Bangalore Environment Trust Newsletter, August 1996
Sat, 03/03/2007 - 19:59 — admin

Rainwater Harvesting

Provision of adequate and safe drinking water is the prime concern of any community. This has been achieved in most of the water scarce villages through tapping of ground sources. Excess tapping, high mineral concentration and pollution of even this water has resulted in it becoming scanty and unsafe.

Women report problems of salinity, excessive consumption of fuel in the cooking of dha1, wastage of soap in laundering and splitting of milk while boiling with this water. Many of them reported walking 1 to 2 kms for fetching potable water for their household consumption.
One of the viable technologies for tiding over such a crisis is harvesting of rain water from the roof surfaces of houses and community buildings.

Rain water collected from roof surfaces during the first rain will be dirty and contaminated with dust, birds droppings and other impurities on the roof surfaces. It would, therefore, be advisable to discard the first sweep off called `foul flush' and collect only the remaining flow for' consumption after proper distillation.

Parameters influencing storage capacity

• Rainfall and its distribution

• Available roof area

• Rate of consumption of water

It is advisable to study the rainfall rate of the area for a period of 3 to 5 years from the available records in order to assess the economic viability of this technology.

Construction and Process

A rain water harvesting unit comprises of the following:

1. Catchment

2. Valley gutters

3. Down-take pipe (conduit)

4. Foul flush seperator

5. Storage tank

Catchment

A roof of any shape forms the catchment area for this water collection.

Valley gutters

The rain pouring on the roof is collected in valley gutters fitted to the caves. Gutters made of asbestos-cement or galvanized iron are suggested for a sloped roofing. This is fixed in such a way as to allow a free flow of water to the side, where it is provided with a downtake pipe.

Down-take pipe

An AC pipe or PVC pipe of 10 cm (4 inches) diameter connected to the gutters, directs the water to the foul flush separator.

Foul-flush separator

This is an interception mechanism provided with arrangements for discarding the flow of water from the roof surface whenever it is not needed and its diversion to the storage tank whenever desired.

Designs that require different types of operation have been explained below.

Design I
 

In figure I, a flexible tube (No.3) connected to the downtake pipe or conduit (No.2) leading to an interception tank (No.4), which is a small surface tank, serves the function. During initial rains, the tube is left outside the tank. When clear water starts flowing, water is collected by introducing the flexible tube (No.3) inside the tank. ,~.s this tank gets filled up, the surplus overflow moves constantly to the bigger storage tank or well.

Design II
 

It is -possible to provide the small collection tank or foul flush (No.2) with a plug (6) that can be left open during initial rains to let off the foul water, and closed when the flow is to be directed to the large storage tank (3). In this case, the down take pipe can be a permanent fixture with exit water going directly to the tank. The wire mesh or filter mesh (1), fixed at the exit from the first tank removes solid particles in the, water stream from the roof surface.

Design III
 

In figure III, the down take pipe (1) allows the water to run into the base of a chamber filled with graded *stones (2). The plug (3) provided at the bottom of this chamber is kept open to discard the initial flow from the roof. Once clean water starts flowing, the plug is closed to permit the surface flow from this unit to move to the storage tank (4) after passing it through the filter media (2).

Design IV
 

The automatic foul-flush separator (3) is shown in figure N. It is a tank with conical shaped top with a mouth and an air vent pipe (1), a float valve (2), a tap (4) at the bottom and a lead to the storage tank (6) The capacity of the tank is worked out at the rate of 5 litres/sq.m roof area, the quantity assumed for cleaning the roof area.

The down-take pipe is connected to the mouth of this tank. As water flows into the tank, the float (2) rises up and finally blocks the passage, directing further flow to the storage tank (6). This tank needs to be emptied after every monsoon to prevent chances of dirty water flowing into the collection tank, during subsequent seasonal rains.

Collection tank
This can be a surface tank or a sub-surface tank.

Its size is to be estimated from the parameters prescribed.

The surface tank occupies space but can facilitate safe and easy tapping.

The sub-surface tank does not occupy any surface space. It is to be covered and provided with a hand pump for facilitating safe distribution.

Dried up wells can also be cleaned and used as storage tanks. This has the duel advantage of charging the ground water sources with pure water and solving the problem of water shortage.

Advantage
It is becoming an urgent need of the day to popularise the non-conventional rain water harvesting techniques in problem areas for augmenting diminishing drinking water sources. Wherever ground water contains a high precentage of mineral salts, this technology is viable for providing soft water.

Care
Chlorination is needed immediately after the first rain and after the last rain. It is desirable to chlorinate the stock of water once in three months, after confirming its chlorine demand.

Cost
It varies according to the design, the area and the materials used for installation.