Rainwater harvesting: a step-by-step guide for plumbers

Installing a rainwater harvesting system can help consumers save on water costs and help the environment.

By: Kelly-Ann Prinsloo – writer

South Africa’s water crisis has created a need for rainwater harvesting technology and the savvy plumber should be looking to fill this need

Rainwater harvesting has been practiced for thousands of years and, as South Africa’s water crisis deepens, consumers are looking more and more to the plumbing industry to provide them with rainwater harvesting solutions.

A reliable water supply is a critical part of our daily lives. And lately, failing infrastructure, mismanagement of resources and the current drought have focused our attention on water, our most precious resource. Most homeowners use potable water to satisfy all of their water needs. But in many instances, treated drinking water can be replaced by captured rainwater. For example, harvested rainwater can be used for irrigation, vehicle washing, and flushing toilets. With special treatment and plumbing, it is even possible for harvested rainwater to become the primary water supply for a home or business.

Currently, the plumbing industry is experiencing something of a slump – if that is the case with your business, rainwater harvesting can provide you with a new source of income. Not only does providing this service to end users mean you can add a vital skill to your repertoire, but you’ll be helping the environment too.

A rainwater harvesting system captures storm water runoff, often from a rooftop, and stores the water for later use. By using harvested rainwater for purposes that don’t require treated drinking water, we reduce the demand on municipal water supplies and increase the sustainability of drinking water supplies. You will also help your clients save on water costs. Rainwater harvesting systems do not require any electrical supply and, after the initial payment, there are no ongoing costs, which is a bonus for any homeowner.

The basic requirements
A rainwater harvesting system consists of four main components (Figure 1):

  • The tank stores runoff for later use;
  • The gutter system collects runoff from the rooftop and directs it into the tank; 
  • The overflow pipe allows excess runoff to leave the tank in a controlled manner; and 
  • The outlet pipe, which is sometimes connected to a pump, draws water near the bottom of the tank for use.

It is important that plumbers consider all of these components and how they work together before installing a rainwater harvesting system. The water tank is the primary component, so select and locate it based on the customers’ anticipated water needs.

In the case of a basic rainwater harvesting system, one that will simply be used to water the client’s garden, wash their cars or fill their swimming pools, a tank, gutters, an overflow pipe and an outlet are all that is needed. Once the tank is installed and connected to the gutters, water will run from the gutters into the tank and, when the consumer wants to use it, they will simply open the tap.

However, should a customer want to feed the water from their tank into their house in order to supply the geyser or certain taps (i.e. bathroom taps, kitchen taps, washing machine etc.), then a pump will be needed in order to pressurise the system. In this case, you would need to consult with a pump specialists to determine what size pump is needed. Depending on where the geyser is located, a pump that is able to operate to at least 300kPa is likely to be needed. Also, if the client wants a pump installed, a strainer will also need to be installed so that the water is free of large debris etc. before travelling through the pump to prevent damage.

Choosing the right tank
For most systems, the tank will have to be ordered and shipped directly to the location where it will be installed. Due to the large size and weight of many tanks, delivery charges can be substantial and it is important to make your client aware of this. A tank should be selected based on its material, size, whether it will be installed above- or below-ground, and where it will be located.

Tank materials
Tanks can be constructed from a variety of materials. The most commonly available tanks are made of plastic, fiberglass, or galvanised metal. Plastic tanks are generally less expensive than other materials and don’t require assembly. They can also be moved into place without much difficulty because they are relatively lightweight, and the plastic material can be easily cut or drilled with standard tools to install the necessary valves and fittings. Metal tanks are often adapted from grain bins, and their basic appearance is generally preferred to plastic tanks. Metal tanks are typically assembled from sections of corrugated galvanised metal, and a plastic liner is installed inside the tank. Because a liner is needed, it may be difficult for an inexperienced installer to construct a metal tank.

If the rainwater harvesting system will be used to supply drinking water, special tank materials are required.

Tank colour is also important for plastic tanks. Translucent or light-coloured materials may permit algae growth within the tank. Plastic tanks are relatively easy to paint or decorate, and paint or other coatings can be used to prevent algae growth.

Tank size
The size of the rainwater tank can have the greatest impact on system cost and performance. Several factors must be considered, including contributing rooftop area, rainfall patterns, and anticipated usage.

Determining contributing rooftop area
Examine the location of downpipes that will drain to the cistern to estimate the rooftop area. Note the slope of the gutter, and visualise what area of rooftop will drain to each downpipe. When measuring the rooftop area, it is important to measure the roof’s horizontal projection (as if you were looking straight down on the roof from above), ignoring the pitch of the rooftop. Because the roof’s slope is not a factor in the area measurements, the contributing area can usually be estimated by measuring the area of the roof at the ground or foundation level.

Above ground or below?
Several factors should be considered when deciding whether a rainwater tank will be installed above or below ground. Above-ground tanks are typically easier to install than below-ground because excavation for the tank and plumbing network are not necessary. And below-ground tanks have limited accessibility after they have been installed.

Adrian Beeslaar, technical representative at Marley Pipe Systems, said, “It is important to remember that rainwater harvesting systems are very end-user-specific.” Every client has different needs and it is important to take all those needs into account and to thoroughly research prices, advantages, requirements etc. before agreeing to install a rainwater harvesting system for a client.

But, all things considered, installing rainwater harvesting systems is a relatively simple process and provides plumbers with an opportunity to expand their skills base and client base. "If he’s a qualified, reputable plumber,” Beeslaar added. “He will be able to install this kind of system.”

Plumbing Africa will continue to focus on how you can harness the power of rainwater harvesting to improve your business. If you would like to be involved in an upcoming feature, please contact Kelly-Ann on 011 579 4940.