Aquaculture in the Classroom

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Re: Ammonia Nitrogen

From: Gordon Mengel, National Council for Agricultural Education
Date: 10/21/98
Time: 9:16:50 AM
Remote Name:



The source(s) of Ammonia Nitrogen or Total Ammonia Nitrogen (TAN) that you are inquiring about are; 1) excretion and 2) decomposition of food and fecal matter. Fish and other aquatic organisms excrete ammonia, a by-product of protein metabolism, from their gills and in their urine. The food that you provide your fish or other organisms has to have a minimal protein content to sustain life and allow for growth and reproduction. The higher the protein content of the feed, the more ammonia that will be produced. Ammonia dissolved in the culture water is typically removed through a biochemical process called nitrication (oxidation of ammonia). Uneaten food and fecal matter release ammonia as they decompose. In a recirculation system, there should be a means (active or passive) to remove the ammonia producing solid particles before they have a chance to decompose.

Ammonia dissolves easily in water where it will be found in 2 forms, NH3 (unionized) and NH4+ (ionized). The total amounts of these 2 forms is the "Total Ammonia Nitrogen." The unionized form (NH3) is toxic to fish, while the unionized form (NH4+) is not. The percentages of unionized to ionized ammonia is a factor of water temperature and pH. Your greatest concern when monitoring the TAN level in your culture system is to determine the level of unionized ammonia (explained below).

The ability to keep and/or grow aquatic organisms (e.g., fish) in a recirculating system or aquarium is dependent on many factors, all or any one of which impact survival, growth and reproduction. In terms of water quality (i.e., ammonia), ideally, your culture system allows for the growth of bacteria (biofilter). In the presence of ammonia, bacteria that colonize the biofilter media will transform (nitrify) ammonia to nitrite (Nitrosomonas spp.) and then to nitrate (Nitrobacter spp.) While (unionized) ammonia and nitrite are harmful to fish, nitrate is not. If your biofilter is functioning optimally and is of adequate size for the amount of fish or other organisms being cultured, it should keep the ammonia and nitrite levels below critical (i.e., stressful or lethal) levels. Please note that under optimal conditions (e.g., temperature, pH) it takes a new biofilter about 3 - 4 weeks to establish an adequate bacterial population. It is best to start your system at a low stocking rate (low fish weight to water volume), increasing the rate as the biofilter becomes more effective (as ammonia and nitrite levels decrease).

If you have a system that has been running for some time and have just started to observe high(er) ammonia levels, there are several things to consider. First, your water test kit probably tests for the Total Ammonia Nitrogen level (TAN). The critical parameter is the unionized portion of the ammonia and therefore you need to determine what percentage of the TAN is in the toxic (unionized) form. This is done by determining the temperature of your water and it's pH. Those 2 numbers are then plugged into a table that has been developed to give the percentage of your measured TAN that is unionized. (I can provide you with a table or you can go to the AQUANIC home page where there is a place where the calculation is done for you). Once you find the unionized ammonia level, and you know what the critical level is for the fish you are raising, then you know if you need to be concerned. Second, you need to make sure that the level of dissolved oxygen in your culture system is adequate. Oxygen is needed by the fish (respiration) and by the bacteria (nitrification of ammonia) on the biofilter. Make sure you measure the DO level of the water going into the biofilter, as well as, the DO of the water in the culture tank. Third, you need to make sure that the solids (uneaten food and fecal matter) are being removed effectively. If not, you may have to syphon them out on a regular basis until you can modify your solids removal technique. Finally, you need to make sure that your biofilter is of adequate size for your production system (more on this below).

What do you do if the unionized ammonia level is high? One thing you can do immediately is exchange water. This is only a temporary solution, however. If the biofilter is new (i.e., in use less than 4 - 6 weeks) or if you took it out of production for a while (bacteria will die off if there's no water with ammonia and oxygen) you will have to continue to exchange small amounts of water until the bacteria recolonize the filter media in large enough quanitities. There may also be a possibility that your present stocking and feeding rate surpasses the capacity of your biofilter. The size of your biofilter should be based on the maximum amount of feed that you will be putting into your culture system at any one time (this will be based on your feeding rate and your expected production). Calculate the size of your biofilter as follows:

1.) determine the pounds of ammonia produced by your fish per day (fish at maximum size):

_____ lbs. of fish food fed/day X 0/03 lbs. of ammonia/lbs. of food = _____ lbs. of ammonia/day

2.) convert this to kilograms/day:

_____ lbs. of ammonia X 0.4536 kg/lb. ammonia = _____ kg ammonia/day

3.) water temperature = _____ degrees C

4.) nitrification rate of system (Kt) = ( ____ degrees C X 0.11) - 0.2 = _____

5.) determine surface area of filter needed:

surface area (S) = ( _____ kg ammonia/day)/Kt = ______

6.) calculate the surface area of the biofilter media you are using* and the amount needed to provide the square meters of surface area from step 5.

* If you are using a biofilter media you purchased from an aquaculture supplier, the surface area (m2/m3) of the media will be listed in the catalogue you used (or you could call the supplier). If you are using a biofilter media that you have "recycled" from a non-commercial source, you will need to calculate or estimate the approximate surface area.

Recirculating aquaculture systems were designed using effective technologies developed for waste water treatment industry. If you have done all of the above, you should not have problems with ammonia. If, however, the level remains high then it is likely that something is limiting the growth and effectiveness of the bacteria in your system. Again, make sure that the water that is going to the biofilter is well oxygenated and that the biofilter remains in contact with the culture water (i.e., don't let it dry out). Your culture water should be circulated completely through the system 1 - 2 times per hour. Make sure that you are dechlorinating (including removal of chloramines) any water your are adding to your system. Make sure that the pH of the water is above 6.0. If the pH is low, due to a decrease in alkalinity, you should be adding sodium bicarbonate at 0.15 lbs. of sodium bicarb. per 1,000 gallons of water. Test your alkalinity at least once a week and maintain it between 75 to 120 ppm (CaCO3). Finally, make sure that your biofilter is not being exposed to unwanted chemicals, including pesticides and insecticides, or an excessive amount of light.

The information above should help you (and your students) with your problem-solving (a great experience for both you and the kids). Let me know if you need any further explanations or additional information. Good luck and Think Fish!

Gordon The Council

Last changed: November 03, 1998