5.5 CHAPTER SUMMARY

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

The nitrification process is a two-step

FIGURE 5.6
Test kit showing low ammonia level (0-0.5 mg/litre) (a) and high ammonia level (4 mg/litre) (b)
FIGURE 5.7
Adding a plant seedling into a media bed during the cycling process

bacterial process where ammonia-oxidizing bacteria convert ammonia (NH3) into nitrite (NO2), and then nitrite-oxidizing bacteria convert nitrite into nitrate (NO).

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 grow th. 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.