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
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.
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
Trickle Tower Filter
(with Bio-Filter directly behind)
Trickle Tower Filter
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.
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
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
the task. I
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
over the topmost tray with the lava rock in. The water
just flows downwards thru each successive lavarock filled
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
- 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.
Top view showing the water sprinkler bar.
You can see the water spouts flowing over the lava
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).
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
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
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
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
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
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
I also cut some square lugs out of the bottom of
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.
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
The sprinkler head sprays, and spreads the water out
over the lava rock.
It is better to have too many holes
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.
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
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
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.
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
other end. Then 4 more are similarly arranged laid
on top of the first set of 4. This lifts the large
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
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
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
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
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
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
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
template first to get the shape and size of the pot
at the bottom of the trickle-tower filter and the base
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
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).
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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