Process Configurations
Types of Activated Sludge Process
The activated sludge process comes in many configurations. They differ mainly in how influent and return sludge are introduced and in their SRT and F/M regimes — which drives nitrification, shock resilience and footprint.
The Main Configurations
Complete-Mix
Influent dispersed uniformly for strong dilution of shock and toxic loads; uniform, lower substrate gradient.
ExplorePlug-Flow
Mixed liquor moves along the basin with a high inlet substrate tapering to the outlet; good settling and effluent quality.
ExploreStep-Feed
Influent introduced at several points to balance the oxygen demand and control the sludge inventory.
ExploreContact Stabilisation
Separates the short contact and longer stabilisation stages to shrink the required aeration volume.
ExploreExtended Aeration
Long SRT, low F/M variant producing stable effluent and low surplus sludge — ideal for small plants.
ExploreOxidation Ditch
Looped-channel configuration operating in the extended-aeration regime with surface or fine-bubble aeration.
ExploreSequencing Batch Reactor (SBR)
Fill-and-draw batch operation cycling react, settle and decant in a single tank.
ExploreCyclic (CAS)
Continuous-inflow cyclic activated sludge technology combining batch-style phases with continuous feed.
ExploreChoosing a Configuration
| Condition | Potential Issue | What to Investigate |
|---|---|---|
| Variable / shock loads | Need dilution capacity | Complete-mix or step-feed |
| Tight effluent BOD/TSS | Need strong settling | Plug-flow or SBR |
| Small / medium plant | Simplicity & low sludge | Extended aeration or oxidation ditch |
| Constrained footprint | Minimise volume | Contact stabilisation or MBBR/MBR |
| Reliable nitrification required | Retain slow nitrifiers | Long-SRT configuration (extended aeration, plug-flow, SBR) |
Compare Across Technologies
Engineering Note
Frequently Asked Questions
Common configurations include complete-mix, plug-flow, step-feed, contact stabilisation, extended aeration, oxidation ditch, sequencing batch reactor (SBR) and cyclic (CAS) systems. They differ mainly in how influent and return sludge are introduced and in their SRT and F/M regimes, which affects nitrification, resilience to shock loads and footprint.
In a complete-mix system the influent is dispersed uniformly, giving strong dilution of shock and toxic loads but a uniform, lower substrate gradient. In a plug-flow system the mixed liquor moves along the basin with a high substrate concentration at the inlet tapering toward the outlet, which favours good settling and effluent quality but is more sensitive to shock loads.
Configurations that provide a long, reliable SRT — such as extended aeration, oxidation ditches and appropriately designed plug-flow or SBR systems — favour nitrification because nitrifying bacteria grow slowly and need to be retained. The right choice depends on load, temperature, footprint and effluent limits, which is what an engineering assessment establishes.
Engineering Assessment
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