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)
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
|Partially above the ground||
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:
|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