Water Technology

• Water


  Nature's most wonderful, abundant and useful compound. Essential for the lives of animals and plants, and occupies a unique position in industries.
• Only 3% of freshwater on the Earth's surface is usable by humans, and 99% of the usable quantity is situated underground
• Uses of water


  • Drinking
  • Bathing
  • Sanitary
  • Washing
  • Irrigation
  • Fire-fights
  • Air-conditioning
  • Production of industrial materials
• In steel industries 30 million liters of water is needed per 1000 tones of production. In paper industries half million liters of water is needed per tone of paper production. In automobile industries 1.5 tones liter of water is required per automobile production.
• Factors to consider when selecting a site for a factory

  • Character & quantity of water available
  • Effects of impurities on the process
  • Quantity of water needed

  
• Impurities of water


  • Suspended impurities
  • Dissolved impurities
  • Colloidal impurities
  • Biological impurities
• Suspended impurities


  Solid particles including clay, mud, sand, bacteria, and algae
• Dissolved impurities


  Salts (Cl-, SO4--, HCO3-) of Ca++ and Mg++ responsible for hardness of water, and gases (CO2, N2, O2, H2S) responsible for acidity
• Colloidal impurities


  Very finely divided silica, clay particles, and aluminum hydroxide responsible for turbidity
• Biological impurities


  Bacteria, pathogens, and vegetable life causing diseases, odour, and colour
• Characteristics of potable (drinking) water

  • Colourless, odourless, and tasteless
  • Free from turbidity and other suspended impurities
  • Free from germs, bacteria, and other pathogenic organisms
  • Free from toxic dissolved impurities
  • pH in the range 7-8.5
  • Moderately soft, with hardness preferably in the range 50-100 ppm
  • Aesthetically pleasant
• WHO standards for drinking water

  • pH: 6.5-9.2
  • BOD: 6
  • COD: 10
  • Arsenic: 0.05 ppm
  • Calcium: 100 ppm
  • Cadmium: 0.01 ppm
  • Chromium: 0.05 ppm
  • Ammonia: 0.5 ppm
  • Copper: 1.5 ppm
  • Iron: 1.0 ppm
  • Lead: 0.1 ppm
  • Mercury: 0.001 ppm
  • Magnesium: 150 ppm
  • Manganese: 0.5 ppm
  • Chloride: 250 ppm
  • Cyanide: 0.05 ppm
  • Nitrate + Nitrite: 45 ppm
  • Polyaromatic hydrocarbons (PAH): 0.2 ppm
  • Selenium: 0.01 ppm
• Methods of purification of water for domestic use

  • Screening
  • Sedimentation
  • Coagulation
  • Filtration
  • Sterilization

  
• Screening


  Removal of floating materials by allowing water to pass through a screen with large number of pores
• Sedimentation


  Removal of suspended impurities like sand and stones by allowing water to settle in large tanks without disturbance
• Coagulation


  Removal of colloidal particles by addition of coagulants like alum, ferrous sulfate, and sodium aluminate
• Coagulation


  Positively charged coagulant species neutralize the charge of colloidal particles, causing them to aggregate and settle down
• Commonly used coagulants


  • Alum (K2SO4, Al2(SO4)3.24H2O)
  • Sodium Aluminate (NaAlO2)
  • Ferrous Sulfate (FeSO4.7H2O)
• Filtration


  Removal of precipitated colloidal particles and bacteria using gravity sand filters or pressure filters
• Gravity sand filter


  Consists of layers of fine sand, coarse sand, and fine gravel. Water is percolated through the layers, with most impurities and germs retained by the top layer.
• Pressure filter


  Consists of a cylindrical vertical tank with three layers of filtering media
• Methods of sterilization


  • Boiling
  • Chlorination
  • Ozonization
  • UV rays

  
• Chlorination


  Killing of germs by chlorine gas, bleaching powder, or chloramines forming hypochlorous acid and nascent oxygen
• Chlorination by chlorine gas

  Cl2 + H2O → HOCl + HCl, HOCl → HCl + [O], Germs + [O] → Killing of germs
• Chlorination by bleaching powder

  CaOCl2 + H2O → Ca(OH)2 + Cl2, Cl2 + H2O → HOCl + HCl, HOCl → HCl + [O], Germs + [O] → Killing of germs
• Chlorination by chloramines


  Cl2 + NH3 → NH2Cl + HCl, NH2Cl slowly decomposes releasing Cl2
• Chlorination


  A method of water treatment
• Advantages of chlorination


  • Economical
  • Less space required
  • Cl2 is easily available
• Disadvantages of chlorination


  • Excess Cl2 produces unpleased taste & odour in water
  • Excessive quantity causes irritation on mucous membrane of mouth
  • The quantity of free Cl2 should not be more than 0.1 to 0.2 ppm in treated water
• Bleaching Powder (CaOCl2)


  A good sterilizer for small water works
• Bleaching Powder treatment


  1. 1 kg of Bleaching Powder is mixed in per 1000 liters of water & the resulting solution is allowed to stand for several hours

  2. CaOCl2 + H2O → Ca(OH)2 + Cl2

  3. Cl2 + H2O → HOCl + HCl

  4. HOCl → HCl + [O]

  5. Germs + [O] → kill the germs
• Chloramines (NH2Cl)


  Produced by mixing Cl2 & NH3 in the ratio of 2:1
• Chloramines treatment


  1. Cl2 + NH3 → NH2Cl + HCl

  2. NH2Cl + H2O → HOCl + NH3

  3. HOCl → HCl + [O]

  4. Germs + [O] → kill the germs
• Ozone


  An effective method of water sterilization

  
• Ozone treatment


  1. Water is allowed to percolate through a tower having perforated partition

  2. Ozone is allowed to enter from bottom

  3. O3 → O2 + [O]

  4. Germs + [O] → killing of germs

  5. Purified water is collected at the bottom of the tank

  
• Ultraviolet rays


  Very effective in killing pathogenic bacteria
• Ultraviolet rays method is not economical for water works, but is used where the mixing of chemical is not permissible for purification of water e.g. food industry, and for disinfection of swimming pool water
• Soft water


  Water which forms lather with soap immediately
• Hard water


  Water which does not form lather with soap immediately but forms lather after consuming more amount of soap
• Hard water contains some dissolved salts of Ca++ & Mg++