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Installing a Rainwater Tank - Part 4 Rainwater Supply

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From the tank to the home: pumps, controllers, top-up systems, backflow prevention and water treatment.
For: Plumbers, apprentices, householders

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John Caley

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John Caley B.E. (mechanical) M.E.S. (environmental) MIEAust CPEng

John Caley is the Principal of Ecological Design, a consultancy providing sustainable design services to building designers, developers and individuals. He trained as an engineer but has also worked in bush regeneration, landscaping and installing rainwater harvesting systems.

John has designed rainwater and greywater systems for many developments from single houses to subdivisions.

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This section focuses on rainwater supply ‐ pumps, filters and backup systems.

This rainwater system runs on gravity. No pump is required. OK for garden taps and filling toiletcisterns.

Pumps

But for most applications you?ll need a pump. The type of pump depends on the number and typeof fixtures and appliances it will be supplying. The main performance criterion for pumps is theflow rate, in litres per minute, at a specified pressure. This pump can deliver a maximum of 20litres per minute at 250 kilopascals ? enough for one or two outlets.

Pump controllers have different mechanisms for turning on and for regulating pressure.

The simplest is a pressure switch that senses a drop in pressure caused by opening a tap andturns the pump on. However, if two or three taps are opened, too much pressure may be lost,but it?s OK for garden irrigation and toilet flushing.

A variable speed pump maintains even pressure at different flow rates by varying pump speed.

Some applications cause the pump to turn on and off at short intervals. For example, 4 litres toflush a toilet; and the start/stop rinse cycle in a washing machine. This can waste energy, and putsadded strain on your pump.

Pressure vessels, or pressure accumulator tanks, store pressurised water, reducing the numberof times the pump needs to turn on.

Your pump supplier should advise you on the right pump and controller for your application.

Pump, installation

The outlet to the pump should be here and prevent the tank level from dropping below here.Pumps are better at pushing water than pulling it, so mount your pump as close as possible tothe tank ? if possible, lower than the tank outlet. Submersible pumps can help achieve this.Avoidhigh spots on the suction line from the tank to the pump which can cause airlocks and preventthe pump operating properly.

If you need to install the pump above the tank outlet level, it can be done but make sure youselect a pump with adequate ?suction head? and fit a foot valve to stop the pump losing prime.

Make sure the pump is protected from the weather and minimise noise and vibration.

Install stop valves and unions on either side of the pump so that the pump can be easily removedwithout needing to drain the rainwater tank or the household plumbing.

Main‐top up systems

To ensure there's always water available to fixtures supplied by rainwater, there are two options:

A top‐up system uses mains water to maintain a minimum level in the tank. They trickle mainswater into the tank when the water drops below a certain level. Alternatively, an interconnectdevice senses when tank water is running low and automatically switches the water supply fromthe tank to the mains. When the next rainfall recharges the tank, supply is switched back torainwater.

Interconnect devices can disguise the fact that the household is running on the mains water, so atank level gauge is recommended to show the householder when the tank is empty. In dryperiods interconnect devices can use less energy as they supply mains water without using thepump.

Backflow prevention

Whatever back‐up system is used, the mains water supply must be protected against backflow ofrainwater into the mains. Top‐ups must be isolated from tank water. There must be a visible gapbetween the mains water top‐up and the top of the tank. Interconnect devices must have dualcheck valves integrated or on the mains inlet. Devices which meet the Australian TechnicalSpecification should have this integrated as part of this device.

In‐ground tanks have a higher chance of contamination from surface run‐off, so if connected tohousehold plumbing, the mains water supply must be fitted with a testable backflow preventiondevice. Many water authorities fit new meters to properties where rainwater and the mainssupply are interconnected and they must be notified of a rainwater connection on the property.

Water treatment

Rainwater from a well maintained well designed system should not need further treatment fornon‐potable uses. However, particles, including the swarf from drilling a hole for the float switchand sediment, can make rainwater appear a bit cloudy and can damage appliances ? such asinterconnect devices, water heaters and washing machines. 20 Micron filters can be used toremove these particles. These should be installed with suitable ball valves to allow easy changingof cartridges. Health authorities recommend that mains water, if available, is used for drinkingand food preparation. Drinking water systems generally include multi‐stage treatment.If tankwater is used for human consumption expert assistance should be sought to ensure you'veaddressed the possible risks.

Your last job is to make sure the occupants understand the importance of maintaining therainwater system by regularly inspecting and cleaning gutters, screens and filters and ensuringpumps, valves and switches are in good working order.

Providing the householders with a maintenance checklist helps get the message across. For moreinformation go to the Trade Secrets website.

 

      


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