Finally Finely Filtered - a Pond Filter

About three years ago I decided to have a Koi and Goldfish pond in my back yard.  I wanted it to be like a small eco system so I would not have to spend a great deal of time on maintenance.  I know now that some sort of filter is required because of algae.  I considered one with a water pump that pulled water through a sponge type material.  It was expensive, and I did not really want to try to keep ahead of the fact that the filter will slowly become clogged with debris, need cleaning, and at the same time will cause the pump to work harder and harder, or even burn up the electric motor of the pump.  So I spent many hours thinking of other alternatives.

I had already purchased an air pump for ponds from pondliner.com, model AP-20 PONDMASTER (worth every penny).  I wanted to insure enough air was available during the hot summer months, as well as keeping the water slowly moving.  Before that I was using an aquarium pump and had a stone bubbler submerged.  I purchased that pump at PetCo, model AC-9903 for 13 – 106 gallon tanks, a great pump but not for ponds.  There is something about the pressure caused by the depth and volume of the water that makes it necessary to have a “for real” pond air pump.  I also realized that when I had kept an aquarium, it was using an under gravel filter system that circulated the water with bubbles from a pump.  I had to figure how I would apply this to my current situation.

I came up with a design using PVC pipe and here is what I did:

Materials list:  

4 inch PVC pipe – 1 piece 5 inches long

4 inch to 2 inch PVC reducer – 1

2 inch PVC pipe – 1 piece 7 inches long      

2    2 inch PVC couples

Now the ½ inch PVC pieces:

   2 plugs (slip) with hex head

   5  T’s (slip)  Slip means smooth inside for gluing, not threaded

   1  T (center is threaded with ends slip)

   1 elbow (slip)

   1 adapter (one end slip / one end threaded outside)

   1 adapter (one end slip / one end threaded inside)

   2 – 8 foot ½ inch pipe

1 PVC cutter (looks like a pair garden pruners)

1 small can of PVC glue

1   ½ inch plastic sprinkler hose attachment piece, threaded

1 Army cot mosquito net cover (to make filter bags)

Plastic zip ties about 14 inches long

Total cost is approximately $35.00, depending where you shop.  I went to Lowe’s and an Army Surplus Store.  The 4 inch pipe is the bubble chamber and the 2 inch pipe is the chimney.  You might get a plumber to give you his discarded pieces.

Please note that my pond has a top measure of 18 feet by 14 feet with 4 sides that slope at approximately 45 degrees down to the bottom.  Here it measures 4 feet square on the flat, and is 4 feet deep with lily pads.  My design is for the filter to sit on a slant rather than flat on the bottom.  This means that the measures and assemble may differ in your situation depending on where it will be positioned in your pond.

To get started I cut two (2) pieces of ½ inch pipe about 5 inches long.  These are to be the chamber supports.  I measured about 1 inch from the end and marked the pieces.  These pieces should be longer if your filter is sitting on a flat bottom which makes it perpendicular to the base.  This position would require the bubble chamber to be raised enough for the air hose to be attached.  (You will probably understand this after you have read my narrative.)

I used my band saw to make this cut.  I attached a fence for a guide because I wanted my blade to be just inside the interior wall of the piece and cut straight.  The cut is length wise up to the mark, remove, and cut across at the mark so that you have that section removed.

This is a chamber support.  Keep the pieces cut away, we will use them later. 

Next, cut a piece of ½ inch PVC pipe about 2 ½ inches long.  This piece is used to join two of the (slip) T’s.  Dry fit the short piece of pipe into the ends of two T’s to make a single unit, and insure that you have about ¾ to 1 inch between the T’s.  It will be critical that the two T’s are aligned properly before gluing.  This glue sets in about 10 seconds, so dry fit and mark alignment on each piece each time from here until fully assembled and finished before any glue is applied.  Here you might use the two chamber support pieces and dry fit them in the center hole of each "T".  Insure they are in perfect alignment with each other.  Then with a  felt tip pin you can mark on the white PVC T's and short piece of pipe joining them.  This will insure that after the glue is applied and you push the pieces together, then these marks will guide you.  Just be sure the short piece of pipe joining them does not get reversed.  When applying the glue, use the ‘applicator top’ of the can and swab glue in the opening of the T that will receive, as well as the end of the piece going into that T.  Once glued & aligned properly, you should dry fit the two chamber support pieces

                                           

and be sure they are in alignment.  The chamber supports are not glued yet because they must be finely tuned in order to get the correct position for attachment to the bubble chamber.  This means that you should turn them so that the cut away area now may be in full contact with the outside of the 4 inch bubble chamber.

Now mark the T’s and supports where they join, and draw the outline of the supports onto the outer wall of the bubble chamber so you will know where to put glue.  A special consideration has to be made at this point.  The top of the chamber will have the reducer added.  Dry fit the reducer top on the chamber and see if the outer lip goes below the mark that is the top of the support.  You may need to shorten the supports. 

Remove one support from the T, add glue, and replace it being very careful to align the support with the marks for that T and let the glue set.  Follow this with the other support and this time, check the alignment by holding the bubble chamber against the two supports to see if any adjustment is needed, then give that a minute to set.

Next apply generous glue to the supports face that will contact the chamber and a generous amount to the chamber within the lines that show where the supports will mate.

Clamp this and give it about an hour to set and dry.  If you are successful in the alignment you will be able to see that the chamber has not only made contact with the supports where you cut away part of the pipe,

but it will sit on the notched part just above the T’s.  Once this has dried you should take the two pieces that were cut from the chamber supports and glue those for additional strength and reinforcement to the supports.

This is not necessary, but may extend the life of your filter by preventing detachment. 

Now cut two pieces of ½ inch pipe 1 1/2 inches long, longer if you want a wider base.  These are used to attach the two (slip) T’s that will accept the feet.

               

This is where you will decide if your filter is to sit on a flat bottom or a slant.  Once the dry fit is accomplished and you position them, then mark and glue them.  I will refer to one side as the ‘outside’ and the other as the ‘air approach side’.  From the ½ inch pipe, cut two (2) pieces 9 1/2 inches and one (1) 5 ½ inches.  These are glued into the T’s with the (2) 9 ½ inches on the back side of the chamber where the supports are, and the 5 ½ inch is on the front of the ‘outside’ T.  Next is the front foot for the air approach side (here it is the front right section assembly).

   

Turn the unit upside down and dry fit a (slip) T and a piece of ½ inch pipe about 2 inches long.  Adjust the length of the piece of pipe until the center exit of the T is in alignment with the center of the air chamber.  Mark the alignment so that the center exit of the T is in the same plane as the T it is joining, and glue. 

Next you will need the elbow and a piece of the ½ inch pipe about 3 ½ inches long so you may adjust length and dry fit to position. 

The elbow and short piece of pipe are fitted into the center exit of that last T that you just added. 

 By adjusting the length of the pipe joining the elbow and the T you will move the elbow to a position that is approximately center of the air chamber.  The dry fit of the elbow can be better aligned if you use a long piece of the ½ inch pipe as a guide (but do not glue this one) through the center of the air chamber.  Once this is accomplished you should mark and then glue the elbow into position.  This is also a good time to add ½ inch pipe that goes into the end of the T and completes the front part of the foot on the ‘air approach side’ (bottom left of photo).  Glue it first and then measure it against the other foot to see where to make your cut. 

Take one of the ½ inch PVC plugs and sand, or cut, the ears of the hex head so that the plug is rounded on the head and will fit into the elbow.  Mark half way on the plug and glue it into one of the slip end of the last T that has the threads in the inside of the center hole.  Make sure to leave half of the plug sticking out of the end.  This is the "air attachment unit".  Those threads in the center exit are so the air attachment can be screwed into it. 

Now get the two adapters, about 2 inches of pipe, and the other plug.  The plug is to be glued into the slip end of the adapter with the threads on the outside.  When they are dry fitted, there should be about 1/4 inch of the plug shaft showing between the hex top and the mouth of the slip end of the adapter. In that area you will drill two 5/64 inch holes slightly angled down on opposite sides after the glue sets.  After you have the system working, you might add more holes or larger ones depending on the power of your pump.  This is ready to glue.  Next screw these two pieces into the mating end of the other adapter with the threads on the inside but do not tighten.  Let the glue dry for about an hour, then drill the 2 holes and add the short piece of pipe.

  This is the "bubbler".

Dry fit the bubbler into the top of the air attachment unit.   As soon as you have the two together, please blow into the T’s center hole and be sure that the bubbler allows the air to flow out the drilled holes and the plug in the bottom prohibits.  Then fit the air attachment unit with the bubbler into the elbow.  You want the top of the bubbler, which is the top of the hex head plug, to be positioned such that the 2 holes will be about half way up the inside of the 4 inch chamber.  If not then remove the assembly and adjust the length of the 1/2 inch pipe to correct.  When this is all correct, you should glue the pipe into the bottom of the bubbler.  Now glue the bubbler into the top of the air attachment unit. 

Now glue the air attachment unit with the bubbler into the elbow and be sure that the center hole of the T faces to the side.  This is where the air hose will be connected.

 The bubble chamber should be about like this photo at this point.  Measure the distance from outside of one foot to outside the other and cut 2 pieces of the ½ inch pipe just about an inch longer (mine were about 10 inches long).

                  

These new pieces are to be notched to lie across and on top of the longer two feet in the back of the chamber, and will support the brick that will be the ballast.  These two should be positioned perpendicular to the two longer feet behind the chamber and marked for the circular notches to be cut.  This is the hardest part since they need to make good contact.  Cut away some and check the fit, then shave some as you adjust.  When you get one right, then put the brick on the feet and let the first notched piece go between the chamber and the brick, then glue and clamp that one.  Now cut, adjust, glue, and clamp the other one.

While that glue sets you can glue the 2 inch reducer on the top of the 4 inch air chamber.  Then the 2 inch PVC pipe glues into the top of the reducer, and the 2 inch couple glues onto the top of the 2 inch pipe.  Again dry fit and remember that another 12 inches will be on top of that when the filter bag is attached, wet, and inflated by air and the flow of water upward.  If needed, you can adjust the length of the 2 inch pipe that is the chimney on top of the reducer.

After placing the brick on the feet, I realized that the brick is just barely short enough that it might slip off the support.  So I cut a 6 inches piece of ½ inch pipe and used my band saw to cut it in half from end to end.

 Turn the unit on its side and glue these two pieces in a position that will further support and stabilize the brick. 

 Now the brick should stay in the desired position.

Notch the brick at the midpoint of each end on top as a guide for the strap that will hold it on the assembly.  

I used plastic zip ties that can be purchased many places for my strap.  Mine were too short so I put them into each other until they were long enough to go around the brick.

Zip it tight and cut the extra off.

 The 2 inch couple on top of the chimney will prevent the bag (filter) from sliding off the top.

The air hose is attached using a piece designed for lawn sprinkler systems.  The air hose slips over one end and the other end screws into the threaded opening of the T.  There is no reason to glue this piece, just tighten by hand.

The filter is made from one of those mosquito nets that you can purchase from your local Army Surplus or Military Surplus stores (or you can find one on the internet).  It is made to be supported at each end and drapes over a metal cot, as in Army barracks beds.  I'm just guessing that it is about 6 1/2' long, 3' wide, and 4' tall, it kinda makes a box to cover a bed so a person can get away from mosquitoes.

I took one and cut along the seams to get it into flat pieces, then cut a long strip about 14 inches wide.  From this strip I cut pieces about 2 ft. long, fold it over to be about 12 x 14 and sew, with my sewing machine, the two outside edges which makes a bag almost square.  Now fold the open end back like a hem of 1 inches and sew it around leaving about 1/2 inch from the folded edge. 

Next I used a needle and some 2 pound fishing line to sew everywhere I had sewn with the sewing machine.  Just to get the most for my effort, I did sew twice with the fishing line.  The reason for this is that

 the sewing thread will deteriorate after about 3 or 4 uses of the filter.  With the reinforcement that the fishing line adds you will get to reuse these bags for a very long time (mine are in their third year).   Turn the bag inside out, snip the edge to make a cut about 1/4 inch, and

I slide a 14 inch Zip Tie into the path to go around the bag and out that same hole again.  The bag will be gathered at spots so even it out and then slip it over the 2 inch couple, making sure that it goes below the couple and zip the tie so that it tightens to the 2 inch pipe of the lesser diameter.  

Here I used a white net bag and red string so you could see how it will look.

These will last depending on leaves, grasses, algae, and such that gets filtered out of the water from the bubbles flowing up the air chamber with the chimney on top.  The indicator will be the air bubbles that you see on the surface of the pond.  Mine last about 3 months, and when I change them, I save the old ones.  I wash them under a faucet, scrub the material against itself and do some cleaning by hand, but no soap.  I hang them in a sunny place to dry out and if needed later I'll use a medium stiff brush for the final cleaning.  They are reusable to the point that since I started using them over 3 years ago I have not had to replace any.

 Above is my personal effort and the bags in the picture are not the right ones.  These are first ones and they did not do the job.  After I tested the first unit I made, I concluded that the pond would get back to clear, and pretty, twice as fast with two filters, so I made another and fitted them as shown.

One concern is a law of physics.  Air (like water) will flow to the path of least resistance.  When I used only one filter, it bubbled like crazy and I felt it was really doing its job.  Then I made another filter and added it to the system.  When I first slid the two down into the pond, I could only get one of the filters to bubble.  As I repositioned the unit by gently moving one higher than the other, I was able to get the other one to bubble, but the first one stopped bubbling.  I thought this is probably a result of one bubbler having more holes than the other, or holes in one bubbler might be larger than the other one. 

I kept checking on the system for about a 2 week period, and found that finally the air pressure in the assembly sort of equalized and as time passed the other filter began to gain momentum.  After being patient and giving it some time, the two filters are currently bubbling about the same amount, and I can live with that.

I took a piece of 1/8 inch nylon twisted rope about 20 feet long and tied one end to the brick on one of the filters and then the end to the other brick.  This is used to pull the filters up out of the pond for cleaning.  Then I made a small bug proof house with vents on each end and an aluminum sheet over the top for the pump.

You could use a 2 inch air chamber with a 1 inch chimney if your pond is not 4 feet deep.  There are many adaptations, combinations, and permutations that could allow you to customize this filter to your particular situation.

After I finished the filter and put it in my pond, I realized that at some time in the future I will have to be able to unscrew the top piece of the bubbler that was inside the 4 inch chamber.  The only access I had was down through the 2 inch chimney.

So, I took one of the hex head plugs and made a trip to a local pawn shop.  I found that they have a wealth of metal sockets usually in a large box for customers to dig through and find what they need.

I found that a 24 mm socket would fit on the hex head plug, and I think it was only a couple of dollars. 

I took this home and found an old yard tool handle that had been broken, so I cut the splintered part off and that left an excellent handle for this new tool.  The first inch after the cut was used to cut into a square tenon that would fit into the drive hole for the socket. 

Then I drilled a hole for a dry wall screw in the center of that tenon. 

Next, I got a slug like the ones you punch out of a metal 4 inch square box mounted in the ceiling for a light fixture to be mounted onto in a home.  I drilled the center of that slug with a hole slightly larger than the one in the tenon. 

Just a little grinding on the slug and it fit down inside the 24 mm socket. 

Now I have a specialty wrench for removing and reinstalling the bubbler for cleaning when I replace the filter bags. 

I also found a 5/64 inch old broken drill bit, so I mounted about a 1 inch piece of 1/4 inch dowel, with a hole drilled in the center, onto the broken bit for another tool to use for cleaning out the holes in the top of the bubbler.

And finally, an old tooth brush that will go up into the inside of the removed part of the bubbler to ensure that the inside is scrubbed out also.  After about a year in the pond the white PVC will be mostly black because of the algae growing on the surface.  This also gets inside the bubbler and will affect the air flow.

UPDATE, JUNE 2020:

After a few years of cleaning the bags and the top of the bubbler, I found that I spend too much time trying to get the two bubblers to stay bubbling.  I get the two working just fine, then a couple of days I check and only one is bubbling.  I thought a long time trying to figure how to correct this.  I pull the small rope attached to the one that is not bubbling, which raises it slightly, and it starts to bubble.  A day goes by and I check it again and the other one is not bubbling.  I came to this conclusion:

1.   The size of holes drilled must be the same for both.

2.   The number of holes must be the same for both.

3.   The air will follow the least amount of restriction

The least amount of restriction means that the two units must be at the same depth in the pond, or the air will flow to the one that is higher than the other one.  Considering the small amount of flexibility in the PVC pipe and the inability to control the mud bottom of the pond from being unlevel, I decided to cut 8 inches from the distance between each bubbler stack and the connection from the air hose.  This moved the two stacks 16 inches closer together and will probably get them closer to being on the same plane at the bottom of the pond. 

Then I considered the depth of the pond and the volume of water will be a major factor in the amount of force that the air pump will have to overcome to get air down to the stacks.  This made me reconsider the number of holes in each bubbler head.  I took a tooth pick, jammed it into one hole in the bubbler head and broke it off, then to the opposite side of the same head and plugged the opposing hole there with the same tooth pick.  The wood will stay if it is tight because once in the water it will swell somewhat and that should ensure the hole remains plugged.  I did the same thing to the other bubbler head in the second stack.  These two changes in the system have made a very good adjustment in getting the two bubbler stacks to produce near equal air flow.  If I have any problem in the future, I will plug another hole in each bubbler head, and if necessary, I can still move the two stacks closer and plug another hole in the heads.

I have added these changes to my article because you might have the same problems if you decide to make this system, and I want to share that the depth and volume of your pond, as well as the size and number on holes you drill in the bubbler heads, and the distance between the two stacks, will be the major variables to the success of your efforts.  The nice thing about this system is that it will ensure an adequate flow of air into the pond for your fish and plants. 

Now if I can only find a treatment for the algae and a good clarifier for the water.  If I do, I will definitely have another addition to this article.

Thanks for looking and good luck, I hope this will help.