People often ask me for tips about installing a rainwater tank, so this webpage is a list of the things I've learnt the hardway wrt rainwater tanks. Because I'm lazy I'm going to use the word 'tanks' instead of typing 'rainwater tanks'.
Size does matter when it comes to tanks because if you undersize your tank you'll run out of water too quickly, and if you oversize your tank you'll never fill it.
The most important thing to remember is that Melbourne only gets 660mm of water a year, on average, and we haven't reached the average for something like 8 years. So, assuming we get 660mm of water, when can we expect it to fall? Well the BoM provides this nice chart (which I've stolen from them):
Something we can be thankful of is that the rain is evenly spread through the whole year. This means that a sensible strategy is to use our rainwater to augment the town's water. Using this data is crucial to optimising the value of our new tank.
The first set is working out the area of the roof, working out the approximate area of each 'catchment' or section of the roof going into a particular downpipe.
You can see from this diagram that the biggest area to collect from is probably the western end of the house. By attaching to only 2 downpipes we get 70m2 collection area, which is 70*0.660 = 46 cubic metres a year, or the entire annual toilet and clothes water use of an average melbourne water efficient home.
Try to minimise your stormwater pipe lengths as stormwater pipe is made from PVC, an environmentally damaging plastic, and is fairly expensive. Also, long runs require significant fall to allow the water to run smoothly. I'd aim for at least 4% fall, higher is necessary if you hang your pipes from the roof, as the pipe sags within a few years and the leaves pile up in the dippy bits.
One solution to long pipe runs is to have multiple tanks. The tanks can be connected with a small polypipe, made from renewable polyethylene. The important thing here is to make sure that the tanks at each are capable of holding the average storm load as the small pipe will take a least a few hours to equalise. We have two tanks connected with 3 meters of 19m polypipe, and it can take 4 hours to equalise. Don't forget that the water will flow until it's level, so your overflows must be aligned.
To size your tank you need to work out how important water is, the rate you will be using the water and the spread of the rainfall. In a place connected to the mains the reliability is much less important than the cost and the total amount of water collected. Because of this a smaller tank set will be adequate, compared with say a farm environment where water cartage is very expensive.
Lets say you intend to use 250l of water a day from the tank. This means you will need 92m3 of water a year. This means you will need 136 m3 of roof space for collection. Our example house had 160m2 of roof, so it would suffice. How big a tank would we need?
Well, what if we were on a farm and were willing to use the whole roof space. We would want to guarantee 250l a day over the average year. The longest time between rains is generally about 60days (although it's been 120days in 2002). This implies that we need 15kl of tank. The web price of an 18kl tank is $1800.00.
Instead, lets be realistic, we're probably using 700l a day. If we can store up a few kl from each rain then we can use that water up in a few days, but because of Melbourne's fairly even rainfall we can use up the water as fast as it falls. This means less pressure on the big dams, and much less capital outlay for about the same amount of water.
Instead, we'll assume we use rainwater preferentially to the mains supply and we'll probably use around 700l. In a single rain you can get up to around 20mm in melbourne, which means that if we only use the two areas on the west side we will get 1.4kl. A single 1700l tank will suffice if we generally get rid of the water before the next heavy rain. This brings our cost down to $465.
So you can see that you are better off using water as quickly as you can, and minimising your tank requirements.
Ken ran across this page and thought of a brilliant way of reducing the installation cost whilst collecting the maximum possible water.
Rather than run gravity fed pvc storm pipe around the house (which is ugly and fairly expensive) Ken reasoned that a bilge pump is going to cost less than the plumbing. So he tracked down where the stormwater exits his property and dug a deep sump around that point. In the sump he carefully cut a hole on the underside of the pipe. The water runs out of the storm pipe into the sump until the water reaches the base of the pipe. At this point the drain returns to normal function.
Now, by adding a bilge pump to the sump, water can be pumped out when the sump is full. A simple water level switch suffices for this. By adding another switch inside your storage tank which cuts off power to the pump the pump will only run if there is water available and the tanks aren't full - thus removing the need for an overflow.
One thing to watch out for - In Ken's design the pipe ran directly from the pump to the top of a water bag. But when the pump turned off the water siphoned straight back out. You will need to insert an airgap or 'snorkel' to break the siphon.
I would recommend against using water from the storm water main drain as this is sure to have various pollutants, such as car oil, car washing up stuff and pool chlorine. Just use the water off your block, as you know where that is coming from. Make sure that the levels are such that water wont back flow except under extreme situations.
The water coming through your stormdrain is no more dirty than that collected directly from your roof (indeed it is probably better as the pipes aren't made of new PVC and will have outgassed most of the monomers and plasticisers).
The most important thing to keep your tank water clean and drinkable is to make sure dirt doesn't get into it. It sounds obvious, but believe me, it's hard work cleaning out a tank that's gone sulphurous because a wattle tree dropped all its flowers in the water.
A simple way to help keep the water clean is to use a so called 'first flush' mechanism. The easiest is a few metres of stormwater pipe hanging vertically near the tank, connected with a standard T junction. Put an end cap on the vertical section and attach a 3mm drip line to the base. Put the other end in a pot that likes lots of water, or contains a lot of soil.
The initial dirty water fills up the vertical section with the possum droppings, the dust from the air, or whatever else has been sitting on your roof for 6 months since the last rain. Once it is full the water flows quite naturally into the tank. When the rain stops the water drains out of the tube through the drip line, and bingo, the system is ready for more rain.
Another important approach to keeping the water clean is to avoid keeping the water around. Melbourne's water is safely kept around because they can use the huge, clear dams of water to UV sterilise the deeper water. UV sterilising a tank is not worth the expense (A cheap UV tube costs around $250). So use the water up as soon as you can.
Melbourne's water seems to contain a floculent, or settling agent. Because of this, you can knock out fine suspended particles out of your tank by putting in a few hundred litres of water after rain.
The simplest technique, and one my grandmother is still quite happy using, is to put a tap at the bottom of the tank and use it to fill watering cans and the like. Personally I find this too much of a hassle. It is cheap and easy, however.
The next easiest thing is to attach a hose to the tank directly. Be warned, however, that common cheap hoses are designed to be used with water in the range 300 to 1000 kpa, and you will not have the patience to even fill a watering can... You can get large diameter hoses, but I've found they cost as much as a cheap diaphragm pump.
Pressure pumps (not water mover pumps, not pool pumps, and not aquarium pumps) come in all sorts and sizes. I bought a waterflo diaphragm pump for $200, and it runs directly off our solar panels. It provides around 11l per minute, which is enough to fill a 9l watering can in about 30 seconds (you can steal a bit of pressure from the fact that the whole system is pressurised when the pump cuts out).
More powerful mains powered pressure pumps come in around the $350 mark, and can provide almost the same pressure and flow rate as town water. If you want to run the whole house off your tanks then these are your only real option.
At a 'sustainable living fair' Lynne and I went to see we saw a brilliant idea for storage. The company sells a giant, tough, plastic bag with some fittings on it. You clear a space under the house and drag the bag under there. once the bag is plumbed in you get a whopping 3500l tank which is totally out of sight. The same idea could be used for greywater (as they say) or even thermal buffering of the house. another idea would be to put them in the ceiling to even out fluctuations in temperature (although you'd want to be careful about insulation and the roof truss strength).
Ok, I've done some tracking down via going solar. It was from a company called polymembrane and pipe. Rain Reviva is now sold by the company "New Water".
Give them a ring - they are very helpful, and will send out flyers and whatnot. Quick summary: the bag itself costs $700, and I think it is 3.5kL, they have a complete deal for $1700 including things like pumps and plumbing. They can make them to any size.
I read about this idea in ATA's ReNew magazine (an excellent magazine!). The basic idea is to balance your pressure pump against the mains through a 'pressure reducing valve'. I have no idea how these work, perhaps by using a strong spring to hold a washer against an adjustable force.
You set the pressure reducing valve (PRV) so that it gives a slightly lower pressure than your pressure pump. When there is water in the tank the pump forces it out preferentially into your house's water. When the tank runs dry the town water seamlessly kicks in.
I have a writeup on an earlier version of this on our house blog.