TDS (TDS)

Total Dissolved Solids (TDS) is the concentration of all dissolved inorganic and organic substances in water — primarily mineral salts (chlorides, sulphates, carbonates, bicarbonates of sodium, calcium, magnesium and potassium) plus dissolved metals and small organic molecules — expressed in mg/L. It quantifies salinity and is the parameter that decides whether an effluent can be discharged to inland water, land or sewer, or whether it must be put through Zero Liquid Discharge (ZLD).

Measurement follows IS 3025 Part 16: a filtered effluent sample (passing 1.5 µm glass-fibre filter to exclude suspended solids) is evaporated to dryness at 180°C in a tared dish; residual mass divided by sample volume gives TDS. A field surrogate, electrical conductivity (EC), is widely used — TDS (mg/L) ≈ EC (µS/cm) × 0.55-0.85, with the multiplier dependent on dominant ion composition. Hand-held EC meters give an instant indication of TDS in the field.

India does not set a single inland-surface-water TDS limit under Schedule VI; the parameter is regulated by industry category, with the binding number set in each plant's CTO. The widely-cited threshold is 2,100 mg/L — above which several states' SPCBs require ZLD, treating the discharge as too saline for inland water. The 2015 CPCB-NGT directive for textile dyeing, distillery, tannery, pulp & paper and pharma plants mandates ZLD; the threshold has progressively spread to chemical plants and battery recycling. Drinking water IS 10500:2012 sets desirable TDS at 500 mg/L and maximum permissible at 2,000 mg/L.

For recycling plants, TDS is the parameter that breaks effluent treatment economics. Battery recycling acid neutralisation wastewater carries TDS of 25,000-60,000 mg/L (mostly sodium sulphate from H2SO4 + NaOH neutralisation). E-waste hydrometallurgy bleed streams reach 80,000-150,000 mg/L TDS. Tyre pyrolysis wash water is mild at 2,000-5,000 mg/L. CBG digestate at 4,000-8,000 mg/L often falls in the must-treat zone. Conventional biological treatment removes none of the TDS — the salts pass through. Reverse osmosis (RO) at 10-25 bar concentrates TDS into a brine (5-15% of feed volume); the brine is then evaporated in mechanical vapour recompression (MVR) or multi-effect evaporators and finally crystallised. Operating cost runs Rs 250-500 per m³ feed for full ZLD on TDS above 25,000 mg/L — the recurring opex line that often makes high-TDS recycling streams uneconomic without a tax incentive.