The unequal distribution of resources in different places around the world has prompted measures to redistribute scarce resources in a fairer and more equitable manner. Usually this means transporting the resources from place to place. Seasonal or periodic scarcity has led to the creation of specially designed storage facilities to hold scarce resources for later use. Water is a perfect example of this. Above ground and underground storage tanks are the two main types of water storage which play a major role in the distribution and storage of harvested rainwater. Water is stored in tanks during periods when water is in abundance and pumped out to where it is needed when water is scarce. Underground tanks are preferred for several reasons. While they may initially be more expensive than the various kinds of above ground water storage tanks, underground tanks save space, particularly in urban areas where land is at a premium. They also provide a more aesthetically pleasing solution to water storage, as they are buried underground and out of sight. Underground tanks are especially suited to submersible pump systems which reduce unwanted noise especially for densely populated urban areas or situations where noise in undesirable such in hospitals or in certain commercial applications. Water from underground storage tanks is always cool. There are many factors to be taken into consideration when designing and installing an underground tank and these factors vary for different applications. Excavation costs, in rock for example, may prohibit the use of deep profile concrete tanks and so the low profile heavy duty polyethylene tanks may be ideal in this situation. Low profile poly tanks can be either a rectangular prism or cube, or toroid in shape (“bagel “or “donut “shaped), or may be constructed of several barrel shapes joined together. Concrete underground tanks are generally cylindrical or are a rectangular prism in shape. All underground tanks are designed with a riser or turret which may be of different heights, depending on how far the tank is buried underground. The location of the tank is another factor which should be taken into consideration. Underground tank installers need to take note of ground water and the resulting hydrostatic pressure and compensate for this by “ballasting” or filling the tank with water as soon as it has been installed. Obviously the excavation should be in an area where there are no underground pipes or electricity or other cables to damage. Most underground tanks are designed to be installed under a garden or lawn which is subject only to foot traffic. If the tank is to be located under vehicle traffic, a special trafficable or heavy duty lid needs to be fitted to the tank. In addition, the tank should be installed under a concrete slab so that the tank is protected from damaging vibrations or pressure loads. Underground tanks poly should be constructed from food grade polyethylene (Australian Standards AS40). Warranties for these tanks are generally 10-15 years, while concrete tank warranties are from 20-30 years although in both cases, the tanks may have more years of useful service. Concrete tanks by the nature of the material, do not affect taste or water quality. Underground concrete tanks may be used for rainwater storage, in grey water management systems or as septic tanks.
Underground tank designsAlan Stringfellow