Question

1. describe to your mate large scale purification from its source till it gets to the consumer

2. explain to your junior the five criteria you will use for the choice of material

3. explain to your junior five ways in which air can be purified without the aid of man

Answer 1

Large scale water treatment often involves seven stages. The steps are given below-

1. screening-

To protect the main units of a traetment plant and to aid in their efficient operation, it is necessary to use screens to remove any large flotating and suspended solids that are present in the inflow. These materials include leaves, twigns, paper, rags, and other debries that could obstruct flow through the plant or damage euipement . These are coarse and final screens.

2. Aeration-

After screening, the water is aerated ( supplied with air) by passing it over a series of steps so that it take in oxygen from the air. This heelps expel soluble gases such as carbon dioxode and hydrogen sulphide and also expels any gaseous organic compounds that might give an undesirable taste of water. Aeration also removes iron or managenese can stain clothing. Once in their insolublr forms, these subtances can be removed by filtration.

In certain instances excess algae in the row water can result in alger growth blocking tha sand filter further down the treatment process. In such situation, chlorination is used in place of , or in addition to , aeration to kill the algae, and this is termed pre- chlorination. This comes before the main satges in the treatment of the water. The pre- chlorination also oxides taste- and odour-causein compounds.

3. Coagulation and flocculation-

After aeration, coaguation takes place to remove the fine particles ( liess than 1microgram in size) that are suspended in the water. In this process a chemical called a coagulant is added to the water, and this neutralises the negative electrical charge of the fine particles. The addition of the coagulant takes place in a rapid mix tank where the coagulant is rapidly dispersed by the impeller.

The next step is flocculation. Here the water is gently stirred by paddles in a flocculation basin and the flocs come into contact larger flocs. The flocculation basin often has a number of compartments with decreasing mixing speeds as the water advances through the basin. This compartmentalised chamber allows increasinly large flocs to form without being broken apart by the mixing blades. Chemicals called flocculants can be added to enhance the process. Organic polymers called polyelectrolytes can be used as flocculants .

4. Sedimentation-

Once large flocs are formed, they need to be settled out, and this takes palce in a process called sedimantation( when the particles fall to the floor of a settling tank). The water is kept in the tank for several hours for sedimentation to take place. The material accumulated at the bottom of the tank is called sludge; this is removed for disposal.

5. Filtration-

Filtration is the process where solids are separated from a liquid. In the water treatment, the soilds that are not separated out in teh sedimentation tank are removed by passing the water through beds of sand and gravel. Rapid gravity filters, with a flow of 4-8 cubic metres per sqare meter of filter surface per hour .

Whwn the filters are full of trapped solids , they are backwashed. In this process, clean water and air are pumped backwards up the filter to dislodge the trapped impurities, and the water carrying the dirt is pumped into the sewage system, if there is one. alternatively, it may be discharged back into the source river after a settlemet satge in sedimentation tank to remove solids.

6. Chlorination-

After sedimentation, the water is disinfected to eliminate any remaining pathogenic microorganisms. The most commomly used disinfectant is chlorine, in the form of a liquide or a gas form. It is relatively cheap, and simple to use. When chlorine is added to water it reacts with the pollutants present, including micro-organisms, over a given peroid of time, referred to as the contact time, The amount of chlorine left after this is called residual chlorine.This stay in the water all the way through the distribution system, protecting it from any micro-organisms that might enter it, until the water reaches the cosumers.

7. Supplymentary treatment-

Supplementary treatment may sometimes be needed for the benefit of the population. One such instance is the fluoridation of water, where fluoride is added to water.. It has been stated by the WHO taht ' fluoridation of water supplies, where possible, is the most effective public health measures for the prevention of dental decay'. The optimum level of fluoride is said to be around 1 mg per litre of water.