Solar Disinfection of Drinking Water

The following instructions are intended primarily for the benefit of housewives in rural areas in developing countries where safe community water supplies are not available. It is assumed that in these areas water-borne diseases are endemic or sporadic. These instructions concern the procedure to be adopted on a routine basis for the proper disinfection of drinking water for household use. The procedure involves exposure to sunlight of water from the usual community source for a minimum period of time in available transparent containers such as colourless or blue-tinted glass or plastic bottles. In order to save time and effort, it is highly desirable for a housewife to make her own arrangements to carry out the routine disinfection operation regularly once a day, or once every other day. For this reason, a housewife should ensure enough containers that would hold the desired quantity of drinking water estimated by her to meet the needs of the family for one or two days. At the end of the sunlight exposure period, a housewife could then transfer indoors the whole set of containers for use. To avoid recontamination, the already disinfected water should preferably be kept in the same containers used in the solar exposure operation. However, if there is a shortage of small containers, then the disinfected water could be transferred from each exposed container to a clean large container reserved for bulk storage of processed water. Once a set of emptied containers becomes available after usage, the refilling and exposure procedures are repeated. If the containers are maintained in a good state of cleanliness, then there would be no reason for having to clean them repeatedly each time they are to be re-used. PROCEDURE 1. Containers: From among those found at home, or purchased from the local market, select a number of containers made of colourless or blue-tinted glass or transparent plastic estimated to hold an amount of drinking water sufficient for household consumption for one or two days. The selected containers could include ordinary bottles, jars, or any other types of vessels provided they are transparent to light. Coloured containers other than blue, or greenish-blue should not be used as they are not as satisfactory. Remove any detachable paper labels from bottles, and wash all the containers with water (and soap, if necessary) to remove dirt and any residue from the previous contents. 2. Water: Fetch water in the usual manner from the common village supply (stream, well, pond, reservoir, etc.). If the water is highly turbid, then clarify the water by allowing the suspended particles to settle. Decant the clear water into other vessels. Carefully fill each of the containers hereafter reserved for the solar disinfection operation with the clarified water. 3. Exposure: Place the containers outdoors in an open space where sunlight cannot be obstructed by houses, walls, trees, or bushes throughout the day. Porches, balconies, roofs, or window sills would be satisfactory if open land is not available. Select places away from dust, children, domestic animals, and pets to avoid contamination and mischief. Individual containers should be spread out to avoid shadows. Keep the containers in their normal upright position. Tilting them at an angle towards the sun (as is commonly advocated for other solar appliances) may diminish the disinfection efficiency. Stoppers for bottles, and original covers for jars may be used to prevent the entry of dust, dirt, or vermin. But, such closures are not essential for the disinfection process. In fact, water exposed to bright sunlight in tightly closed containers could become much warmer than that in open containers. This is because the water vapour escaping from open containers carries with it some of the heat acquired by the water exposed to sunlight. Since it is futile to maintain an exact time for sunlight exposure, it would be a wise arrangement on a routine basis to start the sunlight exposure operation at a convenient time in the morning, and to keep the containers exposed until the late afternoon. The exposed containers may then be kept in place overnight to allow the water to cool, or they may be transferred indoors in readiness for use. However, in such emergencies as when a family runs short of disinfected drinking water, an exposure period of about two hours, especially at noontime, should be adequate for proper disinfection. These practical suggestions will ensure satisfactory results even under moderately cloudy conditions. It would not be practical to carry out the operation under conditions of heavy rainfall. After use, the empty containers can be re-used without the need for rewashing unless they accidentally become dirty. The cycle can now be repeated from the stage of refilling with water through the stage of sunlight exposure. With time and experience, the whole operation becomes a matter of routine. It should be noted that these instructions need to be modified or simplified further by health educators and primary health care promoters to suit local conditions, provided the essential requirements are not altered in any way.