17.3C: Purification of Drinking Water

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Water is purified with filters to remove larger protozoans, and by chemical or UV disinfection to kill bacteria and other small pathogens.

Learning Objectives

Illustrate the steps of drinking water purification

Key Points

Water is first passed through a system of filters and a coagulating agent is added to remove particulate matter.
Water is then passed through a membrane filter to remove large pathogens such as cryptosporidum and giardia.
To finalize the purification process, chemical disinfection (usually with chlorine or ozone ) or UV light is applied to the water to kill bacteria, viruses, and the hardy cysts produced by cryptosporidium and giardia.

Key Terms

ozone: A triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope (O2), breaking down with a half life of about half an hour in the lower atmosphere, to normal dioxygen. Ozone is formed from dioxygen by the action of ultraviolet light and also atmospheric electrical discharges, and is present in low concentrations throughout the Earth’s atmosphere. In total, ozone makes up only 0.6 parts per million of the atmosphere.
coagulation: The precipitation of suspended particles as they increase in size (by any of several physical or chemical processes).
protozoa: Protozoa are a diverse group of unicellular eukaryotic organisms, many of which are motile. Originally, protozoa had been defined as unicellular protists with animal-like behavior, e.g., movement. Protozoa were regarded as the partner group of protists to protophyta, which have plant-like behaviour, e.g., photosynthesis.

Drinking Water Purification

In order to purify drinking water from a source (such as a lake, river, reservoir or groundwater), the water must go through several steps to remove large particles and different types of pathogens.

Pumping and Containment: Water is pumped from the source into holding tanks.
Screening: Water is passed through a screen filter to remove large debris.
Storage: Water is stored in reservoirs, tanks, and water towers in preparation for purification. Sometimes water is “pre-cholrinated” in this system to prevent bacterial growth while it is in storage.
Coagulation and Sedimentation: Although there are many processes by which large particles are removed from drinking water, most water purification systems implement some kind of coagulation system. A chemical that causes particle aggregation is added to the water, and clumps of particles form and settle to the bottom of the reservoir. This is called sedimentation.
Membrane Filtration: Membrane filters are able to remove all particles larger than 0.2 um. Larger pathogens such as giardia lamblia and cryptosporidium are trapped in these filters, but the cysts they produce are small enough to pass through.
Disinfection: Before water is considered potable, it must be disinfected to remove any pathogens that passed through the membrane filter.

Methods of Disinfection

Chlorination is the most common form of disinfection. Chlorine is a strong oxidant, and rapidly kills many microorganisms, especially bacteria. Because chlorine is a toxic gas, it can also be dangerous to sanitation workers. Chlorine based compounds like choloramine are often used. Although chlorine is very effective against bacteria, it is not as effective against the cysts formed by protozoans (like giardia lamblia and cryptosporidium). Chlorine can sometimes leave residual byproducts in water.
Ozone is an unstable molecule that readily gives up one atom of oxygen providing a powerful oxidizing agent. This agent is toxic to most waterborne organisms. Ozone is widely used in Europe, and is an effective method to kill cysts formed by protozoans. It also works well against almost all other pathogens.
Ultraviolet Light is very effective at inactivating protozoan cysts, and will also kill bacteria and viruses. However, it is not as effective in cloudy water. It is sometimes used in concert with chlorination.