NITRIFICATION AND DENITRIFICATION IN ZERO DISCHARGE AQUACULTURE

Many recirculating aquaculture systems rely on nitrification to control ammonia levels.  Disadvantages of nitrification include need to discharge water to reduce accumulating nitrate and acid production from the nitrification reaction requiring continuous replacement of culture alkalinity. The theoretical nitrification reactions yield two equivalents of acid per equivalent of nitrogen oxidized;      

NH₄⁺ + 2 O₂ = NO₃^- + 2 H⁺ + H₂O

This suggests an alkalinity demand of 500 gm of NaHCO₃ per kg of feed added (at 35% protein) however, observed alkalinity demand is only 250 gm/kg-feed. This difference can be explained by ammonia addition from aquatic animals generating alkalinity from the weak-base reaction;

NH₃ + H₂O = NH₄⁺ + OH^-

In aquaculture, in-situ ammonia addition contributes one (unaccounted for) equivalent of alkalinity per equivalent of nitrogen added, whereas in wastewater treatment incoming alkalinity measures the impact of influent ammonia. To illustrate, four 5-gallon nitrification reactors were operated in a laboratory setting. Oxygen was maintained above 4.0 mg/l with bubble aeration and pH was maintained between 8.0 and 9.0 with NaHCO₃ or CO₂ addition. Ammonia was added at 12 mg/l-day as NH₄OH to two buckets and NH₄CL to the two buckets. NaHCO₃ was added as needed to maintain alkalinity at 2 to 4 meq/l. The observed alkalinity destruction illustrates the impact of ammonia hydroxide reducing needed alkalinity supplementation (Figure 1).

Field aquaculture operations have shown that culture systems incorporating denitrification reactors do not require any alkalinity addition. This is the effects of both, of in-situ alkalinity generation from ammonia excretion, combined with alkalinity generation from denitrification;

NO₃^- + organic matter + CHON + OH^- + N₂

Denitrification reactors need not be expensive or complex to operate. At Kent SeaTech, in California, water circulation through constructed wetlands provided NO₃^- reduction, while at Clemson University solids settling tanks provided the same function for zero-discharge marine shrimp culture. Decaying organic matter in wetlands or anaerobic decomposition of settled solids provided organic energy to drive denitrification. RAS units designed around integrated nitrification/denitrification reactors do not required bicarbonate supplementation and need not discharge NO₃ to the environment.