CPCB Source Document
CPCB Comprehensive Industry Document — Man-Made Fibres (Synthetic Fibres) Industry; Schedule VI, Environment (Protection) Rules 1986
Authority: CPCB under Environment (Protection) Act 1986 · Applicable to polyester, nylon, acrylic, viscose rayon, and spandex fibre manufacturing
View effluent standards on cpcb.nic.in ↗CPCB website links may change — search "synthetic fibre effluent standards" on cpcb.nic.in if the link is broken.
Synthetic Fibre Industry Sub-Sectors
India is one of the world's largest synthetic fibre producers. The industry spans several distinct manufacturing processes with very different effluent profiles:
- Polyester fibre and yarn (PET-based): Melt-spun from PET polymer — relatively low-strength aqueous effluent primarily from spin finish washing and utilities. The dominant Indian synthetic fibre by volume.
- Nylon 6 and Nylon 66: Caprolactam (Nylon 6) or adipic acid + hexamethylenediamine (Nylon 66) polymerisation — generates caprolactam-rich extraction wash water and polymerisation condensate.
- Acrylic fibre: Wet-spun from polyacrylonitrile in DMF or DMSO solvent — generates solvent-containing spin water and acrylonitrile-contaminated wash streams.
- Viscose rayon (cellulosic): Technically a regenerated cellulose fibre, not fully synthetic — uses carbon disulphide and NaOH to produce xanthate which is spun in an acid (H₂SO₄/ZnSO₄) bath. Generates the most complex and challenging effluent in the man-made fibre sector.
- Spandex (polyurethane elastane): DMF-wet spun elastic fibre — generates DMF-contaminated spin water requiring solvent recovery before ETP discharge.
CPCB Pollution Category
All synthetic fibre manufacturing plants above specified scale are Red category under CPCB due to their use of hazardous chemicals (CS₂, acrylonitrile, DMF) and high-volume complex effluent. Viscose rayon plants are specifically listed as one of the 17 Grossly Polluting Industries by CPCB. Polyester spin finishing and texturising units may be Orange category if chemical use is minimal.
Key Pollutants by Fibre Type
Each fibre type has distinct key pollutants:
- Polyester: Spin finish oils (anionic surfactants, mineral oil), ethylene glycol residues from PET production — moderate BOD, low toxicity.
- Nylon 6: Caprolactam (BOD 1,000–20,000 mg/L in wash water), oligomers, and hot water — primarily an organic load issue.
- Acrylic: Acrylonitrile (toxic, carcinogenic), DMF or DMSO solvents, and polyacrylonitrile fine fibres — requires strict monitoring for AN and solvent residuals.
- Viscose rayon: Zinc (from spin bath), sodium sulphate (high TDS), sulphuric acid, hydrogen sulphide, carbon disulphide — multi-pollutant complex requiring physicochemical pre-treatment before biological stages.
- Spandex: DMF (dimethylformamide) — a teratogenic solvent requiring recovery to <1% in the waste stream before discharge.
CPCB Discharge Standards
| Parameter | Inland Surface Water | Notes |
|---|---|---|
| pH | 6.5–8.5 | Neutralise acid/alkali before discharge |
| BOD (5-day, 20°C) | ≤ 30 mg/L | Biological treatment required |
| COD | ≤ 250 mg/L | Higher for some viscose streams |
| Total Suspended Solids | ≤ 100 mg/L | Fibre fines require screening |
| Zinc (as Zn) | ≤ 5 mg/L | Viscose rayon spin bath waste |
| Sulphide | ≤ 2 mg/L | Viscose CS₂/H₂S streams |
| Acrylonitrile | ≤ 0.5 mg/L | Acrylic fibre plants only |
| DMF (Dimethylformamide) | ≤ 1 mg/L | Acrylic/spandex plants only |
| Total Dissolved Solids | ≤ 2100 mg/L (irrigation) | Viscose sulphate streams require special treatment |
Viscose Rayon: Complex Effluent Management
Viscose rayon plants generate the most complex synthetic fibre effluent. The spin bath — an aqueous solution of H₂SO₄, ZnSO₄, and Na₂SO₄ — must be carefully managed:
- Spin bath recovery: Cascade-diluted spin bath is recovered, concentrated by evaporation, and replenished with fresh H₂SO₄ and ZnSO₄. Efficient recovery reduces the volume of high-TDS discharge by 80–90%.
- CS₂ recovery: Carbon disulphide vapour from the fibre washing process is recovered by condensation — CS₂ is flammable (flash point -30°C) and toxic; its recovery is both a safety requirement and an economic benefit.
- Zinc removal from dilute bleed: Alkaline precipitation at pH 9–10 removes zinc as Zn(OH)₂. The zinc-bearing sludge is hazardous waste requiring TSDF disposal.
- Sulphide oxidation: Aeration of CS₂/H₂S-contaminated streams oxidises sulphide to sulphate before biological treatment.
ETP Configuration by Fibre Type
ETP configuration varies significantly by fibre type:
- Polyester: Screening → oil-water separator → equalisation → aerobic biological treatment (activated sludge or MBBR) → clarifier. Relatively straightforward.
- Nylon 6: Screening → equalisation → caprolactam recovery (evaporation of high-concentration wash) → aerobic biological treatment → clarifier.
- Acrylic: DMF solvent recovery by distillation → equalisation → activated carbon adsorption (AN polishing) → aerobic biological treatment → outlet AN monitoring.
- Viscose rayon: CS₂ stripping → neutralisation → zinc precipitation → H₂S oxidation → equalisation → anaerobic (optional for BOD reduction) → aerobic biological treatment → clarifier → sulphate stream to evaporator.
Sodium Sulphate Recovery from Viscose Effluent
Sodium sulphate recovery from viscose rayon spin bath effluent is both an economic and environmental imperative:
- The spin bath bleed stream contains 50,000–150,000 mg/L Na₂SO₄ — too high for any biological or conventional ETP to handle.
- Multiple-effect evaporation (MEE) concentrates the stream and crystallises anhydrous sodium sulphate (Glauber's salt) — a commodity chemical sold to glass, detergent, and kraft pulp industries.
- Recovered Na₂SO₄ revenue partially offsets MEE capital and operating costs — making recovery economically viable for large viscose plants.
- CPCB and State PCBs generally require MEE installation for viscose rayon plants above specified capacity under ZLD-adjacent conditions due to the high TDS load.
Compliance and Consent Requirements
Compliance requirements for Red category synthetic fibre plants:
- Consent to Establish and Consent to Operate from State PCB.
- Monthly NABL-accredited effluent monitoring — including specific parameters (AN, DMF, zinc, CS₂) relevant to the fibre type.
- Hazardous waste authorisation for zinc sludge (viscose rayon) and spent activated carbon (acrylic) — TSDF disposal required.
- Air emissions permit for CS₂ and H₂S from viscose rayon plants — separate air consent condition.
- OCEMS at final discharge point if directed by State PCB.
- Annual environment statement under EP Rules 1993.
Need Help with Synthetic Fibre Plant ETP Design?
Spans Envirotech designs ETPs for viscose rayon, nylon, polyester, and acrylic fibre plants — including zinc removal, CS₂ recovery integration, and sodium sulphate MEE systems.
Contact us: bd@spans.co.in · +91-98100 00233
Frequently Asked Questions
What CPCB discharge limits apply to synthetic fibre plant effluent?
CPCB prescribes BOD ≤ 30 mg/L, COD ≤ 250 mg/L, TSS ≤ 100 mg/L, and pH 6.5–8.5 for synthetic fibre plant effluent discharged to inland surface water. Specific parameters vary by fibre type — viscose rayon plants have additional limits for zinc (≤ 5 mg/L) and sulphide (≤ 2 mg/L); acrylic fibre plants have limits for acrylonitrile; nylon plants have limits for caprolactam.
Why is viscose rayon effluent particularly difficult to treat?
Viscose rayon manufacturing uses carbon disulphide (CS₂) and sodium hydroxide to convert cellulose into xanthate and then regenerate cellulose fibres in an acid spin bath. The effluent contains zinc sulphate (from the spin bath), sodium sulphate, sulphuric acid, carbon disulphide traces, hydrogen sulphide, and zinc — creating a complex mix of inorganic salts and toxic organics. Zinc removal (as zinc hydroxide by alkaline precipitation), sulphide oxidation, and acid neutralisation are required before biological treatment.
Is synthetic fibre plant effluent high in TDS?
Yes. Viscose rayon and nylon plants generate very high TDS effluent — viscose spin bath effluent contains 50,000–150,000 mg/L dissolved sodium sulphate. This stream cannot be treated biologically and must be either evaporated to recover sodium sulphate (sold as Glauber's salt) or discharged under special SPCB conditions for inland disposal (not to freshwater bodies). The general TDS limit of ≤ 2,100 mg/L for irrigation use cannot be achieved from this stream without evaporation.
What specific pollutants are regulated for acrylic fibre plants?
Acrylic fibre manufacturing uses acrylonitrile (AN) as the primary monomer. CPCB prescribes: acrylonitrile ≤ 0.5 mg/L in the final discharge (acrylonitrile is a Group 1 carcinogen). Acrylic fibre plant ETPs must include activated carbon adsorption or biological treatment capable of removing AN below this limit. Dimethylformamide (DMF) used as a solvent in some acrylic processes is also regulated: ≤ 1 mg/L in final discharge.
What is caprolactam and why is it regulated in nylon plant effluent?
Caprolactam is the monomer used to produce Nylon 6. During polymerisation and fibre spinning, caprolactam is extracted from the fibre with hot water (extraction wash), generating effluent with caprolactam concentrations of 5,000–20,000 mg/L. Caprolactam is readily biodegradable but at high concentrations inhibits biological treatment and contributes to high BOD/COD. Caprolactam recovery by evaporation and reuse in polymerisation is the primary pollution prevention measure — typically recovering 95%+ of caprolactam from wash water.
This article summarises CPCB norms for synthetic fibre industry effluent for informational purposes. Always verify current standards with your State Pollution Control Board.