Roof Top Rain Water Harvesting

Roof Top only refers to where the water is collected from.  Roof-tops are excellent rain-catchers. After the initial run-off when the roof becomes clean and the water harvestable, considerable amount of water can be caught from the roof tops.Actually the Harvesting cycle involves

There are three basic forms of RTRWH systems:

a. Individual tank based system where the water is stored into brick/stone masonry tank or pre-cast ferro cement tanks or Polypropylene tank (popularly known as Sintex tanks)

b. Surface Storage systems like ponds, tanks, etc.

c. Underground storage systems: mainly charging the existing aquifers through directing the water into hand-pumps, Dug wells, Pits and / or Trenches

The first two stages of the harvesting cycles namely the Collection and the Filtration is common.

 
STAGE 1 -Collection & Carrying

All house roofs whether flat or sloping play the role of the harvester. The quantity of water that can be harvested depends on the total area and on the roofing material.

The most important thing to keep in mind is that the roof should be cleaned of all leaves, dust or other material before the monsoons. The roof should be strictly free of any dangerous materials like chemicals/ pesticides/ fertilizers.

A typical flat-slab house, constructed post-Tsunami is 350 sq.ft built-up area. The roof of such a house would typically have one rainwater outlet.

• Replace existing water-outlet with 4" diameter L-junction, either PVC or Cement.
• The joint around is sealed with cement slurry.
• Check that the water does not stagnate in from of the water outlet. If so redo the surface so as to ensure that water flows out well.

STAGE II - Filteration

Rainwater is a pure form of water, free of any biological & 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 in the form of leaves, dust, sand, mud etc. As the rain hits the roof all these impurities dissolve into the rainwater or are carried along. These impurities can affect the quality of the water collected. To prevent any form of contamination of the water being harvested, these impurities need to be filtered out.

This filtering is carried out in two stages:

1 First-Rain separator:The first rain-separator is a built-in plumbing system that separates the first few minutes of the rain.

2. Filter:  After the first rain separator, the rain water will still carry suspended impurities. These need to be filtered out. This can be done with a simple sand – stone – charcoal filter. Even a clean folded cotton cloth placed in a sieve acts as an effective filter.

First Rain Separator
Most of the dust and organic matter deposited on the roof dissolves and flows within the first few minutes of the rain. This water needs to be separated in order to prevent contamination of water being harvested.

The first rain-separator is a built-in plumbing-system that separates the first few minutes of the rain without any major need of supervision.
A 4" diameter PVC junction pipe is fixed into the lower end of the L-junction that comes from the roof.

A 4" down-pipe is fixed to the lower end of the T-junction. It usually will go upto 30 cm (1 foot) above the ground level. The down-pipe is then provided with an L-junction. This L-Junction has an end cap. This contraption from T-junction below works as the first-rain-separator.

Downpipe: To the lower outlet of the T-Junction we have fixed the first rain separator described above. Fix a 4" L-junction to the other upper outlet. Fix a 4" downpipe to the L-junction that goes to the filter described below.

How it works:
As the rainwater from roof flows down it first fills up the down pipe (described above as the first rain separator). The quantity of water stored in the downpipe equals the first few minutes of rain and will carry with it most impurities. As the downpipe fills up the water starts flowing out of the T-junction (from its upper outlet) into the other downtake pipe which takes the water to the filter.

After every rain, it is advised that the end-cap from the bottom is released and water collected in this empty portion of pipe is flushed out. So that it is ready to store the first few minutes of the rain for next time.

Filter
The water that comes for filtration after the filling up of the first rain separator would still have suspended and dissolved impurities. These need to be filtered out. These filters can be of different types.
We can rely on a simple stone-sand-charcoal filter. It is easy to construct and maintain.

1. Take any 20 litre container or plastic drum or a plastic bucket.
2. Make an opening in the drum cover so that a 4" diameter PVC pipe can enter.
3. On one side of this drum/ bucket (or on the bottom side) make an opening so that a 4" pipe can be fixed as an outlet. This joint should be sealed from the inside.
4. Fill the container with a layer of baby jelly, and then a layer of coarse sand and on the top a layer of charcoal. Ensure that at least 6" at the top is left open. Slow percolation of water through the layers i.e. the filtration medium ensures that the dissolved and suspended impurities are filtered.

STAGE THREE:

A:  TANK BASED RTRWH SYSTEMS

B. SURFACE WATER RECHARGE RTRWH SYSTEMS

C: WELL & BORE RTRWH SYSTEMS

Usually when talking about Roof top Rain water Harvesting, people are referring to the system where the storage is in a tank. 

However, water collected from roof tops are also use for re-charging shallow acquifers through recharge pits and trenches, which help to regenerate local ponds and other water bodies.  They can also be designed to channel water to specific openwells and bore-wells.

TRAPPING

The Lifelines of Tamil Nadu Kanmois are traditional tanks with earthen bunds constructed many centuries ago. They are large surface water stores that collected and stored surface water run-offs. They largely supported and were used for agricultural purposes as well as for drinking water. Ooranis are small ponds that have collected rainwater from rains and from surrounding catchment areas. These Ooranis traditionally were used for various needs of drinking, washing, bathing needs of the villages around it. Oorani is a Tamil word meaning village pond. It is an institution as old as Tamil society. Poet Thiruvalluvar referred to them 2000 years ago. Ooranis were usually endowed by ruling or merchant princes. Beneficiaries were involved in excavation and maintenance. They developed a sense of ownership. After Independence the government departments took over every aspect of village management and Ooranis fell to neglect. read more on this : Dhan foundation site MSSRF site The concept is so interesting that it has inspired the government to launch a scheme called "nammaku namme thittam" (meaning we are our own support) which is based on the Kudimaramathu practice of the ancient times.

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The following table gives us an idea of how much water can be harvested from different sizes of structures and the best method for harvesting rainwater.

" I dont mind staying in the tank "