Page 87 - ICSE Chemistry 6
P. 87

Sedimentation and Decantation

                 Sedimenta  on is a process by which heavier insoluble par  cles of a solu  on se  le down. It is the fi rst
                 step to purify water in water treatment plants. Water from rivers or lakes is allowed to fl ow into basins,
                 reservoirs or sedimenta  on tanks and stand for some   me. Here suspended solids se  le at the bo  om
                 and clear water is obtained at the top. The clear water at the top is decanted. This process of pouring out
                 the liquid without disturbing the sediment is known as decanta  on.


                 Loading

                 The process used to increase the rate of sedimenta  on by addi  on of chemicals is called loading. Potash
                 alum is added to water in the loading tank and is allowed to stand for some   me. This speeds up the
                 rate of sedimenta  on thus making the suspended par  cles heavier which eventually se  le down at the
                 bo  om of the tank.


                 Filtration

                 Filtra  on is the process of separa  ng a solid from a liquid by passing the mixture through a fi lter bed or
                 a fi lter paper.

                 During the process of fi ltra  on at a small scale, the impure water is passed through a fi lter paper. The pore
                 size of fi lter paper is smaller than the size of mud par  cles. In this way, par  culate impuri  es remain on
                 the fi lter paper and only water passes through it. The liquid that passes through the fi lter paper is called
                 fi ltrate while the par  culate impuri  es which remain on the fi lter paper are called residue.

                 In water treatment plants, water from the sedimenta  on tank is fi ltered by passing it through fi lter bed.
                 A fi lter bed consists of diff erent layers of sand, gravel and charcoal.

                 Chlorination

                 The process of adding chlorine to water is called chlorina  on.
                 Filtered water is disinfected by adding chlorine to kill any
                                                                                          Thinking Fountain
                 remaining harmful bacteria. In water supply units of municipal           Find out why swimming pools
                 corpora  on, the fi ltered water is sterilised by adding chlorine to      contain chlorinated water.
                 kill harmful organisms. Chlorinated water is stored in a tank for
                 further supply.


                                   sedimentation tank
                                                                fi ltration tank           chlorination tank
                                                                                                        
                      water from                                                                              chlorine
                      a source                                                                                  cylinder

                                                                                                         
                                     sediment                                                           clean water
                                                             sand   charcoal
                                               Fig. 4.8: Purifi cation of water in a water treatment plant




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