Table of Contents
Every day, India’s 1,600+ urban local bodies and 2,00,000+ industrial units treat over 70,000 million litres of wastewater, yielding approximately 3–4 million tonnes of semi-solid sludge annually. This dark, viscous by-product—often dismissed as mere residue—holds the key to circular economy models that recover energy, nutrients, and water.
For plant operators in Mumbai high-rises, Coimbatore textile parks, or Punjab rice clusters, effective wastewater sludge treatment determines operational costs, odour complaints, and regulatory approvals.
SKF Elixer’s Vulcan STP embeds proven sludge management modules that shrink volumes by 60–70%, generate biogas worth ₹2–3 lakh monthly in mid-sized plants, and produce compost safe for non-food crops.
Physical and Chemical Properties of Sludge
Physical Parameters
- Solids Content: Raw primary 3–7%, secondary 0.5–1.5%, thickened 4–8%, dewatered 18–30%.
- Particle Size: Primary 1–10 mm aggregates; secondary flocs 50–500 μm.
- Rheology: Non-Newtonian, yield stress 50–200 Pa in thickened state—critical for pump sizing (progressive cavity pumps at 1–2 bar).
- Odour: Hydrogen sulphide >5 ppm signals anaerobicity; volatile fatty acids impart rancid smell.
Chemical Parameters
- Volatile Solids (VS): 65–80% in domestic, 50–60% in rice mill (starch, bran).
- pH: 6.0–7.5 fresh; drops to 5.0 in septic primary.
- Nutrients: N 2–4%, P 1–2%, K 0.2–0.5% in digested cake—comparable to farmyard manure.
- Pathogens: 10⁶–10⁸ MPN/g faecal coliforms in raw; <1,000 post-digestion.
- Heavy Metals: <50 mg/kg Cd, <300 mg/kg Cr in compliant plants (CPCB Class A).
Calorific value of dried secondary sludge reaches 12–15 MJ/kg—equivalent to low-grade coal.
Sludge Dewatering Techniques
Thickening
Reduces volume 4–10 fold before digestion.
- Gravity Thickening: 8–12 hour HRT in circular tanks (1.5–2 m side water depth). Primary sludge thickens to 6–8%; secondary to 3–4%. Overflow recycled to inlet.
- Dissolved Air Flotation (DAF): Polymer 2–4 g/kg DS + air saturation at 4 bar floats secondary sludge to 5–7% solids. A 100 KLD STP DAF unit (10 m²) processes 200 m³/day.
- Centrifugal Thickening: Dynamic thickeners achieve 6–8% in single pass with 1–2 kWh/tonne DS.
Digestion
Stabilises organics, reduces pathogens, and generates biogas.
- Anaerobic Mesophilic Digestion (35 °C): Standard for blended sludge. 15–20 day HRT in CSTR digesters. VS destruction 45–55%, biogas yield 0.8–1.0 m³/kg VS destroyed (60–65% CH₄). A 500 m³ digester for 300 KLD STP produces 400–500 m³ biogas/day—enough for 50 kW genset running 20 hours.
- Aerobic Digestion: Extended aeration (20–30 day SRT) for small plants (<50 KLD). No gas recovery but simpler operation.
- Temperature-Phased Anaerobic Digestion (TPAD): 55 °C thermophilic pre-stage + 35 °C mesophilic—achieves Class A biosolids (<1,000 MPN/g).
Dewatering
- Centrifuge: Decanter type, 2,000–3,000 G, 22–30% cake. Power 8–10 kWh/tonne DS.
- Screw Press: Low-speed, 18–22% cake, ideal for fibrous rice mill sludge.
ETPs integrate auto-polymer dosing ( peristaltic pumps + online turbidity feedback) to optimise within ±0.5 kg/tonne.
Slugde Disposal and Reuse Methods
Biogas Utilisation
Raw biogas (60% CH₄, 35% CO₂, 500–1,000 ppm H₂S) is scrubbed to <200 ppm H₂S using iron oxide media (₹150/kg media, 6-month life). A 200 KLD rice mill ETP generates 250 m³ biogas/day, displacing 100 litres diesel (₹9,000/day saving). Dual-fuel gensets or direct boiler injection common. Excess flared via enclosed flare (99.9% combustion efficiency).
Compost Production
Dewatered digested cake mixed with bulking agents (rice husk 1:2 ratio) in windrows. 21-day aerobic phase (55–60 °C) + 30-day maturation yields Class A compost (CPCB Schedule II). A 100 TPD paddy plant produces 15 tonnes compost/month, sold at ₹3,000–4,000/tonne to tea estates. Nutrient profile: N 1.8%, P₂O₅ 2.2%, K₂O 0.8%.
Land Application
Direct application of liquid digested sludge (3–5% DS) via tankers at 10–15 tonnes/ha on cotton fields. Soil injection prevents runoff. pH buffering and slow-release N reduce fertiliser bills by ₹8,000–10,000/ha.
Thermal Options
Incineration in fluidised bed (850 °C) for hazardous industrial sludge. Ash reused in bricks (10% substitution). Capital ₹2–3 crore for 5 TPD unit.
Sewage Treatment Plant Manufacturers
The role of STP plant in pollution prevention is foundational to India’s environmental security. By intercepting 95%+ pollutants, enabling 90% water reuse, and generating compost, SKF Elixer’s Vulcan AABR STP protects soil fertility, groundwater purity, and community health.
Invest in proven sewage treatment benefits for compliance, cost savings, and sustainability. Contact SKF Elixer today.
FAQs
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1. How much biogas can I expect from 1 tonne of dry primary sludge in anaerobic digestion?
At 70% volatile solids and 50% VS destruction, expect 500–600 m³ biogas (62% methane). Energy equivalent: 3,000–3,600 MJ or 90–110 litres diesel.
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2. Is it mandatory to treat tertiary chemical sludge separately in rice mill ETPs?
Not mandatory, but blending >20% alum sludge inhibits methanogens. Pre-neutralise with lime to pH 6.5–7.0 or route to filter press directly for brick-making.
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3. What is the typical polymer dosage for dewatering blended municipal sludge?
3–5 kg active polymer per tonne dry solids for belt press (cationic, 5–6 million MW). Cost ₹600–1,000/tonne DS; optimise via jar tests and online turbidity.
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4. Can digested sludge compost be used on vegetable farms?
Only Class A compost (VS <35%, faecal coliform <1,000 MPN/g, Salmonella absent) per CPCB. Most municipal plants achieve Class B—suitable for cotton, sugarcane, or orchards.
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5. How does automation reduce sludge handling costs in plants?
Auto-polymer dosing cuts consumption 15–20% (₹80,000–1 lakh/year for 200 KLD). Predictive maintenance avoids ₹2–3 lakh breakdown repairs. Remote monitoring saves 2 operator salaries (₹40,000/month).
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