The activated sludge process has been the backbone of biological wastewater treatment for over a century — and despite the emergence of MBBR, MBR, and SBR, it remains the most widely installed biological treatment technology in India. Understanding how it works, what parameters control it, and when it's the right choice is foundational knowledge for anyone managing an industrial ETP.
How the Activated Sludge Process Works
The activated sludge process uses a mixed community of aerobic microorganisms — suspended in the aeration tank as "activated sludge" — to biodegrade dissolved organic matter. The process has three components working in a continuous loop:
Aeration tank: Incoming wastewater mixes with the recycle sludge. Air (or oxygen) is supplied by surface aerators, diffused aerators, or jet aerators to maintain dissolved oxygen at 2–3 mg/L. Bacteria grow by consuming BOD/COD, using oxygen as the electron acceptor. The mixture of treated water and biological sludge ("mixed liquor") flows continuously to the clarifier.
Secondary clarifier: The mixed liquor enters a large, quiescent sedimentation tank where biological sludge settles under gravity. The clarified effluent overflows as the treated water. The settled sludge accumulates at the bottom.
Sludge recycle: A controlled flow of settled sludge (Return Activated Sludge, or RAS) is pumped back to the head of the aeration tank to maintain the target MLSS. Without RAS, the biological mass would wash out of the system in hours. The excess sludge produced by biological growth (Waste Activated Sludge, or WAS) is withdrawn and sent to sludge handling.
Key Design and Control Parameters
Four parameters define ASP performance:
- MLSS (Mixed Liquor Suspended Solids): The concentration of biological solids in the aeration tank — typically 1,500–6,000 mg/L depending on variant. See our MLSS guide.
- HRT (Hydraulic Retention Time): Time wastewater spends in the aeration tank — typically 6–36 hours depending on load and variant. See our HRT guide.
- SRT (Sludge Retention Time): Average age of the biological sludge, controlled by the daily sludge wasting rate. SRT determines organism type: short SRT favours fast-growing heterotrophs; long SRT (10+ days) allows slow-growing nitrifiers to establish. Typical range: 5–30 days for food industry ETPs.
- F/M ratio (Food to Microorganism ratio): Organic load per unit of MLVSS per day — the fundamental driver of sludge growth rate and settleability. Low F/M (extended aeration) produces stable, well-settled sludge. High F/M produces young, fast-growing sludge that can be harder to settle.
ASP Variants: Conventional, Extended Aeration, and Oxidation Ditch
Conventional ASP operates at F/M 0.2–0.5, HRT 6–12 hours, MLSS 1,500–3,500 mg/L. It requires primary sedimentation upstream (or DAF for high-FOG wastewater) but produces less sludge per unit of COD treated than extended aeration. The secondary clarifier must be sized carefully — conventional ASP sludge has higher settling velocity requirements.
Extended aeration operates at very low F/M (0.05–0.15), long HRT (18–36 hours), and long SRT (20–30 days). No primary clarifier is needed because the long aeration time stabilises both incoming suspended solids and biological sludge. The sludge produced is aerobically stabilised and less odorous. This is the preferred variant for most small-to-medium industrial ETPs in India — particularly for food processing plants where simplicity of operation matters.
Oxidation ditch is a loop-form extended aeration system — wastewater and sludge circulate continuously around an oval channel with horizontal shaft aerators. The plug-flow hydraulics of an oxidation ditch create a DO gradient around the loop, supporting both aerobic BOD removal and anoxic denitrification. Common for municipal STPs and larger food industry ETPs.
ASP vs MBBR vs MBR: When to Choose Each
ASP (extended aeration) is the right choice when: the site already has adequate aeration tank and clarifier capacity; the effluent is relatively simple (low FOG, no major shock loads); operational simplicity matters more than treatment intensity; and the capital budget favours using existing civil structures.
MBBR is better when you need to increase biological capacity within existing tank volumes — adding media to an existing ASP aeration tank can 2–3× the biological treatment capacity without new civil construction. MBBR also handles high-strength, variable food industry effluent better than conventional ASP because the biofilm on carrier media is more robust to shock loads.
MBR outperforms ASP when treated water quality must meet reuse standards (BOD <5 mg/L, TSS <1 mg/L) or when severe plot constraints preclude a secondary clarifier. MBR adds significant membrane capital and operating cost but eliminates the clarifier entirely.
Sludge Handling in ASP Systems
Activated sludge systems produce two sludge streams: Primary sludge (from DAF or primary clarifier) contains raw suspended organic matter — it settles well but is odorous and highly putrescible. Waste activated sludge (WAS) is aerobically stabilised biological sludge — it settles less well than primary sludge and requires thickening before dewatering.
Both streams are typically combined in a sludge holding tank, thickened in a gravity thickener or dissolved air flotation thickener (DAFT), and then dewatered by filter press, belt press, or decanter centrifuge. Target moisture content in the cake: 70–80% for filter press, 75–82% for decanter. Dewatered cake is disposed of to a secured landfill, composted for agricultural use (for non-hazardous sludge), or incinerated (for pharmaceutical or heavy metal sludge). Sludge handling is frequently the most costly and operationally challenging component of an ETP — not the biological treatment.
Common ASP Problems and Fixes
Sludge bulking (poor settleability, SVI >150 mL/g): Usually caused by filamentous organism overgrowth from low DO, low F/M, or septic inlet. Fix: raise DO to 2–3 mg/L, add selector zone, controlled RAS chlorination.
Rising sludge (sludge floating in clarifier): Caused by denitrification in the clarifier — nitrogen bubbles lift settled sludge. Fix: reduce SRT (younger sludge nitrifies less), increase RAS rate to reduce HRT in clarifier, add anoxic zone upstream of clarifier.
Foaming: White foamy foam indicates young sludge with high surfactant load — common during plant startup or after a sludge washout event. Brown stable foam indicates Nocardia or Microthrix actinomycetes — associated with high-fat wastewater and long SRT. Fix for brown foam: reduce SRT (waste more sludge); avoid fat and oil spills reaching the biological reactor.
High effluent TSS: Usually from clarifier overloading (increase surface area) or from high SVI sludge (address bulking root cause). Pin floc (very fine non- setteable particles) indicates very old sludge or toxic stress — reduce SRT.
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