Human Environmental Impact » Water Pollution by Sewage

What you'll learn this session

Study time: 30 minutes

The causes and sources of sewage pollution in water
How sewage affects aquatic ecosystems and biodiversity
The process of eutrophication and its environmental impacts
Health risks associated with sewage-contaminated water
Methods for treating sewage and preventing water pollution
Case studies of sewage pollution and successful clean-up efforts

Introduction to Water Pollution by Sewage

Water pollution from sewage is one of the most common and harmful forms of environmental contamination. Every day, billions of litres of wastewater containing human waste, household chemicals and other pollutants enter our water systems. This pollution threatens aquatic ecosystems, drinking water supplies and human health worldwide.

Key Definitions:

Sewage: Waste matter carried away in sewers or drains, containing human excrement, household waste and sometimes industrial waste.
Effluent: Liquid waste or sewage discharged into a river or the sea.
Eutrophication: Excessive richness of nutrients in a body of water, causing dense plant growth and death of animal life from lack of oxygen.
Biological Oxygen Demand (BOD): The amount of dissolved oxygen needed by aerobic organisms to break down organic material in water.

💦 Sources of Sewage Pollution

Sewage enters water bodies from various sources:

Domestic sewage from toilets, sinks and drains in homes
Industrial wastewater containing chemicals and organic waste
Agricultural runoff with animal waste from livestock
Stormwater overflow during heavy rainfall when sewage systems are overwhelmed
Leaking or poorly maintained septic tanks in rural areas

🔬 Composition of Sewage

Sewage typically contains:

Human waste (faeces and urine)
Food waste and cooking oils
Detergents and cleaning products
Harmful bacteria and viruses
Nutrients like nitrogen and phosphorus
Microplastics from clothing and cosmetics
Pharmaceutical residues

Ecological Impacts of Sewage Pollution

When untreated sewage enters aquatic ecosystems, it triggers a cascade of environmental problems that can devastate wildlife and disrupt ecological balance.

The Eutrophication Process

One of the most serious consequences of sewage pollution is eutrophication. This process occurs in the following stages:

💧 Stage 1: Nutrient Enrichment

Sewage adds nitrogen and phosphorus compounds to water. These nutrients act as fertilisers for aquatic plants and algae, causing rapid growth.

🌿 Stage 2: Algal Bloom

Excessive nutrients lead to explosive growth of algae (algal bloom) and other aquatic plants, covering the water surface with a green layer.

☠ Stage 3: Oxygen Depletion

When algae die, decomposers break them down, consuming oxygen in the process. This reduces dissolved oxygen levels in the water (high BOD), suffocating fish and other aquatic organisms.

Effects on Aquatic Life

Sewage pollution harms aquatic organisms in multiple ways:

Oxygen depletion: As bacteria decompose sewage, they use up dissolved oxygen, leading to hypoxic (low oxygen) or anoxic (no oxygen) conditions.
Toxic effects: Ammonia from decomposing sewage is directly toxic to fish and other aquatic life.
Habitat destruction: Sludge deposits can smother bottom-dwelling organisms and destroy breeding grounds.
Biodiversity loss: Only pollution-tolerant species survive, reducing overall biodiversity.
Disruption of food chains: Loss of sensitive species can affect entire food webs.

Case Study Focus: The Great Stink of London

In the summer of 1858, London experienced "The Great Stink" when the River Thames, filled with raw sewage, created an unbearable smell during a heatwave. The crisis led to the construction of London's sewerage system by engineer Joseph Bazalgette. This massive infrastructure project, completed in the 1870s, diverted sewage away from the Thames and dramatically improved public health. It's considered one of the great engineering achievements of the Victorian era and continues to serve London today, demonstrating how addressing sewage pollution can transform urban environments.

Health Risks of Sewage-Contaminated Water

Sewage pollution poses significant risks to human health, particularly in areas with limited access to clean water and sanitation.

🦠 Waterborne Pathogens

Sewage contains numerous disease-causing organisms:

Bacteria: E. coli, Salmonella, Vibrio cholerae (causes cholera)
Viruses: Hepatitis A, norovirus, rotavirus
Parasites: Giardia, Cryptosporidium, intestinal worms

These pathogens can cause illnesses ranging from mild gastroenteritis to severe and potentially fatal diseases like dysentery, cholera and typhoid fever.

😷 Exposure Routes

People can be exposed to sewage-contaminated water through:

Drinking contaminated water
Swimming or bathing in polluted water
Eating fish or shellfish from contaminated waters
Consuming crops irrigated with contaminated water
Contact with contaminated soil or surfaces

Sewage Treatment and Pollution Prevention

Modern sewage treatment processes are designed to remove harmful components before water is released back into the environment.

Stages of Sewage Treatment

🧻 Primary Treatment

Physical processes to remove solid materials:

Screening to remove large objects
Grit removal chambers
Sedimentation tanks where solids settle as sludge

🟩 Secondary Treatment

Biological processes to break down dissolved and suspended organic matter:

Aeration tanks with bacteria to digest organic matter
Activated sludge process
Trickling filters
Further sedimentation

💡 Tertiary Treatment

Advanced processes for further purification:

Nutrient removal (nitrogen and phosphorus)
Filtration through sand or membranes
Disinfection using chlorine, UV light, or ozone
Wetland treatment systems

Sustainable Approaches to Sewage Management

Beyond traditional treatment, sustainable approaches include:

Constructed wetlands: Artificial wetlands that use natural processes to filter and clean wastewater
Decentralised treatment systems: Small-scale systems that treat sewage closer to its source
Water recycling: Treating wastewater for reuse in irrigation, industrial processes, or even drinking water
Composting toilets: Systems that convert human waste into compost, reducing water usage
Separation of grey water: Treating less contaminated household water (from sinks, showers) separately from toilet waste

Case Study: Thames Water Clean-Up

The River Thames has undergone a remarkable transformation from being biologically dead in the 1950s to supporting over 125 fish species today. Key to this recovery was the implementation of stricter sewage treatment standards and infrastructure improvements. The Thames Tideway Tunnel, a massive engineering project currently under construction, aims to prevent millions of tonnes of raw sewage from overflowing into the river during heavy rainfall. This "super sewer" demonstrates the ongoing commitment needed to address sewage pollution in growing urban areas, even in developed countries.

Monitoring and Regulation

Effective control of sewage pollution requires robust monitoring and enforcement of regulations:

Water quality testing: Regular sampling and analysis for indicators of sewage pollution, including coliform bacteria counts, BOD levels and nutrient concentrations
Discharge permits: Legal limits on what can be discharged into water bodies
Environmental standards: In the UK, the Environment Agency sets and enforces water quality standards
Bathing water classifications: Monitoring of coastal waters and beaches for public safety
International agreements: Conventions like OSPAR (for the North-East Atlantic) that address marine pollution

What You Can Do

Everyone can help reduce sewage pollution through simple actions:

Never flush wet wipes, sanitary products, or other non-biodegradable items down the toilet
Dispose of cooking oils and fats in the bin, not down the sink
Use eco-friendly cleaning products with fewer harmful chemicals
Report sewage leaks or pollution incidents to the Environment Agency
Support water conservation efforts to reduce pressure on sewage systems
Get involved in local river or beach clean-up initiatives

Summary

Sewage pollution remains one of the most significant water quality challenges worldwide. While developed countries have made progress through improved infrastructure and treatment technologies, many developing regions still struggle with basic sanitation. Understanding the causes and impacts of sewage pollution is essential for addressing this environmental issue effectively. Through a combination of proper treatment, sustainable management approaches and individual responsibility, we can protect our precious water resources for future generations.

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