Mechanisms of Filtration in Water Treatment
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Questions and Answers

What is the primary mechanism of filtration in a rapid sand filter?

  • Straining (correct)
  • Adsorption
  • Absorption
  • Biological action

    • What is the main advantage of slow sand filters?

  • They are highly effective at removing impurities
  • They are easy to clean and maintain
  • They require minimal land area
  • They are relatively small and compact (correct)

    • What is the purpose of the gravel layer in a slow sand filter?

  • To provide additional filtration
  • To support the sand layer (correct)
  • To reduce the flow rate of water
  • To remove large impurities

    • What is the filtration rate of a slow sand filter?

    <p>0.1-0.3 m3 per hour</p> Signup and view all the answers

    What is the main difference between slow sand filters and rapid sand filters?

    <p>The filtration rate</p> Signup and view all the answers

    What is the common application of slow sand filters?

    <p>Groundwater treatment</p> Signup and view all the answers

    What is the role of absorption in the filtration process?

    <p>It is not very important in the filtration process</p> Signup and view all the answers

    How is a rapid sand filter cleaned?

    <p>Through backwashing</p> Signup and view all the answers

    What is the purpose of sedimentation in water treatment?

    <p>To allow suspended particles to settle out of the water</p> Signup and view all the answers

    Which zone in the sedimentation basin provides a calm area for suspended particles to settle?

    <p>Settling zone</p> Signup and view all the answers

    What happens to the clarified water after sedimentation?

    <p>It drains out of the top of the sedimentation tank</p> Signup and view all the answers

    What is the purpose of pre-chlorination and dechlorination in water treatment?

    <p>To kill algae and remaining bacteria from sedimentation</p> Signup and view all the answers

    What is the purpose of filtration in water treatment?

    <p>To remove suspended particles from water</p> Signup and view all the answers

    What happens to the flocs in the sedimentation tank?

    <p>They settle out of the water and are scraped and vacuumed off the bottom</p> Signup and view all the answers

    What is the function of the inlet zone in the sedimentation basin?

    <p>To provide a smooth transition from the flocculation zone</p> Signup and view all the answers

    What is the function of the sludge zone in the sedimentation basin?

    <p>To store the sludge before it is removed for additional treatment or disposal</p> Signup and view all the answers

    What is the primary mechanism of particle removal in rapid sand filters?

    <p>Adsorption and some straining</p> Signup and view all the answers

    What is the purpose of the underdrain in a rapid sand filter?

    <p>To collect the filtered water</p> Signup and view all the answers

    What is the purpose of backwashing in rapid sand filters?

    <p>To remove trapped particles</p> Signup and view all the answers

    How do rapid sand filters typically operate during filtration?

    <p>The influent flows down through the sand and support gravel</p> Signup and view all the answers

    What is the primary reason rapid sand filters operate more quickly than slow sand filters?

    <p>The influent water is already relatively clear</p> Signup and view all the answers

    What is used to assess when a rapid sand filter needs backwashing?

    <p>Length of the filter run, turbidity of the effluent water, and head loss within the filter</p> Signup and view all the answers

    What is the purpose of the backwash pump in rapid sand filters?

    <p>To force treated water from the system back up through the filter bed</p> Signup and view all the answers

    What is closed during backwashing in rapid sand filters?

    <p>The influent valve</p> Signup and view all the answers

    What is the approximate diameter of a grain of fine sand in a filtration system?

    <p>0.1mm</p> Signup and view all the answers

    Why would a filter made of fine sand not be able to produce safe drinking water?

    <p>Because it would not be able to remove particles smaller than 0.1mm</p> Signup and view all the answers

    What is the mechanism of filtration that involves particles becoming attracted to and sticking to the sand particles?

    <p>Adsorption</p> Signup and view all the answers

    What is the term for the process of passing water through a filter with smaller pores than the particles to be removed?

    <p>Straining</p> Signup and view all the answers

    What is the minimum size of particles that can be removed by a filter with a fine sand layer?

    <p>0.1mm</p> Signup and view all the answers

    What is the term for the gathering of gas, liquid, or dissolved solids onto the surface of another material?

    <p>Adsorption</p> Signup and view all the answers

    What is the mechanism of filtration that involves the breakdown of particles by biological processes?

    <p>Biological action</p> Signup and view all the answers

    What are the four mechanisms of filtration?

    <p>Straining, adsorption, biological action, and absorption</p> Signup and view all the answers

    What is the purpose of collecting dirty backwash water in wash troughs?

    <p>To allow it to settle in a tank, pond, or basin</p> Signup and view all the answers

    What is the typical range of available chlorine in Sodium hypochlorite solution?

    <p>10-14%</p> Signup and view all the answers

    What is the purpose of maintaining a certain amount of residual chlorine in the water?

    <p>To protect against future recontamination</p> Signup and view all the answers

    What is a limitation of chlorine as a disinfectant?

    <p>It is not effective against protozoans that form cysts in water</p> Signup and view all the answers

    What is a potential drawback of using chlorine as a disinfectant?

    <p>It reacts with natural organic compounds in the water to form potentially harmful disinfection by-products</p> Signup and view all the answers

    What is the typical percentage of available chlorine in Calcium hypochlorite granules?

    <p>30-40%</p> Signup and view all the answers

    What is the minimum amount of residual chlorine recommended in the water after half an hour contact time?

    <p>0.2-0.5 mg/L</p> Signup and view all the answers

    What is the common name for Calcium hypochlorite granules?

    <p>Bleaching powder</p> Signup and view all the answers

    Study Notes

    Mechanisms of Filtration

    • Absorption: soaking up of one substance into the body of another substance, but not very important in the filtration process
    • Types of filters: slow sand filtration and rapid sand filtration
      • Slow sand filtration: filtration rate of 0.1-0.3 m3 per hour, pros include reliability, small size, and low maintenance, but requires large land area and manual cleaning
      • Rapid sand filtration: filtration rate of 4-20 m3 per hour, pros include high filtration rate, but requires chemical pretreatment and backwashing
    • Filter media: sand and anthracite coal
    • Filtration mechanism: biological action, straining, adsorption, and absorption

    Sedimentation

    • Sedimentation: accomplished by decreasing the velocity of the water to a point below which particles will no longer remain in suspension
    • Sedimentation basin:
      • Inlet zone: provides a smooth transition from the flocculation zone and distributes the flow uniformly
      • Settling zone: the largest portion of the basin, provides a calm area for suspended particles to settle
      • Sludge zone: provides a storage area for the sludge before it is removed for additional treatment or disposal
      • Outlet zone: provides a smooth transition from the sedimentation zone to the outlet from the tank

    Filtration Process

    • Purpose: to remove suspended particles from water by passing it through filters
    • Filters: consist of filters with varying sizes of pores, often made up of sand, gravel, and charcoal
    • Mechanisms of filtration:
      • Straining: passing water through a filter with smaller pores than the particles to be removed
      • Adsorption: particles becoming attracted to and "sticking" to the sand particles
      • Biological action: breakdown of particles in water by biological processes
      • Absorption: soaking up of one substance into the body of another substance

    Rapid Sand Filter

    • Mechanism of particle removal: primarily adsorption and some straining, no biological action
    • Filter media: sand and anthracite coal
    • Operation: similar to slow sand filter, but influent water is already relatively clear due to coagulation/flocculation and sedimentation
    • Cleaning: through backwashing, which reverses the flow of water through the filter, cleaning out trapped particles

    Disinfection

    • Purpose: to kill remaining bacteria and other microorganisms
    • Methods: chlorination, usually using liquefied chlorine gas, sodium hypochlorite solution, or calcium hypochlorite granules
    • Objective: to add enough chlorine to leave 0.2-0.5 mg/L residual chlorine after half an hour contact time
    • Drawbacks: chlorine has limited effectiveness against protozoans, and can react with natural organic compounds to form potentially harmful disinfection byproducts (DBPs)

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    Description

    This quiz covers the different mechanisms of filtration, including biological action and absorption, and how they are used in water treatment processes.

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