Table of Contents
India’s sewage treatment plants process over 17,000 million litres of wastewater daily, yet the real work happens at a scale invisible to the naked eye. Microorganisms in sewage treatment—bacteria, protozoa, and fungi—form the backbone of the biological treatment process, degrading 85–95% of organic pollutants in compact tanks.
Microbial Biomass Efficiency
In a typical 100 KLD facility, just 2–3 kilograms of active microbial biomass consumes 25–30 kilograms of Biochemical Oxygen Demand (BOD) every day, transforming murky influent into clear effluent suitable for reuse.
SKF Elixer’s Vulcan STP harnesses these natural workers through Attached Growth Bioreactor (AABR) technology, where synthetic media provides a stable home for biofilms that maintain efficiency even during flow surges common in urban apartments or commercial complexes.
Understanding the Microbial Community
Dominant Heterotrophic Bacteria
The journey begins with understanding the diverse microbial community. Heterotrophic bacteria dominate, comprising 70–80% of the biomass in aeration tanks. Species like Pseudomonas and Bacillus excel at breaking down carbohydrates, proteins, and fats. For instance, Bacillus subtilis produces enzymes such as amylases and proteases that hydrolyze starches from kitchen waste—prevalent in hotel sewage—at rates of 1–2 grams per liter per hour under optimal conditions.
Organic Matter Degradation
These bacteria adsorb organic particles onto their cell surfaces, oxidizing them through respiration to produce carbon dioxide, water, and new cell mass. In a 200 KLD plant treating domestic sewage with 300 mg/L BOD, heterotrophs remove 200–250 mg/L in the first 4–6 hours of contact.
Autotrophic Nitrifiers and Nutrient Management
Role of Nitrosomonas and Nitrobacter
Autotrophic nitrifiers play a specialized role in nutrient management. Nitrosomonas converts ammonia (40–60 mg/L in raw sewage) to nitrite, consuming 3.2 kg oxygen per kg nitrogen, while Nitrobacter further oxidizes nitrite to nitrate. This two-step nitrification requires a pH of 7.2–8.0 and dissolved oxygen above 2 mg/L.
Importance of Sludge Retention Time
In extended aeration systems, these slow-growing organisms (doubling time 12–24 hours) need sludge retention times exceeding 10 days to avoid washout. SKF Elixer’s AABR media retains these nitrifiers on fixed surfaces, achieving 85–90% total nitrogen removal without separate anoxic tanks, as denitrifying bacteria like Paracoccus utilize nitrate in low-oxygen zones within the biofilm layers.
Protozoa and Higher Organisms: Biological Quality Controllers
Ciliates and Floc Indicators
Protozoa and higher organisms act as quality controllers. Free-swimming ciliates graze on dispersed bacteria, reducing effluent turbidity, while stalked forms like Vorticella indicate healthy floc formation with Sludge Volume Index (SVI) below 100 mL/g.
Rotifers and Filamentous Bacteria
Rotifers appear in mature systems, consuming 1,000–5,000 bacteria per individual daily, signaling low effluent suspended solids. Filamentous bacteria such as Sphaerotilus natans provide structural support to flocs but can cause bulking if dominant; microscopic counts exceeding 10 intersections per field trigger corrective chlorine dosing at 2–4 mg/L.
Aerobic and Anaerobic Microbial Dynamics
Aerobic Bacteria
Aerobic bacteria thrive in oxygen-rich environments, requiring 1.5–3 mg/L dissolved oxygen maintained by fine bubble diffusers. They yield high energy (32 ATP per glucose molecule) and remove 0.5–0.7 kg BOD per kg biomass produced.
Anaerobic Bacteria and Biogas
In contrast, anaerobic bacteria in digesters operate without oxygen, yielding methane (60–65% of biogas) and reducing volatile solids by 45–55% over 15–20 days at 35 °C. Methanogens like Methanosaeta convert acetic acid to methane at rates of 0.3–0.5 m³ per kg volatile solids destroyed.
A 300 KLD plant’s digester produces 150–180 m³ biogas daily, enough to power a 25 kW generator for 8–10 hours or offset 60–70 kg LPG in boiler operations.
Microbes Across the Treatment Process
Role in Preliminary and Secondary Treatment
Microbes dominate every treatment stage. In preliminary zones, biofilms on screens initiate hydrolysis of detergents. The core secondary biological treatment unfolds in aeration tanks where Mixed Liquor Suspended Solids (MLSS) reach 3,000–4,000 mg/L. Food-to-Microorganism (F/M) ratio of 0.15–0.35 ensures balanced growth; Oxygen Uptake Rate (OUR) of 20–40 mg/L/hour guides blower operation.
AABR Media Advantage
In SKF Elixer’s AABR, media surface area of 220 m²/m³ supports biofilm density of 10¹⁰ colony-forming units per cm², resisting hydraulic shocks up to three times design flow—common during morning peaks in residential complexes.
Tertiary Polishing and Sludge Stabilization
Tertiary polishing relies on residual biofilms in filters to shave final BOD to below 5 mg/L. Sludge digestion completes the cycle: acid-forming bacteria hydrolyze polymers in 48–72 hours, followed by methanogenesis. Pathogen reduction achieves 99.9% kill, yielding cake with less than 1,000 MPN/g faecal coliforms.
Key Factors Governing Microbial Performance
Environmental and Chemical Conditions
Several factors govern microbial performance. Temperature optima range 30–35 °C; activity halves at 15 °C, prompting tank insulation in northern India. pH outside 6.5–8.5 inhibits enzymes—automated lime dosing in Vulcan systems corrects drifts within 30 minutes.
Nutrients and Toxicity
Nutrient balance (C:N 100:5:1) rarely limits domestic sewage but requires urea addition in industrial streams. Toxicants like heavy metals above 1 mg/L copper or phenols over 50 mg/L demand pre-treatment.
Sludge Age
Sludge age controls community structure: 3–5 days for high-rate carbon removal, 15–20 days for nitrification.
Emerging Innovations in Microbial Treatment
Microbial Fuel Cells & Anammox
Emerging innovations amplify microbial potential. Microbial fuel cells integrate exoelectrogenic bacteria like Geobacter on anodes, generating 0.5–1 watt per m² while treating wastewater—pilots power sensors in remote STPs.
Anammox processes cut aeration by 60%, converting ammonia and nitrite directly to nitrogen gas in granular sludge reactors; a 5 MLD demonstration in Surat saves ₹12 lakh annually in power.
Bioaugmentation & Phage Therapy
Bioaugmentation with commercial consortia accelerates startup, reducing commissioning from 30 days to 10–15. Phage therapy targets bulking filaments specifically, resolving SVI issues in 48 hours without chemicals.
Algal-Bacterial and Nano-Enhanced Systems
Algal-bacterial systems in high-rate ponds fix CO₂ while removing nutrients; 1 m² produces 20 grams algae daily, harvestable for biodiesel. Nano-enhanced media with graphene oxide boosts biofilm adhesion by 30%, extending media life beyond 15 years.
AI-Driven Predictive Treatment
Artificial intelligence now predicts microbial shifts: machine learning analyzes OUR trends to forecast bulking 72 hours ahead, auto-adjusting F/M ratios.
Conductive media trials enable micro-current stimulation for 15% faster degradation. Automated bio-dosing ports inject enzymes during shock loads. AI dashboards monitor 1,200 operational taxonomic units in real-time. A 400 KLD installation in Coimbatore can achieve BOD below 5 mg/L, nitrogen removal at 1.2 g/m² media/day, and biogas yield of 180 m³ daily—demonstrating how engineered habitats supercharge natural microbial efficiency.
The Future of Intelligent, Energy-Producing STPs
From carbon crunchers to nitrogen specialists, microorganisms orchestrate sewage purification with remarkable precision. In SKF Elixer’s stainless-steel AABR tanks, these tiny titans deliver consistent performance in 80 m² footprints, recycling 80% of treated water for flushing or landscaping.
As bio-innovations mature, the biological sewage treatment process evolves from reliable workhorse to intelligent, energy-generating system—ensuring cleaner rivers and sustainable urban water cycles across India.
FAQs
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1. What is the primary enzyme produced by Bacillus bacteria in sewage treatment?
Amylases and proteases dominate, hydrolyzing starches and proteins at 1–2 g/L/hour, critical for kitchen waste degradation in hotel STPs.
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2. How does low dissolved oxygen affect aerobic bacteria in aeration tanks?
Below 1 mg/L, facultative bacteria shift to fermentation, producing volatile fatty acids and hydrogen sulfide, leading to odour and reduced BOD removal efficiency by 40–50%.
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3. Why are nitrifying bacteria sensitive to temperature changes?
Enzyme kinetics slow dramatically below 20 °C; activity drops 50% at 15 °C, requiring extended HRT or media retention in AABR systems during winter.
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4. How much biogas can methanogens produce from 1 kg of volatile solids in sludge digestion?
0.8–1.0 m³ at 60–65% methane content, equivalent to 5–6 kWh energy or displacement of 2–2.5 kg LPG.
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5. What indicators show healthy microbial activity in an STP aeration tank?
OUR 20–40 mg/L/hour, SVI 80–120 mL/g, Vorticella density >500 per microscopic field, and MLSS 3,000–4,000 mg/L with 75–80% volatile fraction.
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