How Sewage Treatment Works: Primary, Secondary & Tertiary Explained

Every sewage treatment plant in the world — from the 5,000 m³/day rural plant to the 4.8 million m³/day giants like Boston's Deer Island — relies on the same three-stage process: primary, secondary, and (where required) tertiary treatment. This article explains what each stage does, what it removes, and why most modern plants now include all three.

Primary treatment: settle the heavy stuff

Primary treatment is purely physical. Incoming sewage flows into large rectangular or circular tanks (primary clarifiers or settling tanks) and slows down enough for gravity to do two things at once: heavier solids sink to the bottom as primary sludge, and lighter materials — fats, oils and grease — float to the top as scum.

What primary treatment removes:

• Suspended solids: 50–65%
• Biochemical Oxygen Demand (BOD): 25–40%
• Pathogens: very little (5–15%)

Before primary treatment, larger objects are removed by screening (bars or mesh that catch rags, plastics, sticks) and grit removal (a settling chamber that traps sand and small stones). These are sometimes called "preliminary" treatment, but they're really the first step of a primary stage.

Secondary treatment: biology removes the dissolved organic load

Primary treatment leaves the majority of dissolved organic matter — the BOD that would consume oxygen in a receiving river and kill aquatic life — still in the water. Secondary treatment removes it biologically, using microorganisms that eat the organic matter and convert it to carbon dioxide and new bacterial cells.

The dominant secondary process is the activated sludge process: primary effluent flows into a large aeration tank where blowers inject air and a community of bacteria, protozoa and fungi (the "mixed liquor") metabolises the dissolved organics. The aerated mixture flows into a secondary clarifier where the bacterial mass settles out as secondary sludge — some of which is returned to the aeration tank to maintain the biomass.

Alternative secondary technologies you'll see across the UtilityRadar directory:

• Membrane Bioreactor (MBR) — combines activated sludge with membrane filtration in one step, producing very high-quality effluent. Used at Al Ansab STP (Oman) and the Sharjah Industrial Area STP.
• Moving Bed Biofilm Reactor (MBBR) — plastic media moving through the tank that bacteria colonise, giving more biomass per litre. Deployed at the Dammam WWTP in Saudi Arabia.
• Sequencing Batch Reactor (SBR) — does aeration and settling in the same tank, in cycles. Used at Saudi Arabia's Taif WWTP.
• Trickling filter — older but still common in small US plants; sewage trickles over a stationary medium with biofilm.

For a side-by-side: see MBR vs MBBR vs SBR.

Secondary treatment typically removes 85–95% of BOD and most suspended solids.

Tertiary treatment: polishing for reuse or sensitive discharge

Secondary effluent is clean enough for many uses, but not for two common cases: discharge to a sensitive receiving body (where nutrients would cause algal blooms) and reuse for irrigation, industrial cooling, or aquifer recharge. Tertiary treatment adds one or more polishing steps:

• Nutrient removal — biological or chemical removal of nitrogen and phosphorus, which would otherwise cause eutrophication downstream.
• Sand or media filtration — removes remaining suspended solids and turbidity.
• Disinfection — UV light, ozone or chlorination, kills remaining pathogens. UV is dominant in modern plants because it leaves no chemical residue.
• Activated carbon — removes trace organics, taste, odour, and some pharmaceuticals.

The GCC has been a global leader on tertiary treatment and reuse: in the UAE, the Warsan STP (Dubai) and Jebel Ali STP together produce over a million m³/day of tertiary-treated effluent reused for landscape irrigation. Saudi Arabia's Jeddah Airport 2 ISTP uses Nereda aerobic granular sludge technology to produce reuse-grade water.

Sludge: the other half of the plant

Primary and secondary treatment together produce large volumes of sludge — typically 1–2% of the influent volume. Modern plants treat sludge by anaerobic digestion (microbes break it down without oxygen, producing methane biogas that can power the plant), then dewatering using centrifuges or belt presses, then either land application (as a fertiliser if pathogens have been killed) or incineration with energy recovery.

Boston's Deer Island plant is famous for converting its sludge into commercial fertiliser pellets (Bay State Fertilizer).

Why most modern plants run all three stages

Older plants in many parts of the world still run only primary + secondary, discharging to large rivers or oceans where dilution handles the residual load. But three forces are pushing every new plant to full tertiary treatment:

1. Water scarcity — in the GCC, Australia and the western US, treated effluent is too valuable to discharge.
2. Tighter discharge standards — EU UWWTD, US EPA Clean Water Act and growing nutrient-removal rules.
3. Reuse economics — tertiary effluent for irrigation is cheaper than desalinated water in arid regions.

UtilityRadar tracks the treatment level for over 53,000 plants worldwide. To browse plants by stage, see our primary, secondary, and advanced filter views.