Rainwater harvesting: pump it on

By Tanya Olckers

Rainwater harvesting has been getting a lot of attention recently. We look at pumps.

Backup water tank is unloaded. All images supplied by © Plumbing Africa

With water being a scarce commodity in South Africa, and with the country facing a water crisis, more homeowners are looking to collect rainwater to assist them.

It is quite possible that your business may receive more requests for backup water systems. Not only will this be an environmentally friendly solution and assist in navigating South Africa’s current water issues, it provides a use for water that would most likely otherwise be lost.

A rainwater harvesting system is quite straightforward. To recap from our rainwater harvesting articles from January and March 2024, rainwater is collected from a collection area or surface – usually the roof of the house. The water runs into the gutters and then in a system of pipes into the harvesting or back up tank.

Every 2.5 centimetres of rainwater that is aggregated from the collection area is equivalent to around eight or nine litres of water. For this water to be used, it needs to be pumped out of the tank, and for this purpose one clearly needs to have, you guessed it, pumps.

What is a rainwater pump?

A pump is connected to rainwater storage tanks or collection points. They are essential to rainwater harvesting systems as they create pressure and ensure that a desired flow rate is established to meet water demand. They are usually powered by electricity, but there are pumps available that can be operated manually or by solar energy.

“They can be used for residential, public and commercial properties, and also for water management applications,” says Varun Rajasingh, head of sales excellence at Wilo Pumps. “Wilo products are used in
heating, air-conditioning, cooling, mining, industrial and water supply applications as well as for drainage and sewage.”

There are different kinds of pumps.

• Positive displacement pumps function by trapping and is placing fluid. This means that the fluid – in this case, rainwater – is displaced, moves along the pipe and through to its discharge. The rainwater is continually displaced so that it can keep being discharged from the pump.
• Rotodynamic pumps use motion. This motion generates energy in the rainwater, and this causes it  to move along the pipe. These kinds of pumps usually need a motor to turn the device that increases the pressure on the rainwater, or to increase the flow rate using centrifugal force.
• A centrifugal pump is suitable for most rainwater harvesting applications because of its relative simplicity, range of head and flows, as well as its relative low cost. A centrifugal pump consists of a motor, pump casing and impeller. The pump casing holds the impeller and channels water to and from the impeller. Centrifugal pumps are used to transport fluids by the conversion of rotational kinetic energy to the hydrodynamic energy of the fluid flow.
• The on-demand pump makes sure that the water to a tap or irrigation system stays pressurised. They have built-in sensors that can tell when the pressure drops so that the pump can start up. They are on the more high-end side of the scale, but if your client wants something to be on all the time, this is the type of pump to look at.
• A transfer pump is one of those that you plug in when you need it. They do not run constantly, and they have to have water moving through them while they are on so that they don’t overheat. They are more affordable but do require your clients to flick a switch to switch the pump on or off as needed.

Both the on-demand and the transfer pumps can either be submersible or surface pump. This all depends on the design of the pump. Submersibles go into the water storage tank, while surface pumps are usually mounted to something like the water tank on the outside.

The submersibles often come with the added benefit of being able to shut the pump off if the water level
drops too low because they have built-in dry-run level protection. They also stay hidden, aiding
those clients who are looking for something aesthetically pleasing while the external pumps are outside the tank. For those clients who go for external pumps and who still want things to look pretty, the external pump can be hidden in a hole or under a cover so that it won’t stick out.

There are some things to take into consideration before choosing a pump. “Pumps ensure you have the pressure you need all the time,” says Duncan Keil, CEO of UPP. “There are different types of pumps
with different power and installation requirements, which is why it is always best to talk to a pump specialist about your needs.”

Gareth Manson, sales manager: Stewarts & Lloyds points out that rainwater harvesting systems and backup water systems are different. “Rainwater generally will be used only in the garden for watering. So
we tend to stick with our fixed speed SALFLO range for rainwater systems. Backup systems will be run from a tank fed by council water, with the usage inside the house varying. From one tap to multiple taps and showers, we tend to use our VSD range for these installations. This offers the client a huge energy saving and can work with solar installations, without a huge energy spike on start-up of pump.
The pump will increase and decrease in speed depending on the requirement. This allows a 1.1kw pump to operate a tap only using 250w of power.”

Romeo Giannone, also of Stewarts & Lloyds, says that common problems include using under-sized suction pipes to the pumps, and dry-running of the booster pumps. “Always use suction pipes that are one size bigger than the pump suction port size and use adequate protection starter boxes,” he advises.
When choosing a rainwater pump for your harvesting system, it’s essential to consider several factors. Here are key considerations to keep in mind:

1. Flow Rate

This determines how quickly the pump can deliver water. Take a look at the water demands and choose a pump with a flow rate that matches that demand. Compare peak demand and average use to make sure that the pump can handle the water volume.

2. Power and Pressure

The required pressure or Total Dynamic Head TDH) is as important as flow rate. For instance, if you have a below-ground installation and need to pump to the sixth floor of a building, you need enough TDH to push the water up to compensate for the difference in the height. If you don’t do that, the water is not going to get to that part of the building. Talk to your client about what they want to use the water for because different applications need different kinds of water pressure. Irrigation will need something quite different from a building reticulation application, so keep this in mind. At the same time, look at the power source for the pump – whether electricity, manual or solar – and find a pump that would work best with that power source. To get the right kind of pressure to align with a regular residential system, you’ll need a pump that can produce 45-60 psi.

3. Size and Capacity

Take note of the space you have to fit the pump and make sure that your pump actually fits. Remember also to take the pump’s capacity into account when it comes to the maximum amount of water it can handle.

4. Efficiency

Perhaps it goes without saying but look at pumps that have high energy efficiency ratings. This ultimately saves your client costs and will have a positive effect on the rainwater harvesting system’s performance.

5. Durability

Here you will need to look at corrosion-resistant materials like stainless steel. Your pump needs
to be able to handle exposure to rainwater and all kinds of weather conditions. This means fewer maintenance calls, but durability and reliability.

A DAB pump system.

Getting the copper pipes in place.

6. Technology

There are some pumps on the market that come with some cool features like adjustable
speed settings or automations. This is great for customising a pump’s use according to what the
client needs. Automation is a bonus as this lets your client enjoy the peace of mind that the pump is doing what it needs to, and they don’t have to do much except use the water they are collecting.

7. Power

To obtain higher flow rates and pressure, one might need pumps with bigger kilowatts. Typical domestic applications usually only work with available 1Ph 230V which vary from as little as 370 watts up to 2 200 watts and for commercial applications, one typically finds 3Ph 380V which typically use up to 45KWs or greater if necessary.

8. Check for a Warranty

Make sure there is a decent warranty in place on the pumps so that you and your client are protected against any possible defects. After sales and technical support are big pluses if it comes down to trouble shooting. While budget is a consideration for many households, with a rainwater system that will be in place for many years, it may mean spending a little more for a durable and reliable pump.

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