Chapter 6 - Storage Tank

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Updated on:

21 Dec 2008, 2:03 pm

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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

At or just below a roof slab level
On the ground
Completely or partially below the ground (sump)

DFASD

SFDG

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

Schools

Chapter 6 - Storage Tank

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