Biofiltration & Nitrification

BACTERIAL NITRIFICATION

Aquaculture

Biology of the system

The major metabolic nitrogenous waste product of fish metabolism is ammonia. This product is extremely toxic to fish and if allowed to accumulate in a recirculation system will eventually lead to poor growth rates or fish moralities.

Bacterial nitrification is a biological process which can be applied to recirculation systems in order to control ammonium levels. Two groups of beneficial bacteria are involved, Nitrosomonas and Nitrobacter species. These bacteria first convert the ammonia to a toxic intermediate product (nitrite) and then to a non-toxic end product (nitrate). The process is efficient and reliable when water temperatures are above 10 degrees centigrade. At temperatures below 10 degrees centigrade, bacterial nitrification is greatly reduced, however the loading from the fish is also reduced. At low temperatures it is important that there are no sudden changes to the stocking densities or the amount of feed given to the fish. If these precautions are observed the biofilter will adapt to the low operating temperatures.

The biofilters supplied are trickling down flow systems using plastic packing as the biofiltration media. The system comprises of a tank, one piece or sectional in a variety of sizes either circular or rectangular in glass fibre, Stainless Steel or Polyethylene through which water is evenly distributed over the top surface of the biofilter bed and trickles down through the media to exit through an out-let system at the base of the tank.

Nitrifying bacteria develop on the surface of the plastic packing, and it is these bacteria that are responsible for converting the toxic ammonium to nitrate. In addition to the nitrifying bacteria, there will also be heterotrophic bacteria adhering to the filtration media. These bacteria will digest organic matter such as waste feed and faeces. The biofilter will therefore not only reduce the ammonium levels but it will also reduce the BOD through heterotrophic bacterial degradation of waste matter.

Operating the biofilter as a trickling down-flow system exposes a very high surface area of water to air, as a consequence oxygen will diffuse into the water. The biofilter can therefore be used to aerate the water as it passes through the structure. In cases where additional oxygen is required e.g. to achieve high stocking densities, oxygen may be fed directly into the biofilter. This is a very efficient means of increasing oxygen levels and will utilize more than 90% of the oxygen supplied to the system.

Please note that if the biofilter is used for oxygenation with pure oxygen, it is very important that water should be passing through the biofilter when oxygen is being passed into the tank. It is also important that you do not add too much oxygen since this would not only be wasteful of the oxygen, but could possibly be dangerous. Oxygen should be considered as an explosive gas, and the appropriate signs and precautions should be adhered to at all times.

Some uses for biofilters in aquaculture;

Water treatment in closed systems

Water treatment in partial recycle systems

Treatment of wastewater from fish farms

Aeration of fish culture water

Oxygenation of fish culture water.

Biofilters are manufactured in various sizes from a variety of materials as listed below.

All units are supplied complete including Biofilter packing.

GS = Glass fibre, SS = Stainless Steel, PE = Polyethylene.

Type	FlowrateCubm/Hr	Code	Approx biomass loading of fish @ 15 deg C	Dimensions
Plastic filters
0.5PE	5	4.7.11	100kg	814 Dia. x 1135mm High.
1PE	10	4.7.12	200 kg	1054 x 1321mm
2PE	20	4.7.13	400kg	1360 x 1520mm
Stainless steel filters
5SS	50	4.7.15	1000 kg	1.5m(L) x 1.5m(B) x 3.0m(H)
5SS with lid	50	4.7.18
10SS	100	4.7.16	2000kg	3.0m x 1.5m x 3.0m
10SS with lid	100	4.7.19	2000kg

	Biofilters modular units
	Bacteria starter cultures
	Biofiltration media