envi mod 7

• Purpose of water treatment systems

  To bring raw water up to drinking water quality
• About half of the drinking water in the United States comes from groundwater, and half comes from surface water
• Surface water treatment


  • Focuses on particle removal

  
• Groundwater treatment


  • Focuses on removal of dissolved inorganic contaminants such as calcium and iron
• Producing water free of microbial pathogens is critical for any water source, but surface water has a much greater chance of microbial contamination, so filtration is now a requirement for surface water
• Typical surface water treatment plant

  1. Screening and grit removal

  2. Primary sedimentation

  3. Rapid mixing and coagulation

  4. Flocculation

  5. Secondary settling

  6. Filtration

  7. Sludge processing

  8. Disinfection contact

  
• Typical groundwater treatment plant

  1. Aeration

  2. Flocculation (and precipitation)

  3. Sedimentation

  4. Recarbonation

  5. Filtration

  6. Disinfection

  7. Solids processing

  
• Sedimentation


  Gravitational settling of particles from water

  
• Sedimentation basin/clarifier


  • Large circular or rectangular tank designed to hold the water for a long enough time to allow most of the suspended solids to settle out

  
• Equivalent diameter


  The hydrodynamic diameter when we speak of particles settling in water, and aerodynamic diameter for particles settling in air
• Coagulation


  A chemical treatment process that destabilizes particles (makes them "sticky"), as opposed to a physical treatment operation such as flocculation, sedimentation, or filtration

  
• Colloids


  Particles in the size range of about 0.001 to 1 μm
• Coagulation reactions with alum

  1. Alum ionizes in water, producing Al3+ ions, some of which neutralize the negative charges on the colloids

  2. Most of the aluminum ions react with alkalinity in the water (bicarbonate) to form insoluble aluminum hydroxide, Al(OH)3
• Collision efficiency factor (α)


  The fraction of the number of collisions between particles that result in aggregation
• Coagulation and flocculation


  1. Coagulants are added in a rapid mix/coagulation tank

  2. Flocculation follows in a tank that provides gentle agitation for approximately one-half hour
• Filtration


  One of the most widely used and effective means of removing small particles from water, including pathogens

  
• Rapid depth filtration


  • The filter consists of a layer or layers of carefully sieved filter media, such as sand, anthracite coal, or diatomaceous earth, on top of a bed of graded gravels
• Disinfection


  The final, primary unit operation in most water treatment trains, to kill any pathogens in the water and prevent pathogen regrowth
• Free chlorine disinfection


  The most commonly used method of disinfection in the U.S., using chlorine gas, sodium hypochlorite, or calcium hypochlorite
• Free chlorine is very effective against bacteria, but its effectiveness is less with protozoan cysts, most notably those of Giardia lamblia and Cryptosporidium, and with viruses
• Disinfectant byproducts (DBPs)


  Halogenated compounds such as trihalomethanes (THMs) and haloacetic acids (HAAs) formed when free chlorine combines with natural organic substances
• Hardness


  The presence of multivalent cations, most notably calcium and magnesium ions
• Problems caused by hardness

  • Soap curd formation, scaling of pipes and equipment
• Softening


  The process of removing hardness, typically by the lime-soda ash process or the ion-exchange process