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sand biofilter refugium?
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number6
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Joined: 07 Feb 2003
Location: Ontario, Canada

PostPosted: 2005.01.26(Wed)16:08    Post subject: sand biofilter refugium? Reply with quote

I was reading about sand filtration when I saw this, http://www.davnor.com/images/biosand_operational.gif

It is along the ideas of a project I have been working towards for some time. I wish to build a bio filter.
I want reeds, and aquatic plants on top, with killies
then an fluorite/ soil base, followed by gravel, and finally sand. I had been researching what layer or membrane to put under the sand in order to allow the water to be pulled down through the materials for a full 100% filtration.

Looks like the above silica sand and an underdrain pipe may be fine without clogging?

Any other ideas on how to pump the water out of a bottom layer?
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dale
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Joined: 10 Jan 2005
Location: Vancouver Canada

PostPosted: 2005.01.28(Fri)21:21    Post subject: Reply with quote

Hi Discus man,
The schematic drawing is very similar to the slow sand filtration system that we use in the greenhouse industry to treat our recirculated water. Our tank is concrete, 15' by 15' by 8' deep filled with silica sand. we use a pump to draw water through the sand as shown with a few differences. Here's what I've experienced:
A bacterial scum layer forms on the surface (for us 8"deep). This is where the beneficial bacteria break down harmful pathogens. This scum will become a hard crust that impedes water flow. When this happens water overflows the tank (thousands of gallons if missed). We correct this by periodly breaking up this layer with a pitch fork and once a year removing a layer.
The bottom of the tank has a network of pvc piping with holes in to draw water evenly, wrapped in a synthetic cloth(to prevent biodegrading). As long as the weave is smaller than the silica it is fine. A crust does not form at this depth. If silica gets drawn into your pump it will screw it up so we use a sieve type pre filter on the pump.
I don't know about the other substrates that you want or how they will affect performance as we are only concerned with water filtration. I would be concerned with long roots that will cause water channels to form and bypass the sandfiltration process. I guess it depends on the plants and sand depth.
The biggest problem to occur is if the intake of water overcomes the outflow(if you're filtering another tank). Then you will end up with a big spill. Don't trust float valves as bacterial scum will coat this area and could cause failure.
The best scenario is to have the intake near the top of the tank you are filtering. Then if the outflow slows or fouls the intake will only allow water to enter till it falls below the intake level. Not very much lets hope.
Good luck, I'll check this post later to see if you ask anything else. Very Happy
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