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As of June 2025, India stands as the world’s largest rice producer, harvesting over 149 million tonnes annually from vast paddies stretching across Andhra Pradesh, West Bengal, Uttar Pradesh, and Punjab. Behind every grain on a plate lies an intensive journey through milling units where paddy is husked, polished, and packaged.
Yet, this transformation exacts a hidden toll: each tonne of paddy processed releases 1,200–2,000 litres of murky, organic-laden wastewater. Multiplied across the country’s 1,50,000+ rice mills, the volume rivals medium rivers—enough to submerge entire districts if discharged untreated. An effluent treatment plant (ETP) designed specifically for rice mill wastewater emerges as the linchpin that protects water bodies, ensures regulatory compliance, and unlocks surprising financial gains for mill owners.
How Paddy Processing Generates Wastewater
The pady processing begins with the soaking process for 4–6 hours. Starch gelatinizes, but soluble carbohydrates, proteins, and vitamins leach out, creating a yellowish stew with Biochemical Oxygen Demand (BOD) spiking to 2,000–5,000 mg/L—ten times higher than typical domestic sewage. A modest 50-tonne-per-day paddy parboiling unit thus discharges 60,000–1,00,000 litres of this high-strength effluent daily.
Subsequent polishing scrubs bran layers using water jets and friction, releasing fine rice flour, bran oils, and silica dust. Wastewater from polishers carries suspended solids up to 1,500 mg/L and oils that form stubborn emulsions. Husk separation adds fibrous debris; a single tonne of paddy yields 200–220 kg of husk, much of which washes into drains during cleaning.
Floor washings, boiler blowdowns, and equipment rinses compound the load, pushing Chemical Oxygen Demand (COD) to 4,000–10,000 mg/L. In peak season, a cluster of 20 mills in a single block can collectively release 10–15 lakh litres of untreated effluent into nearby canals, turning them into lifeless, frothy channels within weeks.
How ETPs Prevent Pollution and Ensure Compliance
The Central Pollution Control Board (CPCB) mandates that rice mill effluent meet strict discharge standards: BOD ≤30 mg/L, COD ≤250 mg/L, and pH 6.5–8.5 for surface discharge; even tighter limits apply for land irrigation. Non-compliance invites fines starting at ₹10,000 per day, escalating to closure notices under the Water (Prevention and Control of Pollution) Act, 1974.
An ETP for paddy processing intercepts contaminants at source. Screening removes husk fragments, while oil skimmers extract floating bran lipids, preventing downstream clogging. Equalization tanks (typically 8–12 hours hydraulic retention) dampen diurnal fluctuations—parboiling peaks in mornings, polishing in afternoons—ensuring steady treatment.
Anaerobic digesters follow, where methane-forming bacteria break down 60–70% of organic matter, producing 0.35–0.45 m³ biogas per kg COD removed. This biogas fuels the parboiling boiler, displacing 25–30 litres of diesel per hour in a 50 TPD mill.
Aerobic polishing reduces residual BOD to CPCB limits, while final clarifiers yield clear effluent. Regular monitoring—online pH meters, flow totalizers, and composite samplers—generates data for statutory Form-1 submissions. Mills installing ETPs routinely pass NGT inspections, avoiding legal tangles that can stall operations for months.
Key Stages of Effluent Treatment in Rice Mills
Stage 1: Preliminary Treatment
Bar screens (6–10 mm spacing) trap paddy husk and plastic debris. A detritor removes grit that would otherwise abrade pumps. In a 100 KLD plant, this step recovers 50–80 kg of dry solids daily, saleable as cattle feed supplement at ₹3–5 per kg.
Stage 2: Primary Treatment
Oil and grease traps—tilted plate or dissolved air flotation (DAF) units—skim 100–200 litres of rice bran oil monthly from a medium mill. Coagulants (alum 50–100 mg/L + polyelectrolyte 1–2 mg/L) precipitate suspended starch, dropping TSS by 70%. Primary sludge, at 3–5% solids, is dewatered via centrifuge to 20–25% cake, reducing haulage from 10 tonnes to 2 tonnes per month.
Stage 3: Tertiary Polishing
Dual media filters (sand + anthracite) followed by activated carbon towers adsorb residual color and pesticides from parboiling soak water. UV disinfection eliminates coliforms without chlorine residuals that harm soil microbes during irrigation reuse.
Stage 4: Sludge Management
Filter press yields cake at 25–30% solids; 1 tonne paddy generates roughly 8–10 kg dry sludge. Anaerobic digestion cuts volume by 50%, and the digestate tests as Class B biosolid—safe for application at 2 tonnes per acre on non-food crops.
Benefits of Installing an ETP for Paddy Processors
Environmental stewardship translates into tangible mill-level gains. A 50 TPD unit treating 80 KLD saves 2.5–3 crore litres of freshwater annually by recycling 60–70% effluent for boiler feed and floor washing. Biogas generation offsets 1.5–2 lakh rupees in monthly fuel bills. Recovered bran oil (₹70–80 per litre) adds ₹1–1.5 lakh revenue. Sludge sales and avoided fines further improve the balance sheet.
Operational reliability improves: treated water prevents corrosion in boilers (silica <20 mg/L post-ETP versus 200 mg/L raw), extending equipment life by 5–7 years. Workers avoid exposure to foul drains, reducing absenteeism. Mills could report 8–10% higher paddy throughput after eliminating effluent-related shutdowns.
Cost-Saving and Water Recovery Potential with Modern ETPs
Capital expenditure for a 100 KLD ETP plant hovers at ₹45–65 lakh, including civil works. Payback arrives within 30–36 months via:
- Fuel displacement: 25,000–30,000 litres diesel/year @ ₹90/litre = ₹22.5–27 lakh saving.
- Water recycling: 2 crore litres/year @ ₹40 per 1,000 litres municipal tariff = ₹8 lakh.
- By-product revenue: 1,200 litres bran oil/year @ ₹75/litre = ₹90,000.
- Sludge & solids: 80 tonnes/year @ ₹4,000/tonne = ₹3.2 lakh.
- Avoided penalties: ₹3.6 lakh/year (assuming 1 violation averted).
Total annual benefit: ₹37–43 lakh against ₹12–15 lakh OPEX (power 0.8–1 kWh/m³, chemicals ₹3–4/m³, manpower ₹2 lakh). Net ROI exceeds 60% from year four.
Water recovery hits 70–80% in zero-liquid-discharge configurations using reverse osmosis on tertiary effluent. A 200 TPD mill reclaims 2.5 lakh litres daily—enough to parboil an additional 15 tonnes paddy without tapping groundwater. During summer shortages, such mills operate uninterrupted while competitors idle.
Modular stainless-steel construction allows phased expansion: start with 50 KLD, add parallel 25 KLD skids as production grows ETP in 21 days.
FAQs
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1. How much wastewater does a typical 50 TPD rice mill generate daily?
Parboiling contributes 1,000–1,500 litres per tonne, polishing 200–400 litres, and cleaning 100–200 litres. Total: 65,000–1,00,000 litres (65–100 KLD) depending on process intensity.
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2. Will installing an ETP increase my electricity bill significantly?
Modern AABR-based ETPs consume 0.8–1.2 kWh per 1,000 litres treated. For 80 KLD, monthly power cost is ₹18,000–22,000 at ₹7/unit—offset entirely by biogas savings of ₹20,000–25,000.
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3. Can treated effluent be used directly for irrigation?
Yes, after tertiary treatment, it meets CPCB standards for crop irrigation (BOD <30 mg/L, no pathogens). Many mills supply nearby sugarcane fields, saving ₹5–7 lakh annually on borewell pumping.
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4. What is the lifespan of SKF Vulcan ETP components?
Stainless-steel tanks and piping exceed 20 years. AABR media lasts 10–12 years. Blowers and pumps require overhaul every 5–7 years under recommended maintenance.
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5. Do I need separate permission to sell recovered bran oil?
Bran oil recovered is food-grade if the mill follows FSSAI hygiene norms during skimming. Register with local FSSAI and obtain a no-objection certificate; revenue is tax-exempt under agricultural by-products.
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