# Drinking water/Disinfection

Pathogens in water supplies and disinfection methods for drinking water are following.

### Pathogens in Water Supplies

* Bacteria - Bacteria are very common single-celled microorganisms that generally exist in colonies and almost always exist in water supplies. In fact, in waste-water treatment, the presence of bacteria is essential to the breakdown of chemical contaminants.
* Viruses - A virus is a sub-microscopic (less than one micrometer in size) infectious agent that is unable to grow or reproduce outside a host cell. Each viral particle, or virion, consists DNA or RNA, within a protective protein coat called a capsid. The capsid shape varies from simple helical and icosahedral (polyhedral or near-spherical) forms, to more complex structures with tails or an envelope. Viruses infect all cellular life forms and are grouped into animal, plant and bacterial types, according to the type of host infected.
* Protozoa
** Giardia - Giardia lamblia is a protozoan that is considered a dangerous acute contaminant that commonly exists in water supplies. Within the environment, it is usually present in its dormant form as a cyst, a thick-walled stage within which it can survive transport to a new host through water.
** Cryptosporidium - Cryptosporidium parva is a protozoan that is considered a highly dangerous acute contaminant that occasionally enters water supplies. Within the environment, it is usually present as an cyst or spore, a hardy stage which it uses to survive transport to a new host.

### Disinfection of Pathogens in Water

* Physical - Boiling or freezing water causes changes in phase and density which disrupt the ability of a pathogen to exist within the medium. Cellular cytoplasm (cellular bodily fluid) is mostly water and reacts to physical changes of its medium (human bodily fluids or water supplies). Boiling a fluid into a gas causes a pressure increase that explodes cell membranes. Freezing a fluid completely causes thermal expansion of water (e.g. ice occupying more volume than the equivalent amount of water), which also creates a pressure that bursts cellular membranes.
* Irradiation by Electromagnetic Rays - Electromagnetic waves (light) of a frequency higher than low ultraviolet (i.e. UV, X and Gamma bands) are considered ionizing radiation and may break chemical bonds holding a pathogen together. In the case of microbial DNA, the double helix may be broken, thus inactivating the virus. In the case of any ionizing radiation, the water may be ionized thus creating oxidative conditions.
* Bactericidal metal Ions - Cu$^{2+}$, Ag$^+$ and Hg$_2$$^{2+}$ have been well known to be effective disinfectants for millenia.
* pH Adjustment through Strong Acids and Bases
* Surfactants - Quarternary Ammonium Compounds
* Chemical Oxidants - Cl$_2$, Br$_2$, I$_2$, O$_3$, ClO$_2$, KMnO$_4$ and FeO$_4$$^{2-}$ and halo-organic compunds.

## Wikilinks

* Peruse the appropriate sections of General_Biology/Classification_of_Living_Things
* Peruse the appropriate sections of Biochemistry
* Wikiversity Department of Environmental Engineering
* Wikipedia article:Disinfection
* Wikipedia article:Virus disinfection
* Wikipedia article:Solar water disinfection
* v:Topic:Environmental engineering