Water purification

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Water purification, or drinking water treatment, is the process of removing contaminants from surface water or groundwater to make it safe and palatable for human consumption. A wide variety of technologies may be used, depending on the raw water source, contaminants present, standards to be met, and available finances.

The purification of surface water from sources such as reservoirs or rivers usually requires several phases of treatment. A municipal surface water treatment plant must first screen out large objects such as trash and leaves. Next, the raw water may be flash-mixed with various chemicals to alter its pH, encouraging the flocculation (clumping) and settling of smaller suspended solids. After an additional settling stage, the water can be forced through filter beds composed of sand, garnet, and anthracite to remove even smaller particles. The finished water is then disinfected with chlorine gas, chloramine, sodium hypochlorite, chlorine dioxide, ozone, or ultraviolet light, before it is pumped into the distribution system of water mains and storage tanks on its way to consumers. Some plants also pre-chlorinate their raw water influent after the screening phase. Water utilities may choose to further boost chlorine levels (termed rechlorination) in the distribution system to counteract any pathogens that may occur.

Groundwater from an aquifer not immediately influenced by surface runoff is generally considered to require less rigorous treatment, but must meet the same standards of safety and palatability. Soil and rock layers will have naturally filtered the groundwater to a high degree of clarity even before it is pumped to the treatment plant, but the facility may need to reduce the iron or manganese content of this water to make it pleasant for drinking, cooking, and laundry use. Disinfection is also required.

Many environmental and cost considerations affect the siting and design of water purification plants. Groundwater is cheaper to treat, but aquifers once depleted can take thousands of years to recharge. Surface water sources must be carefully monitored for the presence of unusual types or levels of contaminants. The treatment plant itself must be kept secure from vandalism or terrorism and the presence of large quantities of dangerous chemicals mandates special training for workers and emergency personnel. The facility must responsibly dispose of its settled and filtered solids and prevent them from contaminating the treatment components or the source waters. All facilities disinfect finished water, but the exact method of disinfection can be controversial, and the costs and benefits of different methods must be evaluated.

Water purification techniques

Six popular methods for purifying water are:

  1. Filtering: Water is passed through a sieve that catches small particles. The tighter the mesh of the sieve, the smaller the particles must be to pass through. Filtering is not sufficient to completely purify water, but it is often a necessary first step, since such particles can interfere with the more thorough purification methods.
  2. Boiling: Water is heated to its boiling point long enough to inactivate or kill microorganisms that normally live in water at room temperature. In areas where the water is "hard", (containing dissolved calcium salts), boiling decomposes the bicarbonate ion, resulting in some (but not all) of the dissolved calcium being precipitated in the form of calcium carbonate. This is the so-called "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).
  3. Carbon filtering: Charcoal, a form of carbon with a high surface area due to its mode of preparation, adsorbs many compounds, including some toxic compounds. Water is passed through activated charcoal to remove such contaminants. This method is most commonly used in household water filters and fish tanks. Household filters for drinking water sometimes also contain silver, trace amounts of silver ions having a bactericidal effect.
  4. Distilling: Distillation involves boiling the water to produce water vapour. The water vapour then rises to a cooled surface where it can condense back into a liquid and be collected. 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.
  5. Reverse osmosis: Mechanical pressure is applied to an impure solution to force pure water through a semi-permeable membrane. The term is reverse osmosis, because normal osmosis would result in pure water moving in the other direction to dilute the impurities. Reverse osmosis is theoretically the most thorough method of large-scale water purification available, although perfect semi-permable membranes are difficult to create.
  6. Ion exchange: Most common ion exchange systems use a zeolite resin bed and simply replace unwanted ions (Ca2+ and Mg2+) with benign (soap friendly) sodium or potassium ions. This is the common water softener. A more rigorous type of ion exchange swaps hydrogen (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. The result is essentially deionized water.
  7. 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 de-ionized water results. The water is usually passed through a reverse osmosis unit first to remove nonionic organic contaminants.

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