Leather industry wastewater treatment encompasses the management of highly complex wastewaters generated during the processing of animal hides. Tannery processes such as salting, liming, soaking, chrome tanning, and dyeing create high concentrations of pollutant loads.
The BOD value of leather wastewaters reaches levels of 1,500-6,000 mg/L, the COD value ranges from 3,000-12,000 mg/L, and total dissolved solids (TDS) can reach 10,000-40,000 mg/L. Chromium (Cr+3) content varies between 50-500 mg/L, and sulfur levels range from 200-3,000 mg/L. These parameters make standard biological treatment impossible.
Arsistek's solutions for the leather sector integrate a chromium recovery unit, sulfur stripping column, chemical coagulation, balancing tank, and activated sludge system. The recovered chromium is used in the re-manufacturing process, which is a critical gain both economically and environmentally.
Characterization of Tabakhane Wastewater
Wastewater in a tannery comes from three main workflows: wet processing (liming/shaking), tanning (chrome), and finishing (dyeing-oiling). Each stage produces a different pollutant profile.
Wet processing wastewater has a high pH (10-12) and contains sulfur. Tanning wastewater has a low pH (3-4) and is loaded with chrome. Dyeing wastewater is colored, contains organic solvents, and has a balanced pH value.
Therefore, stream separation is a prerequisite: each stream undergoes different pre-treatment, then they are combined in a collector and sent for common biological treatment. The sulfur-acid mixture produces explosive H2S, making separation essential for safety.
Chromium Recovery System
Chromium plating wastewater is collected separately and sent to the chromium recovery unit. The pH is raised to 8-9 with MgO, causing Cr(OH)3 to precipitate.
Then, the precipitate is dissolved with H2SO4, returning to production as chromium sulfate. The recovery efficiency is between 80-95%. This is critical in terms of both cost and environment.
Sulfur Oxidation
Lime sludge contains high levels of sulfur. Catalytic air oxidation converts S²⁻ to SO₄²⁻.
Manganese or iron salt is used as a catalyst. During a contact time of 2-6 hours, sulfur is removed by over 95%. Thus, the formation of H2S is prevented in the subsequent stage.
Multi-Stage Process Design
Tabakhane wastewater treatment consists of 5 main stages:
- Flow separation: Chromium, sulfur, and general flows are collected separately
- Pre-treatment: Chromium recovery, sulfur oxidation, screening, sand trap
- Chemical treatment: Coagulation-flocculation, primary sedimentation
- Biological treatment: Activated sludge or MBR — adaptation for salt tolerance
- Advanced treatment: Sand filter, activated carbon, ozone if necessary
Using salt-tolerant bacterial culture (halophilic) increases efficiency. Sludge dewatering is done using a belt press or decanter.
Advantages of Solutions for the Leather Industry
Leather Industry Reference Projects
Tabakhane wastewater treatment reference projects.
Frequently Asked Questions
It is not mandatory as a regulation in Turkey, but it is definitely recommended from both an economic (chrome cost) and environmental (toxicity) perspective. A recovery of 80-95% efficiency is achieved.
If sulfur-containing lime wastewater directly mixes with acidic chrome plating wastewater, toxic and explosive H2S gas is generated. Therefore, separation is mandatory for safety.
Standard activated sludge loses efficiency above 5,000 mg/L. Therefore, a halophilic (salt-tolerant) bacterial culture is used, or high MLSS is achieved with MBR. Salt dilution is also an option.
Discharging to the Organized Industrial Zone (OSB) without passing through preliminary treatment for chromium and sulfur is prohibited. A preliminary treatment system that meets the OSB entry limits will be established.
SKKY Table 9 (leather industry) discharge standards: COD < 800 mg/L, BOD < 250 mg/L, TSS < 200 mg/L, Chromium < 1 mg/L, Sulfur < 1 mg/L values are met.
The daily sludge produced from Tabakhane wastewater ranges from 0.5-1.2 kg DM/m³. It is dewatered to 20-30% dryness using a belt press or decanter, and then sent to a licensed disposal company. The sludge remaining after chromium recovery is less hazardous.