CPCB Effluent Standards for Zinc Smelters — Explained

Zinc Smelting Processes and Wastewater Sources

India is the fourth-largest zinc producer globally, with major smelting operations at Chanderiya (Rajasthan) and Dariba. Zinc smelting follows two main routes, each generating distinct wastewater profiles:

• Hydrometallurgical process (Roast-Leach-Electrowin): Zinc concentrate is roasted to form zinc oxide calcine, leached in sulphuric acid, purified to remove cadmium and other impurities, and then electrowon. This is the dominant route globally (about 80% of zinc production). Wastewater arises from the acid plant, leach solution purification, cell room rinsing, and zinc dust purification steps.
• Pyrometallurgical process (Imperial Smelting Furnace): Zinc and lead are co-smelted at high temperature, with zinc vapour condensed in a lead splash condenser. Wastewater arises from off-gas scrubbing, slag granulation, and condenser cooling. This process generates less process water but more complex mixed zinc–lead effluent.
• Zinc die casting and galvanising operations: Secondary zinc operations (galvanising, die casting) generate alkaline rinse waters, fluxing bath effluents, and pickling acids — distinct from primary smelter streams but regulated under the same Schedule VI limits.

The roast-leach process also involves iron removal stages (jarosite process or goethite process) that generate large volumes of hazardous residue, which — while not technically a wastewater stream — creates significant leachate management obligations covered later in this article.

CPCB Classification: 17 Grossly Polluting Industries

Zinc smelting is classified as a Highly Polluting Industry under CPCB's 17 Grossly Polluting Industry (GPI) list, alongside copper smelting, thermal power plants, distilleries, and pulp and paper mills. The GPI designation carries enhanced regulatory obligations:

• Mandatory installation of OCEMS (Online Continuous Effluent Monitoring System) at all ETP discharge points.
• Real-time data transmission to CPCB and the relevant State Pollution Control Board.
• Priority inspection scheduling — GPI industries are inspected more frequently than other Red category industries.
• Any effluent parameter exceedance triggers mandatory reporting and may lead to show-cause notices or closure directions under Section 5 of the Environment (Protection) Act 1986.
• Annual environmental audit by an accredited environmental auditor is standard practice for GPI facilities.

Key Pollutants in Zinc Smelter Effluent

The principal pollutants in zinc smelter wastewater arise from the heavy metal content of zinc ore concentrates (sphalerite — ZnS — commonly contains cadmium, lead, iron, arsenic, and trace mercury as co-present minerals):

• Zinc (Zn²⁺): The primary metal — present at high concentrations (50–500 mg/L) in raw process water from the leach circuit; toxic to aquatic organisms at concentrations above 0.1 mg/L; CPCB inland discharge limit 5 mg/L.
• Cadmium (Cd²⁺): Cadmium is a co-metal in sphalerite (typically 0.1–0.3% Cd in ore); it concentrates in the purification circuit and is highly toxic — the WHO drinking water limit is 0.003 mg/L; CPCB limit 2 mg/L (still requiring significant treatment).
• Lead (Pb²⁺): Present in galena (PbS) impurities in zinc concentrate — particularly significant in Imperial Smelting Furnace plants; CPCB limit 0.1 mg/L.
• Mercury (Hg): Even trace quantities of mercury in ore are problematic — mercury volatilises during roasting and can appear in scrubber water. CPCB limit is extremely stringent at 0.01 mg/L.
• Sulphate (SO₄²⁻): Generated in large quantities by sulphuric acid leaching and the acid plant — raw leach solution may have 50,000–150,000 mg/L sulphate; final effluent must be below 1,000 mg/L.
• Arsenic (As): Present as impurity in zinc concentrates — particularly those from volcanogenic massive sulphide (VMS) deposits; CPCB limit 0.2 mg/L.

CPCB Discharge Standards for Zinc Smelters

ETP Design for Zinc Smelter Wastewater

ETP design for zinc smelter wastewater uses a multi-stage physico-chemical treatment train. Biological treatment alone is not effective because metals are toxic to microorganisms at the concentrations present in raw smelter effluent.

• Stream segregation: Acidic high-metal streams (leach circuit bleed, acid plant condensate) are collected separately from lower-contamination streams (cell room cooling water, floor washings). Segregation allows targeted treatment at lower reagent cost.
• Primary pH adjustment (Stage 1, pH 3.5–4.5): Lime slurry addition to the high-metal stream begins iron precipitation — at pH 3.5–4.5, ferric iron precipitates as iron hydroxide and co-precipitates arsenic. This step is critical before cadmium removal, as high iron interferes with cadmium sulphide precipitation.
• Clarification — Stage 1: Lamella clarifier or thickener to separate iron/arsenic sludge. Clarified overflow proceeds to cadmium removal.
• Cadmium removal (sulphide precipitation, pH 9–10): Sodium sulphide is dosed to the clarified stream — CdS precipitates at pH 9–10. Zinc also precipitates as Zn(OH)₂ and partially as ZnS at this pH. This stage brings cadmium to below 0.05 mg/L, well within the 2 mg/L CPCB limit.
• Clarification — Stage 2: Second clarifier or lamella to collect cadmium sulphide and zinc hydroxide sludge. Overflow is pH-adjusted and polished.
• Final pH adjustment: pH is corrected to 6.5–8.5 for discharge compliance. Excess lime or acid is dosed as required.
• Sand filtration: Removal of residual suspended solids to below 100 mg/L.
• Sludge dewatering: Filter press or belt press dewaters the combined metal sludge to 30–40% solids for hazardous waste disposal.

Cadmium and Mercury Removal: Special Considerations

Cadmium and mercury present the most challenging removal requirements in zinc smelter ETPs due to their extreme toxicity and stringent CPCB limits:

• Cadmium — why hydroxide precipitation is insufficient: Cadmium hydroxide [Cd(OH)₂] has a solubility product of ~2×10⁻¹⁴ — at pH 9, theoretical residual Cd²⁺ is ~0.5 mg/L, which may not reliably achieve the 2 mg/L limit with treatment margins. Sulphide precipitation [CdS, Ksp ~7×10⁻²⁸] achieves residual Cd below 0.01 mg/L, providing a 200× safety factor.
• Cadmium — sulphide precipitation best practices: Sodium sulphide dosing must be carefully controlled — excess sulphide is itself a pollutant. Automated pH and ORP (oxidation-reduction potential) control ensures the correct sulphide dose. The cadmium sulphide sludge is a recoverable cadmium source and is sent to authorised cadmium recovery facilities.
• Mercury removal: Mercury (limit 0.01 mg/L) is removed by sulphide precipitation (HgS, extremely low Ksp ~4×10⁻⁵³) or by activated carbon adsorption for polishing. Where mercury is present in scrubber water, a dedicated mercury scrubber and demister upstream reduces mercury load before ETP treatment.
• Mercury sludge: HgS-containing sludge is classified as a hazardous waste with the highest handling requirements — authorised mercury processors must be engaged for disposal or recovery.

Jarosite and Goethite Residue as Hazardous Waste

The most significant solid waste challenge for zinc hydrometallurgical plants is not ETP sludge but the iron residue from the leach purification circuit:

• Jarosite [KFe₃(SO₄)₂(OH)₆]: Formed when the iron-rich zinc sulphate leach solution is treated with potassium or ammonium sulphate at elevated temperature (90–95°C) — iron precipitates as jarosite. Jarosite contains zinc (3–5%), lead (0.5–2%), cadmium (traces), and sulphate — its TCLP leachate exceeds hazardous waste thresholds for multiple metals. Indian zinc smelters have generated millions of tonnes of jarosite stored in evaporation ponds.
• Goethite [α-FeOOH]: An alternative iron removal process — goethite residue is denser and less soluble than jarosite, containing lower zinc but still classified as hazardous waste (Schedule II, HWM Rules 2016).
• Leachate from jarosite ponds: Rain percolating through jarosite ponds generates acid leachate containing zinc, cadmium, lead, and sulphate — this leachate must be collected and treated in the ETP. Pond liner integrity and leachate collection systems are essential components of the facility's environmental management system.
• Regulatory position: The Supreme Court of India and NGT have issued orders on jarosite management. CPCB expects zinc smelters to pursue jarosite utilisation options (e.g., cement industry co-processing, road base stabilisation) rather than indefinite storage.
• ETP sludge from zinc smelters: Metal hydroxide/sulphide sludge from the ETP contains zinc, cadmium, and lead — classified as hazardous waste under HWM Rules 2016 (Schedule II, Category 35.3). Annual hazardous waste return filing with the SPCB is mandatory.

OCEMS Requirements and Compliance Obligations

As GPI industries, zinc smelters face some of the most rigorous environmental compliance requirements in Indian industry:

• OCEMS installation: pH, flow, temperature, TSS, and select metal sensors at ETP outlet with GPRS/GSM real-time transmission to CPCB and SPCB data servers. Third-party OCEMS audits are required annually.
• NABL-accredited laboratory testing: Monthly (or more frequent) third-party analysis of all scheduled parameters including all heavy metals, sulphate, and TDS. Lab reports must be submitted to SPCB in prescribed formats.
• Effluent measurement: Calibrated flow meters at all discharge points — daily flow records maintained and submitted quarterly.
• Groundwater monitoring: Quarterly sampling of upgradient and downgradient monitoring wells for zinc, cadmium, lead, sulphate, and pH — to detect any groundwater contamination from jarosite ponds or ETP storage.
• Hazardous waste authorisation: SPCB authorisation for storage, transport, and disposal of jarosite, ETP sludge, and cadmium/mercury bearing residues. Annual hazardous waste return (Form 4) filing mandatory.
• Environment clearance conditions: Large zinc smelters typically have MoEFCC environment clearance with additional monitoring conditions — six-monthly compliance reports to regional MoEFCC office.

Frequently Asked Questions