Chapter 6 - Storage Tank

Storage tanks are the most expensive part of a rainwater harvesting systems, so due consideration must be given to its design and construction. The size of the storage tank depends on the amount of rainfall that can be harvested, the demand, aesthetics and budget. The availability of labour and materials / off-the-shelf products, cost, time, and other external factors are important in deciding the tank material and type. They can be

  1. At or just below a roof slab level
  2. On the ground
  3. Completely or partially below the ground (sump)sDSD

SGF

 

 

 

 

 

 

 

 

 

 

Figure 1: Ground level storage                                                     Figure 2: Intermediate slab level storage


General characteristics of storage tanks

  • All storage tanks must be:
  • Durable and water-tight
  • Built / placed on a strong and stable substrate which can support the tank filled with water
  • Opaque, so that light cannot enter the tank. The presence of light can cause algal growth in the stored water.
  • Have a strong, secure and impervious cover which is kept shut at all times, unless being cleaned. Manholes must be present to allow access for cleaning but these too must be secured properly to ensure that nothing can fall into the tank. The cover and manhole should not allow dust, insects or light to enter the tank. Some RWH projects have reported that small animals have fallen into storage tanks and died, increasing risk of bacteriological contamination. Care should be taken to prevent such occurrences.
  • Have all vents, including the overflow pipe, covered with an insect proof mesh to keep out mosquitoes and other insects as well as stray dirt. Care must be taken to ensure that mosquitoes cannot breed in the storage tank, since they are the source of many diseases.
  • Located as close as possible to demand and supply points, so that plumbing is reduced
  • Situated as high as possible (but atleast a couple of feet below the catchment discharge points) to reduce pumping loads.
  • 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.


When going in for underground sumps, some important factors need to be considered. Firstly 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


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

Positioning of the storage tank - Above the ground or below the ground
To help you decide on whether the storage tank should be above the ground, below the ground, or partially below the ground, the table below compares the advantages and disadvantages of each
Comparison of different storage tank positioning

   Advantages  Disadvantages
Above ground
  •  Tanks can be bought off-the-shelf
  • Easy to install and takes lesser time if bought off-the-shelf
  • Easy to inspect
  • Ground level contaminants cannot enter the tank
  • Pumping systems may not be required and gravity flow can be used for water distribution
  • Generally more expensive
  • Can affect the aesthetics of the building
  • Space taken up by the tank cannot be used for any other purpose
  • It is usually not recommended if the storage volumes are very high
  • Cannot be used to capture rainwater from ground catchments
  • Cover can be easily removed and cases of the cover being damaged during rough weather have been reported
 Below ground
  • Generally cheaper than above ground storage tanks
  • Water cannot leak out through an open tap
  • Does not affect the aesthetics of the building and the space above the tank can be utilized for other purposes
  • Most suited for large volume storage tanks (10000 lts and above)
  • Construction is time consuming
  • Can affect the foundation of the building if constructed very close to the building
  • Pumping system required to take out the water·    Not easy to detect or repair leaks
  • Cannot be drained easily and hence cleaning is a problem
  • More prone to contamination
  • If the tank or manhole is left uncovered, it can be a safety hazard
  • The tank can be damaged by tree roots
  • If not constructed properly, the pressure exerted by the Earth can also cause damage to the tank·    Heavy vehicles cannot be driven over the tank, since the exerted pressures can cause damage
 Partially above the ground
  • Generally cheaper than above ground storage tanks
  • Water cannot leak out through an open tap
  • Affects the aesthetics of the building minimally and portion of the tank that is above the ground can be camouflaged easily and used for other purposes
  • Construction is time consuming
  • Pumping system required to take out the water
  • Not easy to detect or repair leaks
  • Cannot be drained easily and hence cleaning is a problem
  • The tank can be damaged by tree roots
  • If not constructed properly, the pressure exerted by the Earth can also cause damage to the tank

Storage tank materials
Storage tanks are commonly constructed with the following materials:

  • Brickwork with cement plastering
  • Reinforced Cement Concrete (RCC)
  • Ferro cement
  • Plastic or polypropylene


Tanks made of galvanized iron are not advised since they are more prone to rusting or paint peel-off. These would be causes for concern, especially in coastal areas where the durability is further reduced. Recycled oil drums are used for water storage in poor communities but these are also not advised since they are prone to contamination.

The cost of construction of storage tanks can vary from Rs3.50 per litre of storage to Rs 5.00 per litre of storage depending on the material and the size of the tank (rates in Bangalore in 2006).
The advantages and disadvantages of the materials which can be used to make storage tanks are given in the Table below:

Comparison of different storage tank materials
   Brick  RCC  Ferro cement  Plastic
 Durability  Durable  Durable Durable Durable
 Water tightness  Prone to leaks Less resistant than Ferrocement and Plastic  Excellent  Very good but leaks can occur near outlet pipe
 Cost  Cheap  Between Ferrocement and Plastic  Cheap  Expensive
 Above or below ground  Above ground  Above or below ground  Above or below ground (above ground preferred) Above ground
 Installation time and ease of installation  Minimal construction skills required. Takes a little time since curing is required  Construction skills required and long curing time required  Construction skills required and time required is between that of brick and RCC  Easiest to install and takes least time
 Material and labour availability  Easy – local materials and labour  Easy – local materials and labour Easy – local materials and labour  Easy - available in most plumbing hardware stores
 Tensile strength to weight ratio  Low. A thick wall is therefore needed  High  Very high  
 Affected by weather conditions  Not affected  Not affected

 May be affected in dry areas  Not affected
 Shape  Suitable only for cuboid  Cylindrical  Different shapes possible, but cylindrical is best  Specific shapes available

Capacity of the storage tank
The sizing of storage tanks depends on the rainwater yield and demand but in many cases is governed by budget and space limitations. This is more common in areas which have heavy rainfall in some months and short dry spells. In cases where it is not possible to install the recommended size of storage tank, it is advised to collect as much as possible for reuse and recharge the rest.

Source: Rainwater Harvesting  - Trainers’ Manual published by Karnataka Urban Infrastructure Development and Finance Corporation

Chapter 7

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