World Aquaculture December 2018

60 DECEMBER 2018 • WORLD AQUACULTURE • WWW.WA S.ORG balance on this system (Table 2) suggests a net alkalinity consumption to nitrogen removal ratio of 0.87 to 1.03, again in agreement with the second rule of thumb proposed by Timmons et al. (2018). ExperiencewithShrimp Culture atClemson University In 2005 researchers at Clemson University operated a fully-closed, paddlewheel- mixed, prototype shrimp production system utilizing suspended-growth, water- column nitrification followed by an anoxic tank that provided denitrification to minimize accumulation of nitrate. Total feed nitrogen application to this 655-m 3 RAS exceeded 7,000 kg of 35-percent protein feed, resulting in an approximately 600 mg N/L addition during the 2005 season. At no time during the 2005 or 2006 season was any alkalinity addition required. Alkalinity declined at midseason from a maximum of 290 mg/L (5.8 meq/L) to a minimum of 110 mg/L (2.2 meq/L), with a concurrent increase in water column nitrate levels from 0 to 50 mg/l (3.57 meq/L) suggesting an alkalinity consumption to nitrogen oxidation ratio of 1.008:1 (Fig. 2). At the end of the 2005 season, denitrification reduced nitrate levels from 50 mg/L (3.57 meq/L) to 20 mg/L (1.43 meq/L). This was observed at a system alkalinity increase from 110 to 260 mg/l, suggesting a net alkalinity gain of 3.0 meq/L per 2.14 meq/L nitrate lost or an alkalinity generation to nitrogen removal ratio of 1.4:1 (Fig. 3). The total system nitrogen removal exceeded 42 meq/L in a system containing at on average 3.0 meq/L of alkalinity, further supporting the stoichiometric relationships observed in the striped bass and trout RAS systems. Nitrification- Alkalinity Stoichiometry: Reactor Study Materials andMethods As a confirmation of field observations, four 5-gal buckets were configured for use as batch nitrification reactors at the Clemson University aquaculture facility in 2006 (Fig. 4). Each reactor was filled with 8 L of water and this volume was maintained thoughout the experimental trials. Each reactor was equipped with a single air diffuser attached to an aquarium type air blower used to provide mixing and maintain aerobic conditions in the reactors. When not being sampled, reactors were maintained in the dark at room temperature. In addition, air diffusers were linked to a CO 2 - injection system that delivered slowmetering of CO 2 into air tubing, providing pH control and oxygen control of the reactors. Air flow rate and CO 2 injection rate was controlled with 3.2-mm needle valves. Airflow rates were adjusted to maintain DO >4.0 mg/L. CO 2 injection rates were adjusted to maintain culture pH between 8.0 and 9.0. The four buckets were operated as batch nitrification reactors for two months. Two reactors received daily additions of concentrated NH 4 OH solution (reactors 1 and 2) and two reactors received daily additions of concentrated NH 4 Cl solution (reactors 3 and 4). The concentrated solutions were adjusted to contain 24.5 mg N/ mL, resulting in a daily addition of 12.26 mg of either NH 4 Cl-N or NH 4 OH-N to each reactor each day. Alkalinity additions were made as needed, depending on measured reactor alkalinity after 24 h of aeration. Alkalinity additions were adjusted to maintain reactor operating alkalinity levels of 2-4.5 TABLE 2. Alkalinity consumption per unit nitrogen removed in a recirculating aquaculture system at the Freshwater Institute with makeup water as the sole alkalinity source. N=10 samples (Summerfelt 2007). Alkalinity added Nitrogen removed Alkalinity consumed per unit (g-equivalents/L) (g-equivalents/L) NH4-N oxidized (eq alk /eq – N) Mean 0.5500 0.5785 0.95 SE 0.034 0.007 0.87 – 1.03 FIGURE 2. Nitrate concentration in Clemson shrimp production system in 2005. FIGURE 3. Alkalinity (mg/L as CaCO 3 ) in Clemson shrimp production system in 2005.