Article written by: Jason Danaher Ph.D.

In order to maintain optimal water quality parameters, specifically with dissolved nitrogen waste products, an aquatic system must have a functioning biofilter. A biofilter is a specific unit within the system’s process flow where water is directed and treated using beneficial microorganisms intentionally grown on specific media. It can be a challenge to jump-start and cycle a biofilter in a new aquatic system, but the following will provide some calculations and practical advice to consider.

This tech talk will briefly discuss the method of “seeding” your biofilter when fish are not present in the system. Seeding a biofilter requires isolating it from the rest of the recirculating aquaculture system and then introducing a source of inorganic nitrogen to the biofiltration unit. The inorganic nitrogen becomes a food source to the nitrifying bacteria to increase their population size in the absence of fish diet entering the system. The bacteria occur naturally in most water sources or can be purchased commercially in concentrated formulations to expedite the colonization process on the biofilter media. In general, Nitrosomonas spp. bacteria convert ammonia to nitrite and Nitrobacter spp. convert nitrite to nitrate. There are a few sources of ammonium safe for the start-up of a biofilter. We are going to use ammonium chloride (NH₄Cl) containing ammonium (NH₄) cations and chloride (Cl) anions as the source of total ammonia for the sample calculations below. The sample calculations assume the ammonium chloride purity is 82%. Alternative biofilter cycling methods and sources of ammonium can be found in the Southern Regional Aquaculture Center Publication 4502, How to Start a Biofilter.

The objective of this article is to create an initial total ammonia concentration of 5.0 mg/L in the biofilter unit, only. For calculation purposes we will assume the biofilter size is 2,000 gallons or 7,570 liters (2,000 gallons x 3.785 liters/gallon). Isolating and seeding the biofilter unit, only, will save on chemical costs rather than treating the entire system and alleviate the need to operate system pumps 24/7 while no fish production is occurring in the system. Also maintaining temperature in only the biofilter is more cost effective than the entire aquatic system.

Step one

First, we determine the molecular mass of ammonium chloride (NH₄Cl). Reviewing the periodic table of elements we calculate the molar mass of NH₄Cl is 14.01 g/mol (Nitrogen) + 4.032 g/mol (Hydrogen) + 35.45 g/mol (Chlorine) = 53.492 g/mol total.

Step two

Second, we determine the percentage of ammonium (NH₄) in ammonium chloride. We divide the molecular weight of ammonium by the molecular weight of ammonium chloride. (18.042 g/mol (NH₄)/ 53.492 g/mol NH₄Cl) * 100 = 33.73%.

Step three

a). Next, we determine the amount of ammonium chloride necessary to create a total ammonia concentration of 5.0 mg/L. We know the percent ammonium in ammonium chloride is 33.73%. Convert the percentage to a decimal equivalent (i.e. 0.3373) and divide the decimal equivalent into the desired concentration of total ammonia nitrogen (i.e. 5.0 mg/L).

5.0 mg/L ÷ 0.3373 = 14.82 mg/L NH₄Cl

Now we have to account for the NH₄Cl purity. In this example the purity is 82%. Convert the percentage to a decimal equivalent (i.e. 0.82) and divide the decimal equivalent into 14.82 mg/L NH₄Cl.

14.82 mg/L NH₄Cl ÷ 0.82 = 18.07 mg/L NH₄Cl is required to obtain a total ammonia concentration of 5.0 mg/L.

b). Now multiply the quantity of NH₄Cl required per liter by the volume of the biofilter which is known to be 7,570 liters.

18.07 mg/L NH₄Cl × 7,570 liters = 136,790 mg NH₄Cl.

Divide the mass by 1,000 to convert milligrams to grams:

136,790 mg NH₄Cl ÷ 1,000 = 136.79 g.

To create a total ammonia concentration of 5.0 mg/L a farmer will need to add 136.79 grams or approximately 0.30 pounds of NH₄Cl to the biofilter.

Final steps

To ensure the bacteria colonize the biofilter media rapidly the farmer can maintain water temperature between 25 – 28oC (77oF to 82oF). This can be accomplished by temporarily adding a properly sized heating element to the biofilter for the seeding process. In addition, it is recommended to maintain a total alkalinity concentration of 75 to 150 mg/L CaCO₃ and pH range of 7.0 to 8.0. Furthermore, it is critical to maintain oxygen concentrations above 5.0 mg/L in the biofilter for bacteria to oxidize the dissolved nitrogen wastes.

The farmer should begin to monitor water quality and determine if total ammonia concentrations are decreasing and nitrite concentrations begin to increase (Figure 1). When total ammonia concentrations decrease down to 1.0 mg/L the farmer can add an additional 109.43 grams or approximately 0.24 pounds of NH₄Cl to increase the total ammonia concentration back to 5.0 mg/L. The farmer will more than likely need to increase the total alkalinity level again as the nitrification process produces a weak acid in the water and decreases the alkalinity concentration. The pH levels will also decrease when alkalinity levels decrease.

Figure 1. Theoretical dissolved nitrogen cycle of a biofilter at start-up.

The monitoring of total ammonia and nitrite concentrations along with the addition of NH₄Cl and total alkalinity should continue until nitrate concentrations are observed to be present and begin to increase (Figure 1). At this point the biofilter unit is seeded, ammonium chloride addition can cease, and the biofilter can be introduced into the recirculating aquaculture system’s process flow when un-ionized ammonia and nitrite levels are in safe ranges for the target species. Fish can be stocked and fed a reduced daily ration until the operator is confident the biofilter can handle increased feeding rates. The commissioning timeline of the biofilter can vary depending on the experience of the operator and whether the system is warm water or cold water and either in a freshwater or saltwater environment. In general, it may take 2 weeks to 6 weeks to cycle your biofilter and this time frame should be incorporated into the system’s commissioning timeline and start-up procedures.

Total ammonia nitrogen, nitrite and nitrate concentrations should continue to be monitored during the fish production process to ensure un-ionized ammonia and nitrite levels remain optimal for your specific fishes’ growth and health. Feel free to call Aquatic Equipment & Design, Inc. staff at 407-995-6490 for additional information and tips on cycling your system’s biofilter.