How The Tanks Work
Rainwater tanks store rainwater run-off from catchment areas like your roof. In most cases, the water from your roof is funneled along your gutters and into downpipes connected to your tank. If you are going to install a rainwater tank, alterations to your guttering may be required. To get the best out of your rainwater tank, it's important to install appropriate screens to stop debris and insects entering the tank.
Water pressure issues and top-up system
If you are connecting your tank to the toilet or washing machine you will need to maintain a minimum level of water in your tank and will therefore need a top-up system. A top-up system will deliver water to your tank through a pipe from the mains water supply. An air-gap is required between this pipe and the rainwater tank. This is to ensure no backflow can go into the drinking water supply from your tank.
Sydney Water, as an example, requires that the flow from your top-up system be limited. If the flow rate is too high, homeowners who are using their tanks for irrigation purposes may think they are using their stored rainwater, when in fact they are using nothing but mains water.
The new BASIX requirements will ensure that there is not overuse of mains water, this is done by restricting the mains water top up speed to 2 litres per minute. With the required 20% minimum top up level in the water tanks, owners will be able to irrigate each day up to 20% of the water tanks volume, before the system will shut down, signaling that mains water is now the major source of water in the tank. Mains water will then trickle top up the tank again to that 20% level, which is in place to enable the household to have water for the toilets and laundry at all times, even in times of low rainfall.
Mains water backup systems
There are two styles of mains water back up for water tanks:
- Trickle top up – which allows the rainwater tanks to be trickle topped up to a maximum capacity of 20% with mains water. The flow rate of this 20% top up is restricted to 2 ltrs/min so that there is no drastic overuse of mains water. The consequence of this is that if the household is irrigating at a rate faster than 2/ltr min for a long period, the tank will run out of water and stop until mains water levels increase and allow the pump to work safely again. A volume of approximately 1000ltrs would need to be used to cause this to occur on the average 5000ltr tank system. One toilet in the house needs to be dual plumbed as if the power is cut, the toilets will not flush.
- Automatic Mains Water Diversion Systems – these systems are very popular as they automatically divert mains water to the household fixtures that are serviced by the tank, when either the tank runs out of rainwater or the power is cut. This is an unrestricted flow of mains water and can lead to over usage of mains water, as the home owner has no idea that the switch has been made. Only purchase systems that have a flashing light or some style of alert for the home owner which tells them when they are on mains water.
This is a device which protects the mains water supply from becoming contaminated by rainwater from the water tanks which mains water is backing up. These devices come in various forms:
- Visible air-gap for trickle top up systems.
- Testable double check valve or RPZD backflow prevention device for automatic mains water diversion systems.
These devices need to be incorporated within the tank systems and in the case of underground tanks another testable device is to be located at the water meter as well.
It is important for us to protect our mains water supply from any risk of contamination through backflow from tanks into our supply. This is why we are fitting new meters incorporating a backflow prevention device to most residential properties.
Sizing of Tanks
Things to consider:
- The number of people living in your home (if you want to use rainwater for toilet flushing).
- The amount of water you currently use.
- The size of your garden.
- Intended use of rainwater (e.g. garden, toilet flushing).
- Available fittings and components to suit your needs.
- The size of the roof catchment area.
- The location of the property (coastal areas are generally wetter than inland areas so a larger tank is required inland to be as effective as a tank on the coast).
- Local council requirements.
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