ETP for Metal Finishing and Electroplating
Heavy metal removal, hexavalent chromium reduction, and cyanide oxidation for electroplating and metal finishing units — CPCB Red Category compliant ETP systems with stream segregation and ZLD options
Industry Overview
ETP for Metal Finishing and Electroplating
Electroplating and metal finishing operations generate some of the most hazardous industrial wastewater in India, containing high concentrations of heavy metals — chromium, nickel, copper, zinc, cadmium, and lead — along with cyanide, strong acids, and strong alkalis. CPCB classifies all electroplating units as Red category, subject to stringent consent conditions, NABL third-party monitoring, and in many urban cluster cases, mandatory connection to a Common Effluent Treatment Plant (CETP). India's electroplating and metal finishing clusters include Faridabad, Ludhiana, Chennai's Ambattur, Mumbai's Chembur, Bengaluru's Peenya, and Coimbatore — all with active SPCB enforcement. Many individual small-scale plating units operate within industrial estates where the CETP handles collective treatment, but large standalone electroplating facilities must operate their own ETP.
Electroplating wastewater treatment is fundamentally different from organic wastewater treatment: the primary concern is not BOD removal but metal removal. Heavy metals cannot be biodegraded — they must be removed by chemical precipitation as metal hydroxides (by pH raise to 9–11), chemical reduction (hexavalent chromium must be reduced to trivalent before precipitation), and cyanide oxidation (alkaline chlorination) before any precipitation step. The order of treatment steps is critical: cyanide oxidation must occur in alkaline conditions; hexavalent chromium reduction must occur in acid conditions; if these streams are mixed before their respective pre-treatment steps, toxic HCN gas can be generated.
Spans Envirotech designs ETP systems for standalone electroplating facilities, CETP operator companies, and large industrial estates with on-site plating operations. Our metal finishing ETP designs always begin with strict stream segregation — separate treatment trains for cyanide waste, hexavalent chromium waste, acid/alkali rinse water, and mixed metal rinse water — before combined clarification and final polishing. Where ZLD is required for industrial estate clients, we integrate ion exchange for metal recovery and ZLD evaporation for dissolved salts. See our chemical and pharmaceutical industries page for related sector information.
Industry Challenges
Key Environmental Challenges
Hexavalent Chromium (Cr⁶⁺) — Carcinogenic and Highly Regulated
Cr⁶⁺ from chrome plating baths is carcinogenic, highly water-soluble, and resistant to precipitation. CPCB discharge limit for Cr⁶⁺ is ≤0.1 mg/L (very stringent). Before precipitation, Cr⁶⁺ must be chemically reduced to Cr³⁺ using sodium metabisulphite (SMBS) or ferrous sulphate at pH 2–3. Only then can chromium be precipitated as Cr(OH)₃ at pH 8–9. Skipping or inadequately performing reduction results in Cr⁶⁺ breakthrough in treated effluent — a serious compliance failure with immediate SPCB action.
Cyanide — Acute Toxicity and Separate Treatment Required
Cyanide from gold plating, silver plating, and some zinc plating baths is acutely toxic. CPCB limit: total cyanide ≤0.2 mg/L. Cyanide must be oxidised to cyanate (CNO⁻) and then to CO₂ and N₂ using alkaline chlorination — chlorine (or bleach) at pH >10. Critical: never mix cyanide-containing wastewater with acid or with hexavalent chromium waste — mixing cyanide with acid generates hydrogen cyanide (HCN) gas, a lethal risk.
Multiple Metals Requiring Different Optimal Precipitation pH
Different metals precipitate as hydroxides at different optimal pH values: chromium Cr³⁺ at pH 8–9, nickel at pH 10–11, copper at pH 8–9, zinc at pH 9.5–10.5 (amphoteric — re-dissolves above pH 11), lead at pH 8–10. A single-stage precipitation at one pH cannot achieve minimum solubility for all metals simultaneously. Multi-stage precipitation or careful pH optimisation for the specific metal mix is required.
High Sludge Generation — Hazardous Waste Classification
Chemical precipitation of metals generates large volumes of metal hydroxide sludge (plating sludge). This sludge is classified as hazardous waste under Schedule 2, Category 12 (metal treatment sludge) under Hazardous Waste Rules 2016. Disposal at an authorised CHWTSDF is mandatory. High sludge generation volumes and TSDF disposal costs are significant operating cost drivers.
Acid-Alkali Rinse Water
The rinsing stages after plating, etching, and cleaning generate high volumes of dilute acid or alkali rinse water. These rinse waters are low in metal content but significant in volume, and their treatment requires neutralisation before combining with more concentrated process streams. Volume reduction through drag-out minimisation (air knives, improved rinse tank design) reduces ETP load significantly.
Our Solutions
Tailored Wastewater Treatment Solutions
Stream Segregation — Cyanide, Chromium, Acid, Rinse
Four separate collection sumps: (1) cyanide waste from plating baths; (2) hexavalent chromium waste from chrome plating rinses; (3) acid rinse water; (4) mixed metal rinse water (nickel, copper, zinc). Each stream enters its own pre-treatment step before combining in a common clarification tank.
Cyanide Oxidation (Alkaline Chlorination)
pH raised to >10.5 with NaOH, followed by sodium hypochlorite (bleach) dosing to oxidise cyanide to cyanate (first stage: pH >10, ORP >+350 mV), then further to CO₂ + N₂ (second stage: pH 7–8, ORP >+600 mV). Two-stage reactor design for complete cyanide destruction to <0.2 mg/L.
Hexavalent Chromium Reduction
Sodium metabisulphite (SMBS) or ferrous sulphate dosing at pH 2–3 reduces Cr⁶⁺ to Cr³⁺ in a dedicated reduction reactor. pH is then raised to 8–9 with lime or NaOH to precipitate Cr(OH)₃ as a floc. ORP-controlled dosing ensures complete reduction before precipitation.
Combined Clarification and Sludge Dewatering
All pre-treated streams combined in a pH-adjusted clarification tank (pH 9–10), with coagulant (alum or FeSO₄) and polymer dosing for metal hydroxide floc formation and settling. Clarified effluent polished through PSF for final TSS and metal removal. Metal hydroxide sludge dewatered in a filter press to 25–30% DS cake for TSDF disposal.
Ion Exchange for Trace Metal Polishing
Cation exchange resin columns for trace metal removal in treated effluent below precipitation detection limits (copper <0.1 mg/L, nickel <1 mg/L, chromium total <0.1 mg/L). Ion exchange also used for metal recovery from rinse water concentrates to reduce metal makeup requirement in plating baths.
Technologies
Proven Technologies for Your Industry
Benefits
Why Choose Spans for Your Industry
- Cr⁶⁺ reduction to ≤0.1 mg/L — most stringent CPCB heavy metal limit met
- Complete cyanide destruction to ≤0.2 mg/L by alkaline chlorination
- All CPCB General Standards met: pH, BOD, COD, TSS, total metals
- Hazardous metal sludge managed with full TSDF documentation
- ORP-controlled process ensures consistent treatment without operator intervention
- Ion exchange polishing enables metal recovery, reducing plating chemical costs
Success Stories
Case Studies
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