Managing Water-related Disease » Water Purification Methods

What you'll learn this session

Study time: 30 minutes

Different methods of water purification and how they work
Physical, chemical and biological water treatment processes
Advantages and limitations of various purification techniques
How water purification helps prevent water-related diseases
Real-world applications and case studies of water treatment

Introduction to Water Purification Methods

Clean water is essential for human health, but many water sources contain harmful substances that can cause disease. Water purification is the process of removing contaminants from water to make it safe for human consumption. Without proper purification, water can spread diseases like cholera, typhoid and dysentery, which affect millions of people worldwide each year.

Key Definitions:

Water purification: The process of removing contaminants from water to make it safe for drinking.
Pathogens: Disease-causing microorganisms like bacteria, viruses and parasites.
Turbidity: The cloudiness of water caused by suspended particles.
Potable water: Water that is safe to drink.

Physical Methods of Water Purification

Physical methods remove contaminants without changing the chemical composition of water. They are often the first step in water treatment.

◼ Filtration

Filtration involves passing water through materials that trap solid particles. Different types include:

Sand filtration: Water passes through layers of sand and gravel that trap particles.
Membrane filtration: Uses semi-permeable membranes with tiny pores to filter out contaminants.
Activated carbon filters: Remove chemicals, tastes and odours by adsorption.

◼ Sedimentation

Sedimentation is the process where heavier particles settle to the bottom of a container due to gravity.

Often used as a pre-treatment step before filtration
Can remove up to 60% of suspended solids
Sometimes aided by chemicals called coagulants that help particles clump together

◼ Boiling

One of the oldest and simplest methods of water purification:

Kills most pathogens by heating water to its boiling point (100°C)
Requires at least 1 minute of rolling boil (3 minutes at high altitudes)
Doesn't remove chemical contaminants or improve taste
Energy-intensive but effective in emergency situations

◼ Distillation

Distillation involves boiling water and collecting the condensed steam:

Removes nearly all impurities including microorganisms, heavy metals and many chemicals
Produces very pure water but requires significant energy
Removes beneficial minerals along with contaminants
Not practical for large-scale water treatment due to energy costs

Chemical Methods of Water Purification

Chemical methods use specific chemicals to kill pathogens or remove contaminants from water.

◼ Chlorination

The most widely used method of disinfection in public water systems:

Chlorine kills most bacteria and viruses
Provides residual protection against recontamination
Inexpensive and easy to implement
Can create harmful by-products and some pathogens are resistant
Typical dose: 0.2-2 mg/L of chlorine

◼ Coagulation and Flocculation

These processes help remove tiny particles that wouldn't settle on their own:

Coagulation: Adding chemicals like aluminium sulphate (alum) to make particles stick together
Flocculation: Gentle mixing that causes the particles to form larger clumps (flocs)
The larger flocs can then be removed by sedimentation or filtration
Effective for removing turbidity and some pathogens

◼ Ozonation

Using ozone gas (O₃) to disinfect water:

Stronger disinfectant than chlorine
Effective against a wider range of pathogens
Doesn't create harmful by-products like chlorination
No residual protection after treatment
Requires complex equipment and higher costs

◼ UV Disinfection

Using ultraviolet light to kill pathogens:

Damages the DNA of microorganisms, preventing reproduction
No chemicals needed and no taste or odour effects
Ineffective in cloudy water (requires pre-filtration)
No residual protection against recontamination
Requires electricity but is energy-efficient

Biological Methods of Water Purification

Biological methods use living organisms to remove contaminants from water.

Slow Sand Filtration

This method uses both physical and biological processes to clean water:

Water slowly passes through a bed of fine sand
A layer of microorganisms (called the "schmutzdecke") forms on top of the sand
These microorganisms break down organic matter and trap pathogens
Low maintenance and can be built with local materials
Requires large land area and works slowly

Constructed Wetlands

Artificial wetlands designed to treat wastewater:

Plants, microbes and natural filtration work together to clean water
Can remove up to 90% of contaminants including pathogens
Environmentally friendly and creates habitat for wildlife
Requires significant land area and careful design

Household Water Treatment Methods

In many parts of the world, people need to treat water at home. Here are some common household methods:

✓ SODIS (Solar Disinfection)

Uses sunlight to kill pathogens:

Fill clear plastic bottles with water
Place in direct sunlight for 6+ hours
UV radiation and heat kill pathogens
Free but weather-dependent

✓ Ceramic Filters

Clay pots that filter water:

Often treated with silver for antibacterial effect
Can be made locally with basic materials
Removes bacteria and parasites
Slow filtration rate but effective

✓ Chlorine Tablets

Chemical disinfection at home:

Add tablets to specified volume of water
Wait 30 minutes before drinking
Inexpensive and widely available
Affects taste but kills most pathogens

Integrated Water Treatment Systems

Modern water treatment plants typically use multiple methods in sequence to ensure water safety:

Screening: Removes large debris using screens
Coagulation/Flocculation: Adds chemicals to clump particles together
Sedimentation: Allows particles to settle out
Filtration: Removes smaller particles through sand or membrane filters
Disinfection: Kills remaining pathogens (usually with chlorine)
Storage: Holds treated water before distribution

Case Study Focus: LifeStraw

The LifeStraw is a portable water filter designed for use in areas without clean drinking water:

Uses hollow fibre membrane technology to filter out 99.9999% of bacteria and 99.9% of parasites
Requires no electricity, batteries, or chemicals
Each straw can filter about 1,000 litres of water
Used in emergency situations and in developing countries
Has helped prevent water-related diseases in countries like Kenya, Haiti and Pakistan
Costs about £15-20 per unit, making it affordable for aid organisations

Challenges in Water Purification

Despite advances in technology, several challenges remain in providing clean water globally:

Cost: Many effective methods are too expensive for developing countries
Infrastructure: Building and maintaining treatment plants requires significant resources
Energy requirements: Many methods need reliable electricity which isn't available everywhere
New contaminants: Pharmaceuticals, microplastics and industrial chemicals pose new challenges
Climate change: Changing rainfall patterns affect water availability and quality

The Future of Water Purification

Emerging technologies are addressing these challenges:

Nanotechnology: Using materials at the nanoscale to remove contaminants more efficiently
Solar-powered systems: Making treatment possible in areas without reliable electricity
Smart monitoring: Using sensors to detect contamination in real-time
Decentralised systems: Smaller, local treatment systems that don't require extensive infrastructure

By understanding and implementing appropriate water purification methods, we can prevent water-related diseases and improve public health worldwide. The choice of method depends on local conditions, available resources and the specific contaminants present in the water.

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