Rainwater harvesting: tank materials

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By Water Research Commission

Rainwater storage containers are available in several materials, each with pros and cons. There is therefore a need to weight their costs and benefits.

The selection of one of these materials for a rainwater storage tank will largely depend on local availability, as well as on cost, storage requirements, site accessibility and/or engineering specifications.

In recent installations, above-ground tanks are often plastic. Another consideration is the potential for chemicals to leach from the tank into the stored rainwater; however, this is primarily a concern if rainwater must be of very high quality for one or more of the connected fixtures. In most cases, installation and operational specifications can be sought from manufacturers.

Above-ground tanks

Corrugated steel and enclosed metal

corrugated tank WebCorrugated steel tanks are often used because of their availability, price, and aesthetic value. They can range in sizes from a few hundred litres to tens of thousands of litres. The large corrugated steel tanks are usually the support structure for a vinyl bladder on the inside which actually stores the water.


Concrete tanks are durable and strong. They can be installed above ground or below ground in various shapes and forms. Credit: Rain Brothers


Because of their size, these tanks are usually assembled on site. An enclosed metal tank is typically prefabricated and assembled off-site. The tank is sealed on the inside with a potable water approved liner or sealant. They are often more expensive than the corrugated tanks because they need to be shipped as a whole unit.

Concrete

Concrete tanks are durable, strong, and heavy. They can be installed above ground or below ground. There are two common types of concrete storage containers: ferro-concrete and monolithic-pour concrete. Ferro-concrete is a relatively new approach where a special concrete mixture is sprayed on and directlycorrugated tank2 Webapplied on a metal frame. This type of approach is common in developing nations.

Monolithic-pour concrete tanks are either poured in place or prefabricated and assembled on site. An advantage to concrete is that they can raise the pH of the stored water. (rainwater is naturally acidic, so it actually neutralises it).


Corrugated tank capacity can range from just a few, to thousands of litres of storage. Credit: Renew Australia


Wooden

Although redwood tanks were once popular, they have become more expensive and less available. If located in a dry climate, the wood will dry and shrink, allowing water to leak out. To prevent leaking, the tank must be kept full, or lined.

Fibreglass

Fibreglass tanks are very versatile as they can be installed above or below ground. They are rigid and fairly lightweight and can be easily repaired. Because their individual strands are very fine and sharp, you should be careful with parts that have been cut.

Polyethylene and polypropylene (plastic)

Plastic tanks are the most common material used for residential rainwater harvesting (RWH) systems in South Africa. This is because they are lightweight, come in many sizes and colours, and are affordable as a solution. Polyethylene is flexible plastic and polypropylene is a rigid plastic. Both can be translucent or opaque. An opaque, solid colour is better for reducing the chances of algae growth.

Underground tanks

Most tanks are above ground; however, if you do not want your tank to be visible or to take up your precious backyard space, you have the option of burying your water tank underground. They are constructed from a variety of materials: polyethylene, steel, fibreglass or concrete.

Plastic underground rainwater tanks are a type of rainwater tank that is designed to be installed underground while still operating effectively. These tanks are made from UV stabilised polyethylene (food grade) and are specifically designed and manufactured for strength, with factors such as large ribs added to the water tank walls. Modern polyethylene water storage tanks that are designed to be installed underground can be positioned underneath driveways, allowing a car to be driven over the top of the tank.

Concrete underground tanks tend to be more expensive to install than polyethylene water tanks, as more excavation is required. Those tanks are also at little risk of rusting, corroding, or damage from tree roots. Because concrete tanks are stronger, they are ideal for placing under driveways, courtyards, sheds, or other areas where they have to take heavy loads. Concrete underground rainwater tanks are either precast or poured on site to client specifications.

In the next issue, we will look at the design and installation of rainwater storage tanks.