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Karmayogi

Something to think about - October 2006
Harvesting Rainwater, Harnessing Life - By Gaurang Damani

I.        WHY RAIN WATER HARVESTING?
The principle of collecting and using precipitation from catchment surfaces is called rain water harvesting. As you read this guide, seriously consider conserving water by harvesting and managing this natural resource by artificially recharging the system.

I.a.     What are the advantages of using rainwater?

• Rainwater is one of the purest sources of water available. Its quality almost always exceeds that of ground or surface water. It does not come into contact with soil or rocks where it can dissolve minerals and salts nor does it come into contact with many of the pollutants that are often discharged into local surface waters or contaminate ground water supplies. However, rainwater quality is influenced by where it falls. Rainfall in areas where heavy industry or crop dusting is prevalent may not have the same purity as rain falling in other areas.

• Rainwater is soft. It can significantly lower the quantity of detergents and soaps needed for cleaning. Soap scum and hardness deposits do not occur. There is no need for a water softener as there often is with well water. Water heaters and pipes are free of the deposits caused by hard water and should last longer.

• Rainwater harvesting promotes self-sufficiency and fosters an appreciation for water as a resource. It also promotes water conservation.

• Rainwater harvesting also conserves energy as the energy input needed to operate a centralized water system is bypassed. Many systems require only a small pump to create water pressure in household pipes.

• Local erosion and flooding from impervious cover associated with buildings is lessened as a portion of local rainfall is diverted into collection tanks.

Rain water harvesting is essential because:

• Surface water is inadequate to meet our demand and we might have to depend on ground water.

• Due to rapid urbanization, infiltration of rain water into the sub-soil has decreased drastically and recharging of ground water has diminished.

II.       How much rainfall can possibly be collected?
An amazing amount of water can be collected. The rule of thumb is 2300 liters of water per inch of rain per thousand square feet of catchment area. Mumbai receives on an average 1800 mm of rainfall every year or about 72 inches per annum. Here is the basic formula for calculating the approximate potential amount that can be collected:

(Catchment area of building¹ x inches of rain x 2300 liters)/ 1000

¹ Calculate the catchment area using the footprint of the building plus the length of the overhangs. In other words if our building is 100 feet by 70 feet the catchment area is 7,000 square feet.

II.a.    How much rainfall can really be collected?
Not all the rain that falls can actually be collected. Several factors affect collection efficiency. A small amount of rain, 3/100s to 1/10th of an inch, will be needed to wet the roof and fill the roof washer. Some of the rain will overshoot the gutters or spill out of gutters during heavy downpours. Once storage tanks are full any additional rain will not be collected. Efficiency is usually presumed to be 75% to 90% depending on system design and capacity.

Sample calculation for Avanti Apartments, Sion (annualized):
Source: Texas Natural resource Conservation Commission

(7,000 sq ft of catchment area x 72 inches of rain x 2300 liters per inch x 0.75 efficiency)/ 1000 = 8,69,400 liters of water collected from 72 inches of rain that Mumbai receives annually. Since monsoon is seasonal in Mumbai, rain falls only for about 4 months in a year. So the rain water harvested will be approximately 2,17,350 liters per monsoon month.

II.b.    Detailed working that needs to be further done:

In Mumbai’s example since rains are seasonal, rainwater collection can only be a supplement. Add the amount already in storage to the amount collected and subtract the monthly demand to determine the balance. Continue doing this calculation for each month to get a picture for the year. This collected water can be used for watering the plants and the garden and washing cars etc.

Rain does not fall evenly throughout the year. You have to know how much water you need each month compared to how much water you can collect and store each month. Can you collect enough during the rainy months to carry you through the dry months? You have to calculate a monthly balance. It's best to calculate the balance using the amount of rain you will get at least 50 percent of the time and the amount of rain you will get at least 75 percent of the time.

III.      RAIN WATER HARVESTING TECHNIQUES:

Rainwater harvesting can be harvested from the following surfaces:

Rooftops: If buildings with impervious roofs are already in place, the catchment area is effectively available free of charge and they provide a supply at the point of consumption.

Paved and unpaved areas: i.e., landscapes, open fields, parks, stormwater drains, roads and pavements and other open areas can be effectively used to harvest the runoff. The main advantage in using ground as collecting surface is that water can be collected from a larger area. This is particularly advantageous in areas of low rainfall.

Waterbodies: The potential of lakes, tanks and ponds to store rainwater is immense. The harvested rainwater can not only be used to meet water requirements of the city, it also recharges groundwater aquifers.

Stormwater drains: Most of the residential colonies have proper network of stormwater drains. If maintained neatly, these offer a simple and cost effective means for harvesting rainwater.

Construction activity in and around the city is resulting in the drying up of water bodies and reclamation of these tanks for conversion into plots for houses. Free flow of storm run off into these tanks and water bodies must be ensured. The storm run off may be diverted into the nearest tanks or depression, which will create additional recharge.

There are two main techniques of rain water harvesting:

1.       Storage of rainwater on surface for future use
The storage of rain water on surface is a traditional techniques and structures used were underground tanks, ponds, check dams, weirs etc.

2.       Recharge to ground water
Recharge to ground water is a new concept of rainwater harvesting and the structures generally used are pits, trenches, dug wells, recharge wells etc.

IV.      COMPONENTS OF A RAINWATER HARVESTING SYSTEM

A rainwater harvesting system comprises components of various stages - transporting rainwater through pipes or drains, filtration, and storage in tanks for reuse or recharge. The common components of a rainwater harvesting system involved in these stages are illustrated here.

1. Catchments: The catchment of a water harvesting system is the surface, which directly receives the rainfall and provides water to the system. It can be a paved area like a terrace or courtyard of a building, or an unpaved area like a lawn or open ground. A roof made of reinforced cement concrete (RCC), galvanised iron or corrugated sheets can also be used for water harvesting.

2. Coarse mesh at the roof to prevent the passage of debris

3. Gutters: Channels all around the edge of a sloping roof to collect and transport rainwater to the storage tank. Gutters can be semi-circular or rectangular and could be made using:

Locally available material such as plain galvanised iron sheet (20 to 22 gauge), folded to required shapes. Semi-circular gutters of PVC material can be readily prepared by cutting those pipes into two equal semi-circular channels. Bamboo or betel trunks cut vertically in half.

The size of the gutter should be according to the flow during the highest intensity rain. It is advisable to make them 10 to 15 per cent oversize. Gutters need to be supported so they do not sag or fall off when loaded with water. The way in which gutters are fixed depends on the construction of the house; it is possible to fix iron or timber brackets into the walls, but for houses having wider eaves, some method of attachment to the rafters is necessary.

4. Conduits: Conduits are pipelines or drains that carry rainwater from the catchment or rooftop area to the harvesting system. Conduits can be of any material like polyvinyl chloride (PVC) or galvanized iron (GI), materials that are commonly available.

The following table gives an idea about the diameter of pipe required for draining out rainwater based on rainfall intensity and roof area:

Sizing of rainwater pipe for roof drainage
Source: National Building Code

5. First-flushing
A first flush device is a valve that ensures that runoff from the first spell of rain is flushed out and does not enter the system. This needs to be done since the first spell of rain carries a relatively larger amount of pollutants from the air and catchment surface.

6. Filter
The filter is used to remove suspended pollutants from rainwater collected over roof. A filter unit is a chamber filled with filtering media such as fibre, coarse sand and gravel layers to remove debris and dirt from water before it enters the storage tank or recharge structure. Charcoal can be added for additional filtration.

(i) Charcoal water filter
A simple charcoal filter can be made in a drum or an earthen pot. The filter is made of gravel, sand and charcoal, all of which are easily available.

(ii) Sand filters
Sand filters have commonly available sand as filter media. Sand filters are easy and inexpensive to construct. These filters can be employed for treatment of water to effectively remove turbidity (suspended particles like silt and clay), colour and microorganisms.

In a simple sand filter that can be constructed domestically, the top layer comprises coarse sand followed by a 5-10 mm layer of gravel followed by another 5-25 cm layer of gravel and boulders.

IV.b.   Filter for large rooftops
When rainwater is harvested in a large rooftop area, the filtering system should accommodate the excess flow. A system is designed with three concentric circular chambers in which the outer chamber is filled with sand, the middle one with coarse aggregate and the inner-most layer with pebbles. This way the area of filtration is increased for sand, in relation to coarse aggregate and pebbles. Rainwater reaches the centre core and is collected in the sump where it is treated with few tablets of chlorine and is made ready for consumption.

to be continued.....

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