MBBR for Tannery Wastewater Treatment

Tannery wastewater is among the most chemically complex industrial effluent streams in Indian manufacturing. The leather tanning process applies a sequence of chemicals — sodium sulphide for dehairing, lime for alkaline swelling, chromium sulphate for tanning, synthetic tannins, dyes, and fatliquoring agents for finishing — each of which contributes distinct pollutant categories to the wastewater. The combination of trivalent chromium (the defining hazardous pollutant of tannery operations), sulphides (toxic to biological treatment organisms), ultra-high TDS from process salts, and variable pH from alkaline liming and acidic neutralisation stages creates a treatment challenge that requires systematic stream segregation and targeted pre-treatment before any biological stage can function.

India's leather industry — the world's second-largest — is concentrated in Kanpur (Uttar Pradesh), Kolkata (West Bengal), Chennai and Ambur-Vellore (Tamil Nadu), Jalandhar (Punjab), and Ranipet (Tamil Nadu). These clusters are served by a combination of individual tannery ETPs and Common Effluent Treatment Plants (CETPs) that serve multiple tanneries within the industrial estate. CPCB has identified tannery clusters as priority pollution control areas — chrome contamination of groundwater near Kanpur's tanneries and the Vellore CETP compliance challenges have been subjects of NGT orders and high-level CPCB intervention. MBBR technology is the current preferred biological treatment platform for tannery ETPs, replacing the conventional activated sludge systems that historically struggled with high-TDS and chrome inhibition.

Chrome management is the foundational design requirement for tannery ETP. Chrome tanning uses basic chromium sulphate [Cr(SO₄)₃·Cr(OH)₃] at 50–80 kg Cr₂O₃ per tonne of hide. Only 60–70% of the chrome applied actually penetrates and binds to the hide; the remainder stays in the spent tanning float at Cr concentration of 3,000–8,000 mg/L. Segregating this spent chrome float (a distinct stream from the rest of the tannery wastewater) is the first treatment design decision. Chrome recovery from the segregated float through NaOH precipitation, acidification, and reuse in tanning reduces total chrome input by 70–80%, significantly reducing the chrome load on the ETP and the volume of hazardous chrome sludge requiring TSDF disposal. For tanneries committed to chrome recovery, the economic benefit (recovered chrome sulphate worth ₹30–60/litre of recovered chrome solution) provides payback within 6–12 months.

Sulphide pre-treatment is required before biological treatment can function in tannery ETPs. Sodium sulphide used in dehairing (10–15 g/L Na₂S in the liming float) creates sulphide concentrations of 500–2,000 mg/L in the liming bath. At pH below 9, dissolved sulphide exists partly as H₂S gas — extremely toxic and a safety hazard for workers. MBBR biological organisms are inhibited by sulphide above 30–50 mg/L; suspended growth ASP is inhibited above 10–20 mg/L. Catalytic air oxidation — injecting compressed air into the segregated liming float with iron salt catalyst at pH 8.5 — oxidises sulphide to thiosulphate and sulphate in 1–2 hours, reducing sulphide from 500–2,000 mg/L to <10 mg/L. After this pre-treatment, the liming float can be combined with the rest of the tannery effluent for equalisation and MBBR treatment.

The MBBR stage for tannery wastewater is designed at extended HRT (18–30 hours) and higher fill ratio (50–60%) compared to food industry MBBR, because: (1) the organic COD is substantially recalcitrant (tannins, synthetic resins, dye molecules) with BOD:COD ratio of 0.2–0.4; (2) the high TDS (8,000–12,000 mg/L after segregation and pre-treatment) reduces biofilm activity compared to lower-TDS streams; (3) trace chrome and sulphide, even after pre-treatment, maintain some residual inhibitory effect on MBBR biofilm that reduces kinetic rates. Two-stage MBBR (first stage for high-load BOD removal, second stage for polishing) with an intermediate settling step between stages is the preferred configuration for achieving CPCB COD <250 mg/L compliance from inlet COD of 3,000–6,000 mg/L after pre-treatment.

Spans Envirotech has designed and commissioned tannery ETPs and CETP systems for leather industry clusters, including the reference installation at the CETP Tannery Complex in Jalandhar. Our tannery ETP designs integrate chrome recovery (reducing hazardous waste generation and recovering economic value), sulphide air oxidation, high-TDS tolerant MBBR design, and sludge management for both chrome sludge (hazardous, TSDF route) and biological sludge (non-hazardous, composting route). We work with individual tanneries and CETP operators across Kanpur, Jalandhar, Vellore, and Kolkata leather clusters.