__NOTOC__[[Image:Underground_tank_icon.png|right|80px]][[Image:Underground_tank.PNG|thumb|right|300px200px|Underground storage of rainwater in Bolivia . Photo: Netherlands Water Project.]]Where access to groundwater is limited, [[Rainwater harvestingHarvesting |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.
These tanks can be constructed of concrete blocksor with other materials. The tanks are partly (1.5 m) built below the soil surface. These tanks can store up to 20 ,000 liters of water, used for domestic use, and crop cultivation.
With regard to the concrete block tanksWater extraction can be done with [[Handpumps|handpumps]] or [[Small and efficient motor pumps|small and efficient motor pumps]]. A [[Rope and bucket|rope and bucket]] can be used but risk of contamination increases. In such a case, 40 community household water tanks (supporting a few families each) were implemented successfully. Following this success, another 80 concrete block tanks will treatment should be built at household leveladvocated.
Practica foundation is testing various options for low-cost storage and groundwater recharge like the "Reed reed cement tank" and the "[[Tube recharge"|tube recharge]].
===Suitable conditions ===The area should be suitable for [[Rainwater Harvesting | rainwater harvesting]]. Depending on the design the capacity of the tank varies from 3 ,500 to 20 ,000 liters.
{{procontable | proborder="1" cellpadding="5" cellspacing="0" align="center"|-! width="50%" style="background:#efefef;" | Advantages! style="background:#f0f8ff;" | Disadvantages|-| valign="top" | - Low cost alternative to metal or ferrocement tanks.<br>
- Good for areas where ground would otherwise be permeable.<br>
- Unaffected by freezing weather. <br>
- Rainwater is generally much lower in salinity than other water. This is good for plants in the garden and fields.<br>
- Using rainwater tanks regularly may reduce storm water run-off from your field. This may reduce flooding, erosion and environmental damage to streams.<br>
- They work well when privately owned and maintained.<br>
| convalign= "top" | - Sub-surface tanks often cannot hold enough water for whole dry season. Making bigger berkeds is possible but more difficult and cost is high (unaffordable tank is not replicable on larger scale).<br>
- Flotation of the tank may occur in areas with a high groundwater table.<br>
- Heavy vehicles driving near to tank can cause damage.<br>
- When built in a remote area, construction is difficult due to lack of water and large distances to transport materials.<br>
- Water quality is likely to be unacceptable for direct consumption, treatment recommended.
|}} <br>===Resilience to changes in the environment=======Drought==== '''Effects of drought''': Badly made concrete and cracked linings (e.g. in tanks, dams, waterways, wells, and other structures). <br>'''Underlying causes of effects''': Less water used for curing; Impure water used for mixing. <br>'''To increase resiliency of WASH system''': Ensure adequate mixing, ratios, purity of ingredients; Minimize water content in mixture; Ensure adequate curing.
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. '''Specific advice''':There are many kinds of underground tanks built, using different materials and different sizes. Below are instructions on how to build a tank as low cost as possible, however cement tanks and prefabricated plastic tanks can be built or bought depending on community needs and suitable environment. ====Design the tank====
Consider five main factors when designing your underground tank.<br>
'''1. Seepage'''<br>
This is related to the above factor but ensure that the crops are hardened to withstand some stress during the dry spell. Determine how much area you would want to save in case of a very long dry spell lasting more than three weeks.<br>
'''5. Construction costs'''<br>
To save costs use as many available local materials as possible as shown below. Work in groups to share labour, as this is a labour intensive system. It takes approximately fours for four to ten people to construct a 10,000 litre tank.
====Materials required====
* To build the tank below: 1 rammer locally made from dry wood, Ant-hill soil (termitaria), water, 5 x 1,5-2m long dry poles, Pick, shovel (depending on labour available), 10 farm/earth bricks, Tape measure or ruler, string 2m, knife/hacksaw/small axe, 50 wooden pegs, old Hessian or plastic bags. 25kg cement.* Care needs to be taken with cheap linings – in some areas with swelling clay and differential settlement, linings can easily crack, as has often been observed in some areas. The areas where cheap linings might work therefore may be site-specific, and depend on the clay content of the soil. For plastic linings, experience from India shows that these can be punctured by rodents, crabs or insects if there is no rodent/insect-proof layer before the plastic.
====Build the tank====
[[Image:TankLiningOptions.jpg|thumb|none|500px|Recommended options for the tank lining when plastering the walls.]][[Image:RootZoneDepth.jpg|thumb|right|200px| Preparing to ram the bottom of the underground tank <br> ©Practical Action/Douglas Gumbo.]][[Image:AddTankLining.jpg|thumb|right|200px| Tank lining / Plastering <br> ©Practical Action/Douglas Gumbo.]]# Along any dug dead level contour (which is your collection area) choose where you can construct your storage tank. All If you would like to consider nearby crops, the storage tanks should be near all soils that allow rapid percolation the closer should be the storage tanks. This is because your soil quickly loses water and that's where your plants are likely to show signs of moisture stress during the dry spell.
# Measure along the channel the length of the tank you want and mark using pegs. Dig the marked area up to a further depth of 500cm or more. The deeper it is the more water will be harvested.
# Mark with a shovel the root zone depth (if considering crops) within your tank.
# Place termitaria-cement mixture at the base of the tank and sprinkle water to facilitate easy ramming.
# Using homemade rammer, start ramming up to a maximum depth of 2-5cm. Sprinkle whenever required.
# Mix termitaria with water to make mortar and plaster up to the marked root zone level of your deep-rooted crops.
[[File:TankLiningOptions====Add a roof to the tank====# Using earth bricks mark the inlet and outlets of your tank from both ends of your tank.jpg | Recommended options # Lay your wooden poles across your tank spaced at 2m. Ensure that the poles are not totally fixed for ease removal during maintenance usually during the dry season.# Decide where you can place two openings at the top for collecting and checking water inside your tank lining when plastering .# Lay old plastic or hessian bags onto the wallspoles. # Use same mortar as referred above to plaster the roof of your tank.]]
====Maintenance====
* Regularly check the tank to ensure there are no unscreened or damaged openings that allow insects, rodents or animals to get into the tank. Keep them clean and free of leaves and debris. * Ensure the tank lid is tight.* Check the tank every two years for sludge and have the tank cleaned if there is a thick layer of sludge on the bottom of the tank.* Check gutters and strainers devices at least twice before and after the rainy season. Removal of debris and overhanging vegetation from gutters and the roof (preventing stagnation of water in the gutters) is required. Physical inspection and repairing cracks with cement is also needed.
* 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.spacepac.com.au/Brochures/WaterTanks/TankMasta/Underground%20Tanks-Installation_8pg_np.pdf Underground tank installation manual] by Tankmasta explains all the steps needed to build an underground tank.* VIDEO: [https://vimeo.com/emas Large collection of EMAS movies on different subjects.]* [http://www.rainwaterharvesting.org/Rural/traditional2.htm Various tanks and irrigation channels in India.] RainwaterHarvesting.org.*[http://www.rainfoundation.org Underground tanks] as part of a project in Burkina Faso.* [http://www.lenntech.com/small-community-water-supplies.htm#ixzz2TsbJyNk6 Water supply in small communities], by Lara Fabrizi. Includes how best to manage the planning, construction, and operation and maintenance of water projects or tanks.* [http://www.aprovecho.net/2009/09/constructing-a-ferrocement-tank/ Constructing a Ferrocement Tank]. A large tank reinforced with metal bars. Abel Kloster, 2009.
==Reference manuals, videos, and links=Acknowledgements===This * Practical Action, [http://wwwpracticalaction.enviro-friendly.comorg/Tankmastauunderground-Undergroundrainwater-Installation.pdf harvesting Underground tank installation ManualRainwater Storage Facilities] by Tankmasta explains all the steps needed to build an underground tank.* Video: [http://emas.blip.tv/ Large collection of EMAS movies The Schumacher Centre, Bourton on different subjects.]* General [http://www.rainwaterharvesting.org www.rainwaterharvesting.org ] [http://www.unep.or.jp www.unep.or.jp ] [http://www.rainfoundation.org www.rainfoundationDunsmore, Rugby, Warwickshire, CV23 9QZ, UK.org ]* Recharge [http://www.practicafoundation.nl www.practicafoundation.nl ]* Storage [http://www.emas-international.de www.emas-international.de]
==Acknowledgements==* CARE Nederland, Desk Study: [[Resilient WASH systems in drought-prone areas]]. October 2010.