Your rainwater harvesting system: Part 2

Installing your tanks

You need to think about the best location for your tank or tanks, tank configuration (if you need multiple tanks), where the water will come from to supply the tank, pump requirements, electricity supply and your particular water-usage requirements.  

Your tank must sit on a level and properly prepared surface called a pad. Once water is in the tank, it’s impossible to move it without emptying the whole tank, so it’s vital to ensure your pad is completely level from the get-go. 

Poor preparation of the pad can result in problems. Creating an uneven pad may lead to a sloping tank, compromising your tank’s capacity and output. Keep in mind that water is heavy, and 1,000 litres weighs 1 tonne. If a large tank is uneven, it becomes heavier on one side and the difference in pressure can result in problems with plumbing and ongoing maintenance. Rainwater tanks are built to be stronger at the bottom to support the pressure exerted by the water, and level water storage is critical to avoid changing the physics and structural integrity of your tank.  

Connecting tanks together

If your tanks are all positioned together on level ground, tanks can be connected with flexible pipes at the bottom. As rainwater flows into the first tank, it will flow through the connecting pipe into the next tank, and this will allow the tanks to fill evenly. Similarly, you will only need one outlet to your pump as the water will always be level. It is important to ensure that isolation valves are installed at each connection point so each tank can be isolated for maintenance or repair without the risk of losing your water.

Dry Pipe Systems

In a dry system, a simple sloping pipe connects the gutter to the tank inlet and water flows downhill from the gutter into the water tank. When the rain stops, the pipe drains into the water tank and with no residual water in the pipe, the system is dry.  

Dry pipe systems are quick and cheap to install, and they require very little maintenance. They are particularly applicable when the water tank is close to the home, and you are happy for the pipework to be visible.  

Wet Pipe Systems

Wet pipe systems are typically used when the water tank is some distance away from the roof and gutters. 

In a wet pipe system, the pipes from the gutters go vertically down the wall and are buried underground before emerging near the water tank as a riser connecting to the tank inlet. Because the underground pipes are below the level of the tank inlets, once the rain stops falling and the flow of water stops, water will remain trapped in the piping system – hence the name ‘wet pipe’  

The main benefit of wet systems is that they may be more aesthetically pleasing than dry systems as pipework is hidden underground and out of sight, and they provide you with the flexibility to position water tanks where you want them.

The key disadvantage is that because water remains trapped in the pipe below the level of the water inlet on the tank, they are particularly vulnerable to mosquito infestations and anaerobic fermentation. This can be prevented by draining water from the pipes after rainfall and your plumber will be able to advise you on methods to drain water-filled pipes and the installation of insect screens. 

Tank fittings

As rainwater is delivered to the water tank, it will pass through a fine strainer on top of the tank that will catch dust, seeds, and organic matter. Your tank should also have a tight-fitting dust cover that prevents wind-blown contaminants entering the tank, but also prevents sunlight entering the water tank and inhibits algae growth and serves as a physical barrier to mosquitoes that lay their eggs in still water.  

A rainwater tank is fitted with an overflow, and a pipe is typically connected to the overflow that directs water a few metres away from the tank. It is important that the discharge-end of the overflow pipe is fitted with an overflow screen that is a barrier to small animals such as frogs and insects.  