Archive for the ‘Rainwater Capture’ category


January 19th, 2011

I was listing to a panel guest on SAFM  – think it was Jeremy Taylor, talking about rain water harvesting. I agreed with much that was said but one of his solutions struck me as a bit odd and it got me thinking that there has to be a better way.

He was talking about the problem of how to handle the tanks running dry when the house is setup to run off an automatic pump. Obviously if you have a Municiple supply the idea would be to connect back to this supply. His solution was to install a ball valve near the bottom of the tank running off the mains. Now whilst this will certainly solve the problem of the tanks running dry, it seems silly to waste the inherent  pressure of the municipal supply by filling up your tank, which you then have to pump out again to make usable. Not only does this waste electricity, but also will wear your pump out quicker. Also during a power cut you will be a bit stuck.

So this begs the question is there a better way? Well a less elegant system would be a set of two valves to enable a switch over between tank and municipal supplies, but that would be too easy and requires user intervention – so this is what I am going to do:

I am going to install 2 solenoid valves (1 ” irrigation type) – one on the municipal supply, the other on the tank. These I will connect via a transformer to a float switch. The float switch I have has 3 wires which effectively gives you 2 states, “tank empty” or “tank has water”. So by simply using this float switch as a kind of relay you have an intelligent system which will automatically switch between the 2 supplies without the need for any fancy control panel.

Another advantage of this system is that the pump can’t run dry and un-prime itself, disadvantage I suppose is the transformer is always pulling electricity. I doubt this is going to be a large cost factor as the solenoid valves are designed to very efficient, so I would be surprised if they use more than Jeremy’s system above. Also in a power-cut you would need to manually operate the municipal solenoid valve to get water – but this is easy enough as most solenoid valves have a manual overide.

Here some pics and a a diagram of what a mean (note non-return valve on municipal supply as precaution):

I am about halfway though my installation so I’ll keep you posted on how well it works, as always if you have a suggestion or comment post away.


August 6th, 2009

I’ve always wanted to collect my own rainwater, I think if you have a garden, to waste the rainwater runoff from your roof almost seems a sin. Obviously the amount of water you going to get from your roof will depend on a few factors, the most important being:

Amount of roof area

The percentage of this roof that you can that you can manage to drain into a tank

The amount and intensity of the rainfall

Seasonal variation in rainfall

To make it simple you can take your annual rainfall and simply multiply it by the area of the roof, in my case that is a annual rainfall of 624 mm or 0.624 m  – multiplied by the roof area (approximately 250 m^2) which gives about 156 m^3 per year. Now 1 m^3 is 1000 litres, so that’s a lot of potential water, problem is if you tend to get all your rainfall in a couple of months of the year, then finding a big enough tank to store it will be problematic.

We are fortunate that Port Elizabeth, rainfall is very nicely spread over the year, so it makes a capture system ideal as the tanks don’t have to massive to capture all the available water.

The next problem I faced was where to put the tanks (I chose 2 x 2500 litre tanks which are quite large, 2.5 m high – you will see why later) and also how to get the water from the gutter into the tanks. A pet hate of mine is big tanks that dominate the garden and even worse a skyhook arrangement of pipes to get the water into the tank.

In your face

In your face piping

So I decided that one option was to bury the tanks (at least partially) and put them under our deck. This has the advantage that they are out of site and also piping the water into the tanks can also be underground, also out of site. It does have the obvious disadvantage that you need to pump the water to get it out, but since I needed pressure anyway this was no issue to me.

How do you bury a tank that isn’t meant to be buried? Well the obvious answer is you shouldn’t try unless you prepared to take the risk and you should get ones that can be buried (quite pricey). So huge disclaimer here – this is what I did, it may not work for you! I dug the hole for the tank about 400 mm bigger than it needed to be – placed the tank in the hole and then filled it with water.

Then once the tank was full, I backfilled around the tank with a dry mixture of sand and cement – ratio about 5:1. I did this in layers and then with water, saturated the mixture each time. I repeated this until I reached ground level. Now the important part, leave the water in the tanks for at least 3 weeks to give the cement time to harden. If done correctly should end up with a lovely compression ring round your tank. I suspect this will only work in good stable soils, if your ground is very clayey and saturated you probably going to come to grief. Also remember if you have a high water table your tanks are simply going to float if empty! It is also better if they stick partially out the ground, as this saves on trying to make a stable roof for your setup.

With the tanks in the ground (see pic) all that was left was connecting it up to the down pipes. Now normally people manage to get one or maybe 2 of their downpipes into the tank, but this leaves a lot of the water going to waste as most houses have more downpipes than this. So I came up with a semi pressure system to pipe the water over longer distances but keeping it neat. I used 50mm (2″) PVC plumbing piping and modified some 110mm gully fittings to make a capture system. As it can handle some pressure you can simply mount the head at about the same level as the top of the tank and run the line at or below ground level. Even if visible it is much neater than down pipes hanging in mid air.

Here are some pics of the capture system with one of the semi buried tanks to the left:

Modifying the gulley so it sits closer to the wall

Modifying the gulley so it sits closer to the wall

Close up of modified gulley

Close up of modified gulley

Capture system with tank to the left

Capture system with tank to the left

Ulitimately the idea is the tanks and most of the piping will be under the deck and hidden (see sketch-up pic below):

Deck layout - sketchup

Deck layout - sketchup