There are a wide variety of cistern materials to choose from. Because they are often smaller in size, residential systems offer more options, but not all will scale to tens of thousands of gallons in a cost-effective manner. This article focuses on commercial and institutional scale projects where storage requirements are thousands of gallons and larger.
1.Fiberglass tanksare a common choice for rainwater harvesting (RWH) tanks because they offer high pressure ratings and resist corrosion. They are mostly used below ground and can be sized up to 60,000 gallons and 12 feet in diameter. While long lasting, they are often not the most economical choice and require special care during handling and installation. Because they are built as continuous tanks, the large sizes require over-sized shipping loads that increase overall costs. Multiple tanks are required for large systems.
2.Polyethylene tankscan be used for underground cisterns and can be a very affordable choice for small systems less than 3,500 gallons. They are lightweight, easy to transport, and easy to handle at the job site. They may be difficult to implement on many sites because they require significant cover, but are limited in depth, which means there is a narrow range where the inlet pipe can be located. Many tanks must be connected together to create even medium size systems, which can offset cost savings.
3.Steel Reinforced Polyethylene (SRPE) cisternsare newer to the market and combine the longevity of polyethylene and the strength of steel in an efficient and economical package. These tanks resist corrosion and can last longer than 75 years. Sections are manufactured as long as 50 feet and multiple sections can be combined and fused together in the field to create one large, leak-proof cistern sealed to a 15 psi rating. SRPE is easily patched in the field if damaged and can handle burial as deep as 30 feet. For cisterns larger than 20,000 to 30,000 gallons, they can also be the lowest cost option.
4.Plastic crateswere once used solely for infiltration, but with a waterproof liner, they are also an entry level cistern option. The liner is installed prior to the crates. Because liners can be prone to damage and installation errors, they often have a much shorter lifespan than other cistern options. They are an economical solution for sites requiring large storage volumes because they do not require heavy equipment for installation. More economical crates are typically not strong enough for more than 6 feet of cover, so increased cover will increase cost. There is no maintenance access so the design life for these systems could be less than 20 years.
5.Concrete structurescan also be used as cisterns. They can be precast for faster installation or cast-in-place to suit specific site constraints. To achieve a watertight system, special care must be used to seal joints, and the engineer should require a coating appropriate for re-use applications because people and the environment will come in contact with the water. Concrete can develop leaks over time from cracks, and in most cases will need to be drained to seal the leak. For small to medium size systems, concrete can be a more expensive option. But for very large systems, concrete structures may offer a better economic value and are worth consideration where the cistern requires a high loading capacity.
6.Fabricated steel tanksare another common option to store fluids and can be extremely strong, with a design life of 50 years or more in the right conditions. They can be painted or coated to increase their life, but can be very expensive and also limited in size.
7.Waterproof corrugated metal cisternsare another newer option for water storage. Thin gage steel with an aluminum alloy coating offers a cost-effective system and can have a design life greater than 75 years in many conditions. These lightweight tanks can be built in diameters up to 12 feet and up to 50 feet long. Because these tanks are newer on the market, engineers should require testing documentation from a reputable supplier before specification. For small and medium size systems, these are the most economical option in most situations.
