The quality of tannery wastewater varies significantly depending on the raw materials used and the production processes employed. For example, wastewater generated from sheep leather processing exhibits low concentrations of grease, chemical oxygen demand (COD), and biochemical oxygen demand (BOD), yet high levels of sulfide ions (S²⁻) which contribute to strong alkalinity. In contrast, wastewater from pig leather processing is characterized by elevated concentrations of suspended solids (SS) and grease. This paper analyzes three methods for treating leather wastewater.
Alkali Precipitation Method
This method involves adding a certain amount of alkali to chrome-tanned wastewater to recover chromium hydroxide, followed by acidolysis of the chromium sludge for reuse. Among precipitants, magnesium oxide (MgO) demonstrates excellent performance but is relatively costly. Calcium hydroxide (Ca(OH)₂) is more economical, yet it generates excessive sludge, which hinders reuse. Sodium hydroxide (NaOH) is considered a more suitable precipitant. However, in practical production, the chromium sludge recovered via alkali precipitation contains refractory proteins, soluble grease, and other impurities. These impurities either cannot be recycled or, if reused, may compromise the quality of leather products.
Solvent Extraction Method
This method utilizes specific extractants and controls the pH of the extraction system at approximately 4.0. Under alkaline conditions, chromium ions (Cr³⁺) in the wastewater are exchanged with hydrogen ions (H⁺) in the extractant at a defined ratio. This approach enables the recovery of high-purity Cr³⁺, thereby exhibiting promising application prospects.
Direct Recycling Method
This method involves subjecting chrome-containing wastewater to water quality testing and filtration before reusing it in the pickling solution for the next batch of raw hides. Alternatively, the wastewater can be adjusted for pH and supplemented with chromium salts for subsequent tanning processes. Direct recycling optimizes the utilization of chromium salts, reduces the consumption of chromium reagents, minimizes the total volume of chrome-tanned wastewater, and lowers chromium content, thereby alleviating the burden of wastewater treatment. However, in practical production, repeated recycling may lead to the accumulation of impurities, which can negatively impact leather quality. Thus, strict control of parameters such as pH and temperature is necessary to ensure product quality.
It is important to note that regardless of the wastewater treatment method employed, monitoring key water quality indicators using water quality detectors is critical, as it directly relates to the safety and compliance of the treated water.
