Summary of Large-Scale Water Purification
The goal of water treatment is to produce clean and safe water. The treatment process
depends on the type of raw water. Groundwater (such as wells and springs) may only
require disinfection, whereas surface water (such as rivers) needs extensive treatment due
to its turbidity and pollution. A typical water purification system consists of storage,
filtration, and disinfection.
I. Storage
Water is first collected from the source and stored in reservoirs (natural or artificial). Storage
serves multiple purification functions:
1. Physical Purification:
○ Settling of suspended particles occurs naturally.
○ About 90% of suspended impurities settle within 24 hours.
○ The water becomes clearer, allowing light penetration, which aids filtration.
2. Chemical Purification:
○ Aerobic bacteria oxidize organic matter, reducing free ammonia levels.
○ There is a rise in nitrates, improving water quality.
3. Biological Purification:
○ A significant drop in bacterial count occurs.
○ Pathogens gradually die off, reducing their count by 90% in 5-7 days.
○ The optimal storage period is 10-14 days to prevent algae growth, which can
cause odor and color changes.
II. Filtration
Filtration is a crucial step in removing bacteria and other impurities, eliminating 98-99% of
bacteria. The two primary filtration methods are slow sand filtration and rapid sand
filtration.
1. Slow Sand (Biological) Filtration
This method, first introduced in Scotland in 1804, remains widely used globally. It consists of:
● Supernatant Water:
○ Depth: 1-5 meters.
○ Provides hydraulic pressure for water flow.
○ Allows pre-purification through sedimentation and oxidation.
● Sand Bed:
○ Thickness: 1 meter.
○ Effective sand diameter: 0.2-0.3 mm.
○ Sand is supported by graded gravel (30-40 cm thick).
○ Water percolates slowly, undergoing mechanical straining, sedimentation,
adsorption, oxidation, and bacterial action.
● Vital Layer (Schmutzdecke):
○ A slimy biological layer forms within days.
○ Composed of algae, plankton, and bacteria.
○ Helps remove organic matter, oxidize ammoniacal nitrogen, and eliminate
bacteria.
○ The first few days of filtration yield non-potable water, which is discarded.
● Under-Drainage System:
○ Composed of perforated pipes at the bottom.
○ Collects filtered water and supports the filter medium.
● Filter Control System:
○ Regulates filtration rate.
○ Uses a Venturi meter to measure resistance (loss of head).
○ When resistance exceeds 1.3 meters, filtration becomes inefficient, requiring
cleaning.
● Cleaning:
○ The filter runs for weeks or months before cleaning is needed.
○ The top 1-2 cm of the sand layer is scraped off when resistance becomes too
high.
○ After 20-30 scrapings, a new sand bed is constructed.
Advantages of Slow Sand Filters:
● Simple construction and operation.
● Cost-effective compared to rapid sand filters.
● Provides high-quality water with 99.9-99.99% bacterial removal.
● Still used in industrialized cities and urban areas.
2. Rapid Sand (Mechanical) Filtration
Developed in the USA in 1885, rapid sand filters are widely used in industrialized nations.
There are two types:
1. Gravity filters (e.g., Paterson’s filter).
2. Pressure filters (e.g., Candy’s filter).
Steps in Rapid Sand Filtration
1. Coagulation:
○ Raw water is treated with a coagulant (e.g., alum, 5-40 mg/L).
○ The dose depends on water turbidity, temperature, and pH.
2. Rapid Mixing:
○ Water is mixed in a chamber to distribute the coagulant evenly.
3. Flocculation:
○ Slow stirring (2-4 rpm) forms a flocculant precipitate (aluminum hydroxide).
○ Larger floc particles settle faster.
4. Sedimentation:
○ Water remains in sedimentation tanks for 2-6 hours.
○ 95% of the flocculant precipitate (with impurities and bacteria) settles.
○ Regular cleaning prevents the growth of mollusks and sponges.
5. Filtration:
○ Water passes through sand and gravel layers.
○ Sand particle size: 0.4-0.7 mm.
○ Sand depth: ~0.5 meters.
○ Gravel layer: 30-40 cm thick.
○ Filtration rate: 5-15 m³/m²/hour.
○ Under-drains collect filtered water.
III. Disinfection
The final step in water purification is disinfection, typically using chlorine to kill remaining
bacteria and viruses before water is distributed for consumption.
Comparison of Slow and Rapid Sand Filtration
Summary of Water Purification Processes
1. Filtration
Filtration removes impurities from water by passing it through a sand bed. The "alum-floc"
that is not removed during sedimentation forms a slimy layer in the sand bed, similar to the
zoogleal layer in slow sand filters. This layer adsorbs bacteria and aids in purification.
Additionally, oxidation of ammonia occurs as water passes through the filter. However, over
time, the filters clog with impurities, leading to decreased efficiency. When the "loss of head"
reaches 7-8 feet, filtration stops, and the filter undergoes a washing process called
backwashing, which reverses the water flow to remove accumulated impurities.
Advantages of Rapid Sand Filters Over Slow Sand Filters:
● Higher efficiency: Can treat raw water directly without preliminary storage.
● Space-saving: Requires less space than slow sand filters.
● Faster filtration: Filtration rate is 40-50 times that of slow sand filters.
● Easy cleaning: Can be backwashed efficiently.
● Operational flexibility: Can handle variable water quality more effectively.
2. Disinfection
For a chemical or agent to be effective as a disinfectant in water, it must:
● Kill pathogenic organisms efficiently within the available contact time.
● Not produce harmful by-products or alter water quality.
● Be readily available, cost-effective, and easy to apply.
● Leave a residual concentration to prevent recontamination.
● Be detectable using simple analytical methods.
Chlorination is the primary method of water disinfection and is used as a supplement to
sand filtration. Chlorine effectively kills bacteria but is less effective against spores and
certain viruses (e.g., poliovirus, hepatitis virus).
Action of Chlorine:
Chlorine dissolves in water to form hydrochloric acid (HCl) and hypochlorous acid
(HOCl). The hypochlorous acid ionizes to form hypochlorite ions (OCl⁻), which have
disinfecting properties. Hypochlorous acid is 70-80 times more effective than hypochlorite
ions. Chlorine is most effective at pH 7; its effectiveness decreases when pH exceeds 8.5.
Principles of Chlorination:
1. Water clarity: Turbidity must be minimal for effective chlorination.
2. Chlorine demand estimation: The amount of chlorine required to disinfect water is
determined.
3. Contact period: Free residual chlorine must be present for at least one hour.
4. Minimum chlorine concentration: At least 0.5 mg/L for one hour to ensure
disinfection.
5. Correct chlorine dose: Includes chlorine demand plus free residual chlorine.
Methods of Chlorination:
● Chlorine gas: Most efficient and commonly used.
● Chloramines: Less effective but produce fewer taste/odor issues.
● High-test hypochlorite (HTH): Calcium-based compound with 60-70% available
chlorine.
Breakpoint Chlorination:
When chlorine is added to water containing ammonia, chloramines form, reducing the
availability of free chlorine. Increasing the chlorine dose further leads to destruction of
chloramines, and beyond a certain point, free chlorine reappears. This is known as the
breakpoint. Chlorination beyond this point ensures complete disinfection.
Superchlorination and Dechlorination:
● Used for heavily polluted waters with fluctuating quality.
● Excess chlorine is added to kill all pathogens, then removed before water distribution.
Orthotolidine (OT) Test:
A rapid test to measure free and combined chlorine in water. The reagent reacts with
chlorine to produce a yellow color, the intensity of which indicates the chlorine concentration.
Orthotolidine-Arsenite (OTA) Test:
A modified OT test to separately measure free and combined chlorine residuals while
eliminating interference from substances like nitrites and iron.
3. Alternative Disinfection Methods
Due to concerns about carcinogenic by-products of chlorination, alternative disinfectants are
being explored:
● Bromine, Bromine chloride, Iodine, and Chlorine dioxide are potential
alternatives but have limitations.
● Ozone (O₃): A powerful oxidant that effectively removes organic chemicals,
pesticides, and bacteria.
○ Ozone is generated by passing oxygen through a high-voltage electric field.
○ Requires 10-20 minutes contact time to be effective.
○ Used along with biological filtration or activated carbon filtration to
remove organic residues.
○ Leaves no residual disinfectant, requiring supplementary chlorine.
● Ultraviolet (UV) Irradiation: Effective against microbes but provides no residual
disinfection, limiting its standalone use.
4. Membrane Filtration Processes
Membrane filtration is used in modern water treatment, particularly for removing dissolved
and colloidal impurities.
High-Pressure Membrane Processes:
● Reverse Osmosis (RO):
○ Uses a semipermeable membrane to remove solutes from water.
○ Requires high pressure (15-50 bar).
○ Used mainly for desalination of seawater and brackish water.
○ Removes monovalent ions and organic molecules >50 Da.
● Nanofiltration:
○ Membrane properties are between RO and ultrafiltration.
○ Removes divalent ions (e.g., calcium, magnesium) and some organic
contaminants.
○ Operates at lower pressures (~5 bar).
Lower-Pressure Membrane Processes:
● Ultrafiltration (UF):
○ Membrane pore sizes: 0.002-0.03 µm.
○ Removes organic molecules above 800 Da.
○ Operates at pressures <5 bar.
● Microfiltration (MF):
○ Filters particles >0.05 µm.
○ Operates at pressures 1-2 bar.
○ Used for removing suspended solids and bacteria.
