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 1.43ppm at pH 9 with similar temperature and TAN levels. Hence, it is important to understand that ammonia toxicity is not dependent only on TAN generated from feeding but also the prevailing conditions in the recirculating aquaculture systems. Furthermore, different species would have different tolerance to ammonia. Sensitive species like pink salmons can only tolerate levels up to 0.08ppm of unionized ammonia, while the common carp can tolerate up to 2.2ppm. Based on experience for mud crabs, TAN should be maintained less than 0.5 ppm.
In theory, there will be no ammonia generated if no feed is given to the crabs. However, that contradicts the purpose of aquaculture, which is to provide feed to the animals to be harvested in the future. The easiest way to reduce ammonia build-up in the water column is by conducting water change. This is commonly done in traditional farms located near water sources. Upon the rising tide, clean water is introduced into the ponds thus displacing the ammonia contaminated culture water. Hence, this restricts the culture of marine species at areas without direct access to seawater. With recent advances in wastewater treatment, recirculating aquaculture systems have enabled the recycling of the cultured water a possibility. This eliminates the need for water replacement and hence removing the location barrier. The treatment equipment that removes the ammonia in recirculating aquaculture systems is known as the biofilters. The biofilters utilize microbes to clean up the excess ammonia in the water and converting them into Nitrite and Nitrate which are less harmful to mud crabs. Note that nitrite can be quite toxic to some freshwater fish in certain conditions. Designing and maintaining an optimal environment for these microbes is no easy feat, and the success of many farms is dependent on the competency in biofiltration. Typical technologies include the moving bed bioreactors, trickling filters and fluidized bed filters. The key concept for bioreactors is to provide surface area and good living conditions for the bacteria to efficiently convert ammonia into a less toxic form.
To run a profitable farm, the key is to optimize feeding rates and ammonia levels. It is paramount that we keep the water quality ammonia-free but is also important to maintain a high feeding rate. Ammonia is a measure daily and the feed rate is determined in advance to ensure that post-feeding ammonia levels do not exceed harmful values.