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Ammonia Chemistry and Removal Techniques in RAS | Aquaculture Technology

Good water quality is essential to the development and growth of the aquatic species. One of the important properties is the ammonia levels in the water. Ammonia is usually measured in the units of parts per million (ppm) and can be measured easily with chemical test kits. Mud crabs are also affected by the presence of ammonia in the system. In this article, we will go through several concepts about ammonia generation and removal methods.

Ammonia is highly soluble in water and originates from nitrogenous waste. Fish and crabs expel ammonia through gill diffusion, gill cation exchange, and urine and feces excretion. Ammonia can also originate from decaying matter from uneaten feed or dead crabs that were not removed from the systems. A well-designed recirculating aquaculture system would be operated in a manner so that leftover feed would be removed as fast as possible. Despite that, there will be still ammonia generation from the crab metabolism alone. The quantity of the ammonia (Total Ammonia Nitrogen-TAN) generated via metabolism is a function of 2 main variables, feed quantity, and protein levels. The higher the feed quantity and crude protein, the higher the generated ammonia. Daily feeding quantity will be about 3-8% of their body weight, dependent on the size of the mud crab. Protein content ranges from 15% for trash-fishes to 40% for formulated feed.

Ammonia can exist in two forms, unionized (NH3) and ionized (NH4+). The unionized form (NH3) is the more toxic form of ammonia due to its ability to move across the cell membrane. However, the proportion of the unionized (NH3) and ionized (NH4+) will depend on pH, salinity and temperature. For example, the unionized ammonia concentration is 0.02ppm, in freshwater temperature of 20 degree Celsius with a pH of 7. However, unionized ammonia can reach up to 1