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Water treatment options

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Jargon[edit]

pH - a measure of the acidity of the water. Most fresh water sources have a pH of 6-8. The ocean's pH is ~8.3. A pH above 7 is considered preferable because enteric pathogens prefer a pH below 7.

Enteric Pathogen - diseases which attack the digestive system such as Typhoid and Cholera

Sedimentation - passing the water through a very calm, pond like structure so that the solids will settle out.

Coagulation - using chemicals such as allum to accelerate sedimentation.

Turbidity - the cloudiness of the water

NTU - a measure of turbidity

Intake Design[edit]

One of the most often neglected elements in the water treatment system is the intake. The intake must be diesigned to limit the amount of dirt and debris taken into the system to reduce problems downstream.

  • Intake channels which do not face upstream are preferable.
  • Fast flowing water will carry more dirt and debris, so seek natural sedimentation basins within the stream.
  • When designing the intake, keep in mind that it must be accessed for maintenance.
  • Custom made perforated pipes and drums can help preserve water quality.
  • Placing the intake .5m below the water surface can avoid algae growth.
  • Avoid suctioning sediments from the lake/river bottom.

Suspended Solids Removal[edit]

Suspended Solids Removal is typically the most difficult element of the water treatment process. There are 2 primary methods of suspended solids removal, sedimentation and coagulation and flocculation.

Sedimentation[edit]

The easiest way to remove suspended solids is through sedimentation.If it is possible to let the water sit for a period of time, or pass slowly through a tank, heavier particles can settle out. See Sedimentation (water treatment)W. Also, if the sedimentation period lasts days to weeks, sedimentation can also have the effect of killing off bacteria. If the water is relatively clear, this step may not be important. Or if the water is turbid (cloudy) but the solids do not settle out easily, this step may be of little use.

A simple test of the efficacy of sedimentation involves, taking a jar of the water and allowing it to sit after being mixed. If the supernatantW water is less than 5NTU after 6-8 hours, then either filtration or coagulation must be employed.

INSERT SOME METHOD FOR DETERMINGING SEDIMENTATION TANK SIZE AND CONTACT TIME

When designing sedimentation tanks, special care should be taken to ensure that inlets and outlets produce minimal disturbance.

Coagulation and Flocculation[edit]

Frequently, the suspended solids will not settle out in a reasonable period of time and sedimentation alone will not work. If this is the case, a two step process of coagulation and flocculation is employed.

Filtering[edit]

A water filter is important as pathogens and other contaminants are mostly attached to particles in the water. This step is sometimes skipped if the water is very clear.

Disinfection[edit]

Disinfection is the backbone of water treatment and required to inactivate pathogens such as viri and bacteria. Filtration is typically employed so that the disinfection phase is more effective.

Removing arsenic[edit]

In cases of arsenic contamination the Sono arsenic filterW should be considered. As it contains sand and charcoal, the filter should also be effective against other impurities. Arsenic containing water is typically groundwater and thus should not usually contain significant pathogens. Iron and manganese oxides can offer a cheap method of removing low levels of arsenic from water. The oxides are able to adsorb arsenic ions and remove them from solution.

See also[edit]

References[edit]

http://www.oxfam.org.uk/resources/downloads/emerg_manuals/water_treatment.pdf