Table of Contents
- Introduction
- Sources of Water Pollution
- What is Water Purification?
- Water Purification Methods
- Purification of Water on a Large Scale
- Purification of Water on a Small Scale
- a. Household Purification
- b. Disinfection of Wells
Introduction
Water is fundamental to sustaining human life on Earth and plays a vital role in ensuring a high quality of life. According to the United Nations, 30% of the global population lacks access to clean, potable, and dependable drinking water. Therefore, water purification is essential to deliver safe and clean water from contaminated sources.
Sources of Water Pollution
Water pollution can be classified into two main categories: natural and anthropogenic (including urbanization and industrialization). The sources of water pollution include:
- Agricultural runoff
- Sewage discharge
- Industrial waste
- Physical pollutants (such as heat and radioactive materials)
What is Water Purification?
- Water purification involves removing impurities, microorganisms (such as bacteria, algae, viruses, and fungi), parasites (including Giardia and Cryptosporidium), toxic minerals (like lead, copper, iron, nitrate, arsenic, and manganese), and other contaminants from raw water.
- Consuming untreated water that contains heavy metals, dirt, and microorganisms can be harmful to health and lead to various health issues.
- After the treatment process, a small residual amount of disinfectant is maintained to minimize the risk of contamination during distribution.
- Purified water is suitable for human consumption or industrial use, ensuring it is clean, free of disease-causing microbes, and improved in taste, smell, and appearance. Therefore, water treatment is essential for providing safe, pure water.
Water Purification Method
Water purification methods can be categorized as follows:
A. Natural Methods
- Aeration
- Sedimentation
- Sunlight
- Dilution
- Oxidation
- Aquatic Plants and Animals
B. Artificial Methods
- Large-Scale Purification: Techniques used for treating water on an extensive scale, typically for municipal or industrial purposes.
- Small-Scale Purification: Methods designed for treating water on a smaller scale, suitable for individual or household use.
Purification of water on a large scale
The primary goal of large-scale water purification is to ensure that water is clean and safe. The treatment process depends on the water type and the desired quality standards.- Groundwater (wells and springs) often requires minimal treatment, typically just disinfection. Bleaching powder or chlorinated lime can be used for purification due to its affordability, ease of use, reliability, and safety.
- Surface water (from rivers, streams, lakes, and reservoirs), which is generally more turbid and polluted, needs extensive treatment. The purification process for surface water involves several stages: Coagulation, Sedimentation, Filtration, and Disinfection.
Large-Scale Water Purification Methods:
a. Storage
- Water is collected from its source and stored in natural or artificial reservoirs. This storage provides a reserve and helps prevent further pollution.
- A significant amount of purification occurs during storage, with an optimal period of 10-14 days.
i. Physical:
- Storage improves water quality as gravity causes about 90% of suspended contaminants to settle at the bottom.
- This method also reduces turbidity and allows light to penetrate, easing the burden on filtration.
ii. Chemical:
- Chemical changes occur during storage as dissolved oxygen helps aerobic bacteria oxidize organic matter.
- This process reduces free ammonia levels and increases nitrate concentrations.
iii. Biological:
- Antibiosis and oxidation lead to a significant reduction in bacterial counts. Pathogenic organisms gradually die off.
- Within the first 5-7 days of storage, the total bacterial count can decrease by up to 90%. However, long-term storage can promote algae growth, which can affect the water's odor and color.
b. Filtration
- Filtration is an ancient and widely used purification method and represents the second stage of water treatment.
- Water is passed through filters consisting of layers of sand and charcoal to remove smaller particles.
- Filtration can reduce bacterial content by 98-99%, turbidity from 50 PPM to 5 PPM, and make water colorless.
Types of Filters:
i. Slow sand filtration (Biological filter):
- Slow sand filters are cost-effective, easy to design, and require less space. They were first used in Scotland in 1804 and are now widely used around the world.
- These filters cover a large area, with a filtration rate of about 100-400 L/m²/hr and a sand grain diameter of 0.2-0.3 mm.
- Pre-treatment includes sedimentation, and filter cleaning is done by scraping. Storage of water prior to filtration is not required.
Mechanism of slow sand filter:
- Sedimentation: Suspended particles settle to the sand surface.
- Mechanical Straining: Particles that cannot pass through sand grains are retained.
- Adhesion: Particles that settle on sand grains are held by adhesion to the biological layer.
- Biochemical Processes: The biological layer removes organic matter, captures bacteria, oxidizes ammonical nitrogen into nitrates, and converts soluble iron and manganese compounds into insoluble hydroxides.
Advantages of slow sand filter
- Simple to construct and operate.
- Less expensive than rapid sand filters.
- Provides high physical, chemical, and bacteriological quality.
- Removes approximately 99.8-99.9% of bacteria, including 99.9% of E. coli and 99.99% of the overall bacterial count.
ii. Rapid sand filtration (Mechanical filter):
- The rapid sand filter, first introduced in the USA in 1885, has gained widespread use, including in developing countries.
- It operates at a filtration rate of approximately 4000-7500 L/m²/hr with sand grains sized between 0.4-0.7 mm.
- Water pretreatment includes coagulation and sedimentation, and backwashing is used to clean the filter.
- Prior storage of water is required, and rapid sand filtration can remove about 98-99% of bacteria.
- There are two types of rapid sand filters: Gravity type (e.g., Paterson’s Filter) and Pressure type (e.g., Candy’s Filter).
Rapid Sand Filtration Process:
i. Coagulation:
- During coagulation, iron or aluminum salts (such as polymers, aluminum sulfate, ferric sulfate, or ferric chloride) are added to the water. These coagulants have a positive charge and help eliminate impurities.
- Raw water is treated with coagulants like alum (5-40 mg/L). The alum dosage varies based on water turbidity and color.
- Coagulants are vigorously mixed with water to form "floc," which attracts dirt particles.
ii. Rapid mixing:
- After coagulation, the water is rapidly mixed in a "mixing chamber" for a few minutes to ensure even dispersion of alum throughout the water.
iii. Flocculation:
- The treated water is then gently stirred in a flocculation chamber for about 30 minutes using rotating paddles (2 to 4 rpm).
- This process forms a thick, white floc of aluminum hydroxide that entangles particulate matter and bacteria. The thicker the floc, the faster it settles.
iv. Sedimentation:
- The coagulated water moves to sedimentation tanks, where it is held for 2-6 hours. During this time, impurities and flocculent precipitates settle by gravity.
- Effective sedimentation requires removing at least 95% of the flocculent precipitate before the water moves to rapid sand filters.
- Sediment (or sludge) at the tank bottom should be removed periodically to prevent issues like mollusc and sponge growth.
v. Filtration:
- The water then undergoes filtration through slow sand filters.
- Components include pretreatment, a filter box with supernatant water, a sand bed, and an under-drainage system.
- Floc that did not settle is retained in the sand bed, creating a slimy layer similar to the Zoogleal layer.
- The filtration process absorbs microorganisms and oxidizes organic matter like ammonia. As filtration progresses, the filter clogs and loses effectiveness, requiring backwashing when the flocculent layer becomes too thick.
Advantage of rapid sand filter
- Can use raw water directly.
- No need for preliminary storage.
- Compact filter bed design.
- Cost-effective in the long run despite initial expense.
- Filtration is much faster (40 to 50 times quicker) than slow sand filters.
- Easy to clean and operate with flexible functionality.
c. Disinfection/Chlorination
Disinfection Criteria:
- Should eliminate pathogenic microorganisms without altering water properties (e.g., pH, temperature).
- Must be safe, colorless, consumable, and inexpensive.
- Should leave a residual concentration to prevent recontamination.
- Must be detectable with simple techniques even at low concentrations.
i. Chlorination:
- Chlorine is commonly used for large-scale disinfection, available as chlorine gas, chloramines, or perchloron. Chlorine gas is preferred for its cost-effectiveness, efficiency, quick action, and ease of use, though it is harmful to the eyes and toxic.
- Paterson’s chloronome is used to measure, control, and administer chlorine gas.
- Chlorination effectively kills pathogenic bacteria, oxidizes manganese, hydrogen sulfide, and iron, improves taste and odor, reduces algae growth, and maintains residual disinfection.
Principle:
- Water should be clear and turbidity-free for effective chlorination. Free chlorine must be present for at least 1 hour for bacterial and viral eradication. Minimum recommended free chlorine concentration is 0.5 mg/L.
- Tests for Residual Chlorine: Orthotolidine test (OT) and Orthotolidine Arsenite (OTA) test.
Advantages of chlorination:
- Cost-effective.
- Easy to apply.
- Kills almost all bacterial contaminants.
Disadvantage of chlorination:
- Forms carcinogenic halogenated compounds.
ii. Ozonation:
- Ozone is a powerful virucidal and oxidizing agent, used previously in Europe and Canada. It has no residual effect and is typically used in conjunction with chlorination.
iii. Other agents:
a. UV rays:
- UV rays are used for disinfection in the UK. This method is expensive and requires clear water, with no residual effect. It is ineffective if water passes through pipes from the treatment plant to the tap.
b. Chloramine:
- Chloramine, a combination of chlorine and ammonia, is less effective than chlorine alone.
iv. Membrane processes:
- Membrane-processes water treatment techniques are required to be promising options for reliable drinking water production.
- It is of two types: High-pressure processes and Lower-pressure processes.
a. High-pressure processes:
i. Reverse osmosis:
- Reverse osmosis is a highly advanced and costly desalination technique.
- It involves applying pressure to a saltwater solution to push it through a semi-permeable membrane. This membrane allows only the solvent (water) to pass through, blocking solutes (dissolved salts).
- Water moves through the membrane from an area of higher salt concentration to one of lower concentration.
- With a pore size smaller than 0.002 μm, reverse osmosis effectively filters out monovalent ions and organic compounds with a molecular weight greater than 50 daltons.
- This process is commonly used to convert brackish or seawater into potable water for community supply.
ii. Nanofiltration:
- Nanofiltration (NF) membranes have a relatively loose structure compared to reverse osmosis, allowing for faster water production with lower energy consumption.
- These membranes permit monovalent ions like sodium or potassium to pass through but block a significant proportion of divalent ions such as calcium and magnesium.
- With a pore size of approximately 0.001-0.01 μm and a molecular weight cut-off of 100 to 1000 Da, NF membranes effectively remove microorganisms, organic compounds, colloidal particles, and suspended solids.
Advantages:
- Easy to automate and compact in size.
- Broad-spectrum removal of various water impurities ensures high water quality.
- Adaptable to different feed water qualities.
- Effective in removing color-forming organic compounds.
b. Low-pressure processes:
i. Ultrafiltration:
- Ultrafiltration membranes filter out organic molecules larger than 800 daltons and have a pore size ranging from 0.002 to 0.03 μm.
ii. Microfiltration:
- Microfiltration can remove particles larger than 0.05 μm, with a pore size between 0.01 and 12 μm.
- It is often used in combination with coagulation to treat water.
Purification of water on a small scale
a. Household purification
i. Boiling:
- Boiling is an effective method for household water purification.
- Boiling water for 10-20 minutes kills most microorganisms, including bacteria, spores, cysts, and ova, and eliminates temporary hardness by removing carbon dioxide and precipitating calcium carbonate.
- Although boiling alters the taste of the water, it ensures sterilization and makes it safe to drink.
- It does not provide long-term protection against microbial contamination, so it is crucial to store boiled water in a clean container to prevent recontamination.
ii. Chemical disinfection:
a. Bleaching powder:
- Also known as chlorinated lime (CaOCl₂), this white powder has a strong chlorine odor and is an unstable compound.
- It is inexpensive, easy to use, and effective. Freshly prepared bleaching powder contains 33% available chlorine but loses potency when exposed to air, light, and moisture.
- It should be stored in a sealed container in a cool, dark, and dry place.
- A 5% solution can be used, with a recommended dose of 3-6 drops per liter of water, allowing for a contact time of 30 minutes.
b. Chlorine solution:
- Prepared from bleaching powder, a 5% chlorine solution can be made by mixing 4 kg of bleaching powder with 25% available chlorine.
- One drop of this solution per liter of water will disinfect it.
- Commercially available chlorine solutions also exist, though they can lose effectiveness over time when exposed to air, light, and moisture.
c. Chlorine tablet:
- Halazone tablets are effective for disinfecting small volumes of water, with one 0.5 g tablet treating up to 20 liters of water.
d. High test hypochlorite (HTH)/ Perchloron:
- This high-strength calcium compound contains 60-70% available chlorine.
- One gram of HTH can disinfect one liter of water and is more stable than bleaching powder, making it less prone to deterioration during storage.
e. Iodine:
- Iodine can be used for water disinfection with 2 drops of a 2% ethanol solution per liter of water, requiring 20-30 minutes of contact time.
- While effective, iodine is less commonly used for municipal water due to its high cost and potential impact on thyroid function.
f. Potassium Permanganate:
- A powerful oxidizing agent, potassium permanganate is not recommended for water disinfection as it can alter the water’s color, taste, and smell. It is effective against Vibrio cholera but not against many other organisms.
iii. Filtration:
- Small-scale water purification can be achieved using ceramic filters such as the Pasteur Chamberland filter, Berkefeld filter, and Katadyn filter.
- The Pasteur Chamberland filter uses porcelain candles, while the Berkefeld filter employs infusorial candles.
- These filters are designed to remove bacteria but may not be effective against viruses, which can pass through the filter.
b. Disinfection of well
- In rural areas, wells are a primary water source and may require disinfection, especially during outbreaks of diseases like cholera or gastroenteritis.
i. Adding bleaching powder:
- A cost-effective method for well disinfection involves adding 2.5 grams of bleaching powder per 1000 liters of water.
- After allowing for a one-hour contact period, the water should be safe for drinking. It is best to disinfect the well at night and use the water the following morning.
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