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Slo sand filter - make it a cake walk for me... strictly from the park textbook spm

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I now have everything. Let me compile the complete, well-organized answer from Park's textbook.

Slow Sand Filter (Biological Filter)

Park's Textbook of Preventive and Social Medicine

Historical Note

Slow sand filters were first used in 1804 in Scotland, then in London. During the 19th century their use spread worldwide. Even today they are generally accepted as the standard method of water purification (Park's, p. 811-813).
Filtration removes 98-99% of bacteria, apart from other impurities.

The 4 Elements of a Slow Sand Filter

Here is the diagram directly from Park's:
Slow Sand Filter - Park's SPM
FIG. 4 - Slow Sand Filter (Park's SPM)
The filter box has 4 components (top to bottom):

(1) Supernatant (Raw) Water

  • Depth: 1 to 1.5 metres above the sand bed
  • Serves two purposes:
    • Provides a constant head of water to overcome resistance and push water downward through the sand
    • Provides a waiting period of some hours for further oxidation and particle agglomeration
  • Level is always kept constant

(2) Sand Bed - THE MOST IMPORTANT PART

Dimensions:
  • Thickness: ~1 metre (1.2 m in the filter box)
  • Sand grain "effective diameter": 0.2 to 0.3 mm (finer than rapid sand filter)
  • Sand must be clean, free from clay and organic matter, preferably rounded grains
  • Below the sand: a layer of graded gravel 30-40 cm deep - supports the sand and prevents fine grains from entering drainage pipes
How purification happens in the sand bed: Water percolates very slowly (takes 2 hours or more) and is subjected to:
  1. Mechanical straining
  2. Sedimentation
  3. Adsorption
  4. Oxidation
  5. Bacterial action
Rate of filtration: 0.1 to 0.4 m³/hour/m² of sand surface
The Vital Layer (Schmutzdecke) - THE HEART OF THE FILTER
This is the single most important examinable concept:
FeatureDetail
Other namesSchmutzdecke, zoogleal layer, biological layer, vital layer
NatureSlimy, gelatinous
ContentsThreadlike algae, plankton, diatoms, bacteria
DepthExtends 2-3 cm into top portion of sand bed
FormationCalled "ripening" of the filter - takes several days
FunctionsRemoves organic matter; holds back bacteria; oxidizes ammoniacal nitrogen into nitrates; yields bacteria-free water
Until the vital layer is fully formed, the first few days' filtrate is run to waste.
When newly laid, the filter acts only as a mechanical strainer - it is not truly "biological."

(3) Under-drainage System

  • Located at the bottom of the filter bed
  • Consists of porous or perforated pipes
  • Dual purpose: (a) outlet for filtered water, and (b) supports the filter medium above
  • Once laid, it cannot be seen

(4) Filter Control Valves

The filter box from top to bottom:
LayerDepth
Supernatant water1 to 1.5 m
Sand bed1.2 m
Gravel support0.30 m
Filter bottom0.16 m
  • Venturi meter - measures bed resistance ("loss of head")
  • When loss of head exceeds 1.3 metres - uneconomical to run the filter
  • Operator opens a regulating valve to maintain steady filtration rate
Filter Cleaning:
  • Filter may run for weeks or even months without cleaning
  • Cleaned by scraping the top 1-2 cm of sand - done by unskilled labourers or mechanical equipment
  • After 20-30 scrapings, sand bed reduces to 0.5-0.8 m - then the plant is closed and a new bed is constructed (no backwashing like in rapid sand filter!)

Advantages of Slow Sand Filter

  1. Simple to construct and operate
  2. Cheaper to construct than rapid sand filters
  3. Physical, chemical, and bacteriological quality of filtered water is very high:
    • Total bacterial count reduced by 99.9 to 99.99%
    • E. coli reduced by 99 to 99.9%
Park's note: "A mistaken idea has grown that slow sand filtration is old fashioned and outdated - this is definitely not the case."

Comparison: Slow Sand vs Rapid Sand Filter

FeatureRapid Sand FilterSlow Sand Filter
1. SpaceOccupies very little spaceOccupies large area
2. Rate of filtration200 m.g.a.d.2-3 m.g.a.d.
3. Effective size of sand0.4-0.7 mm0.2-0.3 mm
4. Preliminary treatmentChemical coagulation + sedimentationPlain sedimentation (storage)
5. WashingBackwashingScraping the sand bed
6. OperationHighly skilledLess skilled
7. Loss of head allowed6-8 feet (2-2.5 m)4 feet (1.5 m)
8. Removal of turbidityGoodGood
9. Removal of colourGoodFair
10. Removal of bacteria98-99%99.9-99.99%
(Park's SPM, Table 3, p. 814-815)
Advantages of rapid sand filter OVER slow sand:
  1. Can deal with raw water directly - no preliminary storage needed
  2. Filter beds occupy less space
  3. Filtration is 40-50 times faster than slow sand
  4. Washing is easy (backwashing vs scraping)
  5. More flexibility in operation

Quick-Recall Memory Aid

Think "SMUDGE" for the Schmutzdecke/vital layer:
  • Slimy gelatinous layer
  • Microbial life (algae, diatoms, bacteria, plankton)
  • Until formed, filtrate run to waste
  • Depth: 2-3 cm into sand
  • Grows during "ripening"
  • Eliminates bacteria + oxidizes ammonia to nitrates
And remember the "0.2-0.3 rule" - slow sand uses finer sand (0.2-0.3 mm) vs rapid sand (0.4-0.7 mm).
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