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Water Filtration: Slow Sand Filter & Rapid Sand Filter

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FILTRATION - Overview

1. Slow Sand (Biological) Filters
2. Rapid Sand (Mechanical) Filters

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PROCESS FLOWCHART: Water Purification

RAW WATER (Source)
        │
        ▼
┌─────────────────┐
│   STORAGE        │  (10-14 days; removes 90% bacteria, suspended matter settles)
│ (Reservoir)      │
└────────┬────────┘
         │
         ▼
┌─────────────────┐
│  FILTRATION      │  ← You are here (Slow Sand OR Rapid Sand)
└────────┬────────┘
         │
         ▼
┌─────────────────┐
│  DISINFECTION    │  (Chlorination, Ozonation, UV)
└────────┬────────┘
         │
         ▼
     SAFE WATER
      (Distribution)

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PART 1: SLOW SAND (BIOLOGICAL) FILTERS

History

• First used in 1804 in Scotland, then London
• Spread globally through the 19th century
• Still accepted as the standard method of water purification

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Elements of a Slow Sand Filter

1. Supernatant (raw) water
2. A bed of graded sand
3. An under-drainage system
4. A system of filter control valves

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(1) Supernatant Water

• Depth: 1 to 1.5 metres above sand bed
• Two purposes:
  • Provides constant head of water to overcome resistance of filter bed and promote downward flow
  • Provides a waiting period of 3-12 hours (depending on filtration velocity) for partial purification by sedimentation, oxidation, and particle agglomeration
• Level of supernatant water is always kept constant

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(2) Sand Bed

• Mechanical straining
• Sedimentation
• Adsorption
• Oxidation
• Bacterial action

The Vital Layer (Schmutzdecke / Zoogleal Layer / Biological Layer)

• When filter is newly laid, it acts only as a mechanical strainer - not truly biological
• Very soon, a slimy growth called "Schmutzdecke" (also called vital layer, zoogleal layer, or biological layer) forms on the sand surface
• Consists of: threadlike algae, plankton, diatoms, and bacteria
• Formation is called "ripening" of the filter - takes several days
• Functions of the vital layer:
  • Removes organic matter
  • Holds back bacteria
  • Oxidizes ammoniacal nitrogen into nitrates
  • Yields bacteria-free, good quality filtered water
• The vital layer is the "heart" of the slow sand filter

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(3) Under-drainage System

• Consists of perforated pipes at the bottom
• Collects filtered water and conveys it to the clear well

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(4) Filter Control

• Controls the rate of filtration
• Flow control valve regulates water output
• Venturi meter measures flow rate

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Flowchart: Working of Slow Sand Filter

RAW WATER enters through inlet (A)
         │
         ▼
SUPERNATANT WATER (1-1.5 m depth)
  → Sedimentation + partial purification (3-12 hrs)
         │
         ▼
VITAL LAYER / SCHMUTZDECKE (top of sand)
  → Biological removal of bacteria, organic matter
         │
         ▼
SAND BED (1 metre thick; grain size 0.2-0.3 mm)
  → Mechanical straining, adsorption, oxidation
         │
         ▼
GRAVEL LAYERS (30-40 cm)
  → Supports sand, prevents fine grain entry into pipes
         │
         ▼
UNDER-DRAINAGE (perforated pipes)
         │
         ▼
FILTERED WATER → Weir and outlet chamber → Clear well

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Cleaning the Slow Sand Filter

• When efficiency drops, the top layer of sand (2.5 cm) is scraped off along with the Schmutzdecke
• The scraped sand is washed, dried, and replaced
• After cleaning, the filter must "ripen" again (new Schmutzdecke forms) before it is fully effective
• A filter runs continuously for 20-30 days before cleaning is needed

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Advantages of Slow Sand Filter

1. Simple to construct and operate
2. Cost of construction is cheaper than rapid sand filters
3. Physical, chemical, and bacteriological quality of filtered water is very high
4. Bacterial removal: total count reduced by 99.9 to 99.99%, E. coli by 99 to 99.9%
5. Does not require chemical coagulants
6. Suitable for smaller communities

Disadvantages of Slow Sand Filter

1. Requires a large land area
2. Rate of filtration is very slow (2-3 million gallons per acre per day)
3. Requires preliminary storage of raw water
4. Cleaning is labour-intensive (manual scraping)
5. Cannot handle highly turbid raw water effectively

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PART 2: RAPID SAND (MECHANICAL) FILTERS

History

• First installed in USA in 1885
• Gained considerable popularity especially in highly industrialized countries
• Two types: Gravity type (e.g., Paterson's filter) and Pressure type (e.g., Candy's filter)

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Steps in Rapid Sand Filtration

RAW WATER
    │
    ▼
┌───────────────────────────────┐
│ STEP 1: COAGULATION            │
│ Add alum (5-40 mg/L or more)  │
│ Dose depends on turbidity,    │
│ colour, temperature, pH       │
└──────────────┬────────────────┘
               │
               ▼
┌───────────────────────────────┐
│ STEP 2: RAPID MIXING           │
│ Violent agitation in mixing   │
│ chamber for a few minutes     │
│ → thorough dissemination      │
│   of alum in water            │
└──────────────┬────────────────┘
               │
               ▼
┌───────────────────────────────┐
│ STEP 3: FLOCCULATION           │
│ Slow, gentle stirring in      │
│ flocculation chamber ~30 min  │
│ Mechanical flocculators:      │
│ paddles rotate at 2-4 rpm     │
│ → Thick white flocculent      │
│   precipitate of Al(OH)₃      │
└──────────────┬────────────────┘
               │
               ▼
┌───────────────────────────────┐
│ STEP 4: SEDIMENTATION          │
│ Coagulated water → settling   │
│ tanks for 2-6 hours           │
│ ≥95% floc must settle out     │
│ before entering rapid filter  │
│ Sludge removed periodically   │
└──────────────┬────────────────┘
               │
               ▼
┌───────────────────────────────┐
│ STEP 5: RAPID SAND FILTRATION │
│ Partly clarified water passed │
│ through rapid sand filter bed │
└──────────────┬────────────────┘
               │
               ▼
          FILTERED WATER
               │
               ▼
          DISINFECTION

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Filter Beds (Rapid Sand)

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Filtration Process (Inside the Filter)

• The alum-floc not removed by sedimentation is held back on the sand bed
• It forms a slimy layer comparable to the zoogleal layer in slow sand filters
• This layer adsorbs bacteria and effects purification
• Oxidation of ammonia also takes place
• As filtration proceeds, suspended impurities and bacteria clog the filter
• When "loss of head" approaches 7-8 feet, filtration is stopped for washing

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Backwashing

• Rapid sand filters require frequent washing - daily or weekly depending on loss of head
• Cleaning by reversing the flow of water through the sand bed = "backwashing"
• Backwashing dislodges impurities and cleans the sand bed
• Stopped when clear sand is visible and wash water is sufficiently clear
• The whole process takes about 15 minutes
• Some filters also use compressed air as part of backwashing

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Advantages of Rapid Sand Filter over Slow Sand Filter

1. Can deal with raw water directly - no preliminary storage needed
2. Filter beds occupy less space
3. Filtration is rapid - 40-50 times that of slow sand filter
4. Washing (backwashing) is easy and quick
5. More flexibility in operation

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PART 3: COMPARISON TABLE (Table 3 from Park's)

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QUICK MEMORY AID

SLOW SAND = "BIOLOGICAL"
  ✦ Speed: SLOW (0.1-0.4 m³/hr/m²)
  ✦ Sand: Fine (0.2-0.3 mm)
  ✦ Key feature: SCHMUTZDECKE (vital layer)
  ✦ No coagulant needed
  ✦ Cleaning: Manual scraping
  ✦ History: 1804, Scotland
  ✦ Best removal of bacteria

RAPID SAND = "MECHANICAL"
  ✦ Speed: FAST (200 mgad - 40-50x)
  ✦ Sand: Coarser (0.4-0.7 mm)
  ✦ Key steps: Coagulation → Mixing → Flocculation → Sedimentation → Filtration
  ✦ Alum REQUIRED
  ✦ Cleaning: Backwashing (15 min)
  ✦ History: 1885, USA

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