Building a Trickle Filter


As the koi in my pond get larger, they naturally put more of a load on the filter.  It was slowly becoming apparent that my present filter was reaching capacity.  While there was no algae in the main pond, string algae was starting to appear on the face of the waterfall and on the surface of the water in the upflow filter.  There are several ways to handle this situation.  One way is to completely redo the filter system, which was rejected as too expensive.  Another option is to get rid of several of the 'lesser' large koi which would reduce the load, but who wants to get rid of old friends.  A third method is to augment the filter in hopes of increasing its 'digestive' capacity.  This last option is the one I took.

My present filter has two stages.  Stage one is a bead filter that does some mechanical and biological digestion.  The output of the bead filter is two large upflow filters filled with Japanese filter mat.  Half the output of the bead filter goes to each upflow filter.  The return waterfall is the output of each of the upflow filters.  What I decided to do was split the output of the bead filter so that a third of the output went to each of the upflow filters and a third of the bead output to the new augmented stage.  This would have one obvious initial beneficial effect on the upflow filters.  It would increase the retention time, the time the water spends in the filter, by fifty percent.  Since the velocity of the flow through the upflow filters was reduced, this resulted in a substantial, though unanticipated, increase in waste mulm setting out of the upflow filter.  This is the same action that the new vortex prefilters are supposed to achieve.

As I searched the internet for help, I found a link that discussed the requirements for a good filter system and hints at achieving this goal.  I was left with the impression that anything I ended up doing must oxygenate the water, get rid of nitrates, and be simple to build and maintain.  I then went to one of my favorite online references, the

Trickle filters are popular overseas.  There are many internet references to commercial trickle filters for sale in Europe and Asia, but it appears that there is little interest here in America.  For example, a nice trickle tower is made and marketed by a company called Nitritech in England, but I could not find an American vender.  The only readily available American trickle filters for ponds were those marketed by Tetra.  This seems odd when you look at the filter system for almost any marine aquarium.  They always include a 'trickle section.'  I searched for reasons why the wet-dry trickle tower is not popular here in the USA and came up with two.  First, because the water drops down in a 'shower' over the filter media, the trickle tower can quickly lower a ponds temperature when the water is cool.  The trickle tower would readily turn into one large icicle in the colder portions of America.  The second reason is one of esthetics.  The output of an upflow filter is at the top, so the output of an upflow filter can be placed at or above the water level in the pond.  Upflow filters can be easily hidden behind the waterfall.  On the other hand, the output of a trickle tower is at the bottom of the filter.  The whole tower is above its output.  If one outputs a wet-dry filter into a waterfall, the entire filter is above the top of the waterfall.

In doing my research, I found out that trickle filters are popular overseas, and they are almost the standard design when a highly efficient filter is needed for the home marine environments.  Because of their ability to purge nitrates and nitrites, they are often called nitrogen towers.  Last, but not least, they have a very simple basic design and they are easy to build.  After I built the filter, I did find a link to an English site that had designed and marketed a similar trickle filter.

The first thing needed was a suitable container.  Basically, the requirements were durable, affordable, taller than wide, UV resistant, and easily available.  After searching around I found several venders that sold a cone bottomed poly container.  I purchased this one locally from William Lim.  The cone bottomed container is shown below.  The white 5 gallon paint bucket can be purchased at most any hardware store.  Note that the illustration shows a hole with a pipe fitting.  The pipe fitting is the filter output.  The cylinder does not come pre-drilled but any standard bulkhead fitting will work.

The next problem was media, but before that, the media had to be kept out of the cone.  Media that gets into the cone will escape from the filter and end up in the pond.  Back to the hardware store for some pvc fittings for a grate that sits inside the cylinder on top of the cone.  Over the grate I put quarter inch some plastic hardware cloth.  

Now that the media support was built, I had to figure out what type of media to use.  Whatever chosen had to be available, have a high ratio of open space for the water to trickle but still have a large surface area, and easy to clean.  My basic choices were lava rock, Japanese filter mat, or bio-balls.  I chose bio-balls over filter mat.  Lava rock was rejected because of poor surface area and hard to clean.  After building a disperser out of pvc, and installing, the final product looked like

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Does it work?  I used to have a coat of slimy algae on the back of my waterfalls.  Three months after installing the trickle filter, the algae was gone.  I assume, indirectly, that the nitrates and nitrites were evaporated out by the trickle filter.