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The benefits of trickle filters

A trickle filter in a pond? Keith Holmes of Koi Water Barn explains why having one is such a good idea.

Five years ago if you asked if someone had a trickle filter on their pond the response would most probably have been “no, why do I need one of those?” or comments to this effect. But over the past year or two trickle towers have become a must-have item for many pondkeepers.

The installation of a trickle tower gives a major boost to the biological filtration already provided by your existing filter system. Due to the nature of the way a trickle tower works, the water is heavily oxygenated which makes it the ideal environment for the growth of the beneficial bacteria required in the biological filtration process of your pond. Actually, the levels of oxygen are so high within a trickle tower, it can make them as much as 20% more efficient at removing nitrite.

Another use of a trickle tower is for gas-stripping, and this involves the use of a blower which forces air against the flow of water. This has the effect of reducing the levels of CO2 and hydrogen sulphide (H2S), which can have an effect on the overall performance of your pond. CO2 and H2S can cause the pH to drop.

A trickle tower can take two forms: Either a single tower of media, such as a large diameter piece of pipe filled with a suitable media, or alternatively a number of trays housing the media through which the water will trickle. Recently stainless steel trickle towers have become very popular, especially units like the Bakki Shower which consist of a number of trays to house the media. In most of these types of units a static spray bar is used to deliver the water to the top of the unit, compared to a rotating spray bar which may be used in a single column tower.

Whatever method is used the aim is the same, and this is to deliver the water evenly over the whole length of the media in the upper tray, before it percolates down to the levels below.

In most instances where trickle filters are installed they are not relied upon as the main method of filtration for the pond, as some form of settlement is still required such as a vortex or a sieve. Due to this, trickle filters are a popular addition to an existing filter system to improve or boost the performance of the filtration system already installed.

One of the few downsides of installing a trickle filter is that due to the nature of the way they work they must be located above the water level – the water is pumped into the top of the unit, then under gravity is allowed to trickle down through the layers of media to return to the pond via gravity. Most trickle filters are installed as the last stage of filtration before the water returns to the pond.

It is perhaps because of this that stainless steel trickle towers have become popular as they are not as obtrusive – an important fact when considering that they must be installed above water, in a position which will allow the water an easy return to the pond. Typically this means it will inevitably end up being sited somewhere around the edge of the pond, and hence in full view, unless you are able to camouflage it behind trees or plants, or build it into a suitable housing.

Along with this, some of these units can be quite noisy – you will continually have the sound of running water through the unit and the media within it. Therefore it is worthwhile considering a location which will reduce the effect of these two down points to a minimum, and hence not spoil the overall appearance and essence of you pond, but allow it to benefit from the addition of a trickle filter.

It is also important to remember if using a trickle filter to think about the colder winter months as you will need to insulate it to prevent rapid water chilling, as the air mixes with the water as it flows down through the media.

So what do you use as media? The choice is literally endless with most media being suitable, but as with everything there are always some which are going to be just that little bit better, and this is certainly the case with some of the media on the market today.

Rock and sintered glass
Perhaps one of the most popular types of media for a trickle filter is Alfagrog, due to its porous nature and thus incredibly large surface area. Alfagrog is also cheap to buy and for its volume relatively light, so is easy to work with, although not as light as plastic media, and comes in numerous grades and sizes.

Canterbury spa or Lytag should be avoided as these do not offer the large surface area which many of the newer, more modern media can boast. Many people may want to consider these due to their relatively inexpensive purchase price, but this should be ignored as not only do they not offer the surface area of other media but they are also heavy and difficult to work with.

An alternative to rock media is sintered glass which boasts an amazingly high amount of surface area. This means only a small volume is required to achieve the same level of biological filtration as a conventional medium. Although this may sound like a plus, the downside is that the majority of sintered glass media is expensive, and if using it in a trickle tower you will require enough of it to fill each tray, or the whole of the trickle tower if using a single column. You can use sintered glass just in the top layers however.

PIP titanium balls
These are a new product from Japan which has only become available in the UK recently. PIP stands for Powder Impact Plating, and it is during this process that a so-called “electromagnetic wave response-type photo catalyst” occurs. The manufacturers claim that when the balls are exposed to light they can help to lower pollution, reduce any odour from the pond and the clarity of the water, though it won’t remove cloudy or suspended solids. For this claimed reaction to occur, the manufacturers say that the titanium balls need to be exposed to light, so having them on the top level is probably the best location.

Here you can either rely on natural daylight to cause the claimed reaction to happen, thus limiting it to only daylight hours, or ideally mount a fluorescent light over the filter, giving them 24-hour exposure to light (some people will disagree with the effectiveness of this, saying that to get the best results from this new and high tech media the balls are best exposed to UV light).

One of the downsides to this media is that it is very expensive, but you do only need one layer – as they need to be exposed to light, it is pointless having several layers as the ones on the bottom will get no light not occur. PIP titanium balls come in 8mm, 10mm and 15mm diameters, and cost around £140 for a 2.5kg bag. Also, think of a method of removing the balls easily should the media onto which you have placed them need cleaning.

Plastic media
Perhaps the most popular choice until recently for the media used within a trickle tower would have been some form of plastic media like Bacballs which have a large surface area in relation to their size. This is achieved not only by the design of the shape of each piece of media, but also due to the fact that if viewed under a microscope the surface of the plastic is not smooth but in fact contoured, which helps to increase the amount of surface area available for beneficial bacteria growth.

Plastic media is relatively inexpensive and easy to work with due to it being very light in relation to its volume, which in turn makes it easy to clean should the need arise.

An alternative is Bio-block: a large section of media which has hundreds of channels running through it. Typically this media can come in numerous sizes, but the main advantage of it is that a trickle tower can be made quite simply by stacking numerous blocks on top of each other. This, combined with its relatively low cost allows for a trickle tower to be constructed quickly and easily for minimum expense.

Each block can be cut very easily down to size, allowing a unit to be made to fit any space, and you are not confined to the dimensions of the trickle filter itself, as the media actually forms the structure of the trickle tower. If constructing a trickle tower in this nature, however, the pipework and means of distributing the water over the media will have to be made by yourself. Or, if you already have a trickle filter you can simply cut it to size to fit your existing unit.

All of the different media have their bonuses, but what you want to look for when making your final choice is a media which has a large surface area, and will allow your pond water to easily flow over it. If you opt for a trickle filter with different layers, you can over time simply change the media you have in each layer.

So all in all trickle filters are a worthwhile addition to your pond's filtration system, and they can offer some benefits which would not be easily achievable in a standard submerged filter.

This article was first published in the August 2006 issue of Practical Fishkeeping. It may not be reproduced without permission.