CPCB Effluent Discharge Standards for Chlor-Alkali Industry — Explained

Chlor-alkali plants — producing chlorine, caustic soda (NaOH), and hydrogen by electrolysis of brine — are among the most tightly regulated industrial water polluters in India. The combination of mercury-bearing waste streams from legacy cell technology, high free chlorine in cooling water blowdown, and elevated TDS and chloride loads places these plants firmly in CPCB's Red Category (Pollution Index ≥60).

This article explains the discharge limits under the Environment (Protection) Rules 1986, distinguishes mercury cell from membrane cell obligations, and summarises CPCB's 2001 direction mandating conversion away from mercury cell technology.

About This CPCB Standard

The effluent standards for the chlor-alkali industry are notified under the Environment (Protection) Rules 1986 (Schedule VI, as amended). CPCB supplements these with sector-specific directions, most notably the 2001 direction on mercury cell phase-out issued under Section 5 of the Environment Protection Act 1986.

State Pollution Control Boards (SPCBs) incorporate these limits into each plant's Consent to Operate (CTO). Where an SPCB sets tighter limits than the national standard, the stricter limit prevails. Plants in ecologically sensitive zones or near waterbodies with existing mercury contamination may face additional restrictions.

Chlor-Alkali Process — Three Cell Technologies and Their Wastewater

The chlor-alkali process electrolyses a saturated sodium chloride (brine) solution to yield three co-products: chlorine gas (Cl₂) at the anode, hydrogen gas (H₂) at the cathode, and sodium hydroxide (caustic soda, NaOH) as the primary product. The cell technology used determines which waste streams are generated and how difficult they are to treat.

Mercury cell technology uses liquid mercury flowing as a cathode across the cell floor. Sodium discharged at the cathode dissolves into the mercury to form sodium amalgam, which is then reacted with water in a separate decomposer to yield caustic soda and hydrogen. This process generates multiple mercury-bearing waste streams:

• Cell room floor washings — periodic flushing of the cell room floor where metallic mercury may have accumulated from spills or leaks.
• Hydrogen gas washing water — water used to scrub hydrogen gas leaving the decomposer, which carries trace mercury vapour.
• Brine purification sludge — the sludge generated during brine softening and filtration, which concentrates any mercury present in the raw salt feed.
• End-of-life mercury inventory — when mercury cell plants are decommissioned, the residual mercury and mercury-contaminated equipment and structural materials require specialised hazardous waste management.

Diaphragm cell technology separates the anode and cathode compartments using a porous asbestos diaphragm. This avoids mercury entirely but introduces an asbestos hazard — both for workers and for the asbestos-containing waste generated during diaphragm replacement. Diaphragm cell technology is now largely obsolete in India due to the asbestos restriction and relatively high energy consumption.

Membrane cell technology uses a perfluorinated ion exchange membrane (such as Nafion) to separate the anode and cathode compartments. Sodium ions pass through the membrane under the electric field while chloride ions and water are excluded, producing high-purity caustic soda on the cathode side. Membrane cell technology is the cleanest of the three — no mercury, no asbestos, significantly lower energy consumption (approximately 2,500–2,700 kWh/tonne NaOH versus 3,200–3,500 kWh/tonne for mercury cells), and simpler effluent treatment requirements. It is now the standard technology for new chlor-alkali installations globally and in India.

Chlor-Alkali Effluent Discharge Limits at a Glance

The following table summarises CPCB's effluent discharge standards for chlor-alkali plants. The numerical limits are identical for mercury cell and membrane cell plants, but the practical treatment challenge differs significantly — achieving ≤0.01 mg/L mercury is straightforward for a membrane cell plant that generates no mercury waste, but requires advanced polishing treatment for a mercury cell plant.

The TDS limit of 2,100 mg/L and chloride limit of 1,000 mg/L reflect the inherently high-salinity nature of brine-based processes. Achieving these limits requires careful management of brine spillages, reject streams, and cooling water blowdown — which contributes the largest volume of effluent at most chlor-alkali plants.

Mercury Cell Plants — The Strictest Limits

While the numerical discharge limit for mercury is the same for mercury cell and membrane cell plants (≤0.01 mg/L), achieving this limit is a fundamentally different engineering challenge for a mercury cell operation. The process generates mercury at multiple points in the plant, and conventional effluent treatment — coagulation, sedimentation, filtration — is not sufficient to reduce mercury to sub-0.01 mg/L concentrations reliably.

Mercury cell plants meeting this limit typically require one or both of the following advanced treatment steps:

• Activated carbon adsorption — granular activated carbon (GAC) columns through which mercury-bearing effluent is passed. Mercury adsorbs onto the carbon surface and is removed from the liquid phase. Spent carbon is classified as hazardous waste and must be disposed of at a TSDF. Carbon breakthrough monitoring is mandatory to ensure consistent performance.
• Ion exchange polishing — chelating ion exchange resins with high selectivity for mercury ions can reduce concentrations to well below 0.01 mg/L. Ion exchange is often used as a final polishing step after primary mercury removal by precipitation or GAC.

Mercury cell plants must also manage the mercury mass balance across the entire plant boundary — tracking mercury inputs (in raw salt), mercury in products (caustic soda), mercury losses to atmosphere (stack emissions from chlorine absorption units), mercury in solid wastes, and mercury in effluent. CPCB and SPCBs require annual mercury mass balance submissions from operating mercury cell plants.

The pH limit of 6.0–8.5 is particularly important for mercury chemistry: at alkaline pH (above 8.5), mercury can precipitate as mercury hydroxide, which may appear to reduce dissolved mercury in grab samples but creates a colloidal or sludge-phase mercury problem. Effluent pH must be accurately controlled within the permitted range before final discharge.

Membrane Cell Plants — Cleaner Technology, Fewer Constraints

Membrane cell plants face the same numerical discharge limits as mercury cell plants, but with a fundamentally simpler compliance pathway. Without mercury in the process, the mercury discharge limit becomes a trace contaminant check on incoming raw materials (principally salt) rather than a treatment engineering challenge.

The dominant effluent concerns for a modern membrane cell chlor-alkali plant are:

• Cooling tower blowdown — the largest volume stream, carrying elevated TDS, chloride, and free chlorine. Blowdown management (concentration ratio control, dechlorination) is the primary effluent treatment challenge.
• Brine saturation and purification reject — the filtration and softening steps in brine preparation generate reject streams with high TDS and sulphate, which must be managed within the TDS and sulphate limits.
• Caustic spillages and floor washings — strongly alkaline (pH can exceed 14) and must be neutralised before inclusion in the effluent stream. Inadequate neutralisation is a common cause of pH limit exceedances at membrane cell plants.
• Chlorine gas absorption blowdown — the tail gas absorption system (sodium hydroxide scrubbing of residual chlorine) generates sodium hypochlorite solution, which is a co-product for sale or degrades to chloride and oxygen. Overflow or spillage can contribute free chlorine to the effluent stream.

Membrane cell plants with well-designed brine circuits and cooling tower management systems can achieve compliance with all discharge limits through conventional treatment — pH adjustment, settling, and dechlorination — without requiring advanced treatment units beyond what is standard for a high-volume industrial effluent plant.

Free Chlorine in Cooling Water Blowdown

Free chlorine in discharge is a distinctly chlor-alkali problem. Cooling towers at chlor-alkali plants are routinely dosed with chlorine (as hypochlorite) to control biological fouling and Legionella. The blowdown from these towers — the concentrated water fraction bled off to control TDS build-up — carries residual free chlorine that must be reduced to ≤1.0 mg/L before discharge.

Standard dechlorination methods used at chlor-alkali plants:

• Sodium sulphite (Na₂SO₃) dosing — reacts stoichiometrically with free chlorine to form sodium chloride and sodium sulphate. Fast reaction, reliable, but adds sulphate load to the effluent — relevant to the ≤1,000 mg/L sulphate limit. Overdosing must be avoided to prevent dissolved oxygen depletion in the receiving waterbody.
• Sodium thiosulphate (Na₂S₂O₃) dosing — an alternative dechlorination agent that reacts with free chlorine without adding sulphate. Often preferred where sulphate in the final effluent is already close to the limit.
• Activated carbon contactors — catalytic dechlorination on granular activated carbon. Capital-intensive but chemical-free operation and no addition of dissolved solids. Used where chemical dosing is operationally problematic.

Free chlorine monitoring at the discharge point is mandatory under OCEMS requirements. Cooling tower biocide dosing schedules should be integrated with OCEMS data — periods of high chlorine dosing to the cooling tower require corresponding increases in dechlorination chemical dosing before discharge.

CPCB's Mercury Phase-Out Directive

In 2001, CPCB issued a direction under Section 5 of the Environment Protection Act 1986 requiring all chlor-alkali plants operating mercury cell technology to phase out mercury cells and convert to membrane cell technology. This direction was driven by:

• Documented mercury contamination of soils, groundwater, and river sediments in the vicinity of long-operating mercury cell plants.
• Evidence of chronic occupational mercury exposure among workers at mercury cell facilities.
• The commercial availability and superior economics of membrane cell technology as a replacement.
• India's obligations under international conventions on mercury pollution reduction.

The majority of Indian chlor-alkali plants that were operating mercury cell technology at the time of the direction have since converted to membrane cell technology. A small number of plants sought and received extensions from SPCBs, but mercury cell operations are now effectively at end-of-life in the Indian industry.

The legacy of mercury cell operation creates an ongoing environmental liability. Mercury contamination of plant soils and groundwater at former mercury cell sites is a site-remediation issue that persists long after conversion. Mercury-contaminated sludge and soils from such sites are classified as Schedule I hazardous waste under the Hazardous and Other Wastes (Management and Transboundary Movement) Rules 2016 and must be consigned to an authorised TSDF. Casual disposal — burial on-site or off-specification landfilling — constitutes an offence under both the Environment Protection Act and the HWM Rules.

Plants that converted from mercury cells to membrane cells are required to submit a post-conversion mercury decommissioning report to the SPCB, confirming:

• Total mercury recovered and its disposal route (typically sold to mercury recyclers).
• Volume and characterisation of mercury-contaminated soil and sludge generated.
• TSDF manifests for all mercury-bearing hazardous waste.
• Post-conversion soil and groundwater mercury monitoring results from the plant boundary.

Monitoring Requirements and Enforcement

Chlor-alkali plants are subject to mandatory Online Continuous Effluent Monitoring Systems (OCEMS) under CPCB's directions on real-time monitoring of Red Category industries. The minimum OCEMS configuration at the final effluent discharge point must include:

• Mercury analyser — continuous or periodic (e.g., 15-minute interval) measurement of total mercury in the discharge stream. Cold vapour atomic absorption (CVAAS) or atomic fluorescence (CVAFS) instruments are typically used. The detection limit must be well below 0.01 mg/L.
• Flow meter — for load calculation (mass per unit time) and for calculating the daily mercury mass discharge — a metric that CPCB uses alongside concentration for compliance assessment.
• pH sensor — continuous in-situ pH monitoring with data logging.
• Conductivity meter — as a proxy for TDS and to detect brine carry-over events in the discharge stream.
• Free chlorine sensor — amperometric or colorimetric online sensor for continuous free chlorine monitoring in the discharge.

All OCEMS data must be transmitted in real time to the CPCB centralised server and the relevant SPCB server. Data gaps exceeding prescribed thresholds are treated as compliance failures. Plants must maintain OCEMS calibration records, with mercury analyser calibration performed against certified reference standards at intervals specified by the SPCB (typically monthly or quarterly, depending on the instrument type).

In addition to OCEMS, chlor-alkali plants are required to submit quarterly self-monitoring data from NABL-accredited laboratory testing of the final discharge. The SPCB conducts independent surprise inspections and sample collection — typically one to four times per year for Red Category plants. Consent conditions specify the parameters to be monitored, the frequency, and the accreditation requirements for the testing laboratory.

For plants still managing legacy mercury contamination — even after cell conversion — the SPCB may require additional groundwater monitoring wells at the plant boundary, with quarterly mercury analysis submissions. Groundwater mercury exceedances trigger remediation directions under the Environment Protection Act.

For the general discharge standards that apply across all industries in India, see our CPCB General Effluent Discharge Standards guide. For context on how chlor-alkali fits within the broader category of heavily regulated industries, see our article on CPCB's 17 Grossly Polluting Industries.