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3.4.2: Other Water treatment processes

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    225685
  • Water Treatment

    The process of water treatment to provide safe drinking water for their communities. There are various combinations and types of processes in water treatment depending on the scale of the plant and quality of the raw (source) water. Today, the most common steps in water treatment used by community water systems (mainly surface water treatment).

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    Figure \(\PageIndex{1}\) Raw water. Image taken from: https://commons.wikimedia.org/wiki/File:2008-09-20_Dirty_water_spilling_from_a_bottle.jpg

    Coagulation and Flocculation

    Coagulation and flocculation are often the first steps in water treatment. Chemicals with a positive charge are added to the water. The positive charge of these chemicals neutralizes the negative charge of dirt and other dissolved particles in the water. When this occurs, the particles bind with the chemicals and form larger particles, called floc.Historically, dirty water is cleaned by treating with alum, Al2(SO4)3.12 H2O, and lime, Ca(OH)2. These electrolytes cause the pH of the water to change due to the following reactions:

    Al2(SO4)3.12 H2O, -> Al3+(aq) + 3 SO42-(aq) + 12 H2O
    SO42-(aq) + H2O -> HSO4-(aq) + OH- (causing pH change)
    Ca(OH)2 -> Ca2+(aq) + 2 OH- (causing pH change)

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    Figure \(\PageIndex{2}\) : By U.S. Government Accountability Office from Washington, DC, United States - Figure 2: Typical Drinking Water Treatment Process, Public Domain, commons.wikimedia.org/w/inde...curid=72268495

    The slightly basic water causes Al(OH)3, Fe(OH)3 and Fe(OH)2 to precipitate, bringing the small particulates with them and the water becomes clear. Some records have been found that Egyptians and Romans used these techniques as early as 2000 BC.

    Suspension of iron oxide particulates and humic organic matter in water gives water the yellow muddy appearance. Both iron oxide particulates and organic matter can be removed from coagulation and flocculation. The description given here is oversimplified, and many more techniques have been applied in the treatment of water. Coagulation is a major application of lime in the treatment of wastewater.

    Other salts such as iron sulfates Fe2(SO4)3 and FeSO4, chromium sulfate Cr2(SO4)3, and some special polymers are also useful. Other ions such as sodium, chloride, calcium, magnesium, and potassium also affect the coagulation process. So do temperature, pH, and concentration.

    Disposal of coagulation sludge is a concern, however.

    Sedimentation

    During sedimentation, floc settles to the bottom of the water supply, due to its weight. This settling process is called sedimentation. The floc particles are then removed from the bottom of the basins.

    Aeration

    Bringing air into intimate contact with water for the purpose of exchanging certain components between the two phases is called aeration. Oxygenation is one of the purposes of aeration. Others are removal of volatile organic substances, hydrogen sulfide, ammonia, and volatile organic compounds

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    Figure \(\PageIndex{3}\) : Aeration at Furman University. Image taken from: en.Wikipedia.org/wiki/File:Furman_library.jpg

    Filtration

    Once the floc has settled to the bottom of the water supply, the clear water on top will pass through filters of varying compositions (sand, gravel, and charcoal) and pore sizes, in order to remove dissolved particles, such as dust, parasites, bacteria, viruses, and chemicals.

    Sanitization

    After the water has been filtered, a chemical disinfectant (chlorine and/or ozone based) or ultraviolet radiation will be applied in order to kill any remaining parasites, bacteria, and viruses, and to protect the water from germs when it is piped to homes and businesses.

    Fluoridation

    This process is not mandatory for water processing plants in the United States. Around 72% of public water systems have this chemical added in order to reduce dental issues.

    Content taken and edited from: chem.libretexts.org/Bookshelves/Introductory_Chemistry/Map%3A_Chemistry_for_Changing_Times_(Hill_and_McCreary)/14%3A_Water/14.6%3A_Making_Water_Fit_to_Drink