Biofiltration Tech Info

Nitrogen is an essential nutrient for all organisms, but a significant percentage of nitrogen in feed ends up as waste. Waste nitrogen exists in many forms, the most important of which are Un-ionized Ammonia (NH3), Ammonium (NH4+), Nitrite (NO2-), and Nitrate (NO3-). In aquaculture systems, nitrogenous waste enters the system through direct excretion from the fish and through the mineralization of solids such as waste feeds and feces.

Ammonia (NH3) is the major end product of the metabolism of nitrogen. Lethal dosages depend highly on species and age. The toxic effects of ammonia include gill damage, the reduction of the bloods ability to carry oxygen, damage to the red blood cells, and increased oxygen demand by tissue (Lawson, 1995).

In hatcheries with flow-through or partial reuse water systems, ammonia is flushed from the system before concentrations build to toxic levels. However, in facilities with recirculation water systems, ammonia must be removed through treatment. Biofiltration uses the metabolic activity of cultured bacteria to convert toxic ammonia-nitrogen to less toxic forms using a process called nitrification. This is a two step process where “Nitrosommonas” bacteria convert ammonia to nitrite, and then “Nitrobacter” bacteria convert nitrite to nitrate. Nitrite (NO2-), the intermediate product between ammonia and nitrate, can cause “Brown Blood Disease” which results in the death of the fish through asphyxiation (Lawson, 1995). Nitrate (NO3-), the end product of the nitrification reaction, is only toxic at vary high concentrations (1000-3000 mg/L).

Nitrifying bacteria grow very slowly and will flush out of a system unless given a media on which to grow. Many media types exist. Fluidized Sand Bed Biofilters use inexpensive sand as the media and provides much more bacteria growing surface per unit volume than other media types.