Storage tanks are the decisive component of the rainwater harvesting system. There are more options than commonly thought!

Many reservoir types are suitable for rainwater storage

• Surface tanks made of ferrocement, bricks/blocks, reinforced concrete, metal, plastic, fibreglass and wood;
• Sub-surface tanks made using ferrocement, concrete, brick and traditional clay linings;
• Communal rock catchment dams, sub-surface dams built of soil, sand river dams, earthdams and hafirs.
  

Essential requirements for an effective rainwater storage tank

• A functional and watertight design with a solid, secure cover
• A screened inlet, overflow pipe and a covered manhole
• An extraction system which protects water quality, and a soakaway
• A maximum height of 2m (unless additional reinforcement is used).

Cylindrical or spherical tanks, e.g. the Thai jar, are stronger and more cost-effective, using less material than square (cuboid) tanks. Construction of the 2 cbm wire-reinforced cement ‘Thai Jar’ can be achieved using any of three types of formwork: a hessian sack, a clay brick mould or a steel frame with bamboo mats.

Care should be taken when siting tanks to avoid risks from pollution sources or damage by erosion. Most common designs, if properly constructed with good-quality materials and good workmanship, are effective.

A solid foundation is essential for surface rainwater tanks and this can also double up as the tank floor. A 13 cm reinforced-concrete slab cast in two layers on the same day is normally sufficient.

Tank walls can be constructed in several ways, including using bricks, blocks or concrete poured in situ, and ferrocement.

The use of organic ‘reinforcement’ such as bamboo, sisal fibres or sticks (in basketwork) as a substitute for steel reinforcement and wire is not recommended for the construction of water tanks.