Biofilter start-up guide

What the biofilter does

Fish excrete ammonia, and uneaten feed and faeces add more. Nitrifying bacteria convert ammonia to nitrite, and other bacteria convert nitrite to nitrate. Ammonia and nitrite are harmful to fish. Nitrate is far less so and is kept in check by water exchange.

What happens during start-up

• Ammonia rises first. The bacteria that convert ammonia are not yet established.
• Nitrite rises next. As ammonia starts to fall, nitrite builds up, because the second group of bacteria lags behind the first.
• Both fall. When both groups are established, ammonia and nitrite stay low and nitrate rises slowly.

The time this takes depends on temperature, salinity and water chemistry. One filter maker reports that in warm freshwater nitrifying bacteria take up to 30 days to establish, with 2–3 weeks more common, and that saltwater systems may need 60–90 days (Aquaculture Systems Technologies). Plan the start date with that range in mind.

Before you start

• Loop running: pumps, drum filter and aeration working, water circulating
• Media moving: in an MBBR, aeration keeps K5 media tumbling
• Test kit ready for ammonia, nitrite, pH and temperature
• Stocking plan agreed, in steps

Feeding the bacteria

Bacteria need an ammonia source to grow. It can be added before fish arrive, or come from a small first stocking. Your engineer sets the method and the dose with you, based on the system and the species.

Stock in steps

When ammonia and nitrite stay low, add fish in steps and keep testing. Each increase in feed raises the load. The biofilter needs time to catch up before the next step.

Keep it running

Solids reaching the biofilter cut its capacity. In a yellow perch trial comparing three biofilter types, all removed less ammonia than expected because of a high organic load (Virginia Tech). Keep the drum filter working well during start-up.

• Do not stop the flow or aeration: bacteria need oxygen.
• Change feed gradually.
• Check the effect of any treatment on the biofilter before using it.
• Keep pH within the range your engineer gives you.

We stay through start-up

Our engineers install, commission and stay on site through biological start-up, so the first stocking is safe. See installation and commissioning and MBBR biofilters.