Managing Pollution of Fresh Water » Sewage Treatment Methods

What you'll learn this session

Study time: 30 minutes

The importance of sewage treatment for freshwater protection
Primary, secondary and tertiary sewage treatment methods
How sewage treatment plants work step-by-step
Sustainable and alternative sewage treatment approaches
Case studies of successful sewage treatment systems
Environmental impacts of untreated sewage

Introduction to Sewage Treatment Methods

Every day, we flush toilets, run showers and drain sinks without thinking much about where all that water goes. But managing sewage properly is crucial for protecting our freshwater resources. Without treatment, sewage can cause serious pollution, harm wildlife and spread diseases.

Key Definitions:

Sewage: Wastewater that contains human waste, food scraps, oils, soaps and chemicals from homes, businesses and industries.
Effluent: Treated or untreated wastewater that flows out of a treatment plant or industrial facility.
Sludge: The solid material that settles during sewage treatment.
BOD (Biochemical Oxygen Demand): A measure of how much oxygen is required by microorganisms to break down organic matter in water.

Why Sewage Treatment Matters

A single litre of untreated sewage can pollute 8-10 litres of freshwater. With the average person producing about 150 litres of wastewater daily, proper treatment is essential for environmental protection and public health.

The Three Stages of Sewage Treatment

Modern sewage treatment typically involves three main stages, each removing different types of contaminants from the wastewater.

💧 Primary Treatment

This first stage is all about physical processes to remove solids:

Screening: Large objects like sticks, rags and plastic are filtered out using screens
Grit removal: Sand, gravel and other heavy materials settle in grit chambers
Primary sedimentation: Wastewater sits in large tanks where solid waste sinks to the bottom forming primary sludge

Primary treatment typically removes 50-60% of suspended solids and 30-40% of BOD.

🧬 Secondary Treatment

This stage uses biological processes to break down dissolved and suspended organic matter:

Activated sludge process: Bacteria and microorganisms consume organic matter in aeration tanks
Trickling filters: Wastewater trickles over beds of media covered with microorganisms
Secondary sedimentation: Microbial clumps settle as secondary sludge

Secondary treatment can remove up to 85% of BOD and suspended solids.

Tertiary Treatment: The Final Polish

For especially sensitive environments or when water will be reused, tertiary treatment provides additional purification:

Nutrient removal: Removes nitrogen and phosphorus that can cause algal blooms
Filtration: Sand or membrane filters remove remaining particles
Disinfection: Chlorine, ultraviolet light, or ozone kills remaining pathogens
Advanced treatments: Activated carbon, reverse osmosis, or constructed wetlands for specific contaminants

After tertiary treatment, water is often clean enough to be safely discharged into rivers or even used for irrigation.

Inside a Sewage Treatment Plant: Step by Step

🚹 Preliminary

Sewage enters the plant and passes through screens that catch large debris. Grit chambers slow the flow, allowing sand and stones to settle. This protects equipment from damage.

📊 Processing

In primary settling tanks, solids sink to form sludge. The water moves to aeration tanks where air is pumped in to support bacteria that consume organic matter. Secondary settling tanks allow bacterial clumps to settle.

✅ Final Steps

The clarified water may undergo additional filtering and disinfection before being released. Collected sludge is treated separately through processes like anaerobic digestion, which can produce biogas for energy.

Alternative and Sustainable Sewage Treatment Methods

Not all sewage treatment relies on large centralised plants. Several alternative approaches can be effective, especially in rural areas or developing countries:

🌱 Natural Systems

Constructed wetlands: Artificial wetlands where plants, soils and microbes filter pollutants
Reed bed systems: Sewage flows through beds of reeds that filter and clean the water
Lagoon systems: Series of ponds where natural processes break down waste

These systems have low energy requirements and maintenance costs but need more land area.

🏠 Decentralised Systems

Septic tanks: Underground chambers where bacteria break down waste; common in rural areas
Package plants: Small-scale treatment systems for individual buildings or communities
Composting toilets: Waterless systems that convert human waste to compost

These systems work well for isolated communities where connecting to main sewers is impractical.

Case Study: Constructed Wetlands in Kolkata, India

The East Kolkata Wetlands cover 12,500 hectares and treat about 750 million litres of the city's sewage daily through natural processes. The wetlands also support fish farming that produces 8,000 tonnes of fish annually and provide livelihoods for 50,000 people. This system demonstrates how natural treatment methods can provide multiple benefits beyond just cleaning water.

Challenges in Sewage Treatment

Despite advances in treatment technology, several challenges remain:

Emerging contaminants: Pharmaceuticals, microplastics and personal care products aren't fully removed by conventional treatment
Energy consumption: Traditional treatment plants require significant energy, contributing to carbon emissions
Ageing infrastructure: Many systems in developed countries are decades old and need upgrading
Population growth: Increasing urbanisation puts pressure on existing systems
Climate change: Extreme weather events can overwhelm treatment systems, causing overflow

The Future of Sewage Treatment

Innovative approaches are transforming how we think about sewage:

💡 Resource Recovery

Modern facilities are increasingly designed to recover valuable resources:

Energy production: Capturing biogas from sludge digestion
Nutrient recovery: Extracting phosphorus and nitrogen for fertilisers
Water reuse: Treating water to standards suitable for irrigation or industrial use

🧪 Advanced Technologies

New technologies are improving treatment efficiency:

Membrane bioreactors: Combining biological treatment with membrane filtration
Advanced oxidation: Using processes like UV/ozone to break down persistent pollutants
Smart systems: Using sensors and AI to optimise treatment processes

Case Study: Singapore's NEWater

Singapore has pioneered water recycling with its NEWater facilities, which purify treated sewage to drinking water standards using advanced membrane technologies and UV disinfection. This ultra-clean water now meets 40% of Singapore's water needs, demonstrating how treated sewage can become a valuable resource rather than waste.

Environmental Impact Assessment

Before building or upgrading sewage treatment facilities, an Environmental Impact Assessment (EIA) is often required to evaluate potential effects on:

Water quality in receiving bodies
Local ecosystems and biodiversity
Air quality (from treatment processes and sludge handling)
Land use and soil quality
Energy consumption and greenhouse gas emissions
Community health and quality of life

These assessments help ensure that treatment solutions solve existing problems without creating new ones.

Summary: The Importance of Proper Sewage Treatment

Effective sewage treatment is essential for:

Protecting freshwater ecosystems from pollution
Preventing waterborne diseases
Conserving water resources through potential reuse
Recovering valuable resources like energy and nutrients
Supporting sustainable development of communities

As our understanding of water pollution grows and technology advances, sewage treatment continues to evolve from simply disposing of waste to recovering resources in a circular economy approach.

🧠 Test Your Knowledge!