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The following are certain operating problems of a rapid sand filter: 1. Sand depth should never be depleted by more than 10 cm. 2. Air binding results due to development of negative head and formation of air bubbles in the filter sand. 3. Water softening with lime-soda leads to incrustation of sand. 4. Bumping of filter bed is caused due to negative head. Which of the above statements are correct?

The following are certain operating problems of a rapid sand filter: 1. Sand depth should never be depleted by more than 10 cm. 2. Air binding results due to development of negative head and formation of air bubbles in the filter sand. 3. Water softening with lime-soda leads to incrustation of sand. 4. Bumping of filter bed is caused due to negative head. Which of the above statements are correct? Correct Answer 1, 2 and 3 only 

Operation Troubles in Rapid Sand Filter

1) Air binding:

  • It happens due to the negative head or formation of air bubbles in the filter media.
  • This could be overcome by more frequent back wasing during these periods. Provision should be made wherever possible to increase the depth by about 15 to 30 cm.
  • The depth 105 m over sand may eliminate air binding problem. If air binding persists, proper washing of filter is done when head loss becomes 2.5-3.5 m or negative head becomes 1-1.2 m

2) Incrustration of media:

  • The problem of incrustation of sand particles may rise as in the case of water softening with lime soda when sand particle get coated with materials that is difficult to remove by normal backwash.
  • The remedy lies in washing the filter ocassionly with sodium hydroxide or bleaching powder.

3) Cracking of filter:

  • Occurs when water is lowered below the surface of the sand, cracks in a filter bed underwater may also arise due to cementing of grains by some materials in the applied water.
  • This can be avoided by use of hand rake or by drawing of bed and removing clogged sand.

4) Bumping of filter beds:

  • The practice of instantaneously changing the filtration rate as opposed to gradually adjusting the filtration rate is referred to as filter bumping. Hydraulic changes, such as those caused by filter bumping, can cause filters to shed particles.
  • This can be controlled by the filter effluent valves changing position in order to maintain the desired flowrate.

5) Mud balls:

  • Caused by the general build up of materials not removed in backwashing. Mud balls accumulate at or near the surface and in course of time, clogs the entire sand media. Can be treated by compressed air scoring for more than 3 minutes.

6) Sand balls:

  • Caused when a disproportionately large discharge of wash water rushes towards expanding the sand and displacing the grave;. This situation arises mainly due to poor distribution of wash water from the underdrain system

Related Questions

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