase from denitrification is equal to 1.0 equivalents per equivalent of nitrogen, or 3.57 g as CaCO 3 per mg of NO 3 -N. As a result, predicted alkalinity increase from complete denitrification is 1.0 equivalent of strong base per mole of NO 3 -N removed or 3.57 g as CaCO 3 per mg of NO 3 -N. Combining equations (1) and (2) suggests that nitrification combined with denitrification will result in a net reduction of 1.0 equivalent of alkalinity per equivalent of nitrogen removed from the system (as nitrogen gas). These equations have been developed and verified for activated sludge wastewater treatment plants. Awastewater composition of 20-30 mg/l ammonia with 100-200 mg/L of alkalinity is common. Equation (1) suggests that complete nitrification of this influent would result in the destruction of approximately 100 mg/L of alkalinity. If this same flowwere subjected to complete denitrification, the predicted alkalinity loss would be 50 mg/L. Despite these theoretical predictions, successfully applied to wastewater treatment, observed alkalinity destruction (and required replacement) in aquaculture systems is half that predicted from sanitary engineering theoretical equations. Such observations are evident as contradictory textbook reports and alkalinity observations reported in field data. Nitrification-Alkalinity Stoichiometry: Literature Recommendations and Field Experience LiteratureContradictions In the textbook Aquacultural Engineering, Timmons et al. (2018) suggest a rule of thumb: “For each gram of ammonia nitrified, 7.14 g of alkalinity (as CaCO 3 ) are required.” This is equal to 2.0 meq alkalinity per meq of TAN oxidized, which is the theoretical prediction described above. However, elsewhere in the same book, a second rule of thumb is reported as: “For every kg of feed, approximately 0.25 kg of NaHCO 3 is needed to replace lost alkalinity,” which is equal to 1.0 meq alkalinity per meq TAN oxidized. This second rule of thumb is based on actual field practice and contradicts the theoretical prediction. FieldExperiencewithStripedBassCulture atKent SeaTech Kent SeaTech Corporation, located in southern California, has produced 1,000-1,400 t/yr of hybrid striped bass for over 20 years in semi-closed RAS units. From 1990-1995, this system operated with a single 3,800-m 3 nitrification reactor and groundwater makeup. From 1994-1995, fish in the systemwere fed an average of 7.7 t/d M ost recirculating aquaculture systems (RAS) rely on nitrification to control ammonia levels. Disadvantages of nitrification include the need to discharge water to reduce accumulating nitrate and the need to replace alkalinity destroyed by the acid produced during ammonia oxidation. Theoretical presentations of summary nitrification reactions predict two equivalents of acid produced per equivalent of nitrogen oxidized: NH 4 + + 2 O 2 = NO 3 - + 2 H + + H 2 O This suggests an alkalinity demand of 500 g of NaHCO 3 per kg of 35-percent protein feed added. However, observed alkalinity demand in recirculating aquaculture systems is only 250 g/kg feed. This difference can be explained by in-situ ammonia excretion from aquatic animals that adds alkalinity to the system from the weak-base hydrolysis reaction: NH 3 + H 2 O = NH 4 + + OH - In aquaculture, in-situ ammonia excretion contributes one (unaccounted for) equivalent of alkalinity per equivalent of nitrogen added, whereas in wastewater treatment the impact of ammonia on alkalinity is accounted for in influent flow. Furthermore, field aquaculture operations have shown that systems incorporating denitrification reactors do not require any alkalinity addition. This is the result of the combined effects of in- situ alkalinity generation from ammonia excretion, combined with alkalinity generation from denitrification: NO 3 - + organic matter = biomass + OH - + N 2 Past system operations have shown that combined nitrification and denitrification processes can be inexpensively provided to RAS facilities that take advantage of internal alkalinity regeneration and eliminate the cost of lime or water additions (for alkalinity control) to the RAS. Depending of the relative costs of fish feed and lime, this reduction can represent a 5-10 percent savings on the cost of feed. Stoichiometry Overview Rittmann and McCarty (2001) presented a balanced reaction for complete bacterial oxidation of NH 4 + -N to NO 3 - -N. This equation, based on an average bacterial cell age of 15 days and microbial yield coefficient of 0.21 g volatile solids/g N, is represented as: NH 4 + + 1.815 O 2 + 0.1304 CO 2 = 0.0261 C 5 H 7 O 2 N + 0.973 NO 3 - + 0.921 H 2 O + 1.973 H + (1) This equation suggests that nearly two strong acid equivalents are produced per mole of NH 4 + -N removed, equivalent to 7.05 mg of alkalinity (as CaCO 3 ) consumed per mg of NH 4 + -N oxidized. This equation differs slightly from similar equations suggested by McCarty (1970) and Ebeling et al. (2006) because of different Impact of Nitrification and Denitrification on Alkalinity in Recirculating Aquaculture Systems David E. Brune