Rooftop: rainwater harvesting systems in Schools

The schools for rainwater harvesting have been selected on the basis of not having any source of drinking water currently. The construction component of the programme is being implemented by the Engineering Department of the various Zilla Panchayat or the District Nirmithi Kendra.

Many schools presently do not have a reliable source of water for drinking and other purposes. The school rooftop rainwater harvesting system seeks to provide a source of water for toilet flushing, cooking, washing hands and feet before eating and after toilet use, hygiene and finally, for drinking purpose – if the water is treated well.

The amount of rainwater that can be collected depends on the rooftop area, the size of the tank and the rainfall at that place. For example, in a place where it rains 500mm and the roof area is 100sqm, the rainwater falling on the roof is 50,000 litres. Some amount of it will be absorbed by the roof and some amount will be lost in the collection process. If we assume 80% can be collected, then 40,000 litres of rainwater is available for collection.

Depending on the size of the rainwater tank and the distribution of rainfall, even a 3000-litre tank may be sufficient to collect this 40,000 litres of rainwater. We must, however, keep on using the rainwater in the tank and not wait for summer to use it. By painting an information board and keeping a small rain gauge in the school, a good school can involve students in monitoring rainfall, total rain in a year, water collected in the rainwater tank and teach them to ensure good maintenance of the system.

System Components

Roof: The existing roof is made use of to collect rainwater. Since rainwater is pure as it falls from the sky, it is necessary that the roof be kept clean for water to remain pure when it is collected. When cleaning the roof with water the first rain separator must be kept open so as not to allow the dirt to enter the filter and the tank.

PVC Gutters: The gutters of PVC collect the rainwater from the roof and transfer it to the filter. On sloping roofs, PVC gutters can pick up leaves, dust, small twigs and other organic matter. The gutters need to be cleaned regularly – at least a week. During the rainy season, the PVC gutters should be inspected and cleaned daily. The gutters are fixed to the roof or the walls with clamps. The clamps hold the gutter or pipes to the wall or the roof firmly and allow a small slope in the system to enable water to flow in one direction. At all times, the PVC gutters or pipes should slope in the direction of the storage tank and not away from it. The PVC gutters should always have an end cap at the end where rainwater begins to flow in the direction of the tank.

Downpipes: PVC down pipes bring the water from the rainwater gutters or pipes vertically down. They should invariably be clamped firmly to the wall and should never be loosely fixed. The down pipe should always be firmly fixed and if necessary, the clamp should be replaced or tightend whenever necessary.

First rain separator: The first rain separator, or a washout pipe as it is called, has a valve or an end cap to allow the first little amount of rainwater to be collected separately. This has most of the dust and dirt in it. The first rain separator is also used when the roof is being cleaned or when rainwater is not to be collected. It is important to ensure that the first rain separator is always kept in the closed position and never left open. After every rain, it should be opened carefully and the waste water allowed to flow out. The pipe should then be cleaned and the valve or the end cap, closed. Sometimes the first rain separator can get jammed due to the dirt or dust in it. In such a situation the valve or the end cap should be carefully replaced by a good plumber.

Leaf trap: Where the roof of a school has lot of leaves falling from a tree or trees, a conical leaf trap can be placed in the vertical down pipe. This has a mesh on top. The mesh prevents small leaves, twigs and other material from entering the pipe and blocking it or choking the filter. The material collected on the leaf trap, if any, must be removed at regular intervals and daily during the rainy season.

Filter: A gravel, sand and ‘netlon’ mesh filter is designed and placed on top of the storage tank. This filter is very important in keeping the rainwater in the storage tank clean. It removes silt, dust, leaves and other organic matter from entering the storage tank.

The filter media should be cleaned daily after every rainfall event. Clogged filters prevent rainwater from easily entering the storage tank and the filter may overflow. The sand or gravel media should be taken out and washed before it is replaced in the filter.

Storage tank: The rainwater storage tank collects all the filtered rainwater and keeps it for future use. The storage tank is made above the ground and on a platform. It can also be an underground sump in some cases.

The tank is invariably painted white on the outside. This is done to keep the water inside cool and prevent the growth of bacteria. Every year the tank must be white – washed neatly.

The tank also will be sealed from the top either with Cuddapah slabs or concrete slabs or any local stone. It must be ensured that the top cover is permanent and always fully covered. This will prevent the growth of algae or bacteria in the tank. If there are small cracks in the joints, they should be sealed with cement mortar immediately. Mosquitoes and dust should never be allowed in to the stored rainwater tank.

The tank should also be completely water tight. If there is any leak in the tank or even dampness, the problem should be addressed immediately with the help of a trained engineer.

Overflow pipe: The storage tank will have an overflow pipe from the top of the tank. In case of heavy rain, the overflow pipe will allow the excess rainwater to be safely disposed off without causing any flooding. The size of the overflow pipe should be the same as that of the inlet pipe. It will have a mesh at the bottom to prevent rats, squirrels and cockroach from coming in.

It should also be ensured that the overflow water is drained away effectively to a pit, plant or storm water drain and not allowed to cause flooding.

Tap: A tap is provided in every tank to draw the rainwater out. Sometimes a tank can have more than one tap. Invariably it is found that children play with the pipe outlet or the tap and it is damaged. Children should be taught not to stand on the pipe or to play with the tap. A broken tap will result in the entire system going to waste as all the collected rainwater will flow out.

Water quality check: If the roof, the gutter, the first rain separator and the filter are kept clean, the collected rainwater will be crystal clear. This is an indication that good maintenance is being followed. If the water is, however, dirty in colour or smells bad, then it means that the system is not being kept clean.

Even if the water is clear and does not smell, still it must be checked for micro-biological contamination. The checking should be daily for the first one month and then, weekly if the water is clear and does’t smell foul. For this, one must use a H2S strip test bottle.

Wash your hands thoroughly with soap. With clean hands the sealed bottle should be opened. From the tap in the rainwater storage tank, fill the bottle to the mark provided. Close the cap tightly. Bring the bottle back to a safe place in a room. Observe for 24 to 48 hours. If the water turns black in the bottle then it is micro-biologically contaminated and requires treatment before being used for drinking. If the water colour stays brown, then the water is fit for drinking.

Water quality treatment: Though rainwater, as it falls from the clouds is very pure, it does pick up dirt, dust and bacteria once it falls on the roof. It is very necessary to therefore, check the quality of the water before using it for consumption. Once it is established that the rainwater is not micro-biologically contaminated, it can be consumed directly. However, if the H2S strip test suggests that water has bacteria in it, it must then be treated before it can be used for drinking. The method suggested for treating for bacteria is chlorination. Liquid chlorine or chlorine tablets are available for treatment of water. Depending on the volume of the rainwater in the tank, chlorine needs to be added to disinfect the water. Chlorination should be carried out every time there is rain and a fresh infusion of water into the tank. Using a chloroscope, residual chlorine of 0.20 mg/litre should be established before the water is used for drinking.

Another form of deactivating bacteria and making water fit for consumption is called Sodis – Solar disinfection of water. In this method, rainwater is kept in a PET bottle or a glass bottle in the sun for six hours. One side of the bottle is painted black. The black surface is kept on the ground. With a combination of UV disinfection and infra red heat sterilisation, the water becomes fit for consumption. In cloudy weather, the bottles need to be kept in the sun longer.