So far there is a way to periodically purge settleable solids from the tank and a way to filter out suspended solids or solids that have not settled yet.

What is also needed in a system that reuses water is a biological filter. That's because fish give of ammonia NH4+, which can poison them regardless of how clean the water looks or is aerated. The killer is actually the unionized form know as NH3, in which the percentage of unionized is dependent on PH and water temperature in relation to total ammonia (NH4). Most is give off by the fishes gills but waste products produce ammonia also.

There are all kinds of biofilters, which are simply a provision for a surface area beneficial bacteria can live on and thrive. But most importantly these bacteria oxidize ammonia an convert it to nitrites (also poisonous but not as much as ammonia) and then to nitrates which are fairly benign with exceptions depending on species and high concentration.

My favorite filter is a rotating biocontact filter (RBC) but the third one I just built for a demonstraton at an aquaculure conference cost me close to $500.00 in materials. From now on any new systems I build for the high schools I set up will use a simpler and cheaper biofilter that will run about $165.00 to build. It also has a smaller footprint than the RBC.

Here's one I'm using now in my winter system.



It's simply a 55 gallon drum with a diffuser mounted in the bottom connected to an air line. There is also an intake where water from the top of the clarifier tank pump runs in which is located close to the bottom of the tank. Near the top of the tank is outlet which dumps back into the fish tank. The outlet is screened to keep plastic media in.

The plastic media you see in the picture is called MB3 and is available for $25.00 per cubic foot from Water Management Technologies. There's another one called Kaldness that runs twice as much and doesn't work as well IMHO as it's much smaller and clogs with film easier.

http://www.w-m-t.com/Products/WaterTek_MB3_Moving_Bed_Media.php

You will need 3 to 4 cubic feet of media for the drum.

Basically bacteria lives and grows on the media as the air keeps mixing the media tunbling it in the tank and provided oxygen to this aerobic bacteria. Old bacteria film which is less productive is scoured off. It's also a self cleaning media vs some other types of systems that have to be periodically cleaned.

There is no drain line on this biofilter drum as it's not needed.

Last edited by Cecil Baird1; 04/21/13 03:04 PM.
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