ZLD for Electroplating and Metal Finishing

Electroplating and metal finishing effluent is low in volume relative to most industrial sectors but among the most hazardous in composition, which is why CPCB classifies the sector Red category — its most stringently regulated tier. Plating bath drag-out and rinse water carry hexavalent and trivalent chromium, nickel, copper, zinc, and cadmium, and units running cyanide-based processes (gold, silver, and copper cyanide plating) additionally carry free and complex cyanide in their rinse streams. None of these contaminants are amenable to conventional biological treatment, and several — hexavalent chromium and cyanide specifically — present acute toxicity and reaction hazards that require dedicated chemical treatment steps performed in a specific, carefully sequenced order before the streams can even be combined.

The foundation of a safe and effective electroplating ZLD system is segregated collection at source — chrome-bearing rinse streams, cyanide-bearing rinse streams, and general (non-toxic metal) rinse streams must run in entirely separate piping from the plating line to their respective treatment stages. This is not a matter of treatment efficiency alone but of acute safety: chromium reduction treatment deliberately acidifies its stream to pH 2.5–3, and if a cyanide-bearing stream were combined with an acidic stream — by accident or by poor segregation design — the reaction releases hydrogen cyanide (HCN) gas, a fast-acting lethal toxin, in an uncontrolled location. Every electroplating ZLD design begins with mapping which plating lines generate which contaminant streams and ensuring those streams never have a physical opportunity to mix before their respective treatment is complete.

Cyanide-bearing streams are treated by alkaline chlorination: sodium hypochlorite dosed while pH is held above 10 with caustic soda, which oxidises cyanide first to cyanate and, with continued dosing, onward to carbon dioxide and nitrogen gas. The alkaline condition is essential — chlorinating cyanide at neutral or acidic pH risks generating cyanogen chloride, itself a toxic gas, rather than completing the oxidation safely. Chrome-bearing streams follow a different path: acidification to pH 2.5–3, dosing with sulphur dioxide or sodium metabisulphite to reduce Cr6+ to the far less toxic and less soluble Cr3+, then pH elevation to 9–10 with lime or caustic soda to precipitate Cr3+ as chromium hydroxide. General rinse streams carrying nickel, copper, zinc, and the now-reduced chromium are similarly precipitated as metal hydroxides at pH 9–10 and removed by clarification.

Because every plating bath is followed by several rinse stages to remove drag-out before parts move to the next process step, electroplating is a water-intensive operation, and drag-out itself steadily concentrates metal salts in that rinse water. Recovering this rinse water — via ion exchange for smaller, higher-value recovery applications or reverse osmosis for larger flows — reduces freshwater intake and, just as importantly, shrinks the volume that must ultimately pass through the evaporation stage of the ZLD train, since MEE/MVR evaporation cost scales directly with throughput volume. Rinse recovery design is therefore not an optional add-on but a primary lever for controlling the capital and operating cost of the entire ZLD system.

The final stage of the ZLD train processes the RO concentrate or residual high-TDS reject through Multiple Effect Evaporator (MEE) or Mechanical Vapour Recompression (MVR) evaporation, recovering clean condensate for reuse and concentrating the remaining stream further, before an Agitated Thin Film Dryer (ATFD) drives off the last of the water to produce a dry mixed salt cake. This sequence is necessary because most electroplating units operating within industrial estates are not permitted to discharge any liquid effluent at all under their SPCB consent conditions — true Zero Liquid Discharge means no liquid stream crosses the plant boundary, only recovered process water and a solid residue. Metal hydroxide sludge generated earlier in the train is classified hazardous waste under Schedule I of the Hazardous Waste Rules and must be dewatered and sent to an authorised TSDF with proper manifesting, a compliance obligation separate from but as important as the liquid effluent consent itself.

India's electroplating capacity is concentrated in dense small-unit clusters — Faridabad and the wider NCR, the Vasai-Bhiwandi belt, Coimbatore, and Jamnagar's brass parts sector — where many individual job-work plating units are too small to economically justify a full standalone ZLD train given the largely fixed capital cost of evaporation equipment. Spans Envirotech designs both standalone per-unit ZLD systems and CETP-linked ZLD configurations, where individual units handle source segregation and primary chemical treatment on their own premises and a shared facility performs combined rinse recovery and evaporation-crystallisation, spreading the high fixed cost of the evaporation stage across the cluster rather than requiring every small platter to absorb it alone.