How to Reduce TDS in Industrial Wastewater

TDS exceedance in final effluent is one of the most frustrating compliance problems an ETP manager can face — because adding more biological treatment capacity, increasing aeration, or optimising chemical dosing has absolutely no effect on it. Dissolved salts pass through biological treatment unchanged. Understanding which part of the system is generating the TDS, and which treatment approach can remove it, is the first step to solving it.

Why TDS Is Different from BOD and COD

BOD and COD represent organic pollution that biological organisms can consume and convert to CO₂, water, and new biomass. TDS represents dissolved inorganic salts — sodium chloride, sodium sulphate, calcium bicarbonate, potassium ions, and others — that are thermodynamically stable and cannot be degraded biologically.

The only proven technologies for reducing TDS are:

• Reverse Osmosis (RO) — physically separates water from dissolved salts
• Electrodialysis — uses electrical potential to migrate ions through selective membranes (rarely used for industrial effluent)
• Thermal evaporation (MEE/MVR) — distillation produces water of near-zero TDS
• Ion exchange — exchanges specific ions (mainly used for softening, not bulk TDS reduction)

All other ETP unit operations — DAF, biological treatment, secondary clarification, sand filtration — do not reduce TDS. They may slightly affect TDS by removing colloidal organic matter that contributes to the analytical measurement, but the effect is marginal.

Major Sources of TDS in Industrial ETPs

Before investing in TDS removal technology, identify where the TDS is coming from. The most common sources in Indian industrial ETPs:

pH adjustment chemicals: Caustic soda (NaOH) for pH raising adds sodium ions — the most common source of increasing TDS in food industry ETPs. Each 10 mg/L of NaOH added contributes approximately 6 mg/L of Na⁺ to the effluent. For a plant dosing 50 mg/L NaOH across 100 KLD of effluent, this is 30 kg of sodium per day flowing to the ETP final effluent.

Coagulant dosing: Ferric chloride (FeCl₃) releases chloride ions; alum (Al₂(SO₄)₃) releases sulphate. Iron and aluminium are partly removed as precipitates, but counter-ions (Cl⁻, SO₄²⁻) remain in solution.

Process wastewater chemistry: Salt used in food processing (pickling, curing, flavour enhancement), sodium in dairy CIP (caustic soda used heavily in dairy), and dyes and auxiliary chemicals in textile effluent all contribute to high TDS.

Utilities blowdown: Cooling tower blowdown at 3–5 cycles of concentration has TDS 3–5× the make-up water TDS. Boiler blowdown is even higher. These should be tracked and quantified separately.

Source Control: The Cheapest TDS Reduction

The most cost-effective TDS reduction strategy is reducing TDS addition at the source, before it enters the ETP:

Switch from caustic soda to lime for pH adjustment: Ca(OH)₂ (lime) raises pH equally to NaOH but adds calcium ions rather than sodium. Calcium is partly precipitated as CaCO₃ and CaHPO₄ in the biological reactor and removed in the clarifier — so net TDS addition from lime is significantly lower than from caustic soda. Lime also costs 50–70% less than caustic soda per unit of alkalinity. A simple jar test protocol can confirm the lime dose for your inlet pH range.

Optimise coagulant dosing: Jar tests quarterly to confirm minimum effective coagulant dose — excess coagulant adds unnecessary ions. Consider switching from ferric chloride to polyaluminium chloride (PAC) which has lower ionic strength at equivalent coagulation performance.

Segregate high-TDS streams: Boiler blowdown and RO reject from water treatment plants often have TDS of 3,000–10,000 mg/L. Routing these directly to ZLD rather than mixing with the general ETP influent reduces the volume of high-TDS stream requiring expensive treatment.

Reverse Osmosis for TDS Reduction

Where source control is insufficient to meet TDS discharge limits, Reverse Osmosis is the most practical treatment technology. RO operating at 75–80% recovery produces:

• Permeate (75–80% of feed volume): TDS 50–200 mg/L
• Reject (20–25% of feed volume): TDS 5–8× feed TDS

The permeate can be discharged (well below TDS limits) or reused in-plant. The reject must be managed separately — either disposed of to an authorised tanker service, blended with other low-TDS streams, or processed through MEE/MVR evaporation as part of a ZLD system.

For plants where discharge compliance requires TDS below 2,100 mg/L and current effluent TDS is 2,500–4,000 mg/L, a partial RO approach is often the most economical route: treat a fraction of the ETP effluent through RO and blend the permeate with untreated effluent to achieve the target final TDS — avoiding the cost of treating 100% of the flow while still achieving compliance.

ZLD: When You Need Very Low TDS

If TDS limits in your CTO are below 1,000 mg/L, or if discharge is prohibited entirely, a full Zero Liquid Discharge system incorporating RO + MEE/MVR + ATFD is required. ZLD produces near-zero TDS in the recovered water (200–500 mg/L TDS as distillate) and captures all dissolved solids as a dry salt cake. CAPEX for ZLD is 5–10× higher than conventional ETP, and OPEX is 5–10× higher — but for plants where discharge is legally prohibited, it is the only compliant path.

Economics of TDS Reduction

The cost-effectiveness of TDS reduction options:

• Lime for caustic substitution: ₹2,000–5,000/month reduction in chemical cost + potential 100–400 mg/L TDS reduction. Payback: immediate.
• Coagulant dose optimisation: ₹1,000–3,000/month chemical saving. No capital cost.
• Partial RO (30% of flow): CAPEX ₹15–30 lakh. OPEX ₹15–25/m³ treated. TDS reduction potential: 30–50% of final effluent TDS.
• Full RO (100% of flow): CAPEX ₹40–80 lakh per 100 KLD. TDS in permeate: <200 mg/L. Reject management adds cost.
• ZLD: CAPEX ₹3–5 crore per 100 KLD. OPEX ₹200–400/m³. Zero TDS in recovered water.