Self Cleaning Rain Water Filter With No Moving Parts

Most designs for filtering rain water pour the water through sand or gravel. In my experience, it doesn't take long for moss and leaves and debris to collect on the surface of the sand. The water in the filter begins to stink, and the filter needs washing out.

This design fills the holding tank, and when the holding tank is full further rain causes the filter tank to cycle through filling and emptying, cleaning the filter tank and the filter substrate. This effect is enhanced by the draining being accelerated by a long drain pipe - in effect the water in the filter tank is quickly syphoned in the opposite direction to which it is filled, hence cleaning the filtrate very thoroughly.

Larger pieces of debris such as whole leaves are held under the perforated aluminium plate which supports the filtrate, and are sucked out with each cleaning cycle.

Rain water that isn't filtered causes blockages in irrigating systems and elsewhere, and causes the stored water to smell like the bottom of a pond.

I have been using this filter system for 4 years, have never needed to empty or clean it, and have never had any smelly water. We use it for the washing machine, toilet flushing and irrigating the garden. It has saved us 000's of pounds in that time.

I run a misting system in summer for propagating cuttings, and since the misters have very small apertures, I've put a standard drinking water filter in before the misters, and have had no problems with the supply.

The water would need further filtering and processing to use it as drinking water, and I don't recommend that.

1500 litre barrel with a lid (it's a barrel that was previously used for importing concentrated orange juice)

100 litre header tank (usually available free or very cheap as people remove them when modernising their houses)

A 3m length of 40mm diameter pipe for the overflow

Perforated aluminium. The piece illustrated here was some scrap lying around an industrial estate. It's commonly used as cladding on industrial buildings.

A short length of downpipe to make the syphon tube. I used square stuff, as I had that available. Round is also fine.

The top of the syphon tube is closed off with an aluminium of plastic plate. It needs to be sealed airtight for the syphon to work, so some sealant such as Sikaflex will do to glue the lid on.

Some lengths of downpipe and connectors to transport the water from the house downpipe to the water filter. The bottom of the last piece of downpipe is cut as shown, so that the pipe can rest on the bottom of the tank, but water can pass through easily.

The only filter medium I've used is LECA - Lightweight Expanded Clay aggregate. It's ideal. I had some left over from another job. Coarse sand should work OK as well.

A bit of forethought will save a lot of trial and error and wasted work. A little bit of maths might help too.

For the tanks, the 1500l storage tank and the 100l header tank for the filter system, I just used what was to hand. I was lucky I got the sizes about right. You need a storage tank that can contain a good downpour before it's full, but not one so big that it rarely reaches capacity. It has to overflow occasionally to clean the filter. So the size of the tank could be smaller or larger than mine depending on your roof collection area and your rainfall pattern. I hit it right first time - purely by chance. The rain tank is the right size to fill completely from empty with a decent thunderstorm or more persistent lighter rain over several days.

I have 90 square metres of roof that I collect water from (the house, the garage and an extension). The house is in South Devon, and so gets around 1000mm of rain annually. Calculate your available roof area and google your rainfall and consider adjusting your collection tank size to suit the amount you'll collect. I think the size of the header tank isn't critical.

If you have a tank you just want to try to see if it might work, bear this in mind. The levels of water in the storage tanks have to match the levels of water in the filter tank. You've got to get it right within a few millimetres. It's not that hard to do, but if you build the system and then decide to change the size of the tanks, you might have a bit of work adjusting the support under the filter tank and adjust pipe lengths.

Take a close look at the design and notice the heights of the top of the filter tank, the height of the drain pipe, the height of the syphon tube and the level of water in the storage tank when it's full. I've drawn it carefully!

Here's the flow of water when the storage tank is filling, how it will be most of the time.

Rain water flows through the downpipe and enters the filter tank at the bottom, under the perforated aluminium sheet. It then flow up through the holes in the sheet and through the filter medium. The water then flows out the outlet pipe into the storage tank.

So the storage tank gets filled until the water level reaches the level of the water in the filter tank. Then the levels in both tanks rise a little more until...

When the water level reaches the top of the waste pipe inside the syphon pipe, it starts pouring down the long waste pipe. The water has a long way to fall, and accelerates, creating a bit of a vacuum behind it - enough to draw down any remaining air at the top of the syphon tube. Then the syphon really starts to flow, like when you flush a toilet. The water is sucked out of the filtrate, dragging with it any debris trapped there and it all shoots down the waste pipe. When the filter tank is empty, air enters the syphon pipe and the drainage stops. While it continues raining, the filter tank is again filled to capacity, and then again, flushed back in the opposite direction until it's empty. It cycles like this for as long as it rains and as long as the storage tank remains full.

That's it - automatic filtering and filter cleaning, with no moving parts. I've had system going for years, and you can see the first thing I'll likely have to replace is the wooden structure that supports the filter tank.

I settled on this design after reading about hydroponics systems where they need to slowly fill a tank and then periodically quickly empty the tank. Some people on the hydroponics forums seemed to be struggling getting the flush action working right.

So to help with that, as I stated above, pay attention to the levels. The top of the waste pipe needs to be about 25mm or and inch below the top of the syphon pipe lid.

Another thing to watch for is getting the pipe diameters right. I calculated the cross section area of the syphon pipe minus the area of the down pipe to be equal to the cross section area of the down pipe. This mean that there were no pinch points in the flow of water - it always had the same area of pipe to pass through.

The bottom of the syphon pipe has cut out sections on each side up to the level of the aluminium sheet to allow the water to flow through easily. You can't see that in any of the photos, but if anyone needs any further illustrations or clarifications, let me know!