Water treatment study ,a method to purify waste water

a water treatment purification method

1. Water treatment and purification Chlorination in water purification Principle Chlorine is used as a disinfectant in water purification. As a strong oxidant , chlorine kills many harmful microorganism in water. Chlorine is used in different forms like gas, or its compounds like hypochlorite, chloramine , chlorine dioxide, the dose of chlorine usually applied is sufficiently high to meet chlorine demand ( i ,e . the tendency of organic substances and ammonia to react with chlorine) and still leave a sufficient concentration ( chlorine residual ) to inactivate microorganisms throughout the distribution system. Reaction mechanism in chlorination When chlorine is added to water, it reacts broadly at two stages : Stage 1 : Chlorine reacts with different organic compounds present in water through a number of reactions. Here only reaction occurs, no disinfection occurs . For example, reaction with hydrogen sulfide (H 2 S) and chlorine in water is shown below :

2. Water treatment and purification Hydrogen Sulfide + Chlorine + Oxygen Ion Elemental Sulfur + Water + Chloride Ions H 2 S + Cl 2 + O 2- S + H 2 O + 2Cl - Hydrogen Sulfide + Chlorine + Water Sulfuric Acid + Hydrochloric Acid H 2 S + 4Cl 2 + 4H 2 O H 2 SO 4 + 8 HCl In the first reaction, hydrogen sulfide reacts with chlorine and oxygen to create elemental sulfur, water and chloride ions. The elemental sulfur precipitates out of the water and can cause odor problems. In the second reaction, hydrogen sulfide reacts with chlorine and water to create sulfuric acid and hydrochloric acid. * Each of these reactions uses up the chlorine in the water, producing chloride ions or hydrochloric acid which has no disinfecting properties . # The total amount of chlorine which is used up in reactions with compounds in the water is known as the chlorine demand . ● A sufficient quantity of chlorine must be added to the water so that, after the chlorine demand is met, there is still some chlorine left to kill microorganisms in the water.

3. Water treatment and purification Stage 2 : Once the precipitation reactions under stage 1 is completed, the leftover chlorine reacts with water to kill the existing microorganisms in water. At this stage, disinfection occurs . ● Chlorine can be used as chlorine gas, hypochlorite , chloramine and chlorine dioxide. For example, reaction of chlorine gas with water is shown below : Chlorine + Water Hypochlorous Acid + Hydrochloric Acid Cl 2 + H 2 O HOCl + HCl (pH : 6-7) The chlorine reacts with water and breaks down into hypochlorous acid and hydrochloric acid. Hypochlorous acid may further break down, depending on pH : Hypochlorous Acid ↔ Hydrogen Ion + Hypochlorite Ion HOCl ↔ H + + OCl - (pH : 8-10) * This hypochlorous acid ( HOCl ) is called the most effective form of free chlorine residual . This means, it is the chlorine available to kill microorganisms in the water.

4. Water treatment and purification Hypochlorites Instead of using chlorine gas, some plants apply chlorine to water as a hypochlorite, also known as bleach. Hypochlorites are less pure than chlorine gas. There are three types of hypochlorite : * Sodium hypochlorite - NaOCl * Calcium hypoclorite - Ca(OC1) 2 * Commercial bleach Chloramines Some plants use chloramines rather than hypochlorous acid to disinfect the water. To produce chloramines, first chlorine gas or hypochlorite is added to the water to produce hypochlorous acid. Then ammonia is added to the water to react with the hypochlorous acid and produce a chloramine .

5. Water treatment and purification Chlorine Dioxide : ClO 2 There is one other form of chlorine which can be used for disinfection – chlorine dioxide. Chlorine dioxide is a very effective form of chlorination since it kills protozoans , Cryptosporidium, Giardia and viruses that other systems may not kill. Breakpoint Chlorination # The breakpoint is the point at which the chlorine demand has been totally satisfied – the chlorine has reacted with all reducing agents, organics and ammonia in the water. # When more chlorine is added past the breakpoint, the chlorine reacts with water and forms hypochlorous acid in direct proportion to the amount of chlorine added. This process, known as breakpoint chlorination , is the most common form of chlorination, in which enough chlorine is added to the water to bring it past the breakpoint and to create some free chlorine residual.

6. Water treatment and purification Disadvantages of chlorination A major shortcoming is that chlorine combines with organic substances in the water to produce chlorinated by-products, some of which are toxic (e.g., trihalomethanes such as chloroform). Chlorine dioxide does not generate significant by-product formation, but its intermediates (chlorite and chlorate) are toxic.