So You Want a Crystal Clear Pond – A (nearly) maintenance-free system for clean, beautiful water
Of course, just because we are building the filter doesn’t mean that we can’t use natural materials for the media. Thus, I’ve chosen to write in detail about a natural media that, in a lot of circles, may be considered a dirty word: rock and gravel.
Almost every gravel area that you see is either an under-gravel grid filter or up-flow gravel filter.
Everything is run off air lifts.
A very old type of filtration media, rock and gravel were used for a long time but have lost favor with many ponders. One of the reasons for the falloff is that the surface area per cubic foot is not very high — or so people think. Yes, rock is a solid material that takes up a lot of space, but it is also a natural material that’s formed on a very fine structure and then is eroded on a microscopic level. With that in mind, the surface would have a fairly high microscopic surface area, which is never included when talking about gravel surface area.Even taking that into consideration, a filter using rock or gravel will require a larger footprint to handle the same size pond as some of today’s newer medias. But the smaller the filter, the more frequently it requires maintenance. Thus, the small amount of maintenance required to maintain many rock or stone filters is not easily accomplished with other media.
When we talk about putting rock and stone on the bottom of the pond, we immediately stir up passionate feelings in some people. There is a lot of debate about whether a pond should have stone on the bottom, and the argument boils down to the buildup of debris in the gravel. To speak to this issue, let me tell you about a pond that I am very familiar with — a pond I built 22 years ago! This pond is six feet deep and has about six inches of gravel on the bottom that has never been cleaned.Now, I know what you’re thinking: “There must be a whole lot of debris in that gravel.” But when I swam in the pond last year, I dove to the bottom and dug in the gravel … and there was no debris to be found! How is that accomplished? The secret is an under-gravel suction grid system. If designed correctly, it performs excellently with very little maintenance.
Under-gravel Suction on a Small Scale ## What is an undergravel suction grid-filter? Well, most of you are familiar with its smaller counterpart: the under-gravel suction filter in aquariums. A filter like this is built with thin slots in the plate, which is supported off the bottom of the aquarium. Small gravel is placed on top of this plate, and water is sucked through the gravel and the slots.
These filters work great for a while, but then they need to have a lot of maintenance done or they fail. By “maintenance” I mean the gravel on top of the filter suction plate has to be cleaned. Anyone who has done this maintenance on his fish tank knows it is not a fun chore. In a busy aquarium full of life, waste and debris quickly build up in the slots, causing the gravel to clog up faster than you can (or want to) clean it.
If we want this type of filter to work
in a pond, we have to look at why it has problems in an aquarium.The real difference between the under-gravel grid in a pond versus in an aquarium is the space in the grid. In the aquarium, the space between the pieces of gravel and in the slots in the suction plate is very small compared to the waste produced. In order for the system to allow water to flow through and not clog up, the waste has to be almost completely eaten by bacteria. This process takes longer than it takes for the gravel to clog up. Therefore, the filtration simply cannot keep up with the waste. The filter clogs up and fails.
To avoid this problem in the pond I built, I designed the under-gravel filter to have a series of pipes on the bottom with ⅜-inch diameter holes drilled in them. The pipes were buried in .- to 1-inch round gravel with about two inches above the pipe. The spacing between the pieces of gravel is fairly large, and the holes in the piping are large compared to the waste to be broken down. All the holes in the suction pipes are six inches apart, providing the waste a lot of area to fill.
In this grid there are six inches in every direction that would have to clog up before this filter would need to be cleaned on every suction line. Of course, if this filter were only a small part of the bottom of the pond, then there could or would be enough waste to clog the grid before the bacteria could break it down enough to get rid of it. Therefore, the larger the area of the pond’s bottom that can be part of this filter, the better.
Will this filter ever clog and need cleaning? The answer is yes, but the better question is: How long will it take? There is no set answer. It all depends on how much debris or waste is being put into the pond (or being made by the pond).
Earlier I wrote a little about the 22-year-old pond with a gravel bed that has never been cleaned. This happens
to be my own koi pond and my design. Located in Batavia, Ill. (about 35 miles west of Chicago), this pond was built to
be as maintenance-free as I could make it.No, it is not completely maintenance-free … but it is close. It is about 18,000 gallons and requires an average of three minutes of maintenance a week. None of the maintenance is spent on the under-gravel suction grid filter. The pond has never been emptied or cleaned since it was built. Based on what I have seen, the filter will not clog up as long as I am alive or as long as the liner lasts. My guess is that the liner will last for another 25 years.
I did make a mistake when I designed and built this pond. I used .- to 1-inch round limestone gravel. The problem is that after 22 years, the limestone gravel is shrinking in size. I believe I may have to remove this gravel and replace it with gravel that takes longer to erode.
Of course, I never thought that the filter would go this long with no maintenance required. Sometimes you stumble on the right combination of ideas and designs and things work far better than expected! Is it working in Illinois because of the climate but possibly would not work else where? I would say no; I just returned from California, where I saw a pond that’s about six years old and has one of these filters. It is over-stocked with koi, and they eat well. But the grid is working great and has not been cleaned. I saw this pond two and a half years ago and it looked good then — but it looks even better now.
Will this design work in every situation?That I can’t answer, because this type of filter hasn’t been used in every possible circumstance there is. But it has worked perfectly every time that I know of it being tried!
I have also used this system as a pressure under-gravel grid filter, and it has performed perfectly for the last seven years. The only complaint from that customer is that the water is too clear. A suction under-gravel grid filter normally uses an external pump or air lift system to run it, but a suction filter can also use a submersible pump. A pressure under-gravel grid is built similarly to the suction grid, but water is pumped through the grid. The pond that is pictured above uses both suction and pressure under-gravel grid filtration. The picture was taken when the pond was five years old. Both systems are being run off the same submersible pump. This is a very formal pond and we kept everything inside the pond.
In conclusion, the under-gravel suction
type of filter I’ve described will give you a
great mechanical and biological filter. It is
hidden in the pond and doesn’t require an area larger than the pond. If done correctly,
it has proved to require little, if any, maintenance.
It can be used with either external
or submersible pumps. It also works great
with the latest air lift technology.
I don’t know if “the perfect filtration
system” really exists. But this one comes
close enough for me.
California Contractors License
C-27 # 991368