How To DIY Fluidized Bed Filter

From: detroit@chopin.udel.edu (Daniel Wi Hofstetter)

Date: 9 Jun 1996 00:07:34 -0400

Newsgroups: rec.aquaria.tech

Subject: How to DIY fluidized bed filter:

Well, I built the filter, and here are the specs as promised, after a good run-in period both off the tank and on...

DISCLAIMER: This is a LONG posting (over 300 lines), if you want to try this I suggest saving it in a file and printing it out for reference. This requires some WORK to do. If you don't enjoy making things like filters yourself, DON'T even attempt this. Go and buy one pre-made from one of the many companies offering them. I attempted to make the instructions as specific as possible; should they confuse you, let me know and I'll try to clarify them for you. Also note that this may not be the best possible way to make this type of filter, but heck, I haven't seen instructions like these posted before... I had a lot of fun doing this, and hope you will as well! Good Luck!

Parts list: -----------

The pipe diameters/lengths can be changed, as well as fitting and hose barb sizes. Depends on what you wish to do. The following parts rounded my filter out:

Now, lets see if I can draw a good diagram of this:

Fluidized Bed Filter

Please note: This was designed to sit on the floor behind my aquarium, strapped to one of the uprights of the stand with velcro strips (it stands pretty well with the tubing in place anyhow). The design can be changed if you want to hang it, or whatever...you just need to come up with a way of hanging it or getting it to sit where you want it.

Instructions for building: --------------------------

1) Cut two holes just big enough to tightly slide the 3/4" pipe through the upper slip cap: One dead center in the top, one with the center offset approx. 1 5/8" from this. Note: Make sure the holes are cut perpendicular to the cap so that the pipe doesn't come out at angles. The middle hole is the most important in this respect, since the inner tube must stay centered inside the outer one. I used a 3/4" hole saw, then medium grit sandpaper to open the holes up a little more (about 1.05" is needed).

2) Cement the bottom cap to the bottom of the 2' section of 4" pipe and let it set. I used the pipe cleaner (purple) to prep the surface since the filter would be under pressure, then applied the cement while the primer was still wet. Follow the directions on the glue/primer cans for proper use.

3) Trace some 4" DIA. circles onto some corrugated cardboard to use as positioning washers for the inner tube. They don't have to be perfect circles, as long as once pushed in the 4" pipe, they hold the 3/4" pipe near center. Cut 1" holes in the center of these discs.

4) Measure the depth from the top of the 4" pipe to the bottom of the inside of the pipe cap (the dome will be a little below the bottom of the pipe). Mark this length on the 3/4" pipe with pencil, and one about 1/4" above it. These are the lines the upper pipe cap will sit within. Still above these, mark off another 2" or so and cut the excess off at this line. The long piece with the lines drawn on it is your intake tube, the short piece is the outlet.

5) Slip the shorter end (with respect to these marks) of the 3/4" pipe through the middle hole in the upper pipe cap, so that the cap is within the lines you drew. Slide the cardboard washers onto the 3/4" pipe about halfway up and about 1" from the bottom. These will position the inner tube correctly. Ideally the inner tube should sit about 1/4" to 1/2" above the bottom (inside) of the lower slip cap. Mine sits 3/8" above this.

6) When you have this positioned correctly, slide the slip cap off the intake tube and prime the surfaces. Then apply the cement and put the pieces back together. Before the cement dries, wipe off any excess that slides down the intake tube (excess on top of the slip cap is good for sealing) and slide the assembly, with washers still attached, into the 4" pipe, and just let it sit for awhile until the glue has set. I let it sit for a good hour or two before messing with it again. Note: do NOT glue the upper cap to the 4" pipe at this time.

7) After the upper cap has cured to the intake tube, remove the upper assembly from the lower one, and push the shorter tube (leftover from cutting the intake tube to length) through the other hole in the upper cap. Keeping it perfectly parallel to the intake tube is nice, but not a necessity. From the underside, while holding the pipe in place (shouldn't be too tough, the hole should keep it fairly snug by itself) trace the outline of the pipe where the slip cap meets it. Next, cut this pipe through the line you traced. This is your outlet tube. The idea here is to make it flush on the inside of the upper cap, so there are no places that air bubbles or water can get hung up in the filter.

8) Next, remove the outlet tube, prime the surfaces, cement together, and put it back together. Hold this in place for awhile until the cement has set pretty well. After it has set, apply a few more coats to the inside, where the pipe meets the cap, to promote better sealing.

9) Once the upper assembly is dry, place it right side up (cap up) and apply PVC cement around the 3/4" pipes where they enter the upper cap, in layers (let one tack up before adding another, and try not to let it run all over the cap as it can be messy...I kept mine about 1/2" around each pipe).

10) Once you are confident you have enough PVC cement around the joints to seal the upper assembly, cut the ends of the pipes sticking through the top of the upper cap at about 1" from the cap level with one another.

11) Check the 3/4" x 1" threaded slip fittings for fit on the 3/4" pipe (mine were a little off, and required some sanding for a tight fit). They should slide on without an enormous amount of effort (you shouldn't have to crack the pipes to get them on). If everything is ok, prime the fittings and pipes, and cement them together. The upper assembly should now consist of the upper cap with an intake and outlet tube and threaded fittings cemented to them.

12) This next step is easy: slide a tape measure or a yard stick into the lower assembly (2' pipe with lower cap), and fill the lower ass'y. with 2" of the quartz gravel. Pour this gravel into a container for final filling of the filter. With the ruler still in the lower ass'y., fill it 6" with your sand. Pour this into the same container as the gravel you measured out, it will all sort out once you run the filter for awhile.

13) Now you're ready to put the upper and lower parts together. Remove the cardboard washers from the intake tube. Prime the surfaces as before, cement them, and slide the upper assembly into the lower one, twisting them slightly as you go (to spread the cement evenly around the pipe).

While the filter is curing, you can plan the tubing lengths and placement of the powerhead and intake/outlet tubes on the back of your aquarium. I ran about 3" of 5/8" Magnum tubing (clear vinyl) from the powerhead outlet to the 3/4" barbed elbow (it took some doing to get the 5/8" hose over the 3/4" barbs, but it won't leak! I heated the tubing under hot water a bit before attempting this, then dried it and went to work twisting and pushing it over the barbs). Make SURE you put the hose bands around the ends of the hoses, sliding them back enough for the fitting they will secure to enter the hose, BEFORE you put the hose on the fitting and powerhead. Otherwise, you'll need to push them over the other end of the elbow and roll them until they get where you need them. These bands are just like hose clamps, but you don't need a screwdriver. You DO need some patience and some strong fingers to stretch the little buggers over the hose ends!

I turned the elbow so that the flow would go upwards. Then, I connected the other barb on the elbow to the Magnum Intake Tube (strainer not used) with a short length of 5/8" hose. I had to modify the intake tube a bit by cutting about 5 or 6" off of the side that the strainer goes on (barb on intake tube goes to outside of aquarium). Again, put the bands on the hoses before attaching them to the fittings. From there, I ran hose to the quick-disconnect valves, and from them to the top of the filter. I had to attach the hose to the 3/4" screw-in fittings before installing them into the filter since the hose is tough to force over the larger barbs (I couldn't find 5/8" barbs at Rickel's).

The Outlet from the filter is similar, going from bottom to top I put the screw-in fitting with hose going to a Magnum double valve, then hose going to the Outlet Tube with Diffuser, which ends the path of the water as far as the filter is concerned. The Tube Strainer Bracket w/Bow was used to hold the intake/outlet tubes in place on back of the aquarium.

Most powerheads come with some hose with a screw-plug in it for venturi aeration. This didn't work for me, and I wound up plugging the port on the outlet of the powerhead with this rubber plug to keep water flow actually going to the filter. Incidentally, this plug makes starting the filter with air in the hoses a lot simpler, since I can remove it until the powerhead pumps water through the hole, then push it in a little at a time until all the flow goes to the filter.

14) Coat the screw-in fittings with aquarium sealant, and thread them into the filter. Tighten by holding the fitting on the filter in one wrench (I used a pipe wrench) while turning the other with another (vise grips here). Turn them until they are nice and tight, and let the sealant cure before putting any pressure through the filter.

If you've got all this put together, you're in good shape. I ran my filter into/from a 6 gallon bucket placed on my kitchen counter, which allowed me to put the tubing/powerhead at the same height as the aquarium roughly. I then filled the bucket with water and started her up to check for leaks, and to clean the filter out in general. I changed the water once during this process. This is also the time to check for leaks. I had one at the outlet port on the filter, but fixed it by following step 9 above (you don't need to figure this step out yourself since I've done it for you, hopefully my instructions in that step will prevent such leaks). If you DO have a leak, dry everything, and apply more cement.

15) Once you are satisfied there are no leaks in your filter, apply aquarium sealant around the pipes that come out of the upper cap and under the slip fittings. Just another safeguard against leaks, can't be too careful :).

16) Now drain the filter, and fill it with sand. This is accomplished by disconnecting the Magnum valves, and using a funnel to pour the sand/gravel mixture in through the OUTLET port of the filter (remember the intake is the center, outlet is on the outside). Shake the filter around a little as you go to try and level it out inside.

18) Now's time to start the filter up again, but slowly. There will probably be air in the tubing and filter that you'll need to displace with water slowly before it can operate normally. You can regulate flow to the filter by turning one of the valves on the inlet side of the filter (this won't hurt the pump since it's the outlet of the pump you are restricting). Adjust the flow rate of your powerhead/pump as needed to keep sand from leaving the filter, but don't worry if a little gets out -- your measured amount is actually a little more than needed (this won't hurt if it doesn't get out, it can only help!) as a safeguard to this "break-in" period. If your pump is not adjustable, use the magnum valve on the intake of the filter -- they're great flow regulators! After you've got it flowing nicely and evenly, let it go for a day or two. The bucket will get a ring around it at the water level if you used colored gravel -- this is the paint coming off from the "sandblasting" it is recieving and is nothing to worry about. After a day, change the water in the bucket and start the filter up again, regulating the filling as before. Remember, any sand that DOES escape can easily be put back in. Test the water with your test kit to make sure the water is clean (I changed water 3 times before the water showed 0 ammonia in the bucket -- I suspect it was residual from the glues/cements/sand/gravel...it does go away with water changes and will not come back on its own).

19) If you're satisfied the filter is clean and operating normally, go ahead and install it on your tank in a place you can get to it for now (I put mine on the front for a day to check and make double sure the water quality was not jeopardized). Remember to regulate flow upon startup. If you run it a day or two and are satisfied it's not hurting things, go ahead and put it in its final resting place and let her rip!

Important Note: These filters operate at their best when they stand upright! If it leans too much, the water will not flow evenly through the entire bed of sand/gravel, and it won't work as well at removing ammonia/nitrite.

The gravel will sift to the bottom of the filter in time, since it is heavier than the sand is. It's pretty gratifying to see this in action when it's completed! Even more fun to listen to the sand moving inside the pipes (it is quiet, so you need to get up close to hear it). You will need some form of prefilter for your pump; I used the QuickFilter from AquaClear to keep debris out of my pump.

If you notice anything that seems to be missing from this, let me know and I'll make the necessary additions. I am keeping this in a file, so it's easy to fix and repost. Also, if you have any other good ideas, feel free to reply with them. I'm currently looking for a nice way to put an air-bleed valve in the Magnum intake/outlet tubes to simplify starting the filter up again after air entrapment at water changes, etc...

Dan