Sources of drinking water - Contents

1. Deep groundwater: The water emerging from some deep groundwaters may have fallen as rain many decades or even hundreds of years ago. Soil and rock layers naturally filter the groundwater to a high degree of clarity before it is pumped to the treatment plant. Such water may emerge as springs, artesian springs, or may be extracted from boreholes or wells. Deep groundwater is generally of very high bacteriological quality (i.e., a low concentration of pathogenic bacteria) but may be rich in dissolved solids, especially carbonates and sulphates of calcium and magnesium. Depending on the strata through which the water has flowed, other ions may also be present including chloride, and bi-carbonate. There may be a requirement to reduce the iron or manganese content of this water to make it pleasant for drinking, cooking, and laundry use. Disinfection is also required. Where groundwater recharge is practised, it is equivalent to lowland surface waters for treatment purposes.
2. Shallow groundwaters: Water emerging from shallow groundwaters is usually abstracted from wells or boreholes. The bacteriological quality can be variable depending on the nature of the catchment. A variety of soluble materials may be present including potentially toxic metals such as zinc and copper. In parts of Bangladesh, many shallow groundwater sources are contaminated with unacceptably high levels of arsenic.
3. Upland lakes and reservoirs: Typically located in the headwaters of river systems, upland reservoirs are usually sited above any human habitation and may be surrounded by a protective zone to restrict the opportunities for contamination. Bacteria and pathogen levels are usually low, but some bacteria, protozoa or algae will be present. Where uplands are forested or peaty, humic acids can colour the water. Many upland sources have low pH which require adjustment.
4. Rivers, canals and low land reservoirs: Low land surface waters will have a significant bacterial load and may also contain algae, suspended solids and a variety of dissolved constituents.

Water treatment methods - Contents

Coagulation

Flocculation

Other water purification techniques - Contents

1. Boiling: Water is heated hot enough and long enough to inactivate or kill microorganisms that normally live in water at room temperature. Near sea level, a vigorous rolling boil for at least one minute is sufficient. At high altitudes (greater than two kilometers or 5000 feet) three minutes is recommended. US EPA emergency disinfection recomendations In areas where the water is "hard" (that is, containing significant dissolved calcium salts), boiling decomposes the bicarbonate ions, resulting in partial precipitation as calcium carbonate. This is the "fur" that builds up on kettle elements, etc., in hard water areas. With the exception of calcium, boiling does not remove solutes of higher boiling point than water and in fact increases their concentration (due to some water being lost as vapour).
2. Carbon filtering: Charcoal, a form of carbon with a high surface area, absorbs many compounds including some toxic compounds. Water passing through activated charcoal is common in household water filters and fish tanks. Household filters for drinking water sometimes contain silver to releases silver ions which have a bactericidal effect. There are two types of carbon filters. One is granular charcoal which is not very effective for removing many contaminants such has mercury, volatile organic chemicals (this is the most prevalent contaminant found in drinking water and is also not removed by reverse osmosis or distillation), asbestos, pesticides, disinfection byproducts (trihalomethanes), mtbe, pcbs etc. A more effective carbon filter is the sub-micrometre solid block carbon filter which removes all of the contaminants listed above. To see if a particular product removes contaminants or to compare such products, see National Sanitation Foundation, or check the California Health Certificate that comes with most filters. Carbon filters are not true filters such as membrane filters; harmful microbes can easily pass right through them so they are often called contacters rather than filters.
3. Distilling: Distillation involves boiling the water to produce water vapour. The vapour contacts a cool surface where it condenses as a liquid. Because the solutes are not normally vaporized, they remain in the boiling solution. Even distillation does not completely purify water, because of contaminants with similar boiling points and droplets of unvaporized liquid carried with the steam. However, 99.9% pure water can be obtained by distillation.
4. Reverse osmosis: Mechanical pressure is applied to an impure solution to force pure water through a semi-permeable membrane. Reverse osmosis is theoretically the most thorough method of large scale water purification available, although perfect semi-permeable membranes are difficult to create. Tight membrane filters like RO or nanofilter membranes will remove salt and colour compounds from water but thorough pretreatment, high pressures and careful cleaning is required leading to high costs per gallon.
5. Ion exchange: Most common ion exchange systems use a zeolite resin bed to replace unwanted Ca²⁺ and Mg²⁺ ions with benign (soap friendly) Na⁺ or K⁺ ions. This is the common water softener. A more rigorous type of ion exchange swaps H⁺ ions for unwanted cations and hydroxide (OH^-) ions for unwanted anions. The result is H⁺ + OH^- → H₂O. This system is recharged with hydrochloric acid and sodium hydroxide, respectively. The result is essentially deionized water.
6. Electrodeionization: Water is passed between a positive electrode and a negative electrode. Ion selective membranes allow the positive ions to separate from the water toward the negative electrode and the negative ions toward the positive electrode. High purity deionized water results. The water is usually passed through a reverse osmosis unit first to remove nonionic organic contaminants.
7. Water conditioning: This is a method of reducing the effects of hard water. Hardness salts are deposited in water systems subject to heating because the decomposition of bicarbonate ions creates carbonate ions which crystalise out of the saturated solution of calcium or magnesium carbonate. Water with high concentrations of hardness salts can be treated with soda ash ( sodium carbonate) which precipitates out the excess salts, through the common ion effect, as calcium carbonate of very high purity. The preciptated calcium carbonate is traditionally sold to the manufacturers of toothpaste. Several other methods of industrial and residential water treatment are claimed (without general scientific acceptance) to include the use of magnetic or/and electrical fields reducing the effects of hard water.
8. Plumbo-solvency reduction: In areas with naturally acidic waters of low conductivity (i.e surface rainfall in upland mountains of igneous rocks), the water is capable of dissolving lead from any lead pipes that it is carried in. The addition of small quantities of phosphate ion and increasing the pH slightly both assist in greatly reducing plumbo-solvency by creating insoluble lead salts on the inner surfaces of the pipes.

Portable water purification - Contents