Process Technology
Activated Sludge Process
The most widely applied biological wastewater treatment process worldwide. Suspended-growth system using aeration and secondary clarification.
Process Description
The activated sludge process maintains a mixed culture of microorganisms (activated sludge) in suspension within an aeration basin. Wastewater and return activated sludge (RAS) are mixed and aerated, allowing bacteria to metabolize dissolved and colloidal organic matter.
After biological treatment, the mixed liquor flows to a secondary clarifier where the activated sludge settles. Clarified effluent is discharged; settled sludge is either returned to the aeration basin (RAS) or wasted (WAS) to maintain the target solids retention time.
Key Design Parameters
MLSS
Mixed Liquor Suspended Solids
SRT
For nitrification
F/M Ratio
Food to Microorganism
DO
For nitrification
Aeration
Aeration serves two functions: oxygen transfer for biological reactions and mixing to keep solids in suspension. Fine bubble diffusers are the most energy-efficient option for conventional activated sludge, achieving 1.5–2.5 kg O₂/kWh standard oxygen transfer efficiency.
Aeration typically accounts for 40–60% of a treatment plant's total energy consumption. DO control strategies — including variable-speed blowers and online DO sensors — can reduce aeration energy by 15–30%.
Troubleshooting Guide
| Condition | Potential Issue | What to Investigate |
|---|---|---|
| SVI > 150 mL/g | Poor sludge settleability | Check for filamentous organisms, F/M ratio, DO levels |
| Effluent NH₄-N > limit | Incomplete nitrification | Verify SRT, DO, temperature, alkalinity |
| Rising sludge in clarifier | Denitrification in clarifier | Reduce sludge blanket depth, increase RAS rate |
| Low MLSS despite no wasting | Solids washout | Check clarifier hydraulic loading, settling properties |
| High effluent BOD | Insufficient treatment | Verify F/M, HRT, oxygen supply, toxic inhibition |
Process Optimization
Optimization of the activated sludge process focuses on maintaining stable biological conditions while minimizing energy consumption. Key control variables include SRT, DO setpoint, RAS rate, and WAS rate.
Engineering Note
Frequently Asked Questions
The activated sludge process is a suspended-growth biological treatment method in which aerated microorganisms (mixed liquor) degrade organic matter. The mixture then flows to a secondary clarifier where the biomass settles; settled sludge is partly returned to the aeration basin (RAS) and partly wasted (WAS) to control the sludge age.
MLSS (mixed liquor suspended solids) is the concentration of biomass in the aeration basin. Conventional activated sludge typically operates around 2,000–4,500 mg/L, while membrane bioreactors run much higher (roughly 8,000–15,000 mg/L). The right MLSS balances treatment capacity against clarifier or membrane performance.
SRT, or sludge age, is the average time biomass stays in the system. It is controlled through the wasting rate (WAS). A sufficient SRT is essential for slow-growing nitrifying bacteria; if the SRT is too short at low temperatures, nitrifiers wash out and effluent ammonia rises.
RAS (return activated sludge) is settled sludge recirculated from the clarifier back to the aeration basin to maintain biomass concentration. WAS (waste activated sludge) is the portion removed from the system to control SRT and prevent excessive solids build-up.
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