__NOTOC__<small-title />[[Image:Underground_tank_icon.png|right|80px]][[Image:Underground_tank.PNG|thumb|right|250px200px|Underground storage of rainwater in Bolivia. Photo: Netherlands Water Project.]]
Where access to groundwater is limited, [[Rainwater harvesting - general | rainwater harvesting]] in underground tanks can be an effective and low-cost alternative. Water stored in the wet season can be used in the dry season and lifted from the tank with a [[rope pump]] or with a [[Deep well pump|deep well pump]], which can elevate water up to 30 m.
Practica foundation is testing various options for low-cost storage and groundwater recharge like the "reed cement tank" and [[Tube recharge|tube recharge]].
===Suitable conditions ===
The area should be suitable for [[Rainwater harvesting - general | rainwater harvesting]]. Depending on the design the capacity of the tank varies from 3,500 to 20,000 liters.
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! width="50%" style="background:#efefef;" | Advantages
! style="background:#ffdeadf0f8ff;" | Disadvantages
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| valign="top" | - Low cost alternative to metal or ferrocement tanks.<br>
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===Resilience to changes in the environment===
====Drought====
For more information on drought: [[Resilient WASH systems in drought-prone areas]]
===Construction, operations and maintenance===
'''General advice on cement''': A common cause of cracks in structures and linings (e.g. in tanks, dams, waterways, wells) is errors in mixing and applying the cement. First of all, it is important that only pure ingredients are used: clean water, clean sand, clean rocks. The materials have to be mixed very thoroughly. Secondly, the amount of water during mixing needs to minimal: the concrete or cement needs to be just workable, on the dry side even, and not fluid. Thirdly, it is essential that during curing the cement or concrete is kept moist at all times, for at least a week. Structures should be covered with plastic, large leaves or other materials during the curing period, and kept wet regularly.
* Basic water quality testing is recommended during the first year, with further testing when water quality is in doubt. E.g. the 'HACH' pillo test, costing about US$ 1 per test. Treatment to manage microbiological quality is also recommended.
===Costs===
* Material costs for [[EMAS Cistern]] tanks consist of sand and four 50 kg bags of cement for each tank.
* Subsurface ferrocement tanks of 60 m3 costs about US$ 1,900.
* Underground tanks can be high and variable in cost per m3 of storage (averages around US$30-40 per m3 of storage, sometimes a lot more depending on various factors). Sub-surface hemispherical tanks made from stone masonry and bricks/cement in Ethiopia cost in the range of 113 - 219 Euro per m3 of storage including all costs such as trucked water for construction and solidity of construction (more solidly built tanks in clay areas cost more). Therefore costs currently limit the replicability of the technology for poorer families and potential to scale things up.
===Field experiences===
* The Brazilian government, with support from organisations such as UNICEF and IRCSA (International Rainwater Catchment Systems Association) started a programme to construct one million rainwater tanks in Brazil. With regard to the concrete block tanks, 40 community water tanks (supporting a few families each) were implemented successfully. Following this success, another 80 concrete block tanks will be built at household level.
* In China, clay has been used to line excavated water cellars (called Shuijiao) in areas where the natural soil (loess) is already fairly impermeable. The lining process is difficult and time-consuming and has been replaced largely by ferrocement or plastic. However, it proves that in some areas it is possible to construct a low-cost tank.
* In Somaliland, similar water cellars were observed that were excavated in impermeable stable soil formations – runoff water entered through a small inlet channel. Other tanks are sealed with a 10cm unreinforced cement lining – it seems that 30m3 is the most economical size as the most volume had that doesn't need reinforcement – such tanks cost US$189 or about US$6.3 per m3 of storage (materials only, presumably).
===Manuals, videos, and links===
* MANUAL: [http://www.enviro-friendly.com/Tankmasta-Underground-Installation.pdf Underground tank installation manual] by Tankmasta explains all the steps needed to build an underground tank.
* VIDEO: [http://emas.blip.tv/ Large collection of EMAS movies on different subjects.]
* [http://www.aprovecho.net/2009/09/constructing-a-ferrocement-tank/ Constructing a Ferrocement Tank]. A large tank reinforced with metal bars. Abel Kloster, 2009.
===Acknowledgements===
* Practical Action, [http://practicalaction.org/uunderground-rainwater-harvesting Underground Rainwater Storage Facilities]. The Schumacher Centre, Bourton on Dunsmore, Rugby, Warwickshire, CV23 9QZ, UK.
* CARE Nederland, Desk Study: [[Resilient WASH systems in drought-prone areas]]. October 2010.
