Rainwater harvesting tanks can be one of the most expensive components of a rainwater harvesting system, but it is probably the most important aspect of a successful installation. It is very important that all the rainwater is recovered, filtered and then is diverted to a central storage tank. Having many small tanks all over the premises defeats the objective of using rainwater effectively from a central point. A key consideration in the design of the system is to ensure that the water flowing into the tank is a clean as possible, it needs to be free of debris and silt before entering the tank as this can affect the lifespan and maintenance requirements for the storage tanks.  There are a number of major factors to consider when choosing the correct tank for the application.

Above or below ground

Making this decision is more than just about aesthetics, it is also about costs, lifespan and water quality. Above ground rainwater storage tanks are 5 to 10 times cheaper than underground rainwater tanks. It is easier and safer to install.  For underground tank installations you need to have an understanding of the subsoil conditions and water table as this might limit the depth of installation. However underground tanks store rainwater better because it is cooler and darker underground with less temperature fluctuations, resulting in improved water quality. Above ground tanks are exposed to the elements resulting in a shorter lifespan than below ground tanks. External environmental factors like temperature and sunlight results in water temperature fluctuations which could affect the water quality.  

Tanks material

Tank material must be solid to prevent any ingress of light into the tank. Ingress of light will allow algae to grow inside the tank

Polyethylene

Polyethylene (plastic) tanks are the most common and affordable tanks available in many sizes and colors. You also get reinforced polyethylene tanks that can be installed underground. Polyethylene tanks don't leak easily and they require little maintenance.  The disadvantage of above ground polyethylene tanks is that the material is UV degradable and after extended periods of exposure to the sun, the tank will need replacement. It can be overcome by covering the tanks or painting them. If you are going to use the tank for drinking water, make sure it is made of food-grade plastic. 

12500l underground polyethelene tank

Fiberglass tanks

They are not freely available in South Africa and are more expensive than plastic tanks. The installation platform construction is critical as a fibreglass tank cannot tolerate any soil movement.

Internal membrane tanks  

These tanks have an external frame with an internal membrane. They are very cost effective and popular in large sizes. Water does not come into contact with the galvanised steel frame. However, when using these tanks, you must ensure water supply is of best possible quality. If not and the tank need to be cleaned, there is always a risk of damaging the internal lining membrane which can be very costly to replace.

Inside of internal membrane tank One million liters of rainwater storage in membrane tanks

 Galvanized steel tanks

They are commercially available but must be constructed above ground, on site. As a result, they can be manufactured to store more rainwater than the biggest plastic tanks on the market. Generally, they have low flat profiles. The disadvantage is that these tanks are prone to corrosion. If they are being used for potable water the tank must be lined with a membrane.          

Concrete tanks

Concrete tanks can be installed above or below ground. They are the most expensive tanks but they are durable, immovable, load bearing and probably provide the best storage conditions for rainwater. Concrete neutralizes the slight acidity that you find in rainwater and thus improves water quality while it is in storage. The disadvantage is that concrete is prone to cracking and leaking. Soil considerations and preparation for installation is more important to prevent cracking. For this you need the advice of a civil and Geo-technical engineer. After installation the concrete must be waterproofed to ensure it does not leak. There is no bigger frustration than a leaking underground water tank!

Underground concrete tank under construction

Pre-manufactured concrete tanks are also available on the market. They are modular and can be installed underground in connected modules to give the required water volume. The benefits of these tanks are that they can be installed very quickly and if time in your project is of essence, these water tanks should be considered.

Pre-manufactured underground concrete rainwater storage tanks

There are a few other materials that can be considered like wood and flat rubber bladder tanks but they are not freely available in the South African market

Pillow or bladder water storage tank

Location

Rainwater tanks are generally gravity fed with the rainwater from the collection surface, thus they must be located below the harvesting surface area.  If however this is not possible a small intermediate storage tank with a pump can be used to collect the water and then pump it at high volume to the rainwater storage tanks.

Minimize exposure of above ground tanks to direct sunlight by covering them or installing them under a roof or trees. It prevents excessive water temperature fluctuations and extends the life of the tank.

Rainwater supply pipes from harvesting surface

For below ground pipes ensure effective installation and protection against heavy vehicles. Above ground pipes must be well supported. When filled with water they will buckle and break if not supported. Make sure all the plastic pipes used have UV protection. It is also a good idea to paint them to protect them from UV degradation

Overflow pipe installation

If designed correctly rainwater tanks should overflow 2-3 time per year. Ensure that the overflow is connected to a storm water drain or soak pit. An uncontrolled overflow will undermine the tank foundations and structure which will ultimately damage your rainwater tank.

Back flow and ingress protection

All tanks should be protected against back flow or ingress of foreign material or animals. Underground tanks should have protection against back flow from storm water systems. During periods of excessive rains and floods the storm water system become flooded and forces dirty water into the rainwater tank. It is critical that tanks are designed in such a manner that it cannot happen and that they overflow effectively.

Above ground tanks should be protected against of insects, frogs, birds and mammals. Ensure the tanks can seal effectively and are well protected. All overflow systems must have a protection mechanism that prevents access to the tanks via the overflow pipe.

Multisyphon overflow protects foreign matter ingress into rainwater tank

Venting

All rainwater tanks require a venting system. It fulfills two functions. As water is introduced into the tanks the air needs to escape. Secondly fresh are needs to enter the storage system to assist with the oxygenation of the water in order to improve water quality.

Tank drainage

Every good tank installation required a drainage point. You never know when you might need to empty the tanks for maintenance purposes and there is no bigger frustration than NOT being able to easily drain a rainwater tank.  

Pump location

It is recommended that you install a pump with a rainwater tank installation. Using gravity to supply water from a tank is not always feasible. Example - If the tank is situated 2m above where the water is required and the tank is filled with 2 m of water, the discharge pressure is only 40KPa. Normal household water pressure ranges between 250KPa and 400KPa. It can thus be a very slow process to drain the water from the tank if you just have a tap connected to it.

Pump location relative to the tank is critical. You should be able to drain the tank easily, thus as the level drop the pumps should still be able to supply the system to the lowest possible level in the tank.