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DIY - Acrylic Fluidized Bed Filter (FBF)
by Brad Newton (aka FeatherfinFan)

Heres a good project for those of you want to try your hand at Acrylic fabrication. FBFs are highly efficient biological filters that have enormous bio-load capacities and take up very little space. The mechanics behind the FBFs are quite simple. Sand is the filters media, and each particle of sand is the surface which the bacteria attach to. When the sand is in motion (the fluidized state) it constantly removes bacteria during its collisions with other sand grains, this provides a replenishment of bacteria and the bacteria colony is able to expand very rapidly to increases (or decreases) in bio-load. Since each surface of each grain of sand is available for bacteria, the total surface area in pounds of sand in a FBF is enormous. One pound of sand is approximately able to handle 100 gallons of an average stocked aquarium. The project that follows contains 15 pounds of media, it has the ability to handle very large (1500gallon) aquariums.

Let's get started!

Tool List:
  • Straight-Cut, Dual flute Router bit (no router reqd)
  • Hole-Saw w/ Drill adapter(size determined by bulkhead selected)
  • Syringe w/25 gauge needle
  • Power Drill
  • Sanding block (optional)
  • Large pipe wrench or welding clamp
  • Circular Saw w/ laminate blade (only if using angled FBF design &/or shop cant pre-cut Acrylic for you)

Part List:
  • ¼ thick Acrylic Sheet (size to be determined by user)
  • 2 threaded bulkheads ½ or ¾, depending on filter size
  • Vinyl tubing ½ or ¾
  • PVC ½ or ¾
  • PVC cement
  • Acrylic solvent cement such as WeldOn 4
  • PVC Check valve ½ or ¾
  • PVC Ball valve ½ or ¾
  • ½ or ¾ MPT to PVC slip adapter
  • assorted ½ or ¾ MPT to hose adaper straight & L fittings
  • Stainless steel hose clamps
  • Teflon plumbing tape
  • MagDrive 5 or 7 pump, depending on filter size
  • 5 lb. Bag of small ¼ aquarium gravel
  • Sack of Silica/Blasting sand

Working With Acrylic:

There are a lot of acrylic fabrication articles, so Ill just stress the basics. Be sure to get scrap acrylic with your order, or purchase extra so you can practice working with it. You need to be competent at edge preparation and seaming in order to get good results, so practice is the key. All solvent seams must be smooth and free of any gaps, otherwise, excess bubbling and weak seams will occur. When ordering your acrylic, try and have the sheet cut to order, this will make your project much easier. Most reputable shops will cut for free or a minimal charge.

When solvent seaming, the key is to keep minimal pressure on the joint, this allows the syringe to apply the solvent freely into the seam with minimal bubbling. Once the seam is full of solvent a slight pressure can be added until the solvent sets, but this isnt always necessary. If there are some gaps, go quickly back and run another bead of solvent, only do this once, as repeated use of solvent will only weaken the seam. Practice will give you a good feel for proper seaming, and just keep trying until youre happy with your results. Do not seam up project until youre confident in your seaming skills.

Now, To The Project:

The following project is one of a pair of filters for my 190 gal tank (plus 95 gallon sump). This is much more biological filtration than is reqd, but I always like to overbuild, plus, I like to have equipment in pairs in case one fails. I used 3/8 thick sheet, but thats ONLY because its some scrap I had lying around, ¼ thick is fine for this project.

Your design may vary, it all depends on the space you have, the size tank youre filtering, etc. Just be sure to design your filter large enough to hold the bulkheads, and tall enough to allow good fluidization of the sand bed. Id recommend a minimum of 20 high and 6 square. My filter is about 24 high, 12 wide at the top, 2.5 at the bottom, and 6 front-to-back. Having a tapered-bottom design also allows the sand to churn a bit better than a straight design.

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If you go with a straight square tube design you wont have any tricky angle edge cuts, much better if youre not experienced with cutting mitered lumber, etc.

Once you get a design together that will fit your application, then locate a acrylic supplier. Hopefully they can cut it for you, otherwise a circular saw will work. If you need to cut the sheet, and you will if you go with ANY angled design such as mine (since the edges have to be angle cut where the top & bottom rest) youll have to get a hollow-ground blade for cutting laminates and clamp a straight edge to your sheet for accurate edges. YOU MUST KEEP BLADE COOL WITH A WATER SPRAY BOTTLE when cutting, this is a good time for a helper. Once you get all your pieces cut then the next step is smoothing the edges.

Edge Smoothing:

The router bit is a great tool for smoothing the edges, even if the shop cut your plastic, youll still need to smooth the saw marks from the edge.

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Just run the router bit down the edge, scraping as you go, dont gouge into the sheet, just scrape. Youll see the marks eventually disappear. Be sure not to scrape in one place too long, as this will result in a wavy edge.

Once the edges are smooth do a trial fitting, if there are no major gaps youre ready to seam. Seam the four sides first, one at a time. Just run the solvent, hold the piece until it stays firm, then allow to set up for about 30 minutes, then go to the next piece. Once the sides are all seamed, make sure the bottom and top resting edges are all even and smooth, if not, get a large sanding block and some 200 grit wet-dri paper and wetsand the top and bottom until theyre smooth and square. Seam the bottom piece. Now is the time to prepare the top piece.

Prepare The Top Piece:

Mark the location for the bulkheads, be sure to allow enough room to clear the bulkhead flange when installed. Drill the proper size hole with your holesaw, but drill as follows: Drill only 1/8 into sheet, flip sheet over and drill rest of the hole, this will prevent splintering. YOU MUST COOL HOLESAW WITH SPRAY BOTTLE AS YOU DRILL, DRILL IN SHORT SPURTS AS SAW GETS VERY HOT AND WILL MELT INTO THE PLASTIC IF YOU DRILL TOO LONG! Trust me, *** learned the hard way.

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Once you get the holes drilled go ahead and LOOSELY install the bulkheads with the flange and rubber washer INSIDE the lid and the holding nut OUTSIDE the lid. Then thread your ½ or ¾ MPT-PVC slip adapter into the incoming bulkhead with a couple wraps of teflon tape, and tighten til snug. Now measure your ½ or ¾ PVC down pipe (if your design is similar to mine and you need some elbows to center the pipe, be sure to allow for this now). Allow for any length of pipe that will be inside any fittings and extend the down pipe to come to ½ from the bottom of the filter if using ¾ PVC, and 3/8 from the botttom if using ½ PVC. This is important, too much of a gap and you get poor flow or channeling in the sand bed, too little of a gap and you restrict your pump output. Once you get your measurements, cut and glue pipe with PVC cement. Now be sure to tighten down your bulkheads, using a large pipe wrench or welding clamp helps, you want these bulkheads pretty darn tight!

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Once youve got everything tight and the pipe is the proper length you can go ahead and seal the top.

Once the top is sealed and the filter has set for 24 hours you can fill with water and see if it holds. If you have good seams youll be fine!

Now its time to fill with gravel & sand. I use 1 part gravel to 5 parts Silica sand, the gravel allows you to maintain higher flow in the filter with less chance of blowing out sand, it also keeps the sand from clumping together. Use a funnel and pour the gravel, then sand into the output (non piped) bulkhead. You want to only fill your FBF so its about 1/3rd full of media. Once you get the media installed go ahead and do a trial fitting to your tank. Youll need various Ls and connectors as per your application. Heres my setup to give you an idea.

As far as sizing your filter. Id recommend going with ¾ PVC / fittings / bulkheads / valves, and a MagDrive 7 pump for tanks over 100 gallons. Id also recommend between 7-15 pounds of media depending on what it takes to get 1/3rd volume level.

Id recommend ½ PVC / fittings / bulkheads / valves, and a MagDrive 5 for tanks under 100 gallons and between 3-7 pounds of media.

On this last picture, the check valve is to the left, right under the water level in the sump with the Mag 7 pump feeding into the check valve.

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Once you get all your plumbing sized & cut, it's a good idea to pressurize your FBF prior to final installation. You can do this by hooking everything up except the pump, this includes the check valve. Put a garden hose on the incoming line and turn the hose on full pressure, this will clear any clogged media in the tube and allow easy startup w/ your pump, the checkvalve will keep the media from clogging the line once you remove the garden hose. Then connect the pump and it should flow just fine. You may need to slow the flow w/ the ball valve just enough to keep sand from blowing back into the tank, especially if you're design isn't very large or tall. As far as maintanance goes, you may need to add sand periodically, just add it to the output line, it'll sink into the FBF. Every 6-12 months it's a good idea to connect the garden hose up to the input of your pump and give it a good "flush" this will release some of the sticky sludge that can clump the sand together. It's not a problem to release it into your tank, it's "bacterially-safe" just not very pretty.

Have fun with your project and let me know if you need more info.

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Disclaimer: By building this DIY project you agree not to hold the author or the owners of this Web site responsible for any injury or bodily harm you may cause to yourself or others. Always wear safety glasses when working with tools and keep chemicals and power tools away from children. Read and understand all safety instructions pertaining to equipment prior to use.
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