Last updated 8:22 pm, Tuesday 5th February 2013
Water tanks, what are they used for?
Water tanks usually have five main uses:
- water for the garden - basically water to do plant watering with, so conserving water and saving you needing to use mains water;
- water for fire fighting - very Australian, but an important usage none the less;
- water for drinking - rain water if stored in the correct tank is quite suitable for drinking;
- water for washing - you can easily use this water for washing your clothes in, and;
- water for flushing the toilet - perfect for the job!
How big does the water storage tank need to be?
Given the above usage cases; how do you 'size' your water storage tank? The trick here is to have enough storage to 'carry you through' the periods of no rain given sufficient prior rain. The above positive/negative figure will give you a steer on how likely being able to carry through will be, the more positive the easier with adequate storage; the more negative the less worthwhile as a goal this is (so don't bother getting such a big rain water tank
The size you need also depends a lot on your rain fall patterns - if you have well defined periods of the year that are 'wet' and 'dry' then you should aim to collect as much as possible to carry across into the dry periods. If its more random and equally distributed across the year, then you just need to hold enough to 'flatten out' the random distribution (i.e. a quarter of a years total rainwater would be more than adequate). Your consumption of the water also comes into play, if you consume it all before it can be saved over the medium term, a large tank will spend most of its life empty!
Although, from talking to many people on this issue, I always hear the same story; namely 'I wish I had got a larger tank'. I think this is more down to the changing weather patterns which have condensed what was previously a months worth of rain into a day!
NEW - we have just completed an online Rainwater harvesting calculator that you can use with you real monthly rainfall figures and average water consumption to get exactly the size of rainwater tank you need.
Water tank type?
Water tanks come on many different types, shapes, sizes and colours; but can be grouped by final insulation location, as follows:
Tanks buried in the ground
These are either plastic or concrete. Plastic water tanks will be supplied as a whole unit that is 'dropped' into the hole and filled over. Concrete water tanks require form work, pouring, topping off and covering over. Plastic is in essence quick and easy, concrete slower but more 'solid' - i.e. should last a lot longer.
Where it is viable to bury your tank is determined mostly by your budget and the ease with which the required hole can be made. Also be aware of ground water
on the site, if this pools in the hole, you can end up with the empty tank literally 'floating' out of the hole...
Tanks under the house
Unless you are building a new house, your only real option here is what are known as 'bladder water tanks' - in effect a big rubber bladder that rests in a special frame in the void space under your house. Big plus is that you don't have to dig holes or take up yard space with a water tank. Downside is that the capacity is usually limited and you pay a premium.
Tanks above ground
This is where you will find by far the biggest variety. Everyone and his wife is either making or selling some form of above ground water tank. The trick here is to use the right 'type' of tank for the size and expected life.Above ground small plastic water tanks
- These go up to around 2000 litres and come in all manner of shapes: round, square, cylinders, etc. They are mostly designed for use on the smaller plot where finding somewhere to 'fit in' the tank can be hard. Often they end up mounted on a small stand. Big downside with these is the lack of water storage and the fact UV will degrade the plastic - so fixed life span (or keep them out the sun and/or paint them).
Above ground big plastic water tanks - These usually go up to 15k litres and are shipped to site whole and ideally 'dropped' right onto a special sand based pad that becomes their home. The big plus here is that its ready to go. The big negatives is that they don't do well in a bush fire and the UV will degrade the plastic over time - so have fixed lifespan. See Nylex Water Solutions for tanks of this type.
Above ground metal water tanks - Basically a big metal 'water butt' with some internal treatments to reduce corrosion effects. These can go up to 150k litres typically. Cheap but the fact the metal is performing two roles (structural and water lining) means they are susceptible to failure due to movement - so they are often mounted on frames or towers as part of the package. Also rusting puts a limited life on them.
Above ground concrete water tanks - Basically a big reinforced concrete box or cylinder. These go up to the millions of litres with a cost to match to boot. No problems with rusting, although be careful on settlement as it could crack the tank. Also no way to move it once its in place, so be darn sure you wanted it there.
Above ground metal water tanks with plastic liner - Think of this as the 'bladder tank' going outdoors. The metal provides the enclosing framework, whilst the liner does the job of keeping the water in. Big win here is that settlement won't loose you water, and rusting doesn't make the tank leak. Sizes up to millions of litres. See Pioneer Water Tanks for more details. Another plus is that the liner is often 'food grade' - so good to drink from direct.
Is it all worth doing??
A whole load of factors come into this: some you can quantify, some down to a lifestyle choice.. The simple factual information is best worked out over the expected lifetime of the various solutions you have available. i.e. look at things over 10 years at least. Most good water tanks should still be functioning well after 10 years with a little bit of TLC. So work out the amount of water 'saved' by the water tank compared to say mains water, i.e. the cost of the tank water if you had it from the mains. Subtract from that the actual full cost of the water tank and you get the nominal cost now amount (no depreciation or price adjustments) you will save (positive) or loose (negative) from utilizing a water tank over 10 years..
Then to that figure add on the cost of replacing any items you will 'loose' or be unable to support without the mostly guaranteed supply of the water tank. i.e. think expensive none drought tolerant plants (and veggies). Difficult to work out precisely but for some people this could be a rather expensive replacement cost.
Another factor to consider, that is very Australian, is that in bush fire prone areas having a large water tank with the right attachments can help the fire services in fighting fires - so reducing the potential risk of fire damage to your property by proxy... In theory your insurance should cover you, but I'd opt for doing without the trouble in the first instance!
Where to put the water tank?
By now you should have an ideal figure for the amount of water you need to store, so next you need to think about where to put it... Remember 1000 litres takes up 1 cubic meter (no getting away from that, its physics!). Also remember most tanks come in height multiples of around a meter (plus 50 cms for the top). Also councils have weird and wonderful restrictions about where the tank can go (think boundary and overshadowing). Plus if its over 10k litre it will usually have to be covered under some form of planning permission.. So think about this hard. Also you can have more than one tank easily enough and that might be cheaper than digging out a massive hole. Also remember when you take the water 'out' of the tank you want its water pressure to be of use to you if at all possible, i.e. put a tank high on your plot under the 'highest' roof, so you can water your garden using just gravity and no pump!
What about the run-off?
No matter which water tank you decide to put in - as soon as it fills up the excess water will come out of an 'overflow' pipe. Now for a small tank and feed-in this is not that much of an issue; but if your tank is feeding off more than about 200m2 of roof area you could end up with quite a volume of water flowing out of that overflow.
Such a volume of water can result in significant localized flooding and erosion
on its own, so it pays to 'manage' this water up front using the following techniques.
- Discharge the water into a 'swale' system - this is a set of humps and curves made in a gentle slope to slow the water down and encourage it to settle into the soil instead of running straight off your land.
- Install an overflow 'weir' basically a normal flow of overflow water just goes into the swale system, but a very high amount goes over the weir and then into a storm-water pipe to carry it away into the storm-water system directly rather than overland. This weir is easy to make with some stone and pipe fittings.
- Get another tank... Seriously, if you have the land space and can 'chain' together tanks in this way you will maximize the amount of water you capture. For instance on our plot we have a tank on the high side whose overflow is plumbed into the rainwater capture for the lower tank which feeds off the house roof. The overflow of that then goes off across the lower paddock and around the chicken coup to the creek.
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