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Identification: Trickling Filter (Percolating Filter)

The image shows a trickling filter (also called a percolating filter) - a component used in the secondary treatment stage of sewage/wastewater treatment. It is a community medicine topic covered under environmental sanitation and sewage disposal.
The key visible feature is the rotating sprinkler arm - hollow pipes mounted on a central pivot that revolve and uniformly sprinkle sewage effluent over the surface of the filter bed.

Structure

FeatureDetail
Filter bedCrushed stones or cinder
Depth1 to 2 metres (4-8 ft)
Diameter2 to 30 metres (6-100 ft), depending on population size
SprinklerRevolving hollow pipes with holes that distribute effluent as a thin film

Where It Fits in Sewage Treatment

Sewage treatment proceeds in stages:
  1. Preliminary treatment - screening and grit removal
  2. Primary treatment - primary sedimentation tank (removes 50-70% of suspended solids by gravity over 6-8 hours)
  3. Secondary treatment - the effluent from the primary tank still contains dissolved organic matter, colloidal material, and living organisms with high oxygen demand. It undergoes aerobic oxidation by one of two methods:
    • (a) Trickling filter method ← this image
    • (b) Activated sludge process
  4. Secondary sedimentation - oxidized sewage from the trickling filter is led into a secondary sedimentation (humus) tank

Mechanism of Action

  • The effluent from the primary sedimentation tank is sprinkled uniformly onto the filter bed surface by the rotating arms.
  • The bed is colonized by a complex biological growth of algae, fungi, protozoa, and bacteria - this is called the "zoogleal layer".
  • As effluent percolates down through the bed, it is oxidized by the bacterial flora in the zoogleal layer.
  • The action is purely biological, NOT mechanical filtration - the term "filter" is a misnomer.
  • Wind blows freely through the open beds, supplying the oxygen needed by the zoogleal flora, so the filter does not need rest pauses.
  • The zoogleal layer lives, grows, and dies. Dead material sloughs off and is washed down as a light green, flocculent material called "humus".

Advantages of the Trickling Filter

  • Very efficient at purifying sewage
  • No rest pauses needed (unlike intermittent sand filters)
  • Natural aeration by wind - no mechanical aeration required
  • Continuous operation

After the Trickling Filter

The oxidized sewage (containing humus) is led into secondary sedimentation tanks (humus tanks) where it is detained for 2-3 hours. The settled sludge is called "aerated sludge" or "activated sludge" and is returned or disposed of. The final effluent can then be safely discharged.
Source: Park's Textbook of Preventive and Social Medicine, Secondary Treatment - Trickling Filter Method
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