The Construction of my Ponds - Page 3 - Building the Filter

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Second Rebuild (2)      Second Rebuild (3)      Rebuilding the Filter      

The Final State


Garden centres have a bewildering variety of filters and as long as you get the right one for your pond, you will have lovely clear water in which to view your fish. However, the majority of them cost far more than it would cost to build a similar one yourself. As you will have seen in my photographs, I have used a children's toy box and a dustbin in the past but when I decided to build my final pond system, I calculated that the total volume of water would be close to 2000 gallons and I decided that I needed a bigger one. It is important that your filter is big enough to allow the water driven by your pump to be flowing around the filter for some time so that the bacteria can do its work. If the water flow through the filter is too fast, the water will simply not be cleaned and you may as well not have a filter in the system.

I started with the design. I wanted to be able to filter out any debris from the water before the water passed into the biological part of the filter. As I would need to clean it out completely at least twice a year, I also wanted to be able to get at each part easily. First I needed the box. For that I chose a cold water storage tank from the local plumbers store. It is 1200mm x 600mm x 600mm deep and is quite rigid and comes with a well fitting lid. I could have just filled it with biological media and covered that with sponge and poured the water in. However, that would not have been too efficient so I chose to divide the filter into 3 chambers by using perspex dividers. In the picture, they are the vertical lines.

Both sheets are fixed at the sides but the right hand one starts from above the water level does not reach the bottom of the filter, whereas the left one is fixed to the bottom but doesn't reach the full height of the water. In that way the water is forced to go down the first chamber, up through the middle one and finally over into the third one, thereby maximising the time that the water spends in the filter. In order to provide support for the perspex I cut 2 pieces of 15mm copper pipe to the exact length of the filter and fed them through holes that I cut in the perspex. I also cut pieces of 21mm waste pipe to the exact lengths that would be between each piece of perspex and the filter ends.  Top

Once these parts were assembled, the whole thing was lowered into the filter and the sides held in place with waterproof adhesive tape. The sides couldn't be sealed properly until it was sunk into its hole and the earth filled in around it. I used perspex, plastic and copper because none of those materials would harm the fish or the filter bacteria.

The next stage was the input and output pipes. The input pipe is a straight forward connection to the pipe from the pump and had to be at the top of the filter. I used standard 35mm plastic waste pipe, available in all hardware stores. My local water garden centre sells the plastic pipe connectors at a very reasonable price. The outlet pipes were a bit more tricky. I didn't want this great big box stood in my garden so I decided to sink it and have the pipes coming out as close to the top as I could. As you can see from the diagram at the top of the page, and from the photo on the right, I've used the same type of pipe and connectors to enable me to sink the filter. As long as the output pipe is lower than the top of the water, gravity will force the water up through the pipes and out of the filter. As you can see, I have use tee pieces to connect the pipes together. The reason is that if the filter should become so clogged up that water cannot get through it (unlikely but theoretically possible) I had to provide a way for the water to get back to the pond. By arranging the system so that the tops of the tee pieces were above the normal water height but below the top of the filter, if the water level rose because of a blockage, it would flow down the tee pieces and out to the pond, rather than leak out over the side of the filter and be lost. You'll see that I have got 3 output pipes. In actual fact, the amount of water that my pump moves would only require 2 pipes, but again I've erred on the side of caution and provided an extra one just in case one of the others gets blocked.  Top

With a filter this big that is almost completely buried, emptying it can be a real pain. Literally! So I also cut a hole into the bottom level of the filter and installed an additional pipe with a valve attached. With the valve in the closed position, the integrity of the filter is maintained but when I need to empty the filter for cleaning, I just switch off the pump and turn the valve. All the dirty water from the filter and any sludge in the bottom drains away. Its much better than lifting dozens of buckets.

Having completed the basic construction of the filter it was time to fill it with media. When I first made my filter, I had very little money for anything else so I just bought some sponge filters. A pack of 3, fine, medium and course. I filled the first chamber with clean flower pots and placed the sponges on top. That would do as a start for the physical filtering. For the bacteria part of the filter, I used anything that I could find that would provide a home for the bacteria. I used old pieces of garden hose, yogurt cartons, juice bottles and any other bits of plastic I could find for the middle chamber and I left the final chamber empty. Those contents did a pretty good job of filtering the water and in time I was able to use more conventional items.

As soon as I could, I got the things that you see here. The brushes replaced the flowerpots under the sponges and the plastic pieces replaced the yogurt pots. Remember that all these parts need to be removed regularly for cleaning at least a couple of times a year and it can be a cold and messy job. If you spend time threading the pieces onto some baling cord or other waterproof string, they can be removed in a fraction of the time. I know because I'd done it many times before I heard about the string idea.   Top

And here is what the completed filter looks like. There is just the sponges to position over the brushes on the right. Note how close the top of the filter is to ground level. When filling the filter for the first time, remember that water is quite heavy so you need to fill up both sides of any fixed sheets of perspex.

The picture on the left shows the right and centre chambers being fed by the pump. But I also have a hose filling the left chamber to equalise the pressure on the perspex. The picture on the right shows the full filter. It also tells me that I have a leak somewhere in the middle perspex divider seal because the water should be flowing through the holes at the top of the divider rather than around the side of it.

As you can see, with the lid on, the filter is not at all obvious. Soon the ivy will grow over the top of it and the disguise will be complete. I have buried the output pipes and extended them so that their outflow is at the beneath the moss covered rock in the foreground of the right hand picture. The earth over the top of the pipes is saturated an makes an ideal location for bog plants. The end result is the appearance of a natural spring feeding a stream.

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