The Trickle Tower Biofilter

Over the past few years, I've become relying more and more on Trickle Tower (TT) technology for biofiltration. My interest in them came from the postings by Chris Neaves (S. Africa) on Doc Johnson's KoiVet web board. Chris wrote a very informative piece on filtration and included some interesting information on the Trickle Tower filter.

Just what *is* a "Trickle Tower?" It is a biofilter who's media is *never* submerged. Water is delivered (preferably) in a spray, that covers the entire top surface area of the media. The water then "trickles" down throughout the media, falling into a collection basin, where it is then returned to the pond.

This does pretty much mean that the bottom of the TT will have to sit higher than the water level of the pond, in order for the water to return from the filter to the pond.

A trickle tower is a pure biofilter or, bioreactor. It cannot by it's very design, be used to mechanically filter water. It needs a good mechanical filter ahead of it, to reach it's maximum potential. And if anyone talks of anaerobic pockets in the filter, then they are discussing a filter with some *serious* design flaws.

That said, I have to point out that almost every hard and fast rule has some exceptions, or hidden qualifiers. Super-fine particles can and do stick to biofilm. This contributes to the death of a layer of biofilm. As that dead film is sloughed off, it carries the trapped super-fines back to the pond, where it should be picked up and trapped out of the system by the mechanical filter. In this way, I guess you could say that a TT can contribute to mechanical filtration.

But in general, a TT should be regarded as a biological filter, only.

I will not go into too much detail on the workings of the TT, as Matt Smith of LS Enterprises (see links page) had done a great job of that. What I will tell you comes from my own experience using this type of filter for several years, now.

Much speculation has been made on the efficiency of the TT compared to a submerged media filter with the same cubic footage of media. Some *purely* theoretical figures which assumed some pretty fanciful translations from the calculator to the "real world," held that it would be "more than a thousand times" as efficient as a submerged media filter, base on the amount of oxygen available to the biofilm.

From what I've seen, I do feel comfortable stating that, give the same total surface area, a TT will easily support *10* times the number of fish that a submerged media filter will.

One version of the TT has come into vogue, lately. And that is the "Lava Rock Fountain." Now, I have absolutely *nothing* against this version of the Trickle Tower. But some have taken my comments on them to read as if I did. The Lava Rock Fountain is indeed a Trickle Tower. But. . . It is a very primitive version of this type of biofilter. The first mention of the Lava Rock Fountain that I recall was as a "water feature." The fact that it did add to the biofiltration of the pond was a "plus." I've seen some that greatly added to the landscape around a pond. And they certainly do help with the biofiltration!

Here is an excellent example of a Lava Rock Fountain:



Click on the picture above and a new window will open with a larger version of it. This picture was sent in by Griff Thomasson, who built it. Very nice job, don't you think?

Now, what I'm looking for in a biofilter is *efficiency*! "Looks" come way down on the list. Make it work and work well, first. Then dress it up.

If you have established biofiltration on your pond and need or what to "take it up a notch," or improve on a marginal system, then a small Trickle Tower, in the form of a Lava Rock Fountain can fill the bill for you. But if you are looking for an efficient biofilter with a tremendous fish-holding capacity, the Lava Rock Fountain is not the one you want. What follows is the post I made on Roark's web board that kind of stirred some folks up:

A Different Approach To Trickle Towers.

I take a different approach to trickle towers. Rocks of any kind are not something I'd use. If a loose media had to be used, bio-balls would be a better choice, but a much more expensive one. That is what they were designed for, in the first place.

Picture what a cut-away of a pair TT's would look like, with the two different materials in them. Those little studs that stick out on the bio-balls keep the larger, round bodies from touching, so they "pack" rather openly. Basically, they have a higher void fraction that lava rock and they are purposely shaped to present the maximum amount of available surface area, when "stacking." There's a number of other type of media that would do well, some designed as filter media, some not.

Bio-barrels is one that comes to mind, another would be bio-strata.

In a TT using this type of media, as long as the water was being delivered properly, that is to say, sprayed fairly evenly across the entire top surface area of the media, much higher flow rates *can* and *should* be used.

The water should "trickle" down over as much of the media's surface area, as possible. Any media, such as lava rocks, that has "flats" on the surface, will tend to stack tightly against each other, greatly reducing the surface area that the water can trickle across.

Using the high void fraction media, than isn't reducing it's available surface area by stacking poorly, with a higher volume of water, evenly distributed through the media, results in a much more efficient biofilter, and one that is virtually self-cleaning.

You can, of course, over-do it. But I have several TTs built with 50-gallon drums, using PVC ribbon as a media. Three cubic feet of it, in the drum, is not quite high enough for the media to pack down, even with the water flow. If I had PVC ribbon more than 2 1/2 feet high, I would use grates every 2 feet, to support it.

But one of those ponds is just a few hundred gallons more than the one in question. It has a single TT, as described above, and it has about 1,000 gallons per hour, running through it. It was only a mediocre filter when I first built it, a small pump put only 500 gph through the filter.

The doubling of the flow rate made it much easier to get a good spray pattern, and the over-all water chemistry improved, markedly.

End of quoted post.

As I keep saying, to learn how a modern Trickle Tower works and how to properly size and design one, read the tech reports on the LS Enterprises' web site. My personal notion of what makes a good TT is as follows:

Use a media that has a high void fraction and a lot of *useable* surface area. Supply it with a water flow that is high enough to cause dead biofilm to slough off and deliver it in a spray that gives complete coverage to the entire top surface area of the media. A taller, narrower stack of media is better than a shorter, wider stack. Why? Longer "contact" time. The notion that contact time does not apply to TT's is nonsense.

What may look like to some as far too much water being fed to the media is deceptive. When the water is evenly distributed across the top of the media, the actual volume that crosses any particular cubic inch of media is quite small. In a "perfect" setting, it would amount to no more than a very thin film.

100 gallons a minute properly distributed across the top of a cube of the corrugated media sold as Biostrata, gives you a nice film of water flowing down it's surface area. The same amount of water crashing down on a pile of lava rocks will channel between spaces in the rocks, creating small "rivers" down through the pile, wetting some of the surface area while leaving other parts of it practically dry.

On a small pond or holding tank of 1,000 to 1,500 gallons, a single 55 gallon barrel containing 3 cubit feet of PVC ribbon (Biofill,) can be supplied with up to 1,200 gallons per hour and work extremely well. A two inch bulkhead fitting in the bottom of the barrel will make a good water return. I put a 6 inch diameter plastic net pot upside-down over the bulkhead fitting inside the barrel to retain the media. Putting a few stand-offs on the bottom with a round metal grill on top would also be a good idea.

Here is what this 55 gallon TT looks like. Click on the thumbnail and a larger picture will open in a new window.



The reason you see the water supply going up to the top of the barrel and then all the way through, with an end cap sticking out on the opposite side, is that I first wanted to use that pipe as a spray bar. I found it difficult to get the even surface area coverage with the homemade spray bar. Not wanting to waste the barrel, I replaced the spray bar with a pipe with an end cap on the end opposite of the 90* fitting leads from the pump, and has a "T" fitting centered over the media in the barrel. The open end of the "T" fitting has a female thread adaptor glued into it and the spray nozzle is screwed into that. It looks something like this: (click on the thumbnail)



The spray nozzle I found to work best at the flow rates in the 55 gallon TT is from LS Enterprises. They have different inserts you can use to change the water flow rate through it, but I cut a disk from some heavy plastic and drilled a hole in the center. The "reduction washer" just slides up into the threaded 1 1/2 inch PVC pipe that the nozzle screws into, and it sits on the nozzle when you screw it into place. Different size holes give different velocities through the nozzle. Here's a picture of that nozzle. As usual, clicking on it will open a new window with a larger picture.



At my flow rates, this nozzle with a reduction washer with a 3/4 inch hole in it, causes the water to leave the spray nozzle in an almost *horizontal* disk pattern. What can happen here is that most of the water hits the side of the barrel and flows down it, rather than through the media. I cured this by adding a "spray shield." This was made from a ten gallon plastic pot, turned upside down, cutting two holes for the spray pipe to pass through. Here is a drawing of what that looks like, inside the barrel.



Now, the water shoots out of the nozzle, hits the spray shield and bounces back, falling down on the media in a pretty even pattern across it's entire surface. This ensures the entire usable surface area of the media is wetted out, with little or no channeling taking place.

One thing to consider, I have a pond with two of these TT's on it. One of the water returns comes out over the surface, near the side of the pond, and has a 45* fitting directing the water down and towards the side of the pond. This sets up something of a circular flow in the pond, itself. The other return comes out over the pond and turns down, with a 90* fitting. The pipe then goes straight down and ends with another 90* fitting, aimed along the side of the pond, enhancing the circular flow.

The added benefit of that sub-surface return is having moving water down deep, and additional aeration. How so? Well, as the water from the filter goes through the pipe, it draws air from the filter in with it. This trapped air is forced down the pipe to the bottom, and out. It takes a bit of weight from the water to push that air out, so it does cause the pipe to fill above the water line a bit, until there is enough water to force the air out. This makes a little "chugging" noise, every few seconds. The fish seem to love playing in both returns, from time to time.

You'll notice that I did not cut any holes in the side of the barrel near the top, to let air in. A lot of people feel you need them, but I do not. The barrel came with a lid. I cut the top out of the lid, leaving just the threaded rim. After the net pot was placed over the outlet in the bottom of the barrel, the media was added, the spray nozzle assembly put together, and the spray shield put in place. Then, I stretched a piece of window screen over the top of the barrel and screwed the cut-down lid into place.

The screen keeps leaves and bugs out of the filter, while the spray shield keeps most of the sunlight out. The water rushing out of the nozzle and finally out the filter at the bottom, draws plenty of air in through the screened top of the filter. Unless your TT is one of the tall, 10 feet or more filters, no additional air is needed. I'd not want to complicate an otherwise simple design by trying to add an airflow that wasn't really necessary.

Remember the DIY rule, "Keep it simple, keep it cheap!"

My "violations" of the DIY rule were these:

I built a wooden stand to mount a pair of these TT's.

I use a 3-way valve that has the pump on one side, another side has a short pipe with 90* fitting to take the water up, then a combination "T" fitting and a pair of 90*'s to split the flow between the two filters, with the third side of the 3-way valve going to a medical by-pass return to the pond. This way I can use the medical by-pass when doing treatments that shouldn't go through the filters, and still circulate water through the pond, or by adding a length of pipe, use it to pump water out of the pond, for water changes. On this particular TT, I put another spray nozzle on the outlet. This nozzle doesn't require a reducing washer.

Let me stress one last time, the Trickle Tower is a *biological* filter! This makes it only *half* of a filter system. To complete the system, there must be a good mechanical filter before the TT, to feed it mechanically clean water.

Accomplish this, and you will have a very good system with tremendous fish-holding capacity.

Mike S.