Andy,
I have been refining my pond filter system for quite some time and I have a few ideas to share with you on this subject.
Using a gravel or sand filter like the one you pulled up in the video works quite well. However, there is a drawback to it that I don't like personally. I see it through my eyes as being higher maintenance than what I have devised so far. The layers of gravel and sand and what-not are excellent filtration media when found in nature on a LARGE scale and I really mean on a HUGE scale like a naturally occuring aquifer which may be acres or hundreds or thousands of acres in size. The water which percolates up and down and through such media will eventually come out clean and pure and pristine. The mechanical filtering of this media works very well, and there are bacteria and minerals which naturally perform chemical and bilogical filtration here. The drawback that I see with this gravel filter system on a small pond scale is that it requires you to frequently purge the system by backflushing it. That may be acceptable to many pond owners, but I believe that there is a much better way. More effective, more efficient and more maintenance friendly as well as much simpler to construct.
I obtained the original notion of this from Paul Van Der Werf who is the host of Earthangroup from Australia and New Zealand. Their site is dedicated to hydroponic gardening and using fish to supply the nutrients to plants, so they are raising both fish and vegetables for food and each of them are providing something, some benefit, to the other.
One of the filters he described in a You Tube video was of what he referred to as a "radial flow" filter for separating solids from the fish pond water. Reviewing the design which he presented, I disagree with the "radial flow" terminology. It is more akin to "axial flow" in the method that it actually does its job of separating solids, but the water does eventually flow radially out through the filter chamber in the end.
I am going to describe in words for you how I percieve a really nice "axial flow - solids separating filter" will operate and how I have engineered the one I am going to use for my pond.
First of all, I designed mine around a gravity flow concept using a bottom drain from the pond, so the filter vessel has to be elevated lower than the pond's bottom. However, you could use a pump if you wish, but I think that gravity flow would allow the solids to remain in tact and be separated out better in the filter if they are not pureed by the impeller of a pump. Use your own desire on this notion as I still believe that even pump fed water will be cleared of fines and filtered out with this design.
So let us begin with a pond which is maybe up to 3,000 gallons with a bottom drain. If you have many fish in this pond, you are going to have a large amount of biological load. Fish poop, uneaten food, leaves from plants and trees or just dust in the pond from the nearby farm or a gravel road or just whatever is in the air, correct?
Why use gravel or sand or filter brushes or some purchased filter media to trap this gunk and have to clean it so frequently when you could use water, gravity and fluid dynamics to trap it all for you? Then just simply open a drain valve and purge it all out into your garden so much more easily.
Going back to the "radial flow" filter or what I desire to refer to as an "axial flow solids separator", here is how it would operate:
Water enters in from the pond through one or maybe two 2 - 3 inch PVC pipes from the lower section of a HDPE barrel, with about 120 gallon capacity. The two incoming pipes then "Tee" in the center of the barrel and the water flows upward through a single PVC pipe. Near the top of the tank, it is dispersed out of a section of the pipe with many holes drilled into it. The water has to make a right angle turn here and now it is inside of another, larger pipe, say a 6" or 8" PVC tube which forces the water to turn downwards and flow to the bottom of the tank where it will lose kinetic energy and drop sediments.
Now the water has to rise back up somehow so that we can tap it off and send it to a more refined mechanical filter unit and a bilogical conversion stage. Therefore, we drop in another HDPE barrel inside the larger 120 gallon main barrel and this smaller barrel will be around the smaller 8 inch PVC pipe. The bottom of this barrel is cut out so that the 8" PVC pipe can drop down through the bottom of it and extend to the lower area of the larger barrel. We will drill many small holes in the bottom of this smaller, center barrel for the water to percolate up through it, between it and the 8 inch PVC pipe.
However, from the outside of the largest, external barrel towards the inside to this smaller barrel, we will install a solid HDPE plastic disc or plate with a rubber gasket attached to the plate so that it seals the space between the outer perimeter of the smaller barrel and the inner wall of the main, large barrel. This is so the water cannot go upwards around the outside of the inner barrel. We want the water to flow upwards between the 8 inch PVC pipe and the inside of the smaller barrel.
Within several inches of the top of the smaller, center barrel, we drill many small holes at the same height around and through the outer perimeter of the smaller barrel for the water to escape into the larger barrel. The water can flow out into the next chamber and eventually out of the larger HDPE barrel through a perforated 2 - 3 inch PVC pipe or some coarse filter tube and into the next filter vessel down the line.
The overall concept is that the dirty pond water has to rise up into the barrel, fall down to the bottom, rise up again, and then evacuate out radially. Each stage of this water changing its direction causes it to lose it's kinetic energy and drop its sediment load which falls out in the lower areas of the tank.
The sediment collected here is then easily expelled by opening the drain valve at the very bottom of the large, outer tank. The flow of the water out through the drain will drag all the gunk with it as it evacuates from every portion of the tank here as it is one assembly.
Because of the "multi-chamber" design of this first vessel, the next vessel won't have to deal with so much fines and crud and the pond owner won't have to spend so much time or labor cleaning the thing.
Atop the next stage, I am going to place a short HDPE barrel (about 12" tall) with A 6" roll of Matala Grey media sitting on top of a 300 micron mesh SS filter screen. If any fines make it into this short barrel and start clogging it up, I will simply disconnect a universal PVC coupling which connects the water feed line from the previous barrel, then lift this short barrel off the next stage and turn it upside down in the yard or driveway and spray it out with a pressure washer.
The drum below this one will contain Kaldness K3 media and an aerator to stir it around and then the water will be pumped back to the pond.
I am close to being able to present an actual scaled diagram of this system, so If you don't get all the concepts from my words, I will try to get these diagrams posted up here.
Catfishnut
