Water may contain pathogens, particularly certain viruses, bacteria, protozoal cysts, and worm eggs. These organisms may be transmitted to water by feces, and may be ingested when water is consumed, potentially causing disease. Although water contamination by human feces is the major concern, animal feces in water may also transmit disease. Water contamination by human urine is a significant threat only in areas where typhoid and urinary schistosomiasis (Schistosoma haematobium) are endemic. By far the greatest risk associated with polluted drinking water is infection with a variety of organisms that cause diarrheal diseases, as well as typhoid and infectious hepatitis (hepatitis A). Diarrheal diseases are caused by a variety of viruses, bacteria, and protozoa. The numbers of viruses and protozoa in water will always decrease over time, with the most rapid decrease occurring at warm temperatures. Bacteria behave similarly, but in exceptional circumstances they may multiply in polluted water. The infectious dose of viruses and protozoa is typically very low, whereas the dose of bacteria needed to establish an infection in the intestine may be high, as in the case of cholera, or extremely low, as in the case of the shigella dysentery organism.
Determining how to treat water on a large scale is best done by experts. Simple and practical measures, however, can be taken before such help is available. All methods require regular attention and maintenance.
In addition to protecting water at its source, five basic methods can be used for water treatment: storage, sand filtration, coagulation and flocculation, chemical disinfection, and boiling. These methods can be used singly or in combination.
Leaving water undisturbed in containers, tanks, or reservoirs improves its quality over time. Storage allows pathogens to die off and suspended particles to settle through sedimentation. If water supplies are unsafe and cannot be easily treated, immediate action must be taken to provide maximum water storage capacity. Storage of untreated surface water for 12 to 24 hours will considerably improve its quality; the longer the period of storage and the higher the temperature, the greater the improvement. In addition, the clarification of turbid or cloudy water can be greatly speeded by the addition of aluminum sulfate (alum). A two-tank system is often used, with the first tank used as a settling tank and the second used to store the clarified water. Further treatment can be done in the second tank as well, and a third tank used for storage, if necessary. While clear water may only require chlorination, turbid surface water will usually require sedimentation and/or filtration before chlorination. Organic matter that causes turbidity reduces the effectiveness of chlorine by reacting with and dissipating chlorine, and also makes it much more difficult for chlorine to maintain necessary contact with microorganisms.
Great care should be taken to prevent the pollution of stored water. This task can be done by covering storage tanks and screening all inlets into them. In addition, the area where storage tanks are located should be fenced off and guarded to prevent children from playing or swimming in the tanks.
b. Sand Filtration
Sand filtration can also be an effective method of treatment. Rapid sand filters, which are suitable for low turbidity waters, operate either by gravity flow or within enclosed pressure vessels. Both methods employ relatively complex backwashing operations to remove suspended particles captured on the filters. A more suitable system for DP camps and rural settlements is the slow sand filter, which develops a thin biologic slime, or "schmutzdecke," on the surface of a sand bed. The schmutzdecke breaks down a large percentage of the bacteria, viruses, and protozoans found in polluted waters, while the sand layers filter out additional organisms, along with suspended sediments and particulate matter. The rate of filtration through a slow sand filter depends on the surface area, depth, and type of sand through which the water is passed, and the pressure of the water. The average size of sand particles is 0.3-1.0 mm. In general, the slower the rate of filtration, the higher the quality of the water.
A packed drum filter can be used for sand filtration and is a good way of providing limited quantities of safe water quickly (e.g., for a health center). In a packed drum, filter water passes down through layers of sand and gravel and is drawn off at a rate not to exceed 60 L per hour for a 200-L drum. Unfiltered water equal to the amount drawn off is continually added to the top of the drum. Other types of sand filters include horizontal sand filters and riverbed filters (suitable only where the riverbed is permeable). These methods can be used to treat larger amounts of water but are likely to be more difficult to set up quickly and effectively. To filter water from a river, a well may be dug close to the bank. This water must still be considered river water, and though it will have been filtered through the bed and bank, further treatment may be necessary (e.g., chlorination).
c. Coagulation and Flocculation
Because chlorine and other forms of chemical disinfection do not work well in highly turbid waters, coagulation and flocculation is used to clarify water through the addition of aluminum sulfate (alum). The alum chemically coagulates the turbidity in the water. Over time, particles begin to stick together in the flocculation process, eventually forming clusters of larger particles that are removed by sedimentation and/or filtration. Coagulation is most effective at pH ranges of 5 to 8.
d. Chemical Disinfection
Chemical disinfection kills pathogenic organisms and is the final stage in the water treatment process. This type of disinfection is most effective when used to purify relatively clear water or to disinfect wells, sand filters, pumps, and piped water systems. Various forms of chlorine can be used as well as iodine, although chlorine is more widely used, cheaper, and often more readily available. The most generally suitable form of chlorine for DP emergencies is calcium hypochlorite powder, which contains 60 to 70 percent available chlorine (the chlorine that remains in water after chemical reactions between the chlorine and the water have occurred). Chlorine tablets are also used in some instances. Sodium hypochlorine (e.g., bleach) in liquid form containing approximate 5 percent available chlorine is also used in many situations. Chlorine gas, which is normally used in large conventional water treatment systems, is highly corrosive and should not be used in emergency situations except under extremely unusual circumstances.
Expert advice is essential for large-scale chlorination, and all systems using chlorine products must be given close attention and regular maintenance. Chlorination generally occurs after sedimentation and filtration, if these processes are needed. At least 30 minutes contact time is needed for chlorine to react with and kill microorganisms.
Water should be tested for chlorine residual levels after each disinfection and before distribution. A pocket-size chlorine test kit is used to test for chlorine residual levels.
After chlorination, there should be at least 0.2 mg/L (or parts per million) of residual chlorine (free available chlorine) in the water to ensure that all bacteria and viruses have been killed, as well as to safeguard against subsequent contamination. Chlorine should be added to the water until the free available chlorine reaches approximately 0.5 mg/L. Highly polluted waters will require greater doses of chlorine than relatively clean waters. The amount of free available chlorine remaining in the water should be no more than 0.6 mg/L to minimize unpleasant taste. If the amount of free available chlorine is above 1.0 mg/L at the distribution point, the water will have a distinct chlorine taste and most people will not want to drink the water.
Chlorine and iodine water purification tablets are available, and are sometimes distributed to DPs for water treatment in the home, but rarely are suitable to treat water for large populations. Tablets, however, may be useful to treat water in health facilities or supplementary feeding centers. Certain new technologies for home-based water treatment such as individual bottles of chlorine, individual packets that both flocculate and disinfect water and small portable filtration units are currently being used on a limited basis, and are under investigation and trial application to evaluate their suitability for emergency use. These technologies may prove useful in specific emergency situations and may be used in the future. The biggest obstacle to their effective use is the difficulty of implementing programs requiring substantial change in human behavior during acute stages of emergencies. Such products may be more suitable for use during more stable situations.
Boiling is the surest and perhaps simplest method of water sterilization but is practical only for small displaced populations. At low altitudes, bringing water to a boil for 1 minute will essentially destroy all pathogens transmitted by drinking water. Boiling times, however, must be increased by 1 minute for every 1,000 m of altitude above sea level, as the temperature at which water boils decreases with altitude. To improve taste, allow the water to cool, and then pour the water from one clean container to another several times. Boiling increases the concentration of nitrates, which are dangerous for very young babies. Care must be taken not to use the boiling option for babies’ drinking water when using ground-water sources that potentially contain elevated nitrate levels. Prolonged vigorous boiling is often recommended but not necessary to destroy fecal-oral transmitted pathogens. Prolonged boiling is not only unnecessary, but it also wastes fuel. In general, boiling is not suitable for camp situations if the energy source is firewood, which is likely to be in short supply. The availability of domestic fuel supplies may be the determining factor, as boiling requires about 1 kg of wood per liter of water. If the displaced people have traditionally boiled their water, however, they should be encouraged to do so. This may make the need for other types of water treatment less urgent.
TOC: C. Water