BACTERIA IN AQUAPONICS

In aquaponics, ammonia must be oxidized into nitrate to prevent toxicity to fish.

• The nitrification process is a two-step bacterial process where ammonia-oxidizing bacteria convert ammonia (NH3) into nitrite (NO2), and then nitrite-oxidizing bacteria convert nitrite into nitrate (NO3).
• The five most important factors for good nitrification are: high surface area media for bacteria to grow and colonize; pH (6-7); water temperature (17-34 °C); DO (4-8 mg/litre); cover from direct exposure to sunlight
• System cycling is the initial process of building a nitrifying bacteria colony in a new aquaponic unit. This 3-5 week process involves adding an ammonia source into the system (fish feed, ammonia-based fertilizer, up to a concentration in water of 1-2 mg/litre) in order to stimulate nitrifying bacteria growth. This should be done slowly and consistently. Ammonia, nitrite and nitrate are monitored to determine the status of the biofilter: the peak and subsequent drop of ammonia is followed by a similar pattern of nitrite before nitrate starts to accumulate. Fish and plants are only added when ammonia and nitrite levels are low and the nitrate level begins to rise.

• Ammonia and nitrite tests are used to monitor the function of the nitrifying bacteria and the performance of the biofilter. In a functioning system, ammonia and nitrite should be close to 0 mg/litre. High levels of either ammonia or nitrite require a water change and management action. Usually, poor nitrification is due to a change in water temperature, DO or pH levels.
• Another class of micro-organisms naturally occurring in aquaponics is that of heterotrophic bacteria. They decompose the solid fish waste, releasing some of the nutrients into the water in a process called mineralization.