World Aquaculture December 2018

WWW.WA S.ORG • WORLD AQUACULTURE • DECEMBER 2018 59 ( C O N T I N U E D O N P A G E 6 0 ) of 45-percent protein feed (Massingill 2007). Average recirculating water flowwas 57 m 3 /min, net newwater addition was 22 m 3 /min with 77 mg/L alkalinity, and water discharge from the systemwas 20 m 3 /min with an increased alkalinity of 95 mg/L. Calcium hydroxide addition of around 1,000 kg/d was required to offset alkalinity destruction from nitrification reactors. System operation was modified in 1995 to include a 16-ha wetland used as a solids polishing and denitrification reactor. From 1996 to 1997, the systemwas fed an average of 7.7 t/d of 45-percent protein feed. As before, average recirculating water flowwas 57 m 3 /min, net newwater addition was 22 m 3 /min with 77 mg/L alkalinity, and water discharge from the systemwas 20 m 3 /min with an average alkalinity of 85 mg/L. Table 1 summarizes the net alkalinity consumed per unit nitrogen added during the two different operational periods. When the systemwas operated as a nitrification-only RAS, lime addition was required, yielding an observed alkalinity consumption of 0.85 equivalent per equivalent of nitrogen removed. This result agrees with the second rule of thumb reported by Timmons et al. (2018). When nitrification and denitrification were both operating, no lime addition was required and no net alkalinity was supplied from makeup water additions. ExperiencewithTroutCulture at theFreshwater Institute In 2005, net consumption of alkalinity and net removal of nitrate and nitrite-nitrogen was measured in a RAS system (Fig. 1, Summerfelt 2007) at the Freshwater Institute, Shepherdstown, West Virginia. Rainbow trout were fed an average of 118 kg of feed per day and were maintained at a density of 73 kg/m 3 . Recirculated water was delivered to the culture tank at 4.5 m 3 /min. Approximately 8 percent of the recirculating water (370 L/min) was discharged from the system. Nearly all (92 percent) of the system water flow exited the tank sidewall drain, where it was passed through a drum filter before entering a recirculation sump. A float valve controlled makeup water addition to maintain a constant systemwater level, balancing makeup water additions with wastewater discharged from the culture tank. The majority of ammonia removal occurring in the systemwas provided by a nitrifying biofilm (Summerfelt 2006). Water exiting the top of the fluidized-sand biofilter passed through a cascade aeration column and a low head oxygenation unit before returning to the culture tank. Makeup water supplied to the recycle system contained an alkalinity of 270 mg/L as CaCO 3 . The rate of alkalinity addition from the makeup water was sufficient to offset daily alkalinity consumption from nitrification. Exit water alkalinity averaged 230 mg/L as CaCO 3 . The RAS was specifically designed and operated to minimize system-wide denitrification. The system flushing rate maintained NO 3 -N concentration of approximately 10 mg/L, further minimizing the driving force for denitrification. A nitrogen and alkalinity mass TABLE 1. Alkalinity consumption per unit nitrogen removed in a striped bass production system at Kent SeaTech Corporation with nitrification and denitrification and nitrification only with lime supplementation (Massingill 2007). System operation Alkalinity added Nitrogen removed Alkalinity consumed per unit (g-equivalents) (g-equivalents) NH4-N oxidized (eq alk / eq N) With denitrification + 0.0 1 -29,769 2 0.0 Nitrification only + 30,675 3 -29,769 2 0.82 - 6,202 4 1 22 m 3 /min water added @ 77 mg /L alkalinity with 20 m 3 /min discharged at 85 mg /L 2 7,700 kg feed/d @ 45 percent protein assuming 75 percent ammonia excretion 3 1,135 kg /d CaOH 2 addition 4 22 m 3 /min water added @ 77 mg /L alkalinity with 20 m 3 /min discharged @ 95 mg /L FIGURE 1. Process flow drawing of recirculating aquaculture system for salmonid grow out at the Freshwater Institute in Shepherdstown, WV (Summerfelt 2007).