ZLD (ZLD)

Zero Liquid Discharge (ZLD) is an effluent treatment configuration that eliminates every drop of liquid waste leaving the plant — process water is treated, the salts are concentrated and crystallised, the recovered water is reused upstream, and only dry solid waste leaves the boundary. ZLD is mandated by CPCB and several SPCBs for highly polluting industries (distilleries, tanneries, pulp & paper, textile dyeing, pharmaceutical bulk drugs) and is increasingly imposed on recycling plants with high-TDS bleed streams — battery recycling, e-waste hydrometallurgy, and tyre pyrolysis wash water.

A standard ZLD train runs in five stages. Pre-treatment: equalisation, pH correction, removal of oils & grease, suspended solids (DAF/clarifier) and heavy metals (sulphide precipitation, hydroxide precipitation). Biological polishing: aerobic ASP/MBBR removes biodegradable organics so they don't foul downstream membranes. Membrane concentration: ultra-filtration to remove residual colloids, followed by reverse osmosis (RO) at 10-25 bar — RO permeate (60-75% of feed) is the recovered water; RO reject (25-40%) carries the concentrated salts. High-pressure RO or HERO (High Efficiency RO) can squeeze the reject to under 15% of feed. Thermal evaporation: the RO reject (TDS now 35,000-80,000 mg/L) goes to a multi-effect evaporator (MEE) or mechanical vapour recompression (MVR) evaporator that boils off water — recovered as condensate, returned to the upstream water tank. Crystallisation: the evaporator concentrate (TDS 200,000-300,000 mg/L) goes to an agitated crystalliser; salt is centrifuged out as a wet cake at 5-15% moisture, dried, and dispatched to landfill or chemical recovery.

The economics are unforgiving. Capex for 100 m³/day ZLD runs Rs 4-9 crore. Opex is dominated by thermal energy for evaporation — MEE consumes 250-350 kWh thermal per m³ of feed, MVR runs at 18-30 kWh electrical per m³ but needs steam economiser support at startup. Total opex of Rs 250-500 per m³ for high-TDS streams (above 25,000 mg/L raw TDS) is typical; for low-TDS streams (under 5,000 mg/L) ZLD looks more like RO + small evaporator at Rs 80-150 per m³.

The trade-off vs conventional ETP is binary. Conventional ETP discharges treated water at Rs 8-25 per m³ opex but releases TDS into the environment. ZLD eliminates discharge but raises opex 10-30x. In states where receiving water bodies are already saline (Gujarat, Tamil Nadu coast) the regulatory push to ZLD is strongest. The salt produced is typically mixed sodium sulphate-chloride — saleable to commodity chemical buyers at Rs 1-3 per kg only if cleanly separated, otherwise landfill at Rs 8-15 per kg disposal cost.