Is a rainwater tank worthwhile?
From the purely financial aspect, think in terms of two to three decades and the answer is yes.
In that time, a rainwater tank will pay for itself in terms of savings in usage of Sydney water.
Our tank cost $1500 after the government rebate, and we have saved $50 in usage of Sydney water
in less than a year.
A big saving is time spent hand watering during water restrictions. Saving $50 on Sydney water
by using rainwater in a watering system instead of hand watering also means saving about 50 hours
of time spent on hand watering. That's quite a few hours, and will build up over the years.
Could use of tank water help prevent the need for a new dam soon?
Warragamba dam was last full in 1998. Any water saved in that time (i.e. by use of tank water instead of Sydney water)
saves water in Warragamba. See the Sydney dams article for how, if everyone had used 20% less water consistently since Warragamba
was last full, the dam would still be just about full.
During a drought, does enough rain fall to bother collecting it?
Even in a drought, it does rain. And even in periods of very low rain, one can collect a reasonable amount
of water in a tank. For example we have used 50,000 litres of water from our rainwater tank in the last year.
See the next section for more information.
How much water will I get in my tank?
Apart from the obvious issue of how much rain falls in your area at a given time, there are three main factors that affect how much
water you will have coming into your tank.
Firstly, there is the roof catchment area. If a tank is collecting water from only one downpipe, you won't capture all that
much water. Try to arrange for as many downpipes as possible to be connected to your tank, and consider what location would be
best for the tank to make this doable.
We live in a large 2 story house, the roof area is about 250 square meters. a small to medium 1 story home has about the same roof area (as every bit
of floor has a corresponding bit of roof). Some of our roof drains on the far side of the house and would be difficult to connect to our tank
but we still managed to get 150 square meters worth going into the tank, with 3 downpipes that go in directly, and by using a Bunnings rainwater diverter kit
we get the rest. So for us, the last 12 month's rainfall (well below average) would give a total of about 100,000 litres. goes off the roof to the tank.
If we only had one downpipe going into the tank, we may only have got a quarter of that figure!
Then there is the size of your tank.
It isn't much good getting 100,000 litres of water if it all comes at once and you only have a 2,000 litre tank!
sometimes you will get 100mm of rain over a couple of days... 15,000 liters... following previous rainfall where the tank was already half full...
so you will never be able to capture all the rain unless you get an enormous tank!
We have a 9,000 litre tank which fills up with about 50mm of rain, which is
about average rainfall for a month in winter or a fortnight in summer. Of course sometimes you do go for a month without rain and need to hand water,
but it hasn't been much for us.
We find almost all of the time that the tank fills at least once per month in summer and every couple of months in winter, and this has been
during quite dry conditions.
There is also the issue of how much you use the water to make room for more to be collected.
A 9,000 litre tank of water hoarded for a year before being used will only save 9,000 litres of water at most - plus a corresponding amount
of time saved in hand watering if you use the water via a watering system. In the meantime, more rain will have fallen, and be wasted due
to the tank overflowing.
Doing some calculations of your own can be useful.
Check out average rainfall figures for your area.
For example, Bankstown airport average rainfall is 917mm. Only 1 in 10 years is it less than 560mm, usually even in a drought there is more than 550mm of rain.
Suppose I get 550mm of rain, how much water do I get in that year?
Well, 1mm of rain over 1 square metre gives 1 litre. 500mm of rain over 200 square metres of roof catchment area is 500x200 litres = 100,000 litres. You get the idea...
What size of tank should I buy?
The short answer - as big as you reasonably can! Then you're not as dependent on frequent rain to fill your tank, and you'll
maximize your savings in water and hand watering time.
A lot also depends on how you will use the water. With a reasonable size garden, roof and a sprinkler system, we suggest a tank of at least 5,000 litres,
preferably 9,000-10,000 litres.
If you are only going to use it for hand watering, try to estimate at least fortnight's hand water usage in summer, about 1,000 litres per hour
with a tap turned full on.
Of course make sure that you can get as much roof space as possible draining to the tank - 100 square metres or more is highly desirable.
There are restrictions on rainwater tank size (see link) and above 10,000 council approval may be required.
It is your responsibility to check the state government regulations, and council regulations.
In our case the NSW government regulations (on the Sydney water website) said we can install our 9,000 litre tank without council
approval. Unfortunately on calling our council they had no idea of these regulations and tried to insist that we do
need council approval for any tank larger than 5000 litres, but that is another story!
I don't want the tank to take up much space.
A taller tank will take up less garden space. Be aware, however, that
this can make installation more difficult. The usual
way to get a tank into position is by rolling it - which requires a means of access whose width matches the height of
the tank. The other way is to place the tank by using a crane, which adds to the cost of installation.
The best way to save space is to get a bladder style tank that fits under your house, or a wall style tank. These, however,
are fairly small, so you may need several to save a significant amount of water. They are also more expensive.
Can I drink the rainwater I collect?
Opinions vary on this, especially since factors such as pollution and the material your roof is made of can
contaminate water. To be on the safe side it's better not to.
Do I need a pump? What kind should I get?
If you're watering using drippers, it depends on the relative height of the tank and the drippers.
With sprinklers, a pump is definitely required, but what type?
Several types of pumps are available for purchase.
- An aquarium pump - approximate cost $50. Over the last few years, aquariums pumps have been getting much more reliable, cheap and efficient.
- A basic pump such as can be bought from Bunnings - approximate cost $200.
- A more expensive pump, again available from Bunnings - approximate cost $500.
Unlike most pumps in category (2), includes protection against the pump running dry.
It also turns itself off and on depending on the pressure in the pipe.
- A good quality farm pump such as those sold at the places that specialize in rainwater tanks,
usually a well-known brand - approximate cost $800.
The aquarium pump may be fine for drippers provided the overall uphill is no more than a couple of metres.
Sprinklers of all types, however, require pressure... 150kPa even for low pressure sprinklers. The aquarium pump
will not do that.
We chose option (3). We find this actually gives more pressure and water flow from the tank than Sydney water.
We elected not to choose option (2) as we calculated that for our place we would need to change all our lawn sprinklers
to low pressure fittings and divide up all our watering zones into 2 zones - a lot of work!
Basically we had optimized our sprinkler system for Sydney water pressure - your system may be different.
Our (3) option works well and I would recommend it to anyone in the same situation. Of course if (2) will do then
go for it!
How do I calculate pressure?
Basically 1m of head is about 10kPa. PSI conversion? Hopefully your sprinklers will come with information
about the pressure they need and how much water they use.
The packaging of the pump should include a graph. This graph tells you how much pressure the pump provides with respect
to flow rate. To calculate whether a pump suits your needs, add the flow rates of your sprinklers in a watering zone and
look it up on the curve. The pump should provide more
pressure than any one of your sprinklers requires. Of course there is the complexity that your piping, elbows and so on
will lose a fair bit of pressure (how much depends on your setup and varies widely). If in doubt get a
higher model, or borrow a pump to test!
Where and how can I get a rainwater tank?
Rainwater tanks come in many sizes, shapes and colours. They are available from garden centres, hardware stores
and specialist rainwater tank suppliers.
We found the specialist suppliers to be the best value. The one we bought our tank from, City Rainwater Tanks (which seems to have disappeared),
offered a free on-site quote where they come and make sure you have access for the tank you want to get in,
calculate the total cost including plumbing, first flush diverter etc. However, we had to prepare the ground
for the place where the tank would go. That meant leveling and compacting the soil.
You may also be interested in reviews of Sydney plumbers if you need or want
a plumber to help you with your installation.
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