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Across India’s urban skyline and industrial corridors, thousands of sewage treatment plants and effluent treatment units hum daily, converting over 40,000 million litres of foul water into clear streams. At the core of most of these facilities lies a century-old yet continually refined technology: the activated sludge process.
From municipal corporations in Delhi to textile clusters in Tirupur, this biological wastewater treatment method remains the gold standard for removing organic pollutants. SKF Elixer integrates advanced variants of this process into its Vulcan STP and ETP systems, delivering consistent performance in compact, Indian-context designs.
Understanding its mechanics reveals why it dominates treatment landscapes and how operators can optimize it for peak efficiency.
Working Principle of the Activated Sludge Process
The activated sludge process operates on a simple yet powerful concept: harness billions of microorganisms to devour dissolved and suspended organic matter in wastewater. Raw sewage or effluent first passes through preliminary screens and grit chambers, shedding large debris. It then enters an equalization tank—typically 6–8 hours capacity—to smooth out flow and concentration spikes common in Indian settings (morning domestic peaks or factory shift changes).
The heart of the system is the aeration tank, a rectangular or circular basin where clarified wastewater mixes with “activated sludge”—a brown, flocculent mass teeming with bacteria, protozoa, and other microbes. Air blowers inject fine bubbles through diffusers at the tank bottom, maintaining Dissolved Oxygen (DO) at 1.5–3 mg/L.
As wastewater flows through (hydraulic retention time 4–8 hours for domestic, 12–24 hours for high-strength industrial), microbes adsorb organic substrates onto their cell surfaces. Enzymes break down complex molecules—starches, proteins, fats—into simpler compounds.
A 100 KLD plant’s aeration tank might hold 400–600 m³ mixed liquor with Mixed Liquor Suspended Solids (MLSS) at 2,500–4,000 mg/L. Roughly 1 kg of BOD requires 0.5–0.7 kg oxygen and generates 0.3–0.4 kg new biomass.
The mixture then flows to a secondary clarifier—a stilling zone where gravity separates clean supernatant (BOD <20–30 mg/L) from settled sludge. A portion of this sludge—rich in active microbes—is recycled via air-lift or submersible pumps back to the aeration tank at 50–100% of influent flow (Recycling Ratio, R). Excess sludge is wasted to maintain steady MLSS.
This continuous cycle mimics a chemostat: fresh food in, waste out, population balanced. In SKF Elixer’s Vulcan systems, PLC-controlled variable frequency drives adjust blower speed and recycle rates in real-time, responding to online DO probes and flow meters.
Role of Aeration and Microorganisms in Organic Breakdown
Aeration serves dual purposes: oxygen supply and mixing. Fine bubble diffused aeration (FBDA) systems—standard in Vulcan plants—transfer oxygen at 20–25% efficiency versus 8–10% for surface aerators. A 100 KLD STP requires 150–200 kg O₂ daily; blowers sized at 3–4 kW deliver 200–250 m³/hr air. Oxygen uptake rate (OUR) spikes to 40–60 mg/L/hr during peak BOD, guiding blower modulation.
Microbial ecology drives treatment. Heterotrophic bacteria (Pseudomonas, Bacillus) dominate organic carbon removal, forming flocs 100–500 μm in size. Filamentous organisms (Sphaerotilus, Thiothrix) provide structural backbone but can cause bulking if excessive. Protozoa—ciliates like Vorticella—graze dispersed bacteria, clarifying effluent. Rotifers and nematodes appear in mature sludge, indicating stable ecosystems. In rice mill ETPs, starch-degrading amylases from Bacillus subtilis reduce COD from 6,000 mg/L to <250 mg/L within 18 hours.
Nitrification adds complexity. Autotrophic Nitrosomonas oxidize ammonia to nitrite, then Nitrobacter to nitrate—requiring 4.6 kg O₂ per kg NH₄-N. In extended aeration systems (common for domestic STP), sludge age >15 days supports these slow-growers. SKF Elixer’s AABR hybrid integrates attached growth media within aeration zones, boosting nitrifier retention even at shorter HRTs.
Factors Influencing Process Efficiency
Dissolved Oxygen (DO): Below 1 mg/L, facultative bacteria shift to fermentation, producing volatile fatty acids and odors. Above 4 mg/L wastes energy. Optimal 2 mg/L balances removal and cost—₹2–3 per 1,000 litres power.
Temperature: Microbial activity doubles every 10 °C rise from 15–35 °C. Winter dips to 12 °C in northern India extend HRT by 20–30%; summer peaks at 38 °C risk thermal shock. Vulcan plants include tank insulation and recirculation loops to stabilize conditions.
BOD/COD Load and F/M Ratio: Food-to-Microorganism ratio (kg BOD applied per kg MLSS daily) ideally stays 0.15–0.4 for conventional systems. Rice mill shock loads (COD 10,000 mg/L) demand equalization volume of 12–16 hours. MLVSS/MLSS ratio >0.75 signals healthy biomass.
pH and Alkalinity: Nitrification consumes 7.1 kg alkalinity (as CaCO₃) per kg NH₄-N. Raw sewage pH 6.5–8.0 suits most; industrial swings require lime dosing (50–100 mg/L).
Toxicity: Heavy metals (>1 mg/L Cu, Cr) or phenols (>50 mg/L) inhibit respiration. Pre-treatment in ETPs—chemical precipitation or oil skimming—protects biology.
Sludge Age: 3–5 days for high-rate, 10–20 days for extended aeration. Longer age yields less sludge (0.4 kg/kg BOD vs 0.6 kg) but larger tanks.
Common Challenges and Operational Control Measures
Bulking Sludge: Filamentous overgrowth causes poor settling (Sludge Volume Index, SVI >150 mL/g). Causes: low DO, nutrient deficiency (N:P 5:1 absent), or septic sewage. Fixes: chlorination (2–5 mg/L Cl₂ to return sludge), polymer dosing (0.5 kg/tonne DS), or selector zones with high initial F/M.
Pin-Floc: Tiny, non-settling particles from over-aeration or young sludge. Increase recycle ratio to 150%, reduce wasting.
Foaming: Nocardia or Microthrix proliferation in fat-rich influents (dairies, rice bran oil mills). Defoamers (silicone-based, 5–10 ppm) or surface sprays control; long-term: anaerobic selectors.
Rising Sludge: Denitrification in clarifiers lifts sludge blanket. Reduce HRT in final zone, add nitrate breathers, or taper aeration.
Operational Controls:
- Daily jar tests for settleability (30-minute settling <400 mL).
- Microscopic examination (filament count <3 intersections/grid).
- Online OUR meters trigger wasting when >25 mg/L/hr.
- SCADA logs track MLSS trends; auto-wasting maintains 3,000 mg/L ±200.
Advantages of Activated Sludge Over Traditional Treatment Methods
Versus Septic Tanks/Imhoff Tanks: Activated sludge achieves 90–95% BOD removal versus 40–60%, producing polished effluent reusable for flushing (saving 40 litres/person/day). No odor nuisance in urban colonies.
Versus Trickling Filters: Higher loading rates (0.8–1.2 kg BOD/m³/day vs 0.3–0.5) shrink footprint—critical where land costs ₹50–100 lakh per acre in peri-urban areas. Better shock resistance; filters clog under oil loads common in food processing.
Versus Lagoons: 80–90% less area (0.2 m²/PE vs 2–3 m²/PE). Year-round performance unaffected by monsoon dilution. Sludge production 30–40% lower in extended aeration.
Versus UASB: Handles lower strength domestic sewage (BOD 200 mg/L) without biogas complexity. Easier retrofitting into existing plants. Zero washout during power failures via settled biomass retention.
Energy and Cost: Though aeration consumes 50–60% of plant power (0.6–1 kWh/m³), biogas recovery in high-strength ETPs offsets 30–40%. A 200 KLD municipal STP saves ₹8–10 lakh annually in freshwater bills through tertiary reuse.
A 50 KLD domestic plant occupies just 80 m²—including clarifier and sludge holding—versus 300 m² for conventional. Relocatability allows labor camps to shift units between projects, preserving investment.
The process’s adaptability shines in mixed applications: a textile ETP treats 400 KLD with 3,500 mg/L COD, achieving <100 mg/L via two-stage aeration; an apartment STP polishes 120 KLD sewage to <10 mg/L BOD for landscape irrigation. Both share the same biological engine, tuned via MLSS, DO, and recycle.
Mastering activated sludge transforms wastewater treatment from mechanical obligation to biological precision. With vigilant monitoring and modern controls, Indian plants deliver environmental compliance, resource recovery, and operational resilience—proving that microscopic workers can solve macroscopic challenges.
FAQs
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1. What is the ideal MLSS concentration in an activated sludge aeration tank for domestic STP?
2,500–3,500 mg/L for conventional systems, 3,500–4,500 mg/L for extended aeration. Maintain via daily sludge wasting of 1–2% tank volume, calculated as: Waste flow = (Influent flow × Influent SS) / (MLSS × (1 – Return ratio)).
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2. How much air is required per kg of BOD removed in the activated sludge process?
1.0–1.2 kg O₂ theoretically, but field transfer efficiency yields 1.5–2.0 m³ air/kg BOD at 2 barg. A 100 KLD plant with 250 kg BOD/day needs 1,500–2,000 m³ air daily, delivered via 5–7.5 kW twin-lobe blowers.
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3. Why does my clarifier effluent turn cloudy after rainfall?
Storm dilution drops influent BOD, reducing F/M ratio and causing dispersed growth. Increase return sludge rate to 100–150% for 4–6 hours to restore floc density. Install flow-paced chemical dosing if needed.
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4. Can activated sludge handle temperature variations in Indian summers and winters?
Yes, within 15–35 °C. Below 12 °C, extend HRT by 20% or insulate tanks. Above 38 °C, increase aeration by 10–15% to compensate for lower oxygen solubility (Henry’s law). These plants include temperature compensation in DO setpoints.
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5. How do these plants reduce energy consumption in activated sludge systems?
Fine bubble diffusers achieve 25% OTE, VFD blowers match air to OUR, and AABR media reduce tank volume by 30%. A 100 KLD STP consumes 80–100 units/day versus 150–180 in coarse bubble systems, saving ₹1.5–2 lakh annually at ₹7/unit.
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