Skip to content
Know.itKnow.it
Create an animated lesson
On request

This article is free to read. Creating a lesson uses your daily or stored credits; review the language and settings before starting.

How does sewage treatment work?

ScienceLesson on requestFree to read

Quick answer

Sewage treatment removes solids, organic matter and harmful microbes from wastewater through a series of physical and biological steps. Primary treatment settles solids, secondary treatment uses bacteria to eat dissolved waste, and tertiary treatment disinfects or removes nutrients before the clean water is released back into rivers or reused.

Sewage treatment works by using physical, biological and chemical processes to remove solids, organic matter and pathogens from wastewater before it is returned to the environment. It typically involves primary settlement, secondary biological treatment and sometimes tertiary disinfection.

Step by step

  1. Screening and grit removal: wastewater passes through bars and tanks that catch large objects and heavy sand.
  2. Primary sedimentation: in large tanks, suspended solids sink to form sludge while fats and oils float and are skimmed off.
  3. Secondary biological treatment: the liquid is aerated so bacteria and other microbes consume dissolved organic pollutants.
  4. Secondary clarification: the microbe-rich sludge settles out, and some is returned to the aeration tank to maintain the process.
  5. Tertiary treatment: filters, chemicals or UV light remove remaining nutrients and pathogens.
  6. Disinfection and discharge: the treated water is disinfected, tested, and released into a river, lake or reused.

Key things to understand

  1. Sewage first passes through screens and grit chambers that trap large debris, rags and sand.
  2. In primary settling tanks, solids sink as sludge while grease and oils float to the surface.
  3. The liquid then enters aeration tanks where bacteria consume dissolved organic matter.
  4. Secondary clarifiers separate the bacteria from the cleaned water, returning some to the aeration tank.
  5. Tertiary treatment may use filters, chemicals or UV light to remove nutrients and pathogens before discharge.

Labelled diagram

  1. Screening & Grit Removal

    Wastewater passes through screens that catch large debris, and grit settles out.

  2. Primary Sedimentation

    In large tanks, solids sink as sludge and floating fats are skimmed off.

  3. Aeration Tank

    Air is pumped in so bacteria can consume dissolved organic matter.

  4. Secondary Clarifier

    Microbes settle out; some are returned to the aeration tank.

  5. Tertiary Treatment

    Filters, chemicals or UV light remove nutrients and pathogens.

  6. Discharge or Reuse

    Clean water is released to rivers or reused for irrigation and industry.

Sewage treatment: from waste to clean water

Real-world examples

  • The Hyperion Water Reclamation Plant in Los Angeles treats roughly 1 billion litres (about 260 million gallons) of sewage on an average day.
  • London's Beckton Sewage Treatment Works, one of Europe's largest, serves about 3.5 million people.
  • In India, the Yamuna River receives treated sewage from several plants in Delhi, though capacity gaps remain.
  • Singapore's NEWater plants further purify treated sewage using advanced membranes and UV, producing high-grade recycled water.

Common mistake

✗ Myth: Sewage treatment just dilutes waste with water until it is harmless.

✓ Fact: Treatment actively removes pollutants through physical separation and biological breakdown. Bacteria consume organic matter, and settling tanks separate solids, so the cleaned water is genuinely different from the original sewage.

People also ask

What is the difference between primary and secondary sewage treatment?
Primary treatment is physical: it settles solids and skims off floating material, removing about half of the suspended solids. Secondary treatment is biological: bacteria consume dissolved organic matter, removing up to 90% of the remaining pollutants. Most plants use both stages together.
How long does sewage treatment take?
The whole process typically takes 12 to 24 hours from arrival to discharge. Primary settling takes a few hours, aeration takes 6 to 8 hours, and secondary clarification adds a few more. Tertiary steps like UV disinfection take minutes.
Can treated sewage water be drunk?
Yes, but only after advanced tertiary treatment that includes reverse osmosis and disinfection. Singapore's NEWater and parts of California's water supply use this approach. Most treated sewage is not directly drunk; it is released into rivers or used for irrigation and industry.
What happens to the sludge produced during sewage treatment?
Sludge is thickened, then digested by bacteria in oxygen-free tanks, which reduces its volume and kills pathogens. The digested sludge can be dried and used as fertiliser or burned, while the methane gas produced can be captured for energy.
Why is sewage treatment important for public health?
Untreated sewage carries bacteria, viruses and parasites that cause diseases like cholera and typhoid. Treatment removes these pathogens, preventing them from contaminating drinking water and recreational waters, which saves lives and reduces illness.
How does biological treatment actually clean the water?
In aeration tanks, a mix of bacteria and other microorganisms feeds on the organic matter in the sewage. They convert it into carbon dioxide, water and more microbial cells, which are then settled out as sludge, leaving the water much cleaner.
What is tertiary treatment and when is it needed?
Tertiary treatment is an extra step after secondary treatment to remove specific pollutants like nitrogen, phosphorus or pathogens. It is used when the treated water is discharged into sensitive environments or reused for irrigation or drinking, and may involve filters, chemicals or UV disinfection.

Part of the guide: Water & Weather Phenomena →