Water Treatment and Disinfection Methods

Disinfection of Drinking Water

• Surface water and groundwater require purification and disinfection; sometimes special treatments are needed.
• Improving water sources is about easily acquiring good water quality, convenient use, and easy hygiene management.
• Water treatment involves four stages: coagulation, sedimentation, filtration, and disinfection.
• A diagram (Figure 2-11) shows the stages of water treatment at a water treatment plant, it shows the sequence.
• Disinfection is a method of killing pathogenic microorganisms.
• Methods of drinking water disinfection in China include physical (boiling, ultraviolet) and chemical (chlorine, chlorine dioxide, ozone) methods.
• Common disinfectants in China include liquid chlorine, bleaching powder, and bleaching powder concentrate.
• Chlorine reacts with water to form hydrochloric acid and hypochlorous acid. Hypochlorous acid then ionizes into hydrogen ions and hypochlorite ions.
• Bleaching powder and bleaching powder detergent turn into hypochlorous acid in water.
• Chlorination has a bactericidal effect. The chlorine compounds have the ability to kill bacteria.
• Chlorine demand is the amount of available chlorine consumed by oxidizing organic and reduced inorganic matters in water.
• Residual chlorine refers to the leftover active chlorine after disinfection. It prevents bacteria from regenerating and reproducing.
• A method to determine the best disinfection concentration is to use different amounts of bleach solution in water samples, wait 30 minutes, then measure the residual chlorine (ideally 0.3mg/L).
• Residual chlorine can react with o-tolidine to create a color change. The depth and color produced are used to quantify the chlorine level.
• The residual chlorine comparator is used to measure color depth.
• Materials needed include Erlenmeyer flasks (250 mL, 5), volumetric cylinders (100 mL, 1), measuring pipettes (5 mL, 1), residual chlorine comparators (1), graduated tubes (10 mL, 5).
• Reagents include 0.01% bleach solution, dilute with water to 100 mL; 0.1% o-tolidine/role biphenyl quinone solution mixed with HCl (3:7).
• Procedure includes adding bleach to water samples, mixing, waiting 30 minutes, adding o-tolidine solution, mixing, waiting 10 minutes, and measuring color depth using a residual chlorine comparator.
• The best bleach solution concentration for disinfection is 0.3 mg/L.
• The experimental report should include principle, procedure, and results.
• To calculate needed bleach solution dose, the formula is: Disinfection(mg/L)=1000CV/100, C (concentration mg/ml) = 0.1 mg/ml, V (mL): Volume of bleach solution needed.
• Residual Chlorine Comparators are rough measuring devices.

Precautions

• Residual Chlorine Comparators are rough measurement instruments.
• The optimal residual chlorine level in disinfected water is 0.3 mg/L.
• Handle the residual chlorine comparators carefully.