Homemade DIY Trickle Filter

A few months ago I had been doing some research into the effectiveness of trickle tower filters, and also while looking through my web site stats I noticed that a lot of people were coming to my web site in search of information on building a trickle filter themselves. So I decided to have a go at researching and then making one using some spare parts I had lying around in my garage. In other words this cost me very little to make!

On this page I detail how I went about it, including a very quick and dirty prototype followed by an improved version which has now been in use since July 2005.

Bear in mind that there were 3 or 4 months of trying out ideas, and this page shows my whole filter arrangement going through several stages of minor improvements.

Perhaps you can start to see why I call my web pages Jim's "Crazy" Bio-Filter, because now it really is starting to look crazy!

Trickle Tower Filter Front View (with Bio-Filter directly behind)

Trickle Tower Filter Side View showing top feed from Bio-Filter

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What is a Trickle Tower Filter

A trickle tower filter can be a very efficient method of reducing ammonia levels in a fish pond, and is particularly useful if you have larger fish varieties such as Koi which produce large amounts of fish waste which if un-checked could produce dangerous levels of ammonia in the pond. It works well because it makes efficient use of high exposure to air (and hence oxygen) to help aerobic bacteria in their life-cycle of converting ammonia into nitrites, and then nitrites into nitrates.

The prime function of a trickle filter is to remove the invisible toxins from the pond water.

It is not intended to remove solids particles from the water, that is the purpose of the pre-filter, and the earlier sections of the bio-filter. However, a trickle filter can have the side-effect that it will help rid the pond of green-water. As well as converting ammonia to nitrates, which can encourage phytoplankton to grow (phytoplankton is the minute water-borne algae that cause green water), the bacteria also produces a certain phytoplankton-killing enzyme. As algae starts to grow in the bio-filter, or on the walls of the pond, the bacteria loves to feed on this algae, and as it does so it releases the enzyme (like an antibiotic) into the water. (Fascinating source article can be found here: http://www.vcnet.com/koi_net/GRENH2O.html).

Prototype & Experimentation

There are commercial trickle filter units available costing from £50 to £150 just for the tower, let alone the additional cost of the filter media. Both my designs of trickle filter cost me virtually nothing because I found all the necessary parts in my bits-n-pieces box. At most it will cost you just the price of some piping and lava rock.

The principle of a trickle-tower filter is so simple. There is nothing complicated because all you are creating is essentially just "a waterfall in a tower".

My first shot at this experiment was to build a rough-and-ready prototype. I used several garden seed-trays which had plenty of drain holes in the bottom, and layered these one on top of another. Each tray had 4 plastic lugs screwed into the sides near the top on the inside, so that the next tray up could rest on the lugs so that it was supported slightly above the lava rock.

I bought the lava-rock from a garden centre for about £8 for a large bag (the kind used in barbeques), and this has been perfectly adequate for the task. I gave the lava-rock a really good wash before putting it into the trays just in case there was any kind of contamination on the lava rock.

The water is fed from the bio-filter outlet pipe through some flexihose to a water sprinkler bar which is simply some white 12mm plastic piping, drilled right through about 12 times from one side to the other with a 3mm diameter drill to let the water flow out and spread over the topmost tray with the lava rock in. The water just flows downwards thru each successive lavarock filled tray.

While this prototype was adequate for a quick test, and added additional surface area for bacteria to live and work their nitrification-cycle magic, I felt there were a few improvements to be made:-

• I felt it needed more height so the water was falling onto and in contact with the lava rock for longer.
• The trays were too close together so preventing adequate airflow between, because I feel that plenty of air around the water as it splashes down thru the lava rock will help oxygenate the water, both for the benefit of the bacteria, and also provide oxygen for my fish because it flows down and back into the pond.
• The prototype was functional but quite ugly.

Trickle-tower prototype.

Top view showing the water sprinkler bar. You can see the water spouts flowing over the lava rock.

New Trickle Tower Filter Design

So after some consideration for the new design this is what I came up with.

The colour is perhaps a bit garish. The acrylic spray paint I chose turned out to be a much brighter, fluorescent colour than I had intended!

The tower is simply a section of 6 inch diameter soil pipe marked at regular intervals and drilled.

The height of the piping will depend on your setup, and where the top of your filter is in relation to the trickle-tower, and also whether the tower will sit inside a pot at the base (see later photos for consideration).

The holes are 10mm diameter to let plenty of natural airflow into the tower.

Yes, some of the water does flow out of these holes and down the outside, but the majority of the water-flow is down through the lava rock on the inside. I don't mind which way it flows as long as it has plenty of air/water contact and the bacteria and algae grow and live on whatever surface they choose to live!

Some people build their trickle towers as a completely enclosed unit, either with no additional airflow or else they put in an airstone at the bottom to pump air up through it. Well thats just additional expense isn't it?

Why not just let the wind in the garden deal with aerating the tower?

The air can go through the holes in the side of the tower, and permeate through in between the lava rock and the water as it passes over the lava rock. So by having it completely enclosed you would be preventing fresh air from getting into it which sort of defeats the purpose, unless you intend to pump air into the base of it using something like a pump and an air-stone.

Also the idea of using a tall tower is that the water will fall a long way down a narrow vertical column, so that the water gets plenty of agitation as it falls down between the lava rock, so it gets plenty of chance to absorb oxygen, which the bacteria living on the lava rock can then absorb. But if you were to use a wide diameter short fat tower then the water will "fall" only a short distance, so lessening the amount of time it will be in contact with air. Also with a wide diameter tower you would need an efficient method of spraying/spreading the water over the wider area. I think it's much simpler and more effective to do this with a narrow tower.

This is why trickle towers are designed like that, because they are literally "towers"!

For a little fun, I got handy with a small drill bit near the top and engraved "TT" for trickle-tower of course, and a downward pointing arrow to show the direction of the water flow!!!! Just in case you couldn't guess ;-)

This is the back of the tower showing a 40mm diameter hole drilled to accept the inlet sprinkler head. The exact height of this hole must be measured to marry up with the bio-filter. It is best not to actually drill this hole until you've got everything ready and can get an accurate measurement when the tower is in place.

You will also notice that on this side I didn't drill any holes. At the time I felt that too many holes might have let too much water escape to flow down the outside of the tower, so it was a compromise really. The tower seems to work quite efficiently without any additional holes.

You can see the sprinkler head on the floor just behind the tower.

This picture shows the sprinkler head fitted in place. Notice that it has an almost 45 degree angle cut on it. I did this to ensure that when it is pushed into the corresponding hole in the top of the bio-filter, that it doesn't block itself because there is an inner lip inside the bio-filter which the pipe would push up against. If the pipe had been cut flat, it might get blocked. The angle makes it easy for the water to escape into the sprinkler head.

This shot is a side-view of the tower showing the way I have drilled holes in the front half only, but not the back half.

Obviously you have to provide a means of stopping the lava rock falling out of the bottom of the tower. This is achieved by some stout plastic coated garden wire threaded through a number of holes around the circumference of the tower.

I also cut some square lugs out of the bottom of the tower as an exit for the water.

Here is an underside view showing the wire grid holding the lava rock in place.

Looking down inside the tower, you can see the lava-rock just below the level of the sprinkler inlet hole.

Try to construct your tower to the maximum height you possibly can with your setup. The more height and lava rock your tower has, the better capacity it has to purify your water.

Sprinkler Head

The sprinkler head sprays, and spreads the water out over the lava rock.

It is better to have too many holes than too few. I used a 4mm drill bit and de-burred the holes to prevent organic matter getting caught up on any burrs, which might block the holes. If your pre-filter and bio-filter are doing their job properly then it is highly unlikely there will be any large particles to block the sprinkler head.

I also have plants in the top of my bio-filter, so the occasional leaf finds its way into the sprinkler, hence the finger-slot in the top for fishing them out.

This side-view of the sprinkler head shows the angle of cut on thr pipe at the right.

You can also see that I have drilled holes well up the side of the sprinkler head. Water very rarely comes up to this height, but this is an additional measure to ensure good water-flow at all times in case the lower holes become blocked by anything.

You certainly do not want this to become blocked, otherwise the bio-filter may overflow and you lose water from your pond. That would not be good!

Top-view showing the finger-slot used for cleaning purposes.

Another shot showing the myriad holes for the sprinkler.

This end view shows a 40mm plastic end cap used to block off the other end of the sprinkler. This was quite a loose fit in the pipe, which I had to make tighter using plumbers PTFE tape.

Fitting The Trickle-Tower to the Bio-Filter

Next I modified my bio-filter by cutting a 40mm hole in the side as a new exit for the water into the trickle-filter.

This is a birds-eye view of the sprinkler head.

Here you can see a slot cut into the top of the sprinkler head. At first I cut this slot too small. In a later photo you will see I have enlarged it slightly. The slot needs to be big enough so you can get the fingers in to remove any debris, such as leaves or algae that might block the sprinkler head.

The idea is to make it easy maintenance!

In both these photos the water is flowing into the sprinkler head. Notice that there is no top to the tower. I have never used a top on it so as to make further use of natural airflows into the tower.

Bio-Filter & Trickle-Filter Support and Base Tray

One of the things I had to do to ensure sufficient height for my trickler-tower (even at the prototype stage) was to raise the level of my bio-filter.

The arrangement in order from ground level up is as follows:-

• A large paving slab on very firm ground. My ground is very heavy, solid clay. Even so I may have to relay this slab on a proper concrete base, because this paving slab supports a lot of weight, with the various house bricks, the water-filled base tray, the water-filled bio-filter, and the trickle-tower filter also filled with lava rock. In total it could be getting on for a ton in weight when filled, so you cannot afford subsidence.
• Next up are 8 house bricks which support a large size thick sturdy plastic garden tray, 2 bricks are laid side by side and on their sides at one end, 2 side by side and on their sides at the other end. Then 4 more are similarly arranged laid on top of the first set of 4. This lifts the large tray above the paving slab by about 8 inches. If you do not need as much height then just lay the bricks flat. There is plenty of room underneath to stow flexible hose between the gaps of the bricks. I have not cemented any of these in place. This allows me to totally dismantle everything if necessary. Bricks are extremely strong and heavy and provide a good base even though they are not fixed. However if you think there is a chance kids might try climbing up or pulling on your bio-filter it may be better to cement the bricks. You can just see the bricks underneath the base tray in the photo below.
• Next up you can see the tray with some lava-rock in the base, and 4 more bricks arranged 2 laid flat, and 2 on their sides to support the large bio-filter pot. This raises the main bio-filter sufficiently so that I could build my trickle-tower to a sufficient height.

Also in the photo below you can see the outlet flange and 25mm flexihose which leads to the water-urn I have at the top of my waterfall. The idea of having the tray was so that I could maximise the available space to make the water flow through the pre-filter and bio-filter, into the trickle-filter, and down into some more lava-rock in the tray, then finally out of the outlet flange in the base tray.

At the opposite end of the base tray is the (green) 40mm pipework that acts as the bio-filter main drain pipe that comes from a shower drain unit in the bottom of the bio-filter. The shower drain unit sits nicely between the house bricks which support the main bio-filter.

Below that you can see a short section of (white) pipe which acts as a cleaning drain for the base tray. Every couple of months when I clean the base tray, I simply undo the flange a little, and rotate the pipe downwards into a bucket placed underneath. This piece of white pipe needs to be as high as it can be to prevent overflow (it just touches the lip of the base tray). As long as the base tray is laid perfectly level I have not once had it overflow.

When I had first built the trickle-tower filter, this was the arrangement.......very simple and straightforward.

Filtration and Settlement of Fine Particles

The next stage in the design was the consideration of filtering out fine particles. As I mentioned earlier, a trickle-tower filter is intended to eliminate ammonia and nitrites, and as a side-effect also improve combatting green-water. It does not remove very fine sediment and silt from the water, which generally passes through the coarse green scrubbies inside the bio-filter.

So now I wanted to make the water flow from the bottom end of the tower, to the far end of the base tray, from where it then would flow slowly between the lava rock making its way back to the tower end and the main outlet from the base tray.

So I installed a plastic plant pot at the base of the tower, which leads the water to the far end of the tray by means of a short length of 40mm piping, as in the two photos below.

Water comes down the tower, then out of the back of the pot through the pipe between the brick supports.

And here is the other end of the pipe opening (the bottom pipe).

The water passes around the back of the bricks, and down the sides and middle of the bricks, being slowed down by the lava rock, then back to the tower end and the base tray water outlet.

This method did work fairly well in so much as the fine particles settle between all the lava rock, however each time I did a cleaning cycle for the bio-filter, which involves turning the water off for a short while, this would disturb the sediment.

Addition of a Foam Filter

So I decided to use some foam to cut into sections act as a fine filter in the base tray. I created a cardboard template first to get the shape and size of the pot at the bottom of the trickle-tower filter and the base tray.

This blue foam is fairly coarse.

The blue foam sections in the above picture were put together before cutting so that the dimples in the foam would marry up correctly when the two pieces are laid on top of each other. There was no particular reason for using this dimpled foam - it was simply what was available at the time.

Some finer foam was then cut in a similar fashion.

And placed one on top of each other.

Here are all the foam pieces placed together in the configuration they will fit around the base of the tower.

Finally another layer of much coarser dimple foam was cut as an upper layer.

And here is all the foam laid as it will be in the base tray.

Here is the fine (green), and medium (blue) foam laid into position around the tower filter base pot. The flow of the water through the foam is from the right to the left of this picture.

Then the coarse (black) foam is laid on top.

Cleaning the Filters

You can find my main cleaning regime for the pre-filter and bio-filter by clicking here.

As for the foam in the base tray, I clean this perhaps once every 2 weeks in the summer, and every month in the winter. To do this I first turn off the water feed to the pre-filter/bio-filter, then block the outlet pipe from the base tray with a cork (to prevent sediment going to the waterfall and pond). Next I rotate the small drain pipe at the other end of the base tray downwards to let all the water out, then remove and rinse out the foam pads. Then I use a hosepipe to wash any gunk out of the base tray through the drain.

That's it! I expect you will have a completely different configuration to mine, but hopefully this will give some ideas for you to develop your own tower trickle-filter. Good luck, and let me know how you get on!

The completed trickle-tower filter is in the background of the photo below. Can you see it?

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