CPCB Source Document
Environment (Protection) Rules 1986 — Schedule VI (General Effluent Standards); CPCB Industry Categorisation List (2016, as amended); CPCB Guidelines for Cotton Textile Industry
Authority: CPCB under Environment (Protection) Act 1986 · Applicable to cotton textile mills with wet processing operations
View effluent standards on cpcb.nic.in ↗CPCB website links may change — search "cotton textile effluent standards" on cpcb.nic.in if the link is broken.
Cotton Textile Industry Structure and Wastewater Sources
India is one of the world's largest cotton textile producers, with a production chain spanning cotton cultivation (Gujarat, Maharashtra, Telangana, Punjab), spinning (Tamil Nadu, Maharashtra, Andhra Pradesh), weaving (Gujarat, Maharashtra, Rajasthan), and wet processing including dyeing and printing (Tiruppur, Surat, Ahmedabad, Jaipur, Panipat). The environmental impact of the industry is concentrated in the wet processing segment, which consumes large volumes of water and discharges complex wastewater.
The cotton textile production chain generates distinctly different wastewater streams at each stage:
- Spinning mills: Primarily domestic wastewater (canteen, toilet), humidification water with cotton fibre fines, and lubricating oil from spinning frames. Wastewater is relatively low-strength and low-volume.
- Weaving mills with sizing: The sizing process applies starch or PVA to warp yarns. Loom shed water (from weaving itself) is minimal, but beam washing and loom cleaning generates water contaminated with sizing agents.
- Desizing: The largest BOD contributor — removal of sizing agents from grey fabric before dyeing generates high-BOD wastewater (starch desizing: BOD 5,000–15,000 mg/L; enzymatic desizing: BOD 2,000–8,000 mg/L).
- Scouring: Removal of natural waxes, oils, and non-cellulosic impurities from cotton using hot alkaline solution (NaOH). Generates high-pH, high-BOD wastewater with significant COD from natural fibre compounds.
- Bleaching: Hydrogen peroxide or sodium hypochlorite bleaching to whiten fabric. Generates wastewater with oxidising agents, surfactants, and optical brightening agent (OBA) residues.
- Dyeing: The most complex wastewater stream. Cotton is dyed primarily with reactive dyes, vat dyes, or direct dyes using salt (NaCl or Na₂SO₄) to fix the dye. Generates high-TDS, coloured wastewater with residual dye, salt, and dye auxiliaries.
CPCB Regulatory Framework and Industry Classification
Under the CPCB industry categorisation system, cotton textile operations are classified across multiple categories based on the specific processes undertaken. The categorisation is critical because it determines whether an ETP is mandatory, what level of consent is required, and whether OCEMS is needed.
- Red Category: Cotton textile units with dyeing. Red Category classification requires mandatory ETP, CTE/CTO from SPCB, and OCEMS for larger units. Major dyeing centres in India (Tiruppur, Jetpur, Erode) are areas with very high concentration of Red Category textile dyeing units, and have seen significant regulatory action by state PCBs due to water body pollution from dyeing effluent.
- Orange Category: Cotton textile wet processing units without dyeing (scouring, bleaching, mercerising only). Require ETP and consent but not OCEMS.
- Green Category: Weaving mills with chemical sizing agents (no dyeing or bleaching). Require compliance with general discharge standards but may be handled under simplified consent procedures in many states.
- White Category: Pure spinning mills (yarn only, no wet processing). Typically exempt from industrial ETP requirements but must manage domestic effluent and cotton fibre waste within applicable regulations.
Cotton Textile Effluent Discharge Limits
Cotton textile wet processing units must comply with the CPCB general effluent discharge standards under Schedule VI of the Environment Protection Rules 1986:
| Parameter | Inland Surface Water | Public Sewer |
|---|---|---|
| pH | 6.5–8.5 | 5.5–9.0 |
| BOD (5-day, 20°C) | ≤ 30 mg/L | ≤ 350 mg/L |
| COD | ≤ 250 mg/L | — |
| TSS | ≤ 100 mg/L | ≤ 600 mg/L |
| Oil and grease | ≤ 10 mg/L | ≤ 20 mg/L |
| Temperature | ≤ 40°C | ≤ 45°C |
| TDS | ≤ 2,100 mg/L | — |
| Sulphates (as SO₄²⁻) | ≤ 1,000 mg/L | — |
| Chlorides (as Cl⁻) | ≤ 1,000 mg/L | — |
| Ammoniacal nitrogen | ≤ 50 mg/L | ≤ 50 mg/L |
| Colour (ADMI units) | No specific general limit | Site-specific consent conditions |
The TDS limit of 2,100 mg/L is a particular challenge for cotton dyeing units using high-salt reactive dye processes. Achieving this limit from dye bath effluent with TDS of 30,000–100,000 mg/L drives ZLD (Zero Liquid Discharge) requirements in water-stressed regions like Gujarat and Rajasthan, where state PCBs have mandated ZLD for textile dyeing clusters.
Sizing Agent Removal — The Dominant BOD Source
Desizing is the removal of sizing agents (starches, PVA, acrylic polymers, CMC) from grey fabric before dyeing. It is typically the highest BOD contributor in the cotton textile wet process chain:
- Enzymatic desizing (starch): Alpha-amylase enzymes are used to hydrolyse starch sizing into soluble sugars. Enzyme desizing liquor BOD: 2,000–8,000 mg/L. Readily biodegradable — activated sludge and MBBR systems achieve excellent starch removal (>95% BOD reduction). The main design consideration is ensuring adequate equalization tank capacity for batch desizing operations.
- Oxidative desizing (starch): Hydrogen peroxide or persulphate used to oxidise starch. Lower BOD than enzymatic desizing but generates oxidised starch fragments with different biodegradability characteristics.
- PVA desizing: Polyvinyl alcohol sizing used in polyester-cotton blended fabrics is poorly biodegradable and does not respond well to conventional biological treatment. PVA removal requires coagulation (with aluminium or iron salts) or the Mather & Platt process (ultrafiltration for PVA recovery and reuse). PVA-containing wastewater streams should be segregated and treated separately from starch desizing streams.
Scouring, Bleaching, and Mercerising Wastewater
Scouring (removal of natural waxes, pectins, protein and ash from cotton) and bleaching generate the second-largest BOD load in cotton textile processing:
- Scouring effluent: Hot alkaline scouring with sodium hydroxide and surfactants at 90–100°C removes cotton waxes (fatty acids, alcohols), pectins, and protein residues. Scouring effluent is characterised by high BOD (2,000–5,000 mg/L from wax and surfactant oxidation), high pH (11–13), and high temperature (80–100°C at source). Cooling and pH neutralisation are required before biological treatment. The natural waxes and surfactants in scouring effluent are biodegradable and do not require chemical pre-treatment beyond pH correction.
- Hydrogen peroxide bleaching: Relatively clean compared to chlorine bleaching (now largely phased out in India). H₂O₂ bleaching generates low BOD effluent but contains residual OBAs (optical brightening agents) and chelating agents (EDTA, DTPA) that are poorly biodegradable and contribute to COD.
- Mercerising effluent: Cotton mercerisation (treatment with concentrated NaOH at room temperature to improve lustre and dye uptake) generates effluent with very high alkalinity (pH 12–14) from sodium hydroxide recovery washings. This requires careful pH neutralisation and often alkali recovery (multiple effect evaporation of dilute NaOH for concentration and reuse) for economic reasons.
Dyeing Wastewater from Cotton Textiles
Cotton dyeing wastewater is the most challenging component of cotton textile ETP treatment for two reasons: colour (even trace concentrations of reactive dye are visually detectable in receiving water bodies) and TDS (salt concentrations from dyeing make discharge to freshwater bodies environmentally harmful).
- Reactive dye wastewater: Reactive dyes react covalently with cotton cellulose but have only 60–80% fixation efficiency — the remaining 20–40% of dye goes down the drain as hydrolysed reactive dye that cannot fix onto fabric. Hydrolysed reactive dye is not removed by conventional biological treatment. Removal requires coagulation-flocculation (for partial colour removal), ozonation, or advanced oxidation for complete decolorisation.
- Vat dye wastewater: Vat dyes (indigo for denim, anthraquinone vats for bright colours) use sodium hydrosulphite as a reducing agent. The reducing conditions generate sulphide in the dye bath effluent, contributing to odour and chemical oxygen demand. Oxidation of sulphide must be managed before biological treatment.
- TDS from salt: Reactive dyeing requires 40–80 g/L sodium chloride or sodium sulphate in the dye bath for dye exhaustion. Combined dye bath discharge + rinse water from a cotton dyeing unit generates effluent TDS of 5,000–20,000 mg/L — far exceeding the CPCB TDS limit of 2,100 mg/L. This drives ZLD requirements through evaporation (MEE, MVR, spray dryers) to concentrate and crystallise the salt for recovery or disposal.
ETP Design for Cotton Textile Mills
A typical cotton textile wet processing ETP treatment train (for dyeing units):
- Screen and collection sump: Bar screen to remove lint and fibre; equalization tank sized for 8–12 hours HRT to buffer batch dyeing operations.
- pH correction: Neutralisation tank with sulphuric acid dosing for alkaline streams; pH control to 6.5–8.5 before biological treatment.
- Coagulation-flocculation + clarification: For colour removal and COD reduction. Alum or ferric chloride coagulation, followed by tube settler or clarifier, can achieve 50–70% colour removal and 30–40% COD reduction.
- Biological treatment (ASP or MBBR): Secondary biological treatment for BOD and remaining COD removal. Achieves BOD < 30 mg/L for compliant discharge if colour-removed coagulated effluent is the biological feed.
- Tertiary treatment for ZLD: For mills under ZLD mandate — multi-effect evaporator (MEE) or mechanical vapour recompression (MVR) evaporator concentrates the biological treatment effluent brine; crystalliser or spray dryer produces solid salt for disposal or reuse.
Need Help with Cotton Textile ETP Design?
Spans Envirotech designs ETPs for cotton textile mills — from desizing BOD removal through colour treatment and ZLD systems for dyeing units requiring SPCB consent compliance.
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Frequently Asked Questions
Do pure spinning and weaving mills (no dyeing) need an ETP under CPCB rules?
Pure spinning mills (cotton yarn production without dyeing or wet processing) generate primarily domestic-type wastewater (toilet and canteen water) and minimal process water. Under CPCB industry categorisation, pure spinning mills are typically classified as White or Green Category industries and may not require a dedicated industrial ETP. However, spinning mills with humidification systems generate wastewater containing cotton fibre fines and oil from spinning lubricants that requires primary treatment before discharge to sewer. Any spinning mill that uses chemical sizing agents (starch, PVA) or carries out beam dyeing, yarn dyeing, or any wet processing is classified as Orange or Red Category and requires ETP consent from the relevant SPCB.
What are the CPCB effluent discharge limits for cotton textile wet processing?
Cotton textile wet processing units (which include scouring, bleaching, mercerising, and dyeing) are subject to the general CPCB effluent discharge standards under Schedule VI of the Environment Protection Rules 1986: pH 6.5–8.5; BOD (5-day, 20°C) ≤ 30 mg/L for inland surface water discharge; COD ≤ 250 mg/L; TSS ≤ 100 mg/L; oil and grease ≤ 10 mg/L; colour — no specific numeric limit in Schedule VI but colour is regulated as a nuisance parameter and SPCBs increasingly set colour limits of 100–200 ADMI units in consent conditions.
What is sizing agent (starch) removal and why is it needed in cotton textile ETPs?
Sizing agents are applied to warp yarns before weaving to improve their strength and reduce breakage on the loom. In cotton textiles, starch is the most common sizing agent. The desizing process (removal of sizing agents before dyeing) generates the single largest BOD load in the cotton textile wet process — starch desizing liquor has BOD of 5,000–15,000 mg/L due to the dissolved starch and oxidised starch degradation products. Biological treatment (activated sludge, MBBR) effectively degrades starch (it is readily biodegradable), but the high organic load requires adequate equalization tank volume and correct aeration capacity sizing. PVA (polyvinyl alcohol) sizing agents used in synthetic blended fabrics are less biodegradable and may require coagulation-flocculation pretreatment.
Are cotton textile mills classified as Red Category by CPCB?
Cotton textile mills are classified based on their specific processes under the CPCB industry categorisation system. Pure spinning mills: White/Green Category (no ETP typically required). Weaving mills with sizing (chemical sizing agents): Green/Orange Category. Cotton wet processing units (scouring, bleaching, mercerising): Orange Category. Cotton textile units with dyeing (including yarn dyeing, fabric dyeing): Red Category. The Red Category classification applies to dyeing operations regardless of whether dyeing is a minor or major part of the production process. Red Category mills require mandatory ETP with CPCB/SPCB consent, and larger mills may require OCEMS.
How does cotton textile ETP differ from synthetic or dyeing textile ETP?
Cotton textile wet processing ETPs differ from synthetic fibre or dyeing ETPs in the following key ways: (1) Colour load: Cotton fabric dyeing (particularly reactive dye processes) generates lower colour loads than synthetic fibre dyeing (reactive and disperse dyes for polyester). (2) Biodegradability: Cotton textile wastewater (starch, natural fibre residues) is more readily biodegradable than synthetic fibre wastewater containing residual disperse dyes, surfactants, and fixing agents. (3) Auxiliaries: Cotton wet processing uses fewer persistent organic auxiliaries than synthetic finishing, which uses silicone softeners, fluorocarbon repellents, and resin finishes that are poorly biodegradable. (4) TDS: Cotton dyeing with reactive and vat dyes uses high salt concentrations for dyeing exhaustion — TDS in cotton dye house effluent can reach 10,000–50,000 mg/L, which is a significant challenge for discharge and drives ZLD requirements in water-stressed regions.
This article summarises CPCB effluent standards for cotton textile industry for informational purposes. Always verify current standards with your State Pollution Control Board.