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Mention the steps involved in the purification of water at waterworks?

Dear Student,

The process of treating water includes:

  1. Bar screen
  2. Sand and grit removal
  3. Clarification
  4. Aeration

Let us understand the different processes involved in the treatment of water. 

Bar screen

 

The purpose of this section of the water treatment plant is to remove large-sized objects from the such as rags, sticks, cans, plastic packets, napkins, papers etc by allowing the water to pass through a bar screen.

Grit and sand removal

 

After the water is treated through the bar screen, it goes into the grit and sand removal tank. In this tank, the flow of the water is slowed down to allow sand, pebbles, and grit to settle down at the bottom of the tank.

Clarification

After the water passes through the grit and sand removal tank, it is allowed to settle down in a large tank, with a slope toward the middle. Solid materials settle at the bottom and are removed with the help of a scraper. The deposited waste material thus collected is called sludge. The contaminants that float over the surface of the water are removed by a skimmer. The water that is treated in this process is called clarified water and the process is called clarification.

 

Sludge in the tank is allowed to decompose with the help of anaerobic bacteria (bacteria which can survive in the absence of oxygen) to produce biogas, that can be used for the generation of electricity.

Aeration

Aeration is the process that follows the process of clarification. Air is pumped into the water so that the aerobic bacteria (bacteria which need oxygen to survive), can grow in it. These bacteria consume human wastes, food wastes, soaps and other pollutants that remain in the clarified water.

This pumping of air into the clarified water goes on for several hours. Eventually, the microbes start to settle down at the bottom of the tank as activated sludge. The process of removal of water starts from the top of the tank. The activated sludge that settles down at the bottom of the tank is about 97% water. This water is removed by machines or sand-drying beds. Dry sludge can be used as manure. The water treated through this process contains low organic material. It is therefore discharged into the sea or river or into the ground for natural purification.

Before releasing the water treated in this manner, chlorination is necessary in order to make it germ-free. Chlorine tablets are thus added to it. This process is known as chlorination. Chlorine acts as a disinfectant. It is harmless to humans and other animals.

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The Eight Steps Involved in Water Purification

Pre-Treatment
Pumps bring ?raw" or untreated water, often from lakes or rivers, into the purification plant through screens that exclude fish, weeds, branches and large pieces of debris. Screening may not be necessary for groundwater. The plant may aerate the water at this point to increase the oxygen content and thus help remove problematic odors and tastes.

Coagulation & Flocculation
The purpose of these two steps is to clear water of the small particles that cause it to be turbid or cloudy. Turbidity renders the water hard to disinfect. The water is rapidly agitated to disperse coagulant chemicals throughout it. The small particles, including many bacteria, begin to form large clumps called flocs or floccules. In flocculation, the water is mixed gently so that these clumps combine and precipitate out further.

Sedimentation
The water and flocs are pumped into sedimentation basins. Here, the flocs settle beneath the water so that they can be removed. About 85 to 90 percent of the suspended particles responsible for turbidity are removed at this point, including large amounts, but not all, of the bacteria.

Filtration
In filtration, the water flows through a multilayer medium such as quartz sand, activated carbon or anthracite coal in order to remove up to 99.5 percent of the solid materials remaining in it, whether flocs, microbes or minerals. This step usually is the last one in the process of removing solids from the water.

Disinfection
Disinfection kills off disease-bearing organisms in the water. Most water treatment plants use chemicals, generally chlorine compounds, as disinfectants. Although chlorine is still one of the most common disinfectants, ultraviolet radiation and ozone gas are becoming more widespread. Chlorine is increasing in cost and has some known toxic effects on humans and fish. In addition, some disease-carrying microbes like Giardia and Cryptosporidium resist chlorine.

Corrosion & Scale Control
The pH of the water is adjusted so that it neither corrodes nor deposits too much scale in pipes. Excessive amounts of scale can disrupt plumbing systems, but small quantities help pipes to function at their best. However, no amount of corrosion in the water distribution system is desirable. As well as causing leaks and other damage, corrosion releases pipe metals like lead and copper that jeopardize human health.

Taste & Odor Control
Unpleasant tastes and odors remaining in the water, such as those from algae, often do not pose any health hazards. Yet consumers prefer to do without them. So water purification plants often remove tastes and odors through additional chemical treatment, ozonation or filtration. At this stage, some municipalities also require the addition of fluoride to the water for dental health.
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