STAGE I: Collection & Carrying

STAGE 2 Filteration

STAGE 3: TAGE

The size/ capacity of the water tank is a critical decision in the design of the rainwater harvesting system. It is determined by various factors including

• Average rainfall in the region and rainfall pattern
• Roof-top area
• Water requirements of the family
• Budget

In rural areas of Tamil Nadu, the cost factor is likely to play the deciding role in determining the tank size. For optimum utilization for the rainwater harvesting system a water tank of 1500 – 2000 liter capacity is suitable.

Keeping in mind the needs of the local economy a masonry tank that employs a local mason is advisable. Another important reason is that the tank top of a masonry tank can be used for various activities like drying food, seating, sleeping etc. The tank shape can also be according to our needs – for example a narrow space can have a vertical tank. This kind of flexibility is absent in Sintex type tanks. It costs more or less the same as a Sintext type tank, but will ensure that the money goes to a local mason and labour instead of going to a material supplier.

Masonry tank can be placed on the ground, partially below the ground or completely below ground level. It should be preferably kept above the ground. The reasons for this are:

• Excavation of the ground for the tank that is partially or completely below ground will be an additional cost and thus increase the cost of the system.
• Keeping it above the ground also allows for quick detection of leaks and their repair.
• It is easier to clean the tank and drain out the water.

Building a ground level water tank :

• The place where the tank is going to be placed is leveled and the ground is well compacted
• Lay a 4" thick 1:4:8 bed concrete to the make the bottom surface of the tank.
• A single brick wall is enough for a water tank that is upto 3 feet high.
• If the tank is higher than 3 feet, the wall should be two-brick thick.
• The corners of the tank should be strengthened.
• The joints should be overlapped properly.
• The interal surface is plastered with 1:3 cement-mortar. This plaster is then properly finished with a rich cement slurry. The external surface above ground is plastered with 1:6 cement-mortar.
• During construction, curing should be carried out regularly.
• Provide a 2" thick band of PCC 1:2:4 on top of the brick wall. The tank needs to be covered with a RCC slab or stone-slabs.
• Once the tank is ready it needs to be filled with water and left for a week to test it for any leaks. Provide access or an option of someone going inside for cleaning purpose.

A molder polypropylene tank (popularly known as Sintex tanks) can also be used. It should be placed on a raised even platform of 6" height made with PCC 1:4:8.

 
Important questions on the Individual Tank based Rainwater Harvesting System

1. Is the water we collect from rain safe?
   Rainwater is a pure form of water, free of any biological or bacterial contamination. However when it rains some suspended impurities in the atmosphere get dissolved in the rain. The roof and gutters have accumulated organic and inorganic matter like leaves, dust, sand, mud etc. They will also have insects etc. As the rain hits the roof most of these impurities dissolve into the rainwater or are carried along. That is why a filtration process is necessary. The charcoal-sand-gravel filter or even cotton cloth helps filter almost all impurities. Care has to be taken to keep the roof and the gutters clean. This will reduce the extent of impurities in the first place. The storage tank should also be kept clean and properly covered at all times. This is important in keeping the water safe and clean.
2. Can we use rainwater for all the daily consumption ?
   The water collected from rain is like any other water so it can be used for all the activities. However we should refrain from using this water for cooking and drinking unless it has been boiled and filtered.
3. Will the Individual Tank based Rainwater Harvesting System provide water needs throughout the year ?
   No. The number of days that this system will serve an individual household depends on various factors like rainfall pattern, the storage capacity of the tank and the area of the roof, household needs etc. The water collected usually is sufficient for a period of 4-6 months in a year.
4. How much does it cost to build the RWH system ?
   We have provided the sample break-up of items for the estimate of the RWH System at domestic level. Water-tank is the most expensive ‘factor’ in the construction of the RWH system. The estimate will vary depending on the cost of the water tank. A rough estimate is for a system with a tank capacity of 1500 litres, it will cost roughly Rs.8000.
5. What all maintenance does this RWH System require ?
   1. Check the gutter system every year. Clean it of any debris, sand or mud that has accumulated. In case you notice that there is blockage then a bucket of water thrown with some force down the pipes could help clear it.
   2. Filter should be cleaned every year, especially before and after the rains. Remove the filtration medium from the filter, wash the medium thoroughly, put the material in sunlight for a few days and refill the filter. Water tank should be cleaned twice every year.
   3. The roof should be cleaned on a regular basis especially before the monsoon.

Catch me .. you can!   I read this in www.rainwaterharvesting.org   Also see: