Water harvesting system

My father put in a water harvesting system when he built his house twenty years ago. It still seems to work. Last time I visited, I helped my mother change the filters and clean out the gutters. Actually we should have done that the other way around. While cleaning the gutters, a certain amount of crap ended up washing down the gutters, and straight into the nice clean filter, so if you ever find yourself cleaning a water harvesting system, clean the gutters first!

​Three drains from the house, and one from the garage to the right, all come into one gutter. This then heads through a drainpipe into the filter butt.


Water goes through a grill to catch leaves and large debris, then a felt filter bag lined with a nylon gauze bag. The filter butt can take something like 100 litres, which quickly fills up if it is raining heavily since the roof has a large area, and it takes time for the water to get through the filter. Another piece of advice: it's a good idea to change the filter when it has not rained for a while, then it is a lot lighter.



When you don't need any more water, the large yellow pipe to the right of the filter butt can be put onto the drainpipe to bypass the filter. In fact when changing the filters, we should probably have put the bypass pipe on.

Filtered water then goes into a tank under the garage. There are two tanks, one being filled and the other supplying water to the house. You can choose which tank to fill by putting the hose into it, and the red taps will choose which tank is supplying water.

Wonderfully low-tech!

You can see how much is in each tank with these two old washing-up liquid bottles, filled with sand, which have floats attached to the other end of the strings.

Detailed calibration has been added.

An electric pump draws water from the tanks, either on demand from a float in a tank at the top of the house, or at the flick of the "over ride" switch.

From the pump the water goes through a chamber with a purification tablet on the left, then another filter on the right.



The pipes then run along the wall to another clearly laid-out junction where the harvested rain water can be switched off, and mains water can be switched on instead. The pipe goes into the house and up to a water tank above the bathroom, which has a ballcock that activates the pump when more water is needed.


This system still works after twenty years, providing most of the domestic water. There are a couple of separate taps of mains water in the house for drinking and cooking. I particularly like the layout and simplicity of this system, and if anything did go wrong with it, any handyman could quickly work out what was wrong and how to fix it. Financially it has probably not paid for itself, since their water is relatively cheap, and in fact my Dad admitted it was a bit of folly, but I'm sure he enjoyed making the system and was very satisfied to see it work.

It does provide some insurance against water shortages, which may now happen with increasing frequency, in and amongst the increasing floods! In many areas there is a cheap, regular and safe supply of water, and harvesting your own water is probably a low priority if you are building on a budget. Rain water harvesting is now mandatory in some Indian and Australian states, and many places around the world offer financial support for people harvesting water. And if you are building on a plot a long way from the infrastructure then a rain water harvesting system could save money.

Using large pipes as rainwater storage

In this interconnected age, whenever you have a great idea and start googling it, you usually find that somebody else has had the same one. In fact, you often find that somewhere there is a lively forum dedicated to implementing that idea, with five-star experts who have been sharing their experience since before the internet was invented.

So when you do have an idea and can't find anyone else who has done it, you start to wonder whether you are just being stupid, and have missed something very obvious that will stop it from working.
For the water storage system, I've been thinking about using pipes, around 15 or 20 cm in diameter, and looking for inspiration and advice on my pan-pipe design, which I think will look a lot nicer than the horizontal pipe rainwater storage system. Either way, I can't find anyone else who has tried it. At least if they have, they haven't blogged, written a book or made a website about it, so it doesn't count.

The closest I can find is the Green Building in Louiseville, Kentucky, which looks to have spiral pipes for rainwater collection.

Then I actually talked to someone about it.

"Wouldn't they be really expensive?" Oli asked. He had laid some pipe before and was shocked by the price he had been charged. I wondered whether that was a supply chain issue, but looking on the internet for white pvc pipes, as thick as possible, the best I could find was 11,130 for 4m lengths of 150mm VU pipe from cut-man.jp. They also had VP, which was a little more expensive. I think the difference is in the wall thickness, 11mm vs 20mm, and there's more information here if you're really interested.

Each 4m length would hold about 70 litres, so the tanks alone would price the system above commercially available ones.

There is a good site recommending low-cost solutions here. But most of them are also low-beauty.

Further over-complications for what is, essentially, a big bucket

Or, instead of a leaky syphon, it could just be a very thin syphon.
Going back to first principles, what I want is to divert the rain water from the roof into the garden, and slow it down as much as possible so that it arrives days, or even weeks after falling from the sky. If I can get a syphon working between each water tank, and make it thin enough, it may be able to do this.
The speed of the fluid in a syphon depends only on the difference in height between the top of the water and the bottom of the outlet pipe. According to wikipedia, it's the square root of the height times 2g. So water in a syphon coming out of a 50cm tank will be moving at around 3 metres per second.

Converting that to volume, if you had a pipe of 1cm diameter, it would drain at 14 litres per minute. A hundred-litre tank would be empty in seven minutes. If the pipe was a millimetre in diameter, it would drain at 8 litres per hour and be empty in 12 hours. Five of these tanks in series and the water is going to get from top to bottom in 2 and a half days.

Of course the rainwater is going to have to be really clean with a pipe this thin as it would be really easy to block it. Also this would have to be in addition to a regular overflow since a pipe this thin is not going to be much help in the event of heavy rainfall.

This syphon is essential going to be the same as having a pipe coming out of the bottom of the tank, leading to the tank below. The differences are that a pipe coming out of the bottom of the tank needs a hole in the bottom of the tank, and the syphon will only start working after the tank has overflowed, and will may not start sending water down the system until it has.

Wikipedia also mentions self-priming syphons with a cotton-filled hose that will suck water up by capillary action, then send it down the other side. These are slower than an open hose, which is even better. The system would then start slowly syphon water down as soon as the bottom of the tank is wet.

Of course it may be a good idea to have a hole in the bottom of every tank since it will make them much easier to drain, unless the tanks are going to be fairly small. The drain could lead to a tap into a lower level of tank, and it would be possible to control the trickle down the system.

A rainwater system with syphons and leaks

I've been trying to work out a system of cascading water barrels. Ideally each barrel would contain enough water for one watering. When it emptied it would fill from the next one up, and then when it was full it would empty and water the garden. This would get regular irrigation for the garden. Better still it would only work in the early morning or in the evening, and would not work when it was raining, but those are separate issues.

Right now I just want to think of a system that will run only on gravity and rainwater.

Each water tank needs up to three pipes connecting to it: water in, water out via a tap at the bottom, and an overflow. Generally I want to get water out of the tank at the bottom, so the tanks further up the system don't necessarily need a water-out tap, or if they do it will normally be closed.

The water-in can go on the lid of the tank, which should be straightforward. The overflow needs to go high up on the side, and usually will be dry, so the seal around the hole is not critical. The water-out needs to be near the bottom, and will be wet and under pressure, so the seal is critical. It would be a good thing if I could get out of having to make these. 

If I use a syphon overflow, so a pipe goes into the higher tank high up on the side, and the pipe reaches all the way to the bottom of the higher tank, then feeds into a lower tank so the other end of the pipe is at or below this end, then it will work as a syphon. When the water goes above the overflow hole and the high point of the overflow pipe, the pipe will fill with water, then water will head down the other side of the pipe, and suck water from the bottom of the upper tank. And because it will work as a syphon, it will continue to suck water out either until all the water has gone into the lower tank, or if the lower tank is only a little lower, then it will empty until it reaches an equilibrium and the water will go to the same level in the two tanks. 

Once it's in this situation, adding more water to the top tank will send some water down to the lower tank, so that it will maintain this equilibrium, even if the level of water in the top tank is below the overflow level. 

I was thinking that I could use the same kind of syphon overflow between each tank, from the top to the bottom. However, because the syphons are going to keep tanks in equilibrium after they have filled up, they are going to try to get to the same level, and all the water will syphon itself out. 

So a syphon overflow may work for two tanks that are on more or less the same level, but will not work so well for the whole array. 

Maybe what I need instead is a leaky system. Rather than trying to make leak-free taps coming out of the tanks, perhaps I should be letting water gradually leak down the cascade.

Or perhaps I need to think about leaking air. The syphon won't carry on trying to keep the two tanks level if air is leaking into it. If I can get a little air hole at the top of the syphon tube, when the water goes above that level, the syphon tube will fill with water and start syphoning out. It will carry on syphoning out while there is still water at the top of the syphon tube, but as more and more air gets in, the flow will decrease and then stop. The bigger the air hole, the faster air will leak in and the less the water will syphon out. This could limit the amount of water going to the tank below each time.

Of course another priority is a low-maintenance, self-cleaning system, and the chances are that if the system relied on leaks to work properly, then rather than leaks getting worse as they do when you don't want them, they would fill themselves in. 

Rain water tanks - buy or build

An early decision as I plant the seeds that will eventually lead to a rain water harvest is whether to buy or build. Either way needs careful consideration of design features of the system.

Panasonic do a few rainwater tanks. They have a 150 litre tank for 55,000 yen, and a 200 litre tank for 70,000 yen. They also have a 340 litre wall tank for 65,000 yen. It actually looks like a wall, although I don't think any walls actually look like this. You'd have to get your house sidings redone to match it! Not sure if this was a translation error.

They are not the most expensive, but certainly not the cheapest. Maruichi have a 140 litre tank for 29,000 yen

There's a whole range of tanks here from Direct Tank, with and without lids. They look very simple, with a basic design, no frills, no included connecting parts. And expensive! For example a 300 litre tank with a lid for 52,000 yen.

Then there is Takiron, with a 120 litre tank for 29,700 yen. They also have a 150 litre wall type for 41,000 yen. You can pay more for something that is less attractive!

The best looking tanks may be either the German Groban Slimline 300 litre at 44,000 yen. Except that it's beige. Only 99 euros on ebay, so somebody is making a profit. It's got the ridges around the edge, which I'm not completely convinced about aesthetically, but I guess they have a pretty good functional rationale. 

Or the Aquatower, 150-200 litres depending where you put the tap for 39,700 yen. Or 100 litres depending which website you look at. They have a very clever looking drainpipe fitting which will stop dirty drizzle from getting into the tank, and also act as an overflow if it is aligned with the top of the tank. 

The best I can get is something like 150 yen per litre of storage. This is very close to the price I pay for each cubic metre. A cubic metre is 1,000 litres, so I would need to fill and empty the tank 1,000 times for it to pay off in purely financial terms. If I tried really hard I could probably fill and empty the tank 20 times in a year, so it would take 50 years to cover the initial cost

Of course everything is not purely financial. If I'd been interested in making money, I would probably have invested in shares of the oil industry or a nuclear power station rather than putting solar panels on my roof. Using the water off the roof will not only have benefits in reducing the demand for local water, and providing my garden with cleaner water than the tap does, it will provide a store of relatively clean water in the case of some natural disaster that destroys the municipal water supply. At least it would if such a disaster did not destroy my water system too. Anyway, I can probably build a system for much less than this cost, provided I spend several hours thinking about it, and not putting any financial value to that time. 

Things to avoid in a water storage system

There are a few things to watch out for in designing a rain water collection system. These probably include mosquitos, algae, freezing, over flow, first flush, noise, leaks and condensation.
watercache.com gives some great information on first flush devices, and there's more here.

The basic issue here is that between rainfall your roof can accumulate a lot of crap. Some literal, of the avian variety, some more in the engineering sense of the word. So when it does rain the first few millimetres will be dirty. This is likely to be a bigger problem when there are longer dry periods, and is probably affected by local pollution.

Research in Tokyo found the first 1-1.5 cm from the roof is dirty, and it seems a rule of thumb is that you should throw away the first 40 litres per 100 square metres of roof. There is no definitive amount of rain needed to clean the roof, and it has been pointed out that rain often starts as a trickle so if the first-flush strategy is to divert a fixed volume of rain at the beginning of a rainfall, then this may all be drizzle, and when the heavy rain comes and stats washing the roof it will go straight into the tank.

The important point is that keeping the water going into the tank as clean as possible will keep maintenance and cleaning easier and make the system last longer. Filters obviously come into this too, and I should probably have put that in the list.

Also obviously, there must be a strategy for the tank overflowing. Since this is likely to happen when it's raining heavily, the overflow pipe should be as big as the pipes going in, and possibly bigger since the pipes going in could be full of water, but the overflow pipe, at least at the beginning, will be full of air.

Even if the tank doesn't leak, there may be a fair bit of condensation on it in the summer, and there is likely to be a pool of water at the bottom, so getting a well drained area below the tank is a good idea.

There's a good site here from the Australian government and lots of information just in this thread on the Alternative Technology Association. They cover most points.

I'm not sure about noise but if there is a lot of water moving around, it's possible that it could be be dripping, hissing or humming. It's probably going to do most moving while it's raining so this is unlikely to be a serious problem.

Freezing is likely to be a challenge when the average temparature is below zero for at least a couple of weeks of the year, so either the system needs to be drained, insulated or set up so the water can freeze without breaking anything. 

Harvesting rainfall

There seems to be a lot to write about water, whether suspended in the air or precipitated.

I've been thinking about harvesting rainfall for a while, with the usual constraints of elegant design, low cost, and aesthetic beauty. In fact it was on the list of things to incorporate into the building of the house, but it soon went onto the back burner along with all the others. It's not a good idea to leave water on a back burner for too long as it will evaporate and you'll burn the kettle.

We need water for the garden and rather than switching on the tap and pouring rainwater down the drain, it would probably save us a fair bit of money to collect rainwater and use it for the plants. 

Just to deconstruct the problem, we'd mostly be looking at collecting water from spring, and using it mostly in the summer. If we could collect enough then, we wouldn't need to worry about the winter, and could use that time to empty the system, which may be preferable to having a collection of ice. 

Looking at the supply side, there's roughly 100 mm rainfall a month, a little bit more from May to July, and we have a total roof area of around 100 square metres. Technically speaking, the roof area is bigger than that, since it's at an angle, but that's the area as far as the rain is concerned, which I guess we have to assume is coming down vertically. 

Anyway, down the four drainpipes we'll get a total of 10 cubic metres of rain water per month, or 10,000 litres. So we'd have no trouble filling a 500 litre tank from one of the drainpipes. 

We could get a big tank somewhere, but would probably want to put it underground to save space, in which case we'd need a pump to get the water out. What would be more useful is something tall and thin that could stand against the wall, right next to the drainpipe. 

A 500 litre tank could be 80 cm in diameter and a metre tall. Or if you had two 250 litre tanks, they could each be 60 cm in diameter and 90 cm tall. At least those are the internal dimensions. You'd probably want to add a bit for the thickness of the walls. 

Since we don't really want water tanks in front of the house, and the gap between the side of the house and the fence is only a little over a metre, we wouldn't really want 80 cm tanks along there. We could make them taller though. 

A 5 metre tall tank of 25 cm diameter would hold 250 litres. We could tip it onto it's side, and run it along the whole width of the house, north to south along the west wall, so that it could fill from two of the drainpipes. Then one tank could be 8 metres long and 20 cm in diameter.

As usual, somebody has already thought of tall this. Rainwater hog make a chunky modular system, and there's Jackson in the UK.

But going back to the tank, as it gets longer it's starting to turn more into a pipe than a tank, so if it is going to be a pipe, then perhaps it could run through the garden towards the plants where it is going to be used. Then it could be 15 metres long and only 15 cm wide. Of course we'd be back to needing a pump again. 

So the best ideas are probably a couple of thin horizontal tanks, or thick horizontal pipes, running along the side of the house, or a few vertical ones, possibly cascading like pan pipes.

So it's time to start looking around for parts and construction techniques. The question is, do they have them in a nice colour? I don't want grey pipes on the side of my house.