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Chemical Contaminant Removal

Sorption

Sorption is the common term used for both absorption and adsorption. Sorption technologies are used for the removal of organics, inorganic contaminants (such as arsenic), taste, and odor. When a substance is incorporated into another substance, the process is called absorption. Adsorption happens when the ions and molecules of one substance physically adhere or bond to the surface of another substance’s molecule. Adsorbents used at drinking water plants include:

  • activated carbon,
  • activated alumina, and
  • iron-based media.

In many cases, it is not always clear which process (or both) is responsible for the removal of a contaminant.

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Water Softening

Water softening is the reduction of the concentration of calcium, magnesium, and certain other metal cations (positively charged ions) in water.

Ion Exchange

Ion exchange is commonly used in drinking water treatment for softening. This technology also removes nitrate, arsenate, chromate, and selenate from municipal water. Ion exchange selectively removes charged inorganic species from water using an ion-specific resin. The surface of the resin contains charged functional groups that hold ionic species by electrostatic attraction. As water containing undesired ions passes through a column of resin or resin bed, charged ions on the resin surface are exchanged for the undesired ions in the water.

There are several advantages to the ion exchange technology:

  • It operates on demand.
  • It has a short contact time; therefore, it is flow insensitive.
  • It is insensitive to pH.
  • The resin can be regenerated.
  • It has a water recovery of over 98 percent.

It is appropriate for small systems.

  • The disadvantages include:
  • Excess oxidant may degrade the resin.
  • Pre-filtration may be required.
  • Sulfate can be a problem.
  • Finished water pH adjustment may be required.
  • It results in large volumes of brine for disposal.

Because of its higher treatment cost compared to conventional treatment technologies, the ion exchange is used mostly by small, medium, and point-of-entry systems.

Other Types of Softening

  • Chemical flocculation transforms dissolved contaminants into an insoluble solid that's easily removed from the water by sedimentation (contaminants settle out of the water and come to rest) or filtration.
  • Chemical precipitation uses treatment chemicals to form particles that remove contaminants and settle.

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Other Technologies

Aeration – Aeration technologies are typically used for removal of volatile organic compounds and excess carbon dioxide. Aeration is the process of mixing water with air by spraying or cascading, wherein the chemical targeted for removal is transferred from the water to the air stream. Closed aeration uses pressure to remove molecules. Open aeration uses gravity to remove gases.

Electrodialysis – Another less commonly used technology for chemical removal is electrodialysis, a process in which ions are transferred through ion-selective membranes by means of an electromotive force from a less concentrated solution to a more concentrated solution. This technology is very effective in removing fluoride and nitrate. it can also remove barium, cadmium, and selenium.

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