Gather, process and analyse info from secondary sources to describe ways in which drinking water can be treated and use available evidence to explain how these methods reduce the risk of infection from pathogens.

Contamination of drinking is a common way for pathogens to enter the body. Therefore treatment is necessary to reduce the risk of infection from pathogens. Examples of treatments include filtration, boiling, distillation, disinfection, chlorination, pre-chlorination and ozone filtration.

Water disinfection means the removal, deactivation or killing of pathogenic microorganisms through the addition of certain chemicals. Microorganisms are destroyed or deactivated, resulting in termination of growth and reproduction.

Chemical inactivation of microbiological contamination in natural or untreated water is usually one of the final steps to reduce pathogenic microorganisms in drinking water. Combinations of water purification steps (oxidation, coagulation, settling and disinfection) filter drinking water after production. As an extra measure many countries apply a second disinfection step at the end of the water purification process, in order to protect the water from microbiological contamination in the water distribution system. Usually a different kind of disinfectant from the one earlier is used during this disinfection process. The secondary disinfection makes sure that bacteria will not multiply in the water during distribution. Bacteria can remain in the water after the first disinfection step or can end up in the water during back flushing of contaminated water, which can contain groundwater bacteria as a result of cracks in the plumbing.

Boiling water is the rapid vaporization of a liquid, which typically occurs when a liquid is heated to its boiling point, the temperature the temperature at which the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environmental pressure.
Boiling water is an effective method of killing most protozoan parasites and bacteria present in the drinking water. Many species of organisms are unable to survive beyond the boiling point of water, as the organism will denature under the intense heat caused.

In many ways, distillation is the reverse of boiling. To remove impurities from water by distillation, the water is usually boiled in a chamber causing water to vaporize, and the pure (or mostly pure) steam leaves the non volatile contaminants behind. The steam moves to a different part of the unit and is cooled until it condenses back into liquid water. The resulting distillate drips into a storage container. This process usually occurs over a long period of time depending on the amount of water that needs to be distilled.

Salts, sediment, metals, bacteria - anything that won't boil or evaporate - remain in the distiller and must be removed. A vapor trap, carbon filter, or other device must be used along with a distiller to ensure the more complete removal of contaminants. If the water is boiled, or heated just short of boiling so that it evaporates, pathogens such as fecal coliform bacteria would also be killed. As long as the distiller is kept clean and is working properly the high quality of treated water will be very consistent regardless of the incoming water - no drop in quality over time.

UV Filtration
Water passes through a clear chamber where it is exposed to the very reactive Ultra Violet (UV) Light. When in contact with any organic compounds UV will start to oxidize by attacking the carbon-hydrogen bonds, and continue on through a chain of reactions ending off with harmless CO2 gas. UV light effectively destroys bacteria and viruses by reacting with microorganism and oxidizing the cell membrane. However, how well the UV system works depends on the energy dose that the organism absorbs. If the energy dose is not high enough, the organism’s genetic material may only be damaged rather than disrupted.

UV is typically used as a final purification stage on some filtration systems. That is, it is usually used to remove microorganisms not kill them because some microbes such as tough cryptosporidia cysts are fairly resistant to UV light. Also UV radiation is not suitable for water with high levels of suspended solids, turbidity, color, or soluble organic matter. These materials can react with UV radiation, and reduce disinfection performance. Turbidity which is the level of transparency of water makes it difficult for radiation to penetrate water and pathogens can be 'shadowed', protecting them from the light and consequently survive.

Chlorination is the process of adding the element chlorine to water as a method of water purification to make it fit for human consumption as drinking water. Water which has been treated with chlorine is effective in preventing the spread of waterborne disease, eliminating almost all bacteria, viruses and amoeba.

Although several methods eliminate disease-causing microorganisms in water, chlorination is the most commonly used. Chlorination is effective against many pathogenic bacteria, but at normal dosage rates it does not kill all viruses, cysts, or worms. When combined with filtration, chlorination is an excellent way to disinfect drinking water supplies. Contact retention time in chlorination is the period between introduction of the disinfectant and when the water is used. A long interaction between chlorine and the microorganisms results in an effective disinfection process. Contact time varies with chlorine concentration, the type of pathogens present, pH, and temperature of the water. Contact time must increase under conditions of low water temperature or high pH (alkalinity). Complete mixing of chlorine and water is necessary, and often a holding tank is needed to achieve appropriate contact time.

Filtration (by Reverse Osmosis)
In reverse osmosis water pressure is used to force water molecules through a membrane that has extremely tiny pores, leaving the larger contaminants behind. Purified water is collected from the "clean" side of the membrane, and water containing the concentrated contaminants is flushed down the drain from the "contaminated" side. The average RO system is a unit consisting of a sediment/chlorine pre filter, the reverse-osmosis membrane, a water storage tank, and an activated-carbon post filter.

Some filters can remove pathogens down to the 0.2–0.3 micrometer range. Most filters of this kind remove most bacteria and protozoa, such as Cryptosporidium and Giardia lamblia, but not viruses except for the very largest, so disinfection by chemicals or ultraviolet light is still required after filtration. In addition not all bacteria are removed by 0.2 micron pump filters; for example, strands of thread-like Leptospira spp. bacteria, (that can cause leptospirosis), are thin enough to pass through a 0.2 micrometer filter. There have been polymer and ceramic filters that incorporate iodine post-treatment in their filter elements to kill viruses and the smaller bacteria that cannot be filtered out, but most have disappeared due to the unpleasant taste imparted to the water, as well as possible adverse health effects when iodine is ingested over protracted periods. While the filtration elements may do an excellent job of removing most bacteria and fungi contaminants from drinking water when new, the elements themselves can become colonization sites and thereby breed more microbes. A new system has been developed that has the ability to removes particle larger than 15 nm, and thus is able to filter-out viruses. Other methods of filtration are also available.