Thursday, December 25, 2008

Drinking water plant and how drinking water is treated

The importance of good drinking water in maintaining human health was recognized early in history. However, it took centuries before people understood that their senses alone were not adequate judges of water quality.

Drinking water is so important for good health. Your body is estimated to be about 60 to 70 percent water. Blood is mostly water, and your muscles, lungs, and brain all contain a lot of water. Your body needs water to regulate body temperature and to provide the means for nutrients to travel to all your organs. Water also transports oxygen to your cells, removes waste, and protects your joints and organs.
The reason for having drinking water treatment is because of the appearance of particles in water. Filtration was established as an effective means of removing particles from water and widely adopted in Europe during the eighteenth century. As so it followed to asia and different parts of the world.

The origin of drinking water to your taps
Our Drinking water comes from the water in lakes or rivers (surface water), or from water that comes from wells (groundwater) and some times the sky (rain water). Many people who live in large cities or towns get their water for drinking from lakes and rivers.

Contaminants that may be found in drinking water

There is no such thing as naturally pure water. People are increasingly concerned about the safety of their drinking water. As improvements in analytical methods allow us to detect impurities at very low concentrations in water, water supplies once considered pure are found to have contaminants. We cannot expect pure water, but we want safe water. The health effects of some contaminants in drinking water are not well understood, but the presence of contaminants does not mean that your health will be harmed.

Drinking water can become contaminated at the original water source, during treatment, or during distribution to the home.
• If your water comes from surface water (river or lake), it can be exposed to acid rain, storm water runoff, pesticide runoff, and industrial waste. This water is cleansed somewhat by exposure to sunlight, aeration, and micro-organisms in the water.
• If your water comes from groundwater (private wells and some public water supplies), it generally takes longer to become contaminated but the natural cleansing process also may take much longer. Groundwater moves slowly and is not exposed to sunlight, aeration, or aerobic (requiring oxygen) micro-organisms. Groundwater can be contaminated by disease-producing pathogens, leachate from landfills and septic systems, careless disposal of hazardous household products, agricultural chemicals, and leaking underground storage tanks.

In general all water contains some impurities
• Erosion of natural rock formations
• Substances discharged from factories
• Discharged from farmlands
• used by consumers in their homes and yards

Drinking water treatment Process

The process flow diagram illustrates a municipal water treatment plant that is used for the removal of taste and odour compounds. Water is pumped from the river into a flotation unit, which is used for the removal of suspended solids such as algae and particulate material. Dissolved air is injected under pressure into the basin through special nozzles. This creates microbubbles which become attached to the suspended solids, causing them to float. The result is a layer of suspended solids on the surface of the water, which is removed using a mechanical skimming technique.

water coagulation & flocculation is used for removal of natural organic matter (NOM) from surface waters.

Ozone is produced on site by passing high tension, high frequency electrical discharges through air in specially designed equipment. Ozone is injected into the water to provide a powerful bactericidal action and to break down the natural humic compounds that are the cause of the taste and odour problem.

Coagulation, flocculation and sedimentation
In traditional water treatment, certain chemicals are added to raw water to remove impurities. While some particles will spontaneously settle out from water on standing (a process called sedimentation), others will not. To cause particles that are slow to settle or are non-settling to settle out more readily, a soluble chemical or mixture of chemicals is added to the water. Such a chemical is called a coagulant and the process is called coagulation.
The coagulant reacts with the particles in the water, forming larger particles called flocs, which settle rapidly.

Flocs can also be effectively removed by passing the water through a filter. The process is controlled so that the coagulant chemicals are removed along with the contaminants.
Coagulation/flocculation processes generally use aluminium sulphate (alum) or ferric chloride as the coagulant.

• Filtration
One of the oldest and simplest processes used to treat water is to pass it through a bed of fine particles, usually sand. This process is called sand filtration. In its simplest form, the water is simply passed through the filter with no other pre-treatment, such as the addition of a coagulant. Usually this type of filter will remove fine suspended solids and also some other particles such as larger microorganisms.

Sand filtration is even more efficient when the water being treated passes through the sand filter very slowly. Over time the sand particles become covered with a thin surface layer of microorganisms. Some might refer to this layer as a slime but water scientists call it a biofilm. Even very small particles stick to this biofilm and are held, while water of greatly improved quality passes out through the filter.

• Disinfection
Disinfection is carried out to kill harmful microorganisms that may be present in the water supply and to prevent microorganisms regrowing in the distribution systems.
Good public health owes a lot to the disinfection of water supplies. Without disinfection, waterborne disease becomes a problem, causing high infant mortality rates and low life expectancy. This remains the situation in some parts of the world.

Key factors considered by a water authority in selecting a disinfection system are:
• Effectiveness in killing a range of microorganisms.
• Potential to form possibly harmful disinfection byproducts.
• Ability of the disinfecting agent to remain effective in the water throughout the distribution system.
• Safety and ease of handling chemicals and equipment.
• Cost effectiveness.

Water is distributed after all this process to our taps.

Introcduction to envirometal engineering, 2nd edition, P.Aarne Vesilind | Susan M. Morgan


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