In western countries there are strict quality standards for water supplies. In the Netherlands, for example, all tap water is suitable for drinking. Water purification plants are charged with the responsibility of maintaining these standards. The treatment process in purification plants is discussed below. In developing countries, however, the water supplies have less high standards and water treatment is left to the private user. The easiest and most effective treatment of water is boiling: water is heated to its boiling point long enough to inactivate or kill microorganisms or pathogens that can live in water at room temperature. Near sea level, a vigorous rolling boil for at least one minute will do. At higher altitudes greater than a kilometer or a mile above sea level, boil for three minutes. This is required due to the lower boiling point at high altitudes.
For the traveler, portable purification systems can be useful. Portable pump filters are commercially available with ceramic filters that will filter 5000 to 50,000 liters per cartridge. A lot of travelers carry iodine, either crystallized, in tablets, or in a solution. Five drops of iodine for a liter of water can kill off many, but not all, of the most common pathogens that may be present in a time span of half an hour. As this method is not completely reliable it is often supplemented by the solar disinfection (“sodis”) method. A transparent plastic bottle of water is oxygenated by shaking, followed by topping-up. It is placed on tile or metal for six hours in direct sunlight. The subsequent temperature rise and extended dose of solar radiation is enough to kill any microbes that may be left.
Water purification plants
Different sources demand different treatment methods to render water suitable for human consumption. Many environmental and financial considerations affect the location and design of water purification plants. Groundwater is cheaper to treat but, once depleted, aquifers can take thousands of years to recharge. Surface water sources must be carefully monitored for the presence of unusual types or levels of contaminants. The treatment plant itself must be kept secure from purposeful contamination or acts of terrorism. The presence of large quantities of dangerous chemicals mandates special training for workers and emergency personnel. The facility must responsibly dispose of its settled and filtered solids and prevent them from contaminating the treatment components or the source waters. All facilities disinfect the water at the end of the purification process, but the exact method of disinfection can be controversial, and the costs and benefits of different methods must be carefully evaluated.
Sources of drinking water
Drinking water is mostly drawn from groundwater aquifers, rivers, canals, and reservoirs.
Groundwater- Groundwater may emerge to the surface as springs, artesian springs, or may be extracted from boreholes or wells. Water can be extracted from deep and shallow aquifers. The quality can be variable depending on the amount of pollution from by-products of industrial manufacture and agricultural production. This is especially a problem in shallow groundwater extraction where the water emerging from deep groundwater aquifers is generally of higher quality. To reach the deep groundwater aquifers water may have travelled for several decades or even hundreds of years, and soil and rock layers will have naturally filtered the groundwater to a high degree of clarity. However, depending on the strata through which the water has flowed, it can also contain potentially dangerous levels of dissolved minerals, such as iron, manganese, or arsenic. In parts of Bangladesh, many ground water sources have unacceptably high levels of arsenic, resulting in significant public health problems.
Rivers, canals and reservoirs- The quality of surface water can vary greatly as well. Lowland surface waters will have a significant bacterial load and may also contain algae, suspended solids, and a variety of dissolved constituents, or “solutes.” Upland reservoirs are usually situated above any human habitation and may be surrounded by some form of protection zone to restrict any chance of contamination. Bacteria and pathogen levels are usually low, but some bacteria, protozoa and algae will be present. Where uplands are forested or peaty, humic acids can color the water brown. Many upland sources have a low pH, which require correction before the water is directed into the supply for human consumption.
The most common treatments methods are one, or a combination, of the following: screening, storage, flocculation, sand filters, and disinfection.
Screening- A municipal surface water treatment plant must first screen out large objects such as garbage, leaves and other debris. The tighter the mesh of the sieve, the smaller the particles must be to pass through. Filtering is not sufficient to completely purify water, but it is often a necessary first step since large particles can interfere with the more thorough purification methods.
Storage- Water from rivers may be stored in bank side reservoirs for periods lasting between a few days and many months to allow natural biological purification to take place. This is especially important if treatment is to be by slow sand filters (see below). The filtered water is then treated to remove any microscopic organisms including protozoa and bacteria. This is generally followed by a disinfection stage to eliminate any residual bacteria and viruses. For water that is particularly difficult to treat such as that from catchments with intensive agriculture, physical and biological treatment methods may need to be combined.
Flocculation- In flocculation, water is treated with small volumes of appropriate chemicals that causes particles to clump together as “flocs.” As a result, the otherwise difficult to remove particles can be easily sifted out. The most common flocculants are aluminium salts such as aluminium sulfate, which is flocculated by a small addition of lime to raise the pH.
Sand Filters- Sand filters can be categorized as rapid and slow sand filters. The use of rapid sand filters is the most common form of physical treatment of water. Passing flocculated water through a sand filter strains out the floc. Sand filters become clogged with floc after a period of use and are then backwashed or pressure washed to remove the floc. This backwash water is run into special settling tanks so that the floc can precipitate out which is then disposed of as waste material. In some countries this may be used as a soil conditioner.
Where land and space are available, water may be treated in slow sand filter beds. These rely on biological treatment processes rather than physical filtration. Slow sand filters are carefully constructed using graded layers of sand with the coarsest at the base and the finest at the top. Drains buried at the base of the filter convey treated water away for disinfection. The actual purification is done by a biological film comprised of bacteria, protozoa, fungi, and algae that build up on the surface of the sand. The slow sand filter produces excellent water quality that natural physical methods of treatment rarely achieve.
Disinfection- At the end of the purification process, the water is disinfected with chlorine gas, chloramines, sodium hypochlorite, chlorine dioxide, ozone, or ultraviolet light. Some water purification plants also pre-chlorinate their raw water influent after the screening phase to reduce the incidence of biological films in the treatment cycle, or to remove excessive iron and manganese from the water. Water utilities may choose to further boost chlorine levels (termed re-chlorinating) in the distribution system to counteract any pathogens that may be present.