What the Color Discharge Limit in India Actually Means
A Karur-based cotton knitting unit failed its quarterly TNPCB grab sample last year on a single parameter: 620 Pt-Co against a consent order cap of 150. The EHS team had been tracking ADMI units in their internal log, the lab had reported "true color" while the inspector had pulled an unfiltered "apparent color" sample — and the two numbers, on the same wastewater, differed by almost 4x. The unit's color discharge number is only as defensible as the unit, the filtration step, and the wavelength method behind it.
Pt-Co (platinum-cobalt) and Hazen units are the same scale, traceable to APHA Standard Methods 2120B: 1 mg/L of platinum as potassium hexachloroplatinate, with cobalt(II) chloride as a stabilizer, defines 1 Pt-Co unit. Indian labs default to IS 3025 Part 4: 2021, which mirrors APHA 2120. ADMI (American Dye Manufacturers Institute) is a tristimulus-filter method developed for dye-bearing streams where a single wavelength cannot capture the hue. The practical conversion for most reactive and azo dye effluents is roughly 500 Pt-Co ≈ 1,000 ADMI units, but the ratio drifts as the chromophore mix changes — never derive one from the other for compliance reporting.
Apparent color is measured on the unfiltered, unsettled sample — what an inspector actually sees in the stream. True color is read after centrifugation or 0.45 µm filtration, removing turbidity that biases the visual reading upward. SPCB consent orders almost always reference "true color" in Pt-Co, but the grab sample taken at the drain is apparent color. Demand both numbers, plus pH and residual chlorine, on every lab report.
The Regulatory Framework Behind Indian Color Discharge Limits
The legal anchor is the Environment Protection Rules, 1986, Schedule VI. Part A sets General Standards for Discharge of Effluents — applied to every industrial sector discharging to inland surface water — and prescribes 400 Hazen units (Pt-Co) as the floor for color. This is the number that appears on most consent orders when the SPCB does not impose a tighter, project-specific limit.
Part B of Schedule VI covers 36 specific industry sectors with parameter lists that often override or tighten the Part A general standard. Textile and dyeing falls under Sector 26, pulp and paper under Sector 28, and distilleries (fermented) under Sector 33. Where a sector entry is silent on color, the 400 Pt-Co Part A value applies by default; where it is specified, the sector value governs.
Consent-to-Operate (CTO) under Section 25/26 of the Water Act, 1974 is where SPCBs — TNPCB, MPCB, KSPCB, GPCB, and others — set project-specific conditions. A dyeing unit applying for CTO in Tirupur or Erode will typically see a tighter cap, often 100–150 Pt-Co for direct inland discharge and 200 Pt-Co for discharge into a Common Effluent Treatment Plant (CETP) inlet, because the CETP cannot accept a member unit's full upstream color load. CETPs in Pali, Ludhiana, Tirupur, and Erode publish their own inlet norms, and non-compliance at the member unit triggers show-cause under the consent order, not just the Water Act.
Zero Liquid Discharge (ZLD) sites — mandated for textile units in Tamil Nadu and Gujarat — have no "discharge" outlet for color, but SPCB consent still imposes a numeric color target on the final recovered-water or cooling-tower-makeup stream, typically ≤50 Pt-Co, to prevent evaporator fouling and to allow downstream reuse without staining.
Color Limits by Industry Sector in India (2026 Snapshot)
The table below consolidates the regulatory floor (CPCB Schedule VI), the project-specific range commonly imposed by SPCBs, and the CETP/ZLD targets that Indian EHS teams are typically measured against. Always cross-check against the unit's own CTO; these are working values, not legal advice.
| Sector | Discharge Route | Pt-Co Limit | Typical SPCB / CETP Example |
|---|---|---|---|
| Textile / Dyeing (Schedule VI Sector 26) | Inland surface water | 400 (CPCB floor); 100–150 typical CTO | TNPCB Erode / Tirupur: 150; KSPCB: 100 |
| Textile / Dyeing | CETP inlet | 200–400 | Tirupur CETP: 200; Pali CETP: 400 |
| Pulp & Paper (Sector 28) | Inland surface water | 400 (CPCB floor); 200 common CTO | Brown-stock wash plants held to 200 |
| Distillery (Sector 33) | Inland surface water / ZLD | 400 (CPCB); ≤100 on MEE/VC condensate | Maharashtra: 100 on condensate |
| Dairy / Food processing | Inland surface water | 100–200 at SPCB discretion | Gujarat dairy CTOs: 100 |
| ZLD reuse stream (any sector) | Recycled to process / cooling | ≤50 internal target | TNPCB ZLD consent: 50 |
For a textile unit on the inland-discharge path, the realistic design number is 100 Pt-Co, not 400 — because the SPCB has the authority, and routinely exercises it, to push the floor down. The 400 Pt-Co number on Schedule VI is the ceiling above which no SPCB can write a consent; the number a unit is held to in practice is whatever the consent order says.
How Color Is Measured: APHA 2120, IS 3025, and the Three-Wavelength Method
Two reports can show different color numbers on the same sample because the lab used different methods, different filtration, and different wavelengths. Indian labs default to IS 3025 Part 4: 2021, which mirrors APHA 2120; the inspector's grab sample may have used a different procedure. Demand the method code on every report.
| Method | Instrument | Wavelengths / Standard | Typical Range | Best Use |
|---|---|---|---|---|
| APHA 2120B / IS 3025 Part 4 (visual Pt-Co) | 50 mL Nessler tube, visual comparison | Pt-Co standards 0–500 | 0–500 Pt-Co | Routine SPCB reporting, low-color effluents |
| Spectrophotometric ADMI (3-wavelength) | UV-Vis spectrophotometer | Absorbance at 436, 525, 620 nm → ADMI via tristimulus weighting | 0–2,000+ ADMI | Dye-bearing textile effluent, hue variation |
| Tristimulus filter method | Filter photometer with CIE XYZ filters | CIE XYZ weighting → ADMI directly | 0–1,500 ADMI | Field screening, multi-hue dye mixes |
| True color (filtered) vs. Apparent color (unfiltered) | Same as above, before/after 0.45 µm | Reported together | — | Dispute resolution with SPCB |
The three-wavelength ADMI method is the one Indian textile labs use when the chromophore mix is unknown or varies lot-to-lot — absorbance is read at 436 nm (yellow region), 525 nm (red), and 620 nm (blue), then combined with CIE tristimulus weighting to produce an ADMI number that is more reproducible across hues than a single-wavelength reading. For compliance against a Pt-Co consent order, the ADMI reading must be converted using the method-specific factor published in the lab's SOP — and the conversion is not 2:1 universally; it ranges from roughly 1.5:1 to 2.5:1 depending on the dye class.
Always report pH and residual chlorine alongside the color number. Chlorination bleaches color and a sample that flashes 50 Pt-Co in the lab may revert to 300 Pt-Co at the drain once residual chlorine decays. The reverse is also true: a SPCB sample preserved with acid can shift hue and read higher than the in-stream value.
Treatment Engineering: Removing Color to Meet Indian Limits
A typical Indian textile dyeing influent arrives at the ETP at 2,000–5,000 Pt-Co and 800–2,000 mg/L COD. A standalone MBBR or Zhongsheng MBR system removes 20–30% of color — biologically effective on COD and BOD, but ineffective on the azo, anthraquinone, and reactive dye chromophores that drive visible color. The numbers that follow are anchored to CPCB Schedule VI Sector 26 and to SPCB consent orders at 100–150 Pt-Co.
The defensible process train for a 100–150 Pt-Co inland-discharge consent is:
- Equalization + pH correction — neutralize to 6.5–7.5, blend batch-to-batch variation, dose via a Zhongsheng automatic chemical dosing skid.
- Coagulation-flocculation + DAF — alum or PACl (polyaluminium chloride) at 100–300 mg/L with anionic polymer; a Zhongsheng ZSQ DAF system typically delivers 60–80% color removal, taking 3,000 Pt-Co to ~600 Pt-Co and removing the suspended dye mass that fouls downstream membranes.
- Fenton oxidation — Fe²⁺ at 50–200 mg/L with H₂O₂ at 200–800 mg/L, pH 3.0–3.5, 30–60 min residence; this is the workhorse for azo and reactive dye chromophores, delivering 80–95% decolorization and breaking aromatic rings that biological treatment cannot touch.
- Biological polishing (MBBR or MBR) — bring COD and residual organics below 100 mg/L, residual color to 100–200 Pt-Co.
- Ozone-based AOP or activated carbon (PAC) — ozone at 1.5–3 g O₃ per g of residual color for the final polish to ≤100 Pt-Co; or PAC at 50–200 mg/L as a polishing step. For ZLD sites, RO concentrate is recycled upstream of the Fenton stage rather than discharged.
Capital and operating cost for a 500 m³/day dyeing ETP targeting 100 Pt-Co typically runs INR 4–7 crore capex and INR 35–55 per m³ opex, dominated by H₂O₂, FeSO₄, ozone power, and sludge disposal (Zhongsheng field data, 2026). For a sector-by-sector cost comparator and reuse-stream economics, the 2026 water reuse market outlook is a useful secondary read. For pre-treatment solids handling, the suspended-solids removal guide for industrial wastewater covers DAF and primary clarifier sizing in more detail.
Frequently Asked Questions
What is the difference between Pt-Co and ADMI color units in Indian effluent reporting?
Pt-Co (Hazen) is a visual comparison scale defined by 1 mg/L platinum as chloroplatinate, traceable to APHA 2120B and IS 3025 Part 4: 2021. ADMI is a tristimulus spectrophotometric method developed for dye-bearing streams, read at 436, 525, and 620 nm. For most Indian textile effluents, ~500 Pt-Co ≈ 1,000 ADMI, but the ratio varies with dye class. Always report which method was used (per IS 3025 Part 4: 2021).
What is the color discharge limit for textile effluent in India?
400 Pt-Co is the CPCB Schedule VI Part A floor for inland surface water discharge, but TNPCB, KSPCB, and GPCB routinely impose 100–150 Pt-Co in the Consent-to-Operate. CETP inlet norms in Tirupur and Pali typically cap member units at 200–400 Pt-Co (per CPCB Schedule VI, Sector 26, and individual SPCB CTOs).
What color limit applies to a Zero Liquid Discharge (ZLD) site?
ZLD has no discharge outlet, but SPCB consent orders impose an internal numeric color target on the final recovered water or cooling-tower makeup stream — typically ≤50 Pt-Co — to prevent evaporator fouling, staining in cooling towers, and visible color in any emergency bypass. Fenton + RO polish is the standard train (per TNPCB and GPCB ZLD consent conditions, 2025–2026).
Which lab method should an Indian SPCB-approved lab use to report effluent color?
IS 3025 Part 4: 2021, which mirrors APHA 2120. The lab should report apparent color, true color (after 0.45 µm filtration), pH, and residual chlorine on the same report. If a single Pt-Co number is quoted without the method code and the filtration step, the result is not defensible in a consent dispute (per IS 3025 Part 4: 2021).
How much does it cost to treat textile dye wastewater to 100 Pt-Co in India?
For a 500 m³/day ETP, indicative capex is INR 4–7 crore and opex is INR 35–55 per m³, dominated by Fenton reagents (H₂O₂, FeSO₄), ozone power, and sludge disposal. The biggest cost driver is whether the site targets direct discharge (100–150 Pt-Co) or ZLD reuse (≤50 Pt-Co), which adds RO and MEE stages (Zhongsheng field data, 2026). For regional supplier benchmarking, see the sewage treatment equipment suppliers in Jharkhand guide.
