HydrionisGlobal Water Intelligence
Wastewater

How sewage is treated

Wastewater treatment is mostly a controlled biological process: micro-organisms are fed the pollution and then removed.

introductoryUpdated 23 August 2026

Sewage treatment removes solids, then dissolved organic matter, then — where the receiving water requires it — nutrients. The middle step, which does most of the work, is biological.

Wastewater treatment

Sewage treatment removes solids, then organic matter, then — where required — nutrients, before returning the water to a river or the sea.

Wastewater treatmentRaw sewage passes through preliminary treatment removing rags and grit, then primary settlement removing settleable solids, then secondary biological treatment by activated sludge where micro-organisms consume dissolved organic matter, then final settlement, then optionally tertiary treatment for nutrients or disinfection, before discharge. Sludge from the primary and secondary stages goes to anaerobic digestion, producing biogas and biosolids.sludgesurplus sludgewhen capacity exceededRaw sewagePreliminaryscreens, gritPrimarysettlementSecondaryactivated sludgeSecondary: Aeration typically consumes more electricity than any other process at the works.Final settlementTertiarynutrients, UVDischargeto river or seaAnaerobic digestionBiogasenergy recoveryBiosolidsto agricultureStorm overflowin heavy rain
Population equivalent
Works are sized in population equivalent, a measure of organic load rather than of people — one food factory can equal a small town.
Why storm overflows exist
Combined sewers carry rainfall as well as sewage. Without a relief point, heavy rain would back sewage up into homes.
Sludge is a resource
Anaerobic digestion produces biogas that can power a large share of the works, and biosolids used as agricultural fertiliser.

The stages

Preliminary
Screens remove rags, wipes and debris; grit channels remove sand and grit that would abrade pumps.
Primary
Settlement tanks let the heavier solids sink and the fats and oils float. Removes roughly half the suspended solids and a third of the organic load with no energy input beyond gravity.
Secondary
The biological stage. A suspended or attached community of micro-organisms is supplied with oxygen and consumes the dissolved organic matter. Removes over 90% of the remaining organic load.
Tertiary
Where required: nutrient removal for nitrogen and phosphorus, sand filtration, or ultraviolet disinfection before discharge to a bathing water.
Sludge treatment
Anaerobic digestion of the solids removed at each stage, producing biogas and a stabilised biosolid.

What activated sludge actually is

The activated sludge process is the most widely used secondary treatment in the world. Settled sewage enters an aerated tank containing a dense suspended community of bacteria, protozoa and rotifers. Air is blown in, the organisms consume the dissolved organic matter, and they flocculate into settleable particles.

The mixture then goes to a final settlement tank. Most of the settled biomass is returned to the aeration tank to maintain the population; the surplus is removed as waste sludge. The operator is, in effect, farming micro-organisms and harvesting them.

What does not get removed

Conventional treatment was designed for organic load, solids and pathogens. It removes those very well. It was not designed for pharmaceuticals, hormones, PFAS or microplastics, and it removes those partially at best.

PFAS in particular pass through essentially unchanged and concentrate in the sludge, which is then often spread on agricultural land — moving the contamination rather than destroying it. This is an active and unresolved regulatory question in several jurisdictions.

Sludge is a resource

Anaerobic digestion breaks down the organic content of sludge in the absence of oxygen, producing biogas that is typically 60% methane. Many works generate a large share of their own electricity from it, and some export to the grid. The digested biosolid is used as an agricultural fertiliser, subject to controls on metals and pathogens.

Sources

More on wastewater

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