CPCB Effluent Standards for Glass and Ceramics Industry — Explained

Glass and Ceramics Manufacturing: Processes and Wastewater Sources

The glass and ceramics sector in India spans a wide range of products and processes — from large float glass plants supplying the construction industry to small tile and tableware manufacturers. The effluent profile differs significantly by product type:

• Container glass (bottles, jars): Large-scale melting furnaces using soda-lime glass composition. Wastewater arises from cullet washing, batch plant dust suppression, furnace cooling water, and tin oxide coating operations (for container glass). Generally lower toxicity but high TSS from cullet fines.
• Float glass (flat glass for windows and mirrors): Uses the Pilkington float process where molten glass floats on a bath of molten tin. Wastewater from tin bath nitrogen purge condensate and tin oxide coating sprays — tin (as Sn²⁺) is a pollutant of concern. Mirror manufacturing adds silver nitrate and formaldehyde to the effluent profile.
• Lead crystal glass: High-value decorative glass containing 24–36% lead oxide (PbO). Grinding, polishing, and washing of lead crystal generates lead-bearing wastewater — most stringently regulated.
• Optical and specialty glass: Lenses, prisms, and specialty optical components may use arsenic, antimony, bismuth, lanthanum, niobium, and other rare earth elements as refining agents or dopants — each requiring individual effluent characterisation.
• Ceramic tiles and sanitaryware: Body preparation (clay mixing), glaze application, kiln firing, and surface grinding are the main processes. Glaze wastewater is the primary concern — glazes contain lead, cadmium, barium, zirconium, and chromium compounds as colorants and opacifiers.
• Technical ceramics (electrical insulators, advanced ceramics): May use alumina, zirconia, silicon carbide, and various rare earth dopants — wastewater from machining and polishing contains fine ceramic particles that are difficult to settle.

Key Pollutants in Glass Industry Effluent

Glass manufacturing wastewater pollutants depend on the glass composition and process steps:

• Lead (Pb²⁺): From lead crystal grinding and polishing slurry — lead concentration in polishing wastewater can reach 20–50 mg/L. The CPCB limit is 0.1 mg/L, requiring hydroxide precipitation at pH 9–11. Lead sludge is classified as hazardous waste.
• Tin (Sn²⁺/Sn⁴⁺): From float glass tin bath nitrogen vent condensate and atmospheric oxide overspray. Tin precipitates as Sn(OH)₂ above pH 6 — generally achievable with simple lime treatment.
• Antimony (Sb): Used as a refining agent (fining agent) in optical and specialty glass — antimony trioxide (Sb₂O₃) is added to remove bubbles. Antimony CPCB limit 0.1 mg/L; treatment by co-precipitation with iron hydroxide at pH 5–7.
• Arsenic (As): Arsenic trioxide (As₂O₃) was historically used as a glass refiner in optical glass — largely phased out but still present in some specialty operations. Strict regulatory attention at 0.2 mg/L CPCB limit.
• Fluoride: From fluorite-based opalescent glass and from glass batch fluoride compounds used for certain optical properties. Two-stage treatment (lime + alum) required to achieve 2 mg/L limit.
• High TSS from glass cullet: Glass cullet (recycled broken glass) washing generates high-TSS wastewater — sharp glass particles are not amenable to biological treatment and require physical separation (vibrating screens, settling) before ETP.

Key Pollutants in Ceramics and Tile Industry Effluent

The ceramics sector generates a more complex wastewater profile than glass due to the diversity of glaze chemistries used across product categories:

• Suspended colloidal silica: Arising from clay body preparation, grinding, and polishing of ceramic ware. Colloidal silica (particle size 1–100 nm) does not settle by gravity — requires coagulation-flocculation for removal. TSS in raw ceramics wastewater can be 500–5,000 mg/L.
• Lead and cadmium from ceramic glazes: Traditional ceramic glazes used lead silicate frits (PbO·SiO₂) as fluxes and cadmium sulphide/cadmium selenide as yellow/red colorants. Although lead and cadmium glazes are regulated and largely replaced in tableware (due to food safety concerns), they remain in use in some industrial ceramics, tiles, and decorative applications. Glaze preparation and application washing waters contain dissolved lead and cadmium.
• Barium from glaze flux: Barium carbonate (BaCO₃) is used as a flux and to improve whiteness in some tile bodies. Barium is toxic — CPCB limit 1 mg/L; treatment by sulphate addition to precipitate barium sulphate (BaSO₄, very low solubility).
• Zirconium from opacifiers: Zircon (ZrSiO₄) and zirconium oxide (ZrO₂) are the dominant white opacifiers in modern tile glazes. Zirconium itself is of low toxicity, but zircon contains trace fluoride (0.1–0.2%) and trace uranium/thorium (naturally occurring radioactive material — NORM) — requiring periodic radioactivity monitoring at large zircon-using facilities.
• Chromium from ceramic colorants: Chrome-tin pink, chrome-alumina pink, and chrome-green pigments are used in tile decoration. If hexavalent chromium Cr(VI) is present, it requires reduction with sodium bisulphite at pH 2–3 before precipitation — Cr(VI) limit is 0.1 mg/L, much stricter than total chromium (2 mg/L).
• Fluoride from kiln off-gas: Fluorine-containing compounds in ceramic bodies (from feldspars, frits, and fluorite flux) volatilise during kiln firing. Where wet scrubbers are used on kiln exhaust, fluoride-bearing scrubber water must be treated before discharge.

CPCB Discharge Standards for Glass and Ceramics

ETP Design: Coagulation for Silica and Heavy Metal Precipitation

Glass and ceramics ETPs typically combine physico-chemical treatment steps addressing both the high colloidal silica/TSS load and the diverse heavy metal contamination:

• Screening and pre-treatment: Coarse glass particles (from cullet washing) and ceramic body fragments are removed by vibrating screens or a coarse filter before the ETP. Removing these particles protects pumps and reduces TSS load entering the main treatment train.
• pH correction: Ceramic effluent is often alkaline from raw material dissolution. pH correction to 6–7 before coagulation is necessary for optimal coagulant performance.
• Coagulation for colloidal silica: Alum [Al₂(SO₄)₃·18H₂O] at a dose of 100–500 mg/L, or ferric chloride (FeCl₃) at 50–200 mg/L, is dosed and flash-mixed at pH 6.5–7.5. Both coagulants form positively charged hydroxide species that adsorb onto and destabilise the negatively charged silica colloids.
• Flocculation: After coagulation, a polyanionic or cationic polyelectrolyte (1–5 mg/L) is added in a slow-mix flocculation basin to aggregate the destabilised silica into settleable floc. Flocculation contact time of 20–30 minutes is typical.
• Heavy metal precipitation: For lead-bearing effluent (lead crystal, lead glaze), a lime dosing stage at pH 9–11 precipitates Pb(OH)₂. Cadmium co-precipitates as Cd(OH)₂; for very low cadmium targets, sulphide polishing is added. Chromium (if Cr⁶⁺ is present) must be reduced to Cr³⁺ before pH adjustment — add a sodium bisulphite reduction stage at pH 2–3 before lime addition.
• Clarification: Lamella clarifier or inclined plate settler achieves rapid settling of the combined coagulated silica and metal hydroxide sludge. Clarifier surface overflow rate is critical — colloidal silica floc is fragile and requires gentle hydraulics.
• Sand/multimedia filtration: Polishing step to remove residual TSS to below 100 mg/L. Some plants add a pressure sand filter after the clarifier.
• Sludge dewatering: Ceramic ETP sludge (colloidal silica + metal hydroxides) can be difficult to dewater due to the fine silica particles — belt press or filter press at 6–10 bar pressure is typical, achieving 25–35% solids cake.

Glaze Wastewater Management: A Special Challenge

Glaze preparation and application generates the most concentrated and hazardous wastewater in the ceramics sector. Special attention is warranted:

• Glaze preparation area: Ball mills grinding glaze frits generate glaze slip containing high concentrations of lead, cadmium, barium, zirconium, and colorant metal oxides in fine suspension. Ball mill washings are the most concentrated heavy metal stream in a ceramics plant — should be collected separately for targeted treatment.
• Glaze application (spray glazing): Spray booths generate overspray mist — wet collectors or water curtain spray booths capture the overspray. The booth wash-water is a high-solids, heavy-metal-bearing stream requiring segregated collection.
• Glaze line floor washings: Daily or shift-basis floor washing of glaze application areas — lower concentration than spray booth water but higher volume.
• Best practice — segregation and concentration: CPCB's guidance encourages segregation of concentrated glaze wastewater from dilute rinse water. Concentrated glaze streams should be filtered or centrifuged to recover valuable pigment for reuse — reducing both raw material cost and heavy metal load to ETP.
• In-glaze kiln decoration: On-glaze decals and in-glaze printing in tableware decoration use ink systems that may contain lead, cadmium, or bismuth compounds as low-melt flux media. Decal printing rinse waters from the decoration area require the same heavy metal treatment as glaze line washings.

Kiln Scrubber Water and Fluoride Treatment

Kiln-related effluent is relevant for ceramics facilities using wet scrubbers on their kiln off-gas treatment systems — dry electrostatic precipitators or bag filters are more common for modern kilns, but older kilns may use wet scrubbers:

• Kiln scrubber water fluoride: At firing temperatures (1100–1300°C for porcelain tiles), fluorine-bearing compounds in the ceramic body and glaze volatilise as HF and SiF₄. Wet scrubber water from such kilns may contain 20–100 mg/L fluoride — requiring two-stage treatment (lime precipitation + alum coagulation) to meet the 2 mg/L CPCB limit.
• Heavy metals in kiln scrubber condensate: Volatile metal compounds (cadmium oxide, lead oxide) can vaporise at kiln temperatures and condense in the scrubber — kiln scrubber water from kilns firing cadmium or lead-glazed ware can have significant metal concentrations.
• Sulphur dioxide from sulphur-bearing clays: Some raw clay materials contain pyrite (FeS₂), which oxidises during kiln firing to release SO₂. Wet scrubbers remove SO₂ but generate sulphate-bearing acidic scrubber water — requiring pH adjustment before ETP or discharge.
• Temperature of kiln cooling water: Kiln shell cooling water (used to cool the kiln casing and conveyor systems) is generally clean but may be above the discharge temperature limit — CPCB specifies that cooling water discharge should not raise the temperature of the receiving water body by more than 5°C above ambient. Cooling towers may be needed.

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