Commercial Products

Biological Filtration: Choosing the Correct Biological Filter

Biological filtration refers to filtration methods that utilise living organisms to neutralise or remove undesirable substances from a process liquid. It is the removal of waste metabolites that would otherwise accumulate from keeping, feeding and growing aquatic animals in a closed recirculating system that is considered here.

After oxygen, it is the accumulation of ammonia - the waste metabolite from protein digestion - that is probably the most limiting factor affecting the success of operating closed recirculation systems. The removal of ammonia (NH₃) by oxidation to nitrite (NO₂) and finally nitrate (NO₃) is carried out by the bacteria Nitrosomonas sp. and Nitrobacter sp. respectively. This process is known as nitrification.

Un-ionised ammonia and nitrite are both toxic to fish at relatively low concentrations whereas nitrate, the end product of nitrification, is relatively non-toxic at even quite high concentrations.

The rate of nitrification is proportional to temperature, proceeding particularly slowly below 10°C and is optimal around 30°C. However, the metabolic rate of aquatic animals is also proportionally lower at reduced temperatures and it is perfectly feasible to consider recirculation at cooler temperatures.

The particular size and type of filtration equipment for a given application will depend on a number of factors including, but not limited to, the type of livestock being held, the total stocking density, the amount of food that is added to the system and the temperature of the water. Use the table below to calculate which size of a particular piece of equipment your application will require.

Most of our biological filters are sized to a given water volume to be filtered at application level 3 and at a tropical temperature range.

Appl. Level	Comparable Livestock Holding Applications			Stocking Density (kg/m³)	Tropical Species 25 - 27°C	Temperate Species 18-20°C	Coldwater Species
Appl. Level	Comparable Livestock Holding Applications			Stocking Density (kg/m³)	Tropical Species 25 - 27°C	Temperate Species 18-20°C	13-15°C	6-8°C
	O	R	A		Number of Units Required
1	Display	Larval	Incubation	< 1	0.25	0.3	0.4	0.5
2	Retail	Juvenile	Hatchery	1 – 2	0.4	0.5	0.6	0.7
3	Wholesale	Adult	Broodstock	3 – 5	1.0	1.1	1.5	1.7
4		Wet Lab	Nursery	6 - 9	2.0	2.5	3.0	3.5
5			Growout	10 – 15	3.0	4.0	5.0	5.5

O = Ornamentals, R = Research, A = Aquaculture

How to calculate the number of units needed to filter a different water volume at a different application level and/or temperature range:-

1. Choose item from the relevant pricing table e.g. 7910 FST 5200A Fluid Sand Filter, and note the maximum system volume at application level 3 e.g. 10,000 litres.

2. Divide your actual system volume by maximum system volume to determine the number of units required e.g. 20,000 litres / 10,000 litres = 2.

3. Find your actual system application level from the table above e.g. Retail /Juvenile/Hatchery or refer to the stocking density. Then, read across row to the factor given under your appropriate temperature range e.g. Retail, 25-27°C = 0.4. Multiply this figure by the number calculated in 2 above to determine the appropriate number of filter units recommended for your application.

- If the calculated value is less than 0.5 then check if there is a smaller unit from our range.

- If the calculated value is between 0.5 and 1.5 then one unit will be required.

- If the calculated value is more than 1.5 then up to two units will be required.

- If the calculated value is more than 2.5 then check to see if there is a larger unit from our range.