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PURIFICATION OF WATER

PSM - University Exam Oriented Answer

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INTRODUCTION

• Ground water (wells, springs): may need only disinfection
• Surface water (rivers): turbid and polluted - requires extensive treatment

1. Purification of water on a large scale
2. Purification of water on a small scale

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1. PURIFICATION OF WATER ON A LARGE SCALE

• I. Storage
• II. Filtration
• III. Disinfection

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(I) STORAGE

Benefits of Storage:

• Optimum storage period for river water: 10-14 days
• Prolonged storage may cause algal growth, imparting bad smell and colour

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(II) FILTRATION

• Slow Sand (Biological) Filter
• Rapid Sand (Mechanical) Filter

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A. SLOW SAND OR BIOLOGICAL FILTER

Elements of a Slow Sand Filter:

1. Supernatant (raw) water - depth 1 to 1.5 m; provides constant head for downward flow; allows 3-12 hours of partial purification by sedimentation, oxidation and particle agglomeration
2. Sand bed - 1 metre thick; sand grains with effective diameter 0.2-0.3 mm; provides ~15,000 sq. m surface area per cubic metre; filtration rate 0.1-0.4 m³/hour/m²; filtration processes include mechanical straining, sedimentation, adsorption, oxidation and bacterial action
3. Under-drainage system - porous/perforated pipes at the bottom; provides outlet for filtered water and supports the filter medium
4. Filter control valves - Venturi meter measures "loss of head"; when loss of head exceeds 1.3 metre, it is uneconomical to run the filter

The Filter Box (top to bottom):

The Vital Layer (Schmutzdecke):

This is the "heart" of the slow sand filter - the most important examinable concept.

• When newly laid, the filter acts as a mechanical strainer only
• Very soon, the surface of the sand bed gets covered with a slimy growth called "Schmutzdecke" (also called vital layer, zoogleal layer, or biological layer)
• It is slimy and gelatinous and consists of threadlike algae, plankton, diatoms and bacteria
• Formation of the vital layer is called "ripening" of the filter - takes several days
• When fully formed, it extends 2-3 cm into the top of the sand bed
• Functions: removes organic matter, holds back bacteria, oxidizes ammoniacal nitrogen to nitrates, yields bacteria-free water
• First few days' filtrate is run to waste until the vital layer is fully formed

Filter Cleaning:

• When the filter clogs, the top 1.5-2 cm of sand is scraped off and the filter is allowed to drain, then resanded
• This is called "harrowing" or "resanding"
• After cleaning, the filter needs several days to re-ripen (reform the Schmutzdecke)

Advantages of Slow Sand Filter:

1. Simple construction and operation
2. No coagulants needed
3. Produces biologically safe water
4. Standard method for water purification

Disadvantages:

1. Needs large land area
2. Slow rate of filtration
3. Requires preliminary settling for turbid water
4. Costly to clean; needs re-ripening period after cleaning

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B. RAPID SAND (MECHANICAL) FILTER

Key features:

• Sand grain effective diameter: 0.4-0.7 mm (coarser than slow sand)
• Rate of filtration: 5-15 m³/hour/m² (40-50 times faster than slow sand)
• Does not develop a biological layer (Schmutzdecke)
• Action is purely mechanical (not biological)
• Bacteria removal: ~98-99% (but requires pre-coagulation and subsequent disinfection)

Backwashing:

• Rapid sand filters need frequent washing (daily or weekly), depending on loss of head
• Washing is done by reversing the flow of water through the sand bed = "backwashing"
• Backwashing dislodges impurities and cleans the sand bed
• Process takes about 15 minutes
• In some filters, compressed air is used as part of backwashing

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 faster)
4. Washing is easy (backwashing)
5. More flexibility in operation

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COMPARISON TABLE: Slow Sand vs Rapid Sand Filter

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(III) DISINFECTION

Criteria for an ideal disinfectant:

1. Should destroy pathogens within available contact time, not unduly influenced by temperature, pH, or mineral content
2. Should NOT leave toxic products or impart colour/taste
3. Should be readily available at reasonable cost
4. Should leave residual concentration to deal with recontamination
5. Should be detectable by simple analytical techniques

In water works practice, disinfection is synonymous with chlorination.

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CHLORINATION

• Kills: pathogenic bacteria
• Does NOT kill: spores, certain viruses (e.g., polio, viral hepatitis) except in high doses

Chemistry of Chlorination:

Cl₂ + H₂O → HOCl + HCl HOCl → H⁺ + OCl⁻

Chlorine Demand:

Break-point Chlorination:

• As chlorine is added progressively to water, residual first rises, then falls (due to chloramine destruction), then rises sharply again
• The point at which residual starts to rise sharply again = break-point
• Beyond the break-point, residual is free chlorine (most germicidal)
• Standard practice: add enough chlorine to produce 0.5 mg/litre free residual chlorine after 1 hour contact time at the consumer's tap

Methods of Chlorination:

1. Plain chlorination - chlorine gas or chlorine compounds added directly
2. Super-chlorination - adding larger doses of chlorine (up to 5-10 mg/litre) then followed by dechlorination with sodium thiosulphate or activated carbon
3. Pre-chlorination - added before filtration (reduces algal growth, oxidizes iron/manganese)
4. Post-chlorination - after filtration (most common)
5. Double chlorination - both pre- and post-filtration

Agents used:

• Chlorine gas (Cl₂) - most efficient; used in large municipal supplies
• Bleaching powder (CaOCl₂): contains ~33% available chlorine; unstable on exposure to air/light/moisture
• High Test Hypochlorite (HTH/Perchloron): 60-70% available chlorine; more stable than bleaching powder
• Chloramines (chlorine + ammonia): produce combined chlorine; less powerful but more persistent; used in long distribution systems

Testing for Residual Chlorine:

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OTHER DISINFECTION METHODS

Ozonation:

• Ozone (O₃) is a powerful oxidant
• Formed by passing dry air/oxygen through a high-voltage electric field
• Dosed via porous diffusers at base of contactor tanks (~5 m deep; 10-20 minutes contact time)
• Dose for organic chemical oxidation: ~0.5 mg/l residual after 20 min contact
• Advantage: no toxic by-products at low doses; also removes taste/odour
• Disadvantage: no residual protection; expensive; carcinogenic by-products (bromate) may form

UV Irradiation:

• Destroys bacteria and viruses by disrupting DNA
• No residual protection
• Useful as a complementary agent to chlorine
• Limited usefulness as sole disinfectant in large-scale supply

Membrane Filtration:

• Reverse Osmosis (RO): operates at high pressure (>15 bar); removes dissolved solids, bacteria, viruses
• Ultrafiltration: pore size 0.002-0.03 μm; pressure <5 bar; removes molecules >800 daltons
• Microfiltration: pore size 0.01-12 μm; pressure 1-2 bar; removes particles >0.05 μm; used with coagulation or PAC

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2. PURIFICATION OF WATER ON A SMALL SCALE

(a) BOILING

• Bring to a "rolling boil" for 10-20 minutes
• Kills all bacteria, spores, cysts, and ova - yields sterilized water
• Also removes temporary hardness (drives off CO₂, precipitates CaCO₃)
• Disadvantage: No residual protection against subsequent contamination; alters taste; expensive for large quantities
• Water should be boiled and stored in the same container to avoid recontamination

(b) CHEMICAL DISINFECTION

(c) FILTRATION METHODS

(d) OTHER HOUSEHOLD METHODS

• Solar disinfection (SODIS): clear plastic PET bottles, place in sunlight for 6-8 hours (48 hours if cloudy); UV + heat kills pathogens; used in disaster/field conditions
• Ceramic filters: porous clay filters with colloidal silver coating; removes bacteria and protozoa

(e) MULTI-STAGE REVERSE OSMOSIS (RO):

• Combination of pre-filtration + activated carbon + RO membrane + UV stages
• Removes bacteria, viruses, dissolved solids (heavy metals, fluoride, nitrates)
• Used widely for household purification in urban areas of India
• Disadvantage: wastes water (reject water ~50-70%); removes beneficial minerals; requires electricity

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COAGULATION AND SEDIMENTATION (Pre-treatment)

• Coagulant added: Alum (aluminium sulphate) is most commonly used
• Alum reacts with natural alkalinity of water to form Al(OH)₃ floc - a gelatinous precipitate
• Floc enmeshes fine particles, bacteria, and colloids - they settle by sedimentation
• This is called "flocculation" followed by "sedimentation"
• Reduces turbidity, colour, organic matter, and bacterial count dramatically before filtration

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QUICK COMPARISON SUMMARY (High-yield for exam)

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HIGH-YIELD EXAM POINTS (Frequently Asked):

1. Schmutzdecke = vital layer of slow sand filter = "heart of the filter" - composed of algae, diatoms, bacteria; removes organic matter, oxidizes ammonia to nitrates
2. Break-point chlorination = point beyond which only free chlorine residual remains
3. Free residual chlorine at tap = 0.5 mg/litre after 1 hour contact
4. OT test = Orthotolidine test; reading within 10 seconds for free chlorine
5. Bleaching powder = 33% available chlorine; stabilized with excess lime
6. HTH/Perchloron = 60-70% available chlorine; most stable chlorine compound
7. Chlorination is supplement NOT substitute for filtration
8. Boiling = only method that kills spores and cysts; no residual protection
9. Optimum storage for river water = 10-14 days
10. Backwashing = cleaning method for rapid sand filter; takes ~15 minutes
11. Harrowing/Scraping = cleaning method for slow sand filter (top 1.5-2 cm scraped)
12. Ozonation = no residual protection; no taste/odour; forms bromate (carcinogenic)

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