Fluidized Bed Filter

[Fishn']

Hello,

I am curious if anyone has experience with a fluidized bed filter using sand as the medium? I've read claims that these filters/medium have the greatest contact area for biological filtration.

Any thoughts or experience?

Lief

 

[JLW]

Hey Lief,

I use them, a lot. They were "fashionable" about, oh... 10-15 years ago, especially among the saltwater crowd, but have lately gone out of fashion. You can often pick them up at auctions for a couple of bucks, at best. They work by having a column of fine, biological media of some sort -- often sand, but sometimes beads or other media -- suspended in a constantly flowing source of water. The media "fluidises," or becomes like a liquid. It's similar to how you (should) run things like GFO, carbon, silicate absorber, etc.

They produce a lot of nitrate -- which means they're breaking down a lot of protein in the water -- and I think the marine folks have discarded them in favour of better defractioners. Most freshwater people think they're unnecessary, but even a small one adds a tremendous amount of biological filtration.

 

[Frank Cowherd]

In theory fluidized bed filter never needs cleaning since the sand or whatever is fluidized and keeps bumping into other pieces of sand and knocking off the growing biofilm on the particle. In practice this does work and if you keep large stuff/debris out of the filter, using a prefilter, the fluidized bed filter can be a very good biofilter for a very long time. I have used them without cleaning for close to a year.
I have seen them on 2000 gallon aquariums with pumps that could handle 10,000 gallons and hour and perfiltered with the socks that remove debris down to microns. Very nice. You could dial in the flow to keep the fluidized bed at whatever expansion the manufacturer recommended. River rock the size of your fist was used about 12 inches at the bottom of the column so when you turned off the flow, the sand would not settle back n the pipes and make it hard to restart.
In reality you cannot keep some debris from getting into the filter. This material slowly builds up in the fluidized bed and means the water flow gets slower with time. For this reason, you are always better off using a pump sized for about twice the flow as actually needed. Then when the flow slows, you just open the valve a bit more to compensate.

 

[Fishn']

Josh,

Thank you for the answer. I like the fact that their biological filtration capacity in comparison to other media is very high. Is breaking down the protein in freshwater a good thing? I assume it is.

So just remove the nitrate through water changes and assist the nitrate removal with nitrate absorbing plants?

What are the drawbacks compared to a canister when it comes to biological? I've seeing a few steups where a fluidized bed filter with sand media follows the canister.

Lief

 

[Fishn']

Frank,

That is great insight into setting one up. Thank you.

Lief

 

[Fishn']

Frank,

Was there mechanical filtration outside of the sock on these large tanks?

 

[Frank Cowherd]

I believe the flow from the tank just went directly to the socks, though the top of the sock tank may have had a screen on it with big holes.