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Purification of Town Water Supplies | Water Treatment
Town water supplies go through some purification and
treatment processes before it is made available for use by the public.
Water treatment can be defined as any procedure or method used to alter the chemical composition or natural “behaviour” of a water supply. Water supplies are classified as either surface water or groundwater.
The majority of public or municipal water comes from surface water such as rivers, lakes, and reservoirs. The majority of private water supplies consist of ground water pumped from wells.
Most municipal water found in a city or community today has been treated extensively.
Specific water treatment methods and steps taken by municipalities to meet local, state, national, or international standards vary. Below are the processes which water may be subjected to, in its purification at the water works before being distributed for human use:
1. Coagulation, Flocculation, and sedimentation - These are the processes of rapid mixing of chemicals known as coagulants to make the solid particles in the water clump together (coagulation), and then the gentle mixing to form larger groups of particles known as floc (flocculation). Alum (aluminium sulphate), polyaluminium chloride and a group of chemicals known as polyelectrolytes (these are polymers with ionizable groups that can dissociate in solution, leaving ions of one sign bound to the chain and counterions in solution) are the materials currently used for this purpose. This thicker, denser floc floats down by gravity and settle out of the water in large tanks (sedimentation) or is removed during filtration.
2. Filtration - In this stage the remaining floc, other chemicals and physical impurities, and most of the biological impurities (bacteria, etc.) are removed. The water flows by gravity through filters. There are different types of filters which may be employed. A type is the tank-type pressure filter which consist of: oxidizing filters; activated carbon filters; and dual or multi-media filters.
Oxidizing filters - these use a medium treated with oxides of manganese as a source of oxygen to oxidize and precipitate Iron, manganese, hydrogen sulphide, and others.
Activated carbon filters - these are similar to ion exchange resins in density and porosity. They absorb low molecular weight organics and reduce chloride or other halogens from water, but do not remove any salts.
They must be change periodically to avoid bacterial growth, but are not easily reactivated in the field.
Dual or multi-media filters - these remove suspended solids to as low as 20 microns in size, but not dissolved solids. The top layer is coarse anthracite followed by fine sand.
Note: the type of filters used depends primarily on the nature of the water.
3. Disinfection or Sterilization - The destruction of disease-causing organisms in the raw and treated water through the addition of the chemical, chlorine is the most important step in the water treatment process. The chemical is added to water at different points in the treatment process. When chlorine is added to the raw water as it enters the plant, the process is known as pre-chlorination.
When it is added after filtration, it is known as post-chlorination. Additional chlorine can be added when the levels of bacteria are high, through a process known as super chlorina- tion. Sulphur(IV) oxide is then added to the water- it combines with the excess chlorine to reduce the chlorine residual to an acceptable level before the ammoniation stage. Sterilization of water can also be done using ozone, but this is very expensive.
4. Fluoridation - Drinking water with the mineral, fluoride, present in low concentrations will strengthen the outer layer (enamel) of teeth, making them more resistant to tooth decay. Natural water sources already have
some concentration of fluoride present in low amount, so water treatment plants add
additional fluoride after the filtration stage to raise the level to the required amount.
5. Ammoniation - Ammonia is added to combine with the remaining chlorine. This stabilizes the chlorine so that it remains dissolved in the treated water for longer periods of time, keeping
the water safe during its long trip through the distribution system. Ammoniation also prevents minute amounts of chlorine from evaporating out of drinking water (causing smells).
6. Lime Treatment (Ca(OH)2) - The addition of lime and soda ash (Na2CO3)
reduces the level of calcium and magnesium in the water, and is referred to as “lime softening”. The purpose of lime softening is to remove hardness, and then clarify the water and improve its taste.
7. pH Adjustment - Municipal water may be pH adjusted to a pH of an approximately 7.5 to 8.0 to prevent corrosion of water pipes, particularly to prevent dissolution of lead into the water supply. In the case of excessive alkalinity, the pH may be reduced by the addition of CO2. Also, pH adjustment is important because certain chemicals,
membranes, ion exchange resins and other materials used in water treatment are sensitive to specific pH conditions; and is to prevent acid corrosion in boiler feed water by adjusting the pH to be between 8.3 and 9.0 .
Other Chemicals used in Water Treatment Processes:
Dispersants are added when scaling may be expected due to concentration of specific ions in the stream. Dispersants disrupt the scale formation, preventing growth of precipitated crystals.
B. Sequestering (chelating) Agents
Sequestering agents, e.g., polymetaphosphate, (NaPO3)n and tetrasodium diphosphate, Na4P2O7 are used to prevent the negative effects of hardness, preventing the deposition of Ca, Mg, Fe, Mn and Al.
C. Oxidizing Agents
Oxidizing agents have two distinct functions: as a biocide, or to neutralize reducing agents. Example, potassium permanganate - potassium permanganate (KMnO4) is a strong oxidizing agent used in many bleaching applications.
It will oxidize most organic compounds and is often used to oxidize ferrous iron to ferric for precipitation and filtration.
D. Reducing Agents
Reducing agents, like sodium metabisulphite (Na2S2O5), are added to neutralize oxidizing agents such as chlorine or ozone. In membrane and ion exchange systems, they prevent the degradation of certain membranes or resins, which are sensitive to oxidizing agents.
Water - Composition and Reaction
Removal of Hardness from Water
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