[[Image:sump storage tank.jpg|thumb|right|200px|A sump tank under construction – Every building in Bangalore has one. Source: [http://rainwaterharvesting.wordpress.com/2013/10/31/on-the-sump-tanks-of-bangalore/ On the sump tanks of Bangalore]]]
[[Image:sump tank 2.jpg|thumb|right|200px|Source: [http://www.rainwaterharvesting.org/Urban/Components.htm rainwaterharvesting.org]]]
===Suitable conditions===
The ground should not be rocky close to the surface, which can hinder the digging and construction process. Secondly the groundwater table in the area should not be very high and should ideally be several metres below the bottom of the tank. Underground sump tanks must not be built very close to underground sewerage chambers or lines trees like the coconut which have strong roots that spread widely existing foundations, especially those that may not be very deep waste water treatment systems.
{| border="1" cellpadding="5" cellspacing="0" align="center"
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! width="50%" style="background:#efefef;" | Advantages
! style="background:#f0f8ff;" | Disadvantages
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| valign="top" | - Generally cheaper than above ground storage tanks <br>
- Water cannot leak out through an open tap <br>
- Does not affect the aesthetics of the building and the space above the tank can be utilized for other purposes <br>
- Most suited for large volume storage tanks (10,000 litres and above)<br>
| valign="top" | - Construction is time consuming<br>
- Can affect the foundation of the building if constructed very close to the building<br>
- Pumping system required to take out the water<br>
- Not easy to detect or repair leaks <br>
- Cannot be drained easily and hence cleaning is a problem<br>
- More prone to contamination<br>
- If the tank or manhole is left uncovered, it can be a safety hazard <br>
- The tank can be damaged by tree roots<br>
- If not constructed properly, the pressure exerted by the Earth can also cause damage to the tank <br>
- Heavy vehicles cannot be driven over the tank, since the exerted pressures can cause damage
|}
===Construction, operation and maintenance===
* The overflow pipe should be just below the tank / sump cover so that there is no ‘dead space’ and the tank can be used to its maximum capacity. The overflow pipe should divert the excess water away from the sump and the foundations of other buildings and structures. This water should be directed to gardens or into the storm water drain and should not create slushy areas. A reverse U bend should be installed at the end of the overflow pipe so that light and other impurities do not enter the tank.
* Water should be extracted from the tank / sump only by means of a tap or pump. Taps or draw off pipes should not be very close to the bottom of the tank, since there is likelihood of sedimentation and the dirt should be allowed to settle and remain in the tank until cleaned. If the dirt enters the water outlet pipe and the plumbing network it can cause blockages.
* Sumps could have level indicators, so that the pumps don’t run dry. Level indicators can be as simple as a dip-stick or can be electronic. Dip-stick systems have the disadvantage of being a conduit for light and impurities to enter the storage tank
===Bangalore example===