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: 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: Chlorine tablets are easily available in the market. One tablet of 0.5 g is enough to disinfect 20 litres (a bucketful) of water.
Boiling: 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: This 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. The processes involved are indicated in the illustration.
Clean the bottle well
Fill the 3/4th of the bottle with water and cap it
Shake the bottle well
Keep the bottle on black iron sheet for minimum 6 hours in sunlight before consumption
Functioning of Sodis depends on the following factors:
Weather and climate: Sodis requires sun's radiation and temperature to purify drinking water. The container needs to be exposed to direct sunlight for about six hours. If the temperature raises above 50 degrees Celsius, the disinfection process is three times faster. If weather conditions are not optimal the efficiency of the disinfection process can be increased by using half-blackened plastic bottles, which achieve approximately 5 degrees C higher water temperatures than fully transparent, non painted bottles. Placing the plastic bottles on a black corrugated zinc sheet would also help in achieving disinfection
• Water Turbidity: Suspended particles in the water reduce the penetration of solar radiation into the water and protect microorganisms from being irradiated. Sodis requires relatively clearer water with a turbidity ofless than 30 NTU (Naphelometric Turbidity Unit - naphelometer is a modern commercial instrument used to measure turbidity)
• Material: Various types of transparent plastic materials are good transmitters of light in the UV and visible range of the solar spectrum. Plastic bottles made of PET (PolyEthylene Terephtalate) are preferred because they contain less UV- stabilizers than PVC (Poly Vinyl Chloride) bottles.
• Shape of containers: More the area of bottle exposed to sunlight, more would be efficiency in achieving disaffection.
• Oxygen: Sodis is more efficient in water containing high levels of oxygen. Shaking a water container which is about 3/4th full for about 20 seconds before they are filled completely could increase oxygen levels. On reacting with this water, sunlight produces highly reactive forms of oxygen (oxygen free radicals and hydrogen peroxides). These reactive forms of oxygen kill microorganisms
Although Sodis can neither treat turbid waters nor change the chemical quality of water, this method is ideal to disinfect small quantities of water used for consumption. For more details visit http://www.sodis.ch
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/catchmet 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