wastewater treatment

Introduction & Top Questions
Historical background
- Direct discharge of sewage
- Developments in sewage treatment
Sources of water pollution
Sewage characteristics
- Types of sewage
- Principal pollutants
  - Organic material
  - Suspended solids
  - Plant nutrients
  - Microbes
Sewerage systems
- Combined systems
- Separate systems
- Alternative systems
- Pumps
- Flow rates
Wastewater treatment and disposal
- Primary treatment
- Secondary treatment
  - Trickling filter
  - Activated sludge
  - Oxidation pond
  - Rotating biological contacter
- Tertiary treatment
  - Effluent polishing
  - Removal of plant nutrients
  - Land treatment
- Clustered wastewater treatment systems
- On-site septic tanks and leaching fields
- Wastewater reuse
Sludge treatment and disposal
- Treatment methods
  - Thickening
  - Digestion
  - Dewatering
- Disposal
Emerging technologies
- Improved treatment methods
- Automation
- Environmental considerations

References & Edit History Quick Facts & Related Topics

Images & Videos

Aerial view of wastewater treatment basins with circular clarifiers; overlaid toilet graphic and arrow with text reading: “Where does it go?”

Does global warming affect the ocean's oxygen levels?

How is sewage and industrial wastewater reclaimed for use?

Diagram illustrating sewage treatment using the activated sludge process. Raw sewage passes through screens and grit chambers in primary treatment, then flows to a primary clarifier. The primary effluent enters an aeration tank for secondary treatment, where air is added and activated sludge helps break down waste. The mixture moves to a secondary clarifier, with some sludge returned to the aeration tank and excess sent for disposal. Disinfected secondary effluent is released to surface water or

Diagram illustrating tertiary wastewater treatment, showing two tanks: one for filtering, where water passes through filter media to produce treated water, and one for backwashing, where air and water clean the filter using sand, gravel, and anthracite layers. Key components and flow directions are labeled.

A black plastic septic tank partially buried in an excavated pit, with surrounding dirt and exposed pipes, during installation.

A detailed diagram shows the process of sludge treatment and disposal in wastewater treatment, with labeled tanks, pipes, and equipment illustrating steps like flotation, pulping, hydrolysis, digestion, dewatering, and biogas production.

A large pile of plastic trash collected from the ocean.

Related Questions

How can methane from sewage sludge be used at a wastewater treatment plant?
How can tertiary treatment remove more than 99 percent of impurities from sewage?
How can treated wastewater be reused for irrigation or groundwater recharge?
How do bacteria, viruses, and protozoa enter wastewater?
How do coagulation and flocculation help remove small particles from water?
How do effluent standards regulate wastewater treatment plants?
How do grit chambers remove sand, coffee grounds, and eggshells from sewage?
How do microorganisms clean wastewater during secondary treatment?
How do nitrates and phosphates cause eutrophication in lakes?
How do sewer system inflows and infiltration affect wastewater treatment plants?
How do sludge-drying beds remove water from sewage sludge?
How do trickling filters use microorganisms to clean wastewater?
How do wastewater treatment plants protect water quality?
How does a comminutor process debris in wastewater?
How does aerobic digestion differ from anaerobic sludge digestion?
How does ammonia stripping remove ammonia from wastewater?
How does biochemical oxygen demand measure wastewater pollution?
How does land treatment use soil and vegetation to further clean wastewater?
How does nitrification-denitrification remove nitrogen from sewage?
How does the activated-sludge process treat sewage?
How does untreated wastewater affect public health?
How much suspended material and biochemical oxygen demand does primary treatment remove?
What are the main biological methods used for secondary wastewater treatment?
What factors determine the size of a wastewater treatment system?
What happens in a primary clarifier?
When is tertiary wastewater treatment needed?
Which pollutants are commonly found in wastewater?
Which water-quality measurements are regulated by stream standards?
Why are biosolids not spread on crops grown for human consumption when toxic chemicals are present?
Why are grease and floating materials skimmed from primary clarifiers?
Why are nitrogen and phosphorus removed from some wastewater?
Why are rags, wood, and other large debris removed from sewage?
Why are suspended solids removed from sewage?
Why can grit damage wastewater treatment equipment?
Why do large cities often discharge treated wastewater into surface water?
Why do rural areas often use subsurface wastewater disposal?
Why does primary treatment not remove most dissolved impurities?
Why does secondary treatment help protect oxygen levels in rivers and lakes?
Why is air added to an activated-sludge aeration tank?
Why is ammonia in wastewater harmful to fish?
Why is dechlorination sometimes needed after chlorine disinfection?
Why is ocean dumping of sewage sludge prohibited in the United States and many coastal countries?
Why is some activated sludge returned to the aeration tank?
Why is tertiary wastewater treatment more expensive than secondary treatment?
Why is wastewater disinfected before it is discharged?
Why must wastewater be treated before it is released into rivers, lakes, or oceans?
What are the common pollutants present in wastewater?
How is wastewater processed at a sewage treatment facility?
Why is wastewater resource recovery important?

Technology Engineering Civil Engineering

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External Websites

Chemistry LibreTexts - Wastewater and Sewage Treatment
Food and Agriculture Organization of the United Nations - Wastewater treatment
National Center for Biotechnology Information - PubMed Central - Wastewater treatment in 2050: Challenges ahead and future vision in a European context
USGS - Water Science School - Wastewater Treatment Water Use
RSC Publishing - Environmental Science Advances - Innovative approaches to sustainable wastewater treatment: a comprehensive exploration of conventional and emerging technologies (PDF)
United States Environmental Protection Agency - How Wastewater Treatment Works...The Basics

wastewater treatment

environmental engineering

Also known as: sewage treatment

Written by

Jerry A. Nathanson

Professor of Engineering, Union County College, Cranford, New Jersey. Author of Basic Environmental Technology: Water Supply, Waste Disposal, and Pollution Control.

Jerry A. NathansonAll

Fact-checked by

Britannica Editors

Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....

Britannica Editors

Last updated

Sep. 26, 2026 •History

wastewater treatment, the removal of impurities from wastewater, or sewage, before it reaches aquifers or natural bodies of water such as rivers, lakes, estuaries, and oceans. Since pure water is not found in nature (i.e., outside chemical laboratories), any distinction between clean water and polluted water depends on the type and concentration of impurities found in the water as well as on its intended use. In broad terms, water is said to be polluted when...

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