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Karmayogi

Something to think about - December 2006
Harvesting Rainwater, Harnessing Life (PART II) - By Gaurang Damani

V.   Quality of stored water

To prevent leaves and debris from entering the system, mesh filters should be provided at the mouth of the drainpipe. Further, a first-flush device should be provided in the conduit before it connects to the storage container.

If the stored water is to be used for drinking purposes, a sand filter should also be provided. Methods to protect rainwater quality include appropriate system design, sound operation and maintenance and use of first flush devices and treatment. Treatment is mainly appropriate as a remedial action if contamination is expected. First flush devices can be effective in reducing levels of contamination if properly maintained. Good system design, operation and maintenance are generally the simplest and most effective means of protecting water quality.

a.       System design: The best initial step to protecting water quality is to ensure good system design. Water quality will generally improve during storage provided sunlight and living organisms are excluded from the tank and fresh inflows do not stir up any sediment. The design should include

Clean impervious roof made from smooth, clean non-toxic material. Over hanging branches above the catchment surface should be removed

Taps or draw-off pipes on tanks should be atleast five centimeters above the tank floor (more if debris accumulation rates are high). A tank floor sloping towards the sump can greatly aid tank cleaning as will a well-fitting access manhole.

Wire or nylon mesh should cover all inlets to prevent any insects and other creatures from entering the tank. The tank must be covered and all light excluded to prevent growth of algae and other organisms. The grill at the terrace outlet for rainwater arrests most of the debris carried by the water from the rooftop like leaves, plastic bags and paper pieces.

A coarse filter and/or foul flush device should be fitted to intercept water before it enters the tank for removing leaves and other debris.

b.       Operation and maintenance: Proper operation and maintenance of rainwater harvesting systems helps to protect water quality in several ways. Regular inspection and cleaning of catchment, gutters, filters and tanks reduce the likelihood of contamination. Water from other sources should not be mixed with that in the tank.

c.       Treatment: Treatment of stored rainwater only makes sense if it is done properly and if hygienic collection and use of the water will ensure it does not suffer from re-contamination. There are several types of treatment possible, the most common being chlorination, boiling, filtration and exposure to ultraviolet or natural sunlight.

Chlorination is most appropriately used to treat rainwater if contamination is suspected due to the rainwater being coloured or smelling bad. It should only be done if the rainwater is the sole source of supply and the tank should first be thoroughly inspected to try to ascertain the cause of any contamination. Chlorination is done with stabilised bleaching powder (calcium hypochlorite - CaOCl2) which is a mixture of chlorine and lime. Chlorination can kill all types of bacteria and make water safe for drinking purposes. About 1 gm (approximately 1/4 tea spoon) of bleaching powder is sufficient to treat 200 litres of water. Chlorine tablets that are easily available in the market can also be used. One tablet of 0.5 g is enough to disinfect 20 litres (a bucketful) of water.

Boiling is a very effective method of purification and very simple to carry out. Boiling water for 10 to 20 minutes is enough to remove all biological contaminants.

Direct sunlight can also be used to kill many of the harmful bacteria in water by exposing it in clear glass or plastic bottles for several hours. Although feasible in some circumstances, the water must be clear, the weather fine and the water cooled overnight before consumption.

SODIS: Solar disinfection method uses sun's ultra-violet (UV) radiation to improve the microbiological quality of drinking water. It has been proven that synergies induced by radiation and thermal treatment have a significant effect on the die-off rate of microorganisms.

V.a.   Tips to ensure quality of harvested rain
It is extremely important to maintain the rainwater harvesting systems regularly for high quality performance. Following aspects should be taken care of:

• Just before the arrival of monsoon, the rooftop/ catchment area has to be cleaned properly.

• The roof outlet on the terrace should be covered with a mesh to prevent entry of leafs or other solid waste into the system.

• The filter materials have to be either replaced or washed properly before the monsoon.

• The diversion valve has to be opened for the first 5 to 10 minutes of rain to dispose off the polluted first flush.

• All polluted water should be taken away from the recharge structures.

• The depth of bores (of recharge structures) shall be finalised depending on the actual site condition

VI. ATTRIBUTES OF GROUNDWATER:

• There is more ground water than surface water
• Ground water is less expensive and economic resource.
• Ground water is sustainable and reliable source of water supply.
• Ground water is relatively less vulnerable to pollution
• Ground water is usually of high bacteriological purity.
• Ground water is free of pathogenic organisms.
• Ground water needs little treatment before use.
• Ground water has no turbidity and colour.
• Ground water has distinct health advantage as art alternative for lower sanitary quality surface water.
• Ground water is usually universally available.
• Ground water resource can be instantly developed and used.
• There is no conveyance losses in ground water based supplies.
• Ground water has low vulnerability to drought.
• Ground water is key to life in arid and semi-arid regions.
• Ground water is source of dry weather flow in rivers and streams.

Urbanization effects on Groundwater Hydrology:

• Increase in water demand
• More dependence on ground water use
• Over exploitation of ground water
• Increase in run-off, decline in well yields and fall in water levels
• Reduction in open soil surface area
• Reduction in infiltration and deterioration in water quality

VII. ARTIFICIAL RECHARGE TO GROUND WATER:

Artificial recharge to ground water is a process by which the ground water reservoir is augmented at a rate exceeding that obtaining under natural conditions or replenishment. Any man-made scheme or facility that adds water to an aquifer may be considered to be an artificial recharge system.

Recharge of groundwater through storm run off and roof top water collection, diversion and collection of run off into dry tanks, play grounds, parks and other vacant places are to be implemented by Municipal Corporations and other Government Establishments with special efforts. The Municipal Corporations will help the citizens and builders to adopt suitable recharge method in one's own house or building through demonstration and offering subsidies for materials and incentives, if possible.

Methods of artificial recharge in urban areas:

• Water spreading

• Recharge through pits, trenches, wells, shafts

• Rooftop collection of rainwater

• Road-top collection of rainwater

• Induced recharge from surface water bodies.

Recharge is usually done through the following mechanisms:

Pits: Recharge pits are constructed for recharging the shallow aquifer. These are constructed 1 to 2 m, wide and to 3 m. deep which are back filled with boulders, gravels, coarse sand.

Trenches: These are constructed when the permeable stram is available at shallow depth. Trench may be 0.5 to 1 m. wide, 1 to 1.5m. deep and 10 to 20 m. long depending up availability of water. These are back filled with filter materials.

Dug wells:- Existing dug wells may be utilised as recharge structure and water should pass through filter media before putting into dug well.

Hand pumps :- The existing hand pumps may be used for recharging the shallow/deep aquifers, if the availability of water is limited. Water should pass through filter media before diverting it into hand pumps.

Recharge wells :- Recharge wells of 100 to 300 mm. diameter are generally constructed for recharging the deeper aquifers and water is passed through filter media to avoid choking of recharge wells.

Recharge Shafts :- For recharging the shallow aquifer which are located below clayey surface, recharge shafts of 0.5 to 3 m. diameter and 10 to 15 m. deep are constructed and back filled with boulders, gravels & coarse sand.

Lateral shafts with bore wells :- For recharging the upper as well as deeper aquifers lateral shafts of 1.5 to 2 m. wide & 10 to 30 m. long depending upon availability of water with one or two bore wells are constructed. The lateral shafts is back filled with boulders, gravels & coarse sand.

Spreading techniques :- When permeable strata starts from top then this technique is used. Spread the water in streams/Nalas by making check dams, nala bunds, cement plugs, gabion structures or a percolation pond may be constructed.

Benefits of Artificial Recharge in Urban Areas:

• Improvement in infiltration and reduction in run-off.
• Improvement in groundwater levels and yields.
• Reduces strain on Municipal Corporation water supply
• Improvement in groundwater quality
• Estimated quantity of additional recharge from 100 sq. m. roof top area is 55,000 liters.

VII.       COST

Rainwater harvesting methods are site specific and hence it is difficult to give a generalised cost. But first of all, the major components of a rainwater harvesting system - rain and catchment area - are available free of cost. A good proportion of the expenses would be for the pipe connections. By judiciously fixing up the slopes of roofs and location of rainwater outlets, this could be brought down considerably. However the cost varies widely depending on the availability of existing structures like wells and tanks which can be modified and used for water harvesting.

Typically, installing a water harvesting system in a building would cost between Rs 2,000 to 30,000 for buildings of about 300 sq. m. The cost estimate mentioned above is for an existing building. For instance, water harvesting system in the CSE building in Tughlakabad Institutional Area, Delhi, was set up with an investment of Rs 30,000 whereas those in the model projects ranged between Rs 70, 000 and Rs 8 lakh. The costs would be less comparatively if the system were incorporated during the construction of the building itself.

Some basic rates of construction activities and materials have been given here, which may be helpful in calculating the total cost of a structure. The list is not comprehensive and contains only important activities meant to provide a rough estimate of the cost.

a. Unit cost of construction activities:

b. Ferrocement tanks with skeletal cage
Source: Action for food Production and United Nations Children's Fund, Rooftop rainwater harvesting systems

c. Plastic tanks:
Available as finished products in various capacities. The cost of these tanks ranges from Rs 2/litre to about Rs 3.5/litre. Other brands available in the market.

VIII.      Other water conservation techniques:

Xeriscaping is a form of landscaping that is very thrifty with water. These are four of the basic ideas behind Xeriscaping:

• Use plants that are native or well-adapted to your area

• Water efficiently using mechanized means if possible

• Improve the soil quality

• Use mulches to moderate soil temperatures and conserve moisture.

• By putting these ideas into practice, you can dramatically reduce your water use, especially during hot, dry weather--and you won't have to sacrifice a healthy landscape to do it!

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