Jargon
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
Contaminants
| Contamination types | Contamination Agents | Comments |
| Physical | Particles and suspended solids | It is common to experience large seasional variance in quantity of particles and suspended solids. When choosing intake location, consideration of the natural features such as flood terracing should be made to optimize the placement of the intake |
| Biological | Faecal waste | Faecal contamination is detected via the methods described in Water Quality Field Testing and approprate treatment methods determined based on those results. Common diseases spreat through Faecal contaminataion include cholera and typhoid fever. |
| Algae | Algae can result in bad taste. It is difficult to remove with coagulants and can speed up deterioration of sand filters. Bankside filtration can help to avoid these problems | |
| Chemical | Minerals, soil type | High salinity (due to the presense of Sodium ions) is treated with expensive procedures such as reverse osmosis and distillation. pH (due to the presence or lack of Hydrogen ions) must also be considered as its levels influence coagulant and chlorine dosages and contact times
The pH and salinity of different sources can vary, even though the sources may be in a close proximity. pH is an important factor where treatment involves the addition of coagulants (alum etc) as the quantity to be added is influenced by pH, as is the contact time for chlorine. |
Turbidity
Turbidity is measured in Nephlometric Turbidity Units (NTU). A Turbidity Tube is required for this and can be built from scratch
If it is found that your water has NTU greater than 5, a sedimentation test should be conducted. The results of the sedimentation test will determine if sedimentation alone is adequate and if so, how large a sedimentation basin is required. Also, if sedimentation is ineffective at removing the suspended solids, coagulation may be necessary.
Some information can be obtained visually. If the water is greenish, organic material is present. If large particles are present, sedimentation is likely to be effective. If the particles are microscopic, coagulation is more likely to be necessary.
pH
pH levels can be tested with a pool tester and should be between 6.5 and 8.5. Chlorine reaction times are slowed above a pH of 8. Allum effectiveness is reduced for pH outside of
Total Dissolved Solids
T
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 W.) 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.
Filtering
This is sometimes skipped if the water is very clear. This is important as pathogens and other contaminants are mostly attached to particles in the water.
Disinfection
Disinfetion 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.
- Boil the water for 10 minutes.
- W
- Chlorine or iodine.
Removing arsenic
In cases of arsenic contamination the W 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.
References
http://www.oxfam.org.uk/resources/downloads/emerg_manuals/water_treatment.pdf