Wastewater Discharge Standards for Tanneries

Leather manufacturing is one of the most pollution‑intensive industrial sectors. The tanning process generates large volumes of wastewater containing a complex mixture of pollutants: high concentrations of organic matter (measured as COD and BOD), heavy metals (especially chromium), sulphides, chlorides, salts, suspended solids, and dyes. 

In China, the discharge of such wastewater is subject to a dedicated mandatory standard – the Discharge Standard of Water Pollutants for Leather and Fur Making Industry (GB 30486‑2013), which came into effect on 1 March 2014. This standard applies to both existing facilities and new construction projects, covering environmental impact assessment, treatment facility design, and compliance monitoring.

Segregated Collection and Treatment

A key requirement of the standard is the segregation of wastewater streams according to their pollutant characteristics. Chromium‑bearing wastewater, mainly from the tanning process, must undergo separate physico‑chemical and biochemical treatment – often followed by membrane systems for reuse – before the concentrated brine can be discharged. 

Sulphide‑bearing wastewater, generated during unhairing and liming, is collected separately to remove hair and solid residues; it may be recycled within the liming process and treated only after several cycles. Only after these individual pre‑treatment steps can the various streams be combined with general production wastewater and domestic sewage for integrated treatment. This segregated approach is critical because chromium and sulphide, if mixed prematurely, can form precipitates that interfere with biological treatment.

Discharge Limits for Conventional Pollutants

GB 30486‑2013 establishes differentiated limits depending on three factors: the type of enterprise (leather tanning versus fur processing), the age of the facility (existing versus new), and the discharge route (direct versus indirect). Direct discharge – releasing treated effluent directly into natural water bodies – is subject to stricter limits. Indirect discharge – sending pre‑treated wastewater to a municipal treatment plant – allows somewhat higher concentrations.

For key conventional pollutants, the limits are as follows:

COD: 80 mg/L (existing) or 60 mg/L (new) for direct discharge; 200 mg/L for indirect discharge.

BOD₅: 20 mg/L for direct discharge; 50 mg/L for indirect discharge.

SS: 50 mg/L for direct discharge; 70 mg/L for indirect discharge.

Ammonia‑nitrogen: 12 mg/L (existing) or 10 mg/L (new) for direct discharge; 30 mg/L for indirect discharge.

Total nitrogen: 20 mg/L for direct discharge; 40 mg/L for indirect discharge.

Total phosphorus: 0.5 mg/L for direct discharge; 1.0 mg/L for indirect discharge.

pH: 6–9 for both routes.

Colour: 50‑fold for direct discharge; 80‑fold for indirect discharge.

Limits for Characteristic Pollutants

The standard pays particular attention to pollutants that are specific to leather processing:

Total chromium: 1.0 mg/L at the workshop or production facility discharge point (the strictest node control); 1.5 mg/L at the overall enterprise outlet.

Hexavalent chromium: must be kept below the detection limit, typically <0.1 mg/L.

Sulphides: 1.0 mg/L at both workshop and enterprise outlets.

Animal and vegetable oils: 5 mg/L for direct discharge; 15 mg/L for indirect discharge.

Chloride ions: 4,000 mg/L for indirect discharge – a parameter added to address the high salt content from preserved raw hides.

It is important to note that for highly toxic pollutants such as total chromium and hexavalent chromium, the same strict limits apply regardless of whether the discharge is direct or indirect.