The Activated Sludge Process: How It Works and Why It's Still Dominant

If you asked an engineer in 1920 what would be running the world's wastewater treatment plants in 2026, "activated sludge" wouldn't have been a wild prediction — they invented it in 1913. The process has been refined dozens of times but the core idea is unchanged: get a community of bacteria to eat the dissolved organic matter in sewage, then settle the bacteria out and recycle some of them back to keep the population alive.

The basic flow

Primary-treated sewage enters the aeration tank, where it's mixed with returning activated sludge (RAS) — a concentrated population of bacteria from the previous batch. Air is injected from the bottom, providing oxygen for the bacteria to metabolise organics. After 4-8 hours of contact, the mixture flows into a secondary clarifier where the sludge settles by gravity.

Three things happen with the settled sludge:

• Most is returned to the aeration tank (Returned Activated Sludge, RAS) to keep the bacterial population high — typically 50–100% of inflow volume.
• A small portion is wasted (Waste Activated Sludge, WAS) to control the population — typically 1–3% of inflow. This balances bacterial growth with cell death.
• Clear supernatant flows to the next treatment stage (typically tertiary or discharge).

Key control parameters

• MLSS (Mixed Liquor Suspended Solids): The biomass concentration in the aeration tank. Target: 2,000–4,000 mg/L for conventional plants, up to 12,000 mg/L for MBR.
• F:M ratio (Food-to-Microorganisms): BOD load divided by MLSS. Controls whether bacteria are starved (low F:M = small bugs, good settling) or feasting (high F:M = fast growth, poor settling).
• SRT (Solids Retention Time / Sludge Age): Average days a bacterial cell spends in the system. Determines which species dominate — short SRT favours fast-growing organic eaters, long SRT favours slow-growing nitrifiers (for ammonia removal).
• DO (Dissolved Oxygen): Target 2 mg/L. See our aeration guide for how this is delivered.

Major variations

• Conventional plug flow: Long, narrow tanks where sewage flows from inlet to outlet, gradually getting cleaner. Common in larger plants.
• Complete mix: Circular or square tanks with uniform conditions throughout. Better for shock loads, common in industrial treatment.
• Extended aeration: Long SRT (20–30 days) means less sludge is produced and ammonia is fully removed. Used at smaller plants and packaged STPs.
• Step feed: Sewage is added at multiple points along the aeration tank. Maintains MLSS at low levels in the early stages.
• Sequencing Batch Reactor (SBR): Activated sludge in time rather than space — see our MBR vs MBBR vs SBR comparison.

Why it still dominates

A century after invention, activated sludge still handles the majority of municipal sewage worldwide for three reasons:

1. Scales beautifully: From 1,000 m³/day rural plants to 4.8 million m³/day giants like Deer Island, Boston.
2. Cheap to build: Concrete tanks, blowers, basic instrumentation. No exotic materials.
3. Robust: Recovers from shock loads, weather changes, operator mistakes.

Newer technologies (MBR, MBBR, aerobic granular sludge) all build on activated sludge — they just modify how the bacteria are retained or how the carriers are arranged. The fundamental "feed bacteria, let them eat, then settle them out" approach hasn't changed.