Ammonia Removal

Ammonia is toxic to aquatic animals and is also the waste product of their protein metabolism. The sensitivity of aquatic animals to ammonia concentration depends on species, live stage, level of stress, and on other environ mental conditions. Ammonia exists in water as the equilibrium product of ammonium ion and unionized ammonia:

Unionized ammonia (NH3) is the toxic form of ammonia (5,14). Approximate estimates of the concentration of unionized ammonia in fresh water can be obtained form measures of total ammonia (NH3 plus NH4 concentrations) and pH as (5):

where [NH3] is the unionized ammonia concentration (mol/L) and [TA] is the total ammonia concentration (mol/L).

Removal of ammonia from aquaculture systems may be achieved by ion exchange or by biological filtration. Ion exchange is carried out with a natural zeolite (clinoptilolite) that has a high affinity for ammonium ions. Removal capacity of the resin varies widely, but a common design value is around 1 mg NH4 (g clino)-1 (22). Clinoptilolite resin may be regenerated with brackish or salt water, making the use of the resin suitable for fresh water applications only. Ion exchange columns need to be designed to incorporate the downtime involved with the recharging and reconditioning. In addition, unless properly maintained, these columns have a tendency to be colonized by bacteria and become biological filters.

The biological removal of ammonia by the process of nitrification is carried out by two groups of bacteria: nitro-somonas, which take ammonia to nitrite; and nitrobacter, which complete the reaction to nitrate. Nitrate is toxic to fish only at very high concentration and therefore is a suitable end product for ammonia and nitrite, forms of nitrogen toxic to fish. The nitrification process is relatively slow and sensitive to temperature, pH, and ammonia concentration. To achieve practical nitrification rates, high bacterial biomass must be maintained, which is normally achieved with some form of attached growth filter (23,24). Many types of filters are used for ammonia removal: down-flow, upflow, rotating disks, submerged, trickling, fluidized beds, and soon (23,24). Removal rates are highly variable, and this is an area of active research at the present time.

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