As is usual in Australia, we’ll be collecting water off the roof and storing it in a tank. There are loads of websites to calculate the required size of the water tank. The tankulator provides a good calculation and allows you to play with first flush volumes etc and will load your rainfall data automatically (if you live in Australia):
Be careful about your selected rain gauge it can make a big difference in the calculations. The Bureau of Meteorology has all the rainfall data. http://www.bom.gov.au/climate/data/
We also need to reserve 10,000 l for the CFA for fire fighting water. This means that we need to have a CFA fitting at the base of the tank for them to hook their fire trucks & hoses up to, and that the tank needs to be within a few meters of the 16m fire truck turning circle, so that they can easily access it. The water takeoff for our use will be elevated so that we can not access the bottom 10,000l of water without using the CFA fitting eg. the water is reserved for their use. In reality, they will likely use the water from our dam, and just drop their hose in.
Regardless of whether you need to reserve water for fire fighting, it’s a good idea not to draw water from the bottom of the tank to avoid pumping sludge into your water supply. So we would have specified an elevated offtake anyway. With such a large volume below the offtake, we should have less maintenance on desludging – longer time before we have to do it.
We’ve opted for a much larger tank than recommended, because increasing tank size does not add a lot of cost. Using the driest years forecast on tankulator, (and assuming we didnot modify water use for the drought), the storage that gave us the fewest days without water (and no overflow) was around 55,000l, so 65,000l capacity tank.
Indicative pricing for heritage tanks now (from their website):
For around $2k we could double our water storage, so we decided to spend the money. The tank will fill in wet years since we have ample water catchment area, with the house and shed. Once filled, it will give us some buffer volume for dry years.
There are now also recommendations to disconnect your water catchment from your rainwater tank in the event of bushfires in the area – a lot of people had to drain their tanks after the bushfires because the water was contaminated with smoke and ash – tasted terrible. I can use the flush valve (see below) to divert the water away from the tank if we spend another summer blanketed in smoke. Another reason to have a large tank – we can cope for a prolonged period with no water capture.
We remember the millennial drought and how sad my mother was when they had to get a water tanker in with “smelly” town water. And with climate change, we didn’t want to rely too much on historical data. Dry years likely to be drier, wet years wetter…. Running out of water is not fun.
So, in the end, we did ignore the results of the tank size calculator…..
A lot of people recommend two tanks – so if you have a problem with one, you have another to fall back on. It’s probably a good idea, but we’ve not come across people with a problem that affects just one tank, so we opted to save the money and just install one big one. Hopefully we won’t regret it.
I’ve always just drunk unfiltered tank water, but we did fit a first flush diverter to our shed water tank. It was a complete pain – always blocking up and ineffective. So I was interested in what was recommended for filtering/sterilisation and what the best systems were. There are lots of site promoting various systems, and a lot of conflicting advice. The current Australian advice is: https://rainharvesting.com.au/wp-content/uploads/sites/2/2020/06/enhealth-raintank.pdf?msclkid=80fa4542aefa11ecb810a2bbc2e74d18
Unless you’re immuno-compromised, unfiltered, unsterilised tank water is safe for consumption according to the health guidance, as long as the water is captured in a well managed water catchment and tank system. No overhanging trees etc. With our bushfire overlay, we definitely won’t have any trees near by – no leaves etc, no animals living on top of the roof, so we should be in a pretty good position for a healthy tank. They do recommend first flush devices.
For our roof size, to get a decent first flush, we’d need to dump a large volume, and first flush devices don’t dump that much. I looked at rigging up an automatic system with a moisture sensor operating a solenoid valve to dump water until a certain amount of rain had fallen (some garden moisture sensors can do this), but in the end I’ve decided to see how our water quality goes, partly because we have a flush valve and drain built into the tank supply because we are running a wet system.
The piping runs underground to the tank and it is recommended that this is flushed periodically to avoid the stagnant water, so there is a flush valve and drain built into it in a pit, (standard installation for this type of system, I believe) that you should use periodically to flush the system. When it looks like rain after a dry spell, I’ll go out and manually operate it and get a first flush that way. In summer anyway….
One really annoying thing I remember while growing up was getting “wrigglers” in the tank – mosquito larvae. Disgusting seeing them in your drinking water. Fine mesh screening is required into the tank to ensure that you don’t get mosquitoes in there – the health document above has some details on what to install. Definitely going to do this.
I was amused to read that they are still recommending my father’s approach to deal with this – putting some kerosene (or paraffin) into the tank – it spreads across the surface then evaporates or overflows. We’re having a metal tank, with a polymer liner, so we may not be able to do this. There is an approved larvicide. Way, way better not to get them in there in the first place.