Tannery industry production processes typically include degreasing, soaking and unhairing, softening, tanning, dyeing, drying, and finishing. Various chemicals are added during these processes, resulting in wastewater containing pollutants and toxic substances such as fats, collagen, animal and plant fibers, organic and inorganic solids, sulfides, chromium, salts, surfactants, and dyes. The overall water quality characteristics of tannery wastewater include CODcr (Chemical Oxygen Demand) levels of 3000-4000 mg/L, BOD5 (Biochemical Oxygen Demand) levels of 1000-2000 mg/L, SS (Suspended Solids) levels of 2000-4000 mg/L, and a pH range of 8-11.
The main sources of wastewater are pre-tanning preparation, tanning, and other wet processing stages. The most heavily polluted wastewaters are degreasing wastewater, soaking and unhairing wastewater, and chromium tanning wastewater, which account for approximately 50% of the total wastewater volume but contain the majority of pollutants. The mass fraction of various pollutants in these wastewater streams is as follows: CODcr 80%, BOD5 75%, SS 70%, sulfides 93%, sodium chloride 50%, and chromium compounds 95%.
The characteristics of tannery wastewater are as follows:
Large fluctuations in water quality and quantity.
High biodegradability.
High concentration of suspended solids, prone to putrefaction, and generates a large amount of pollution.
Presence of toxic compounds, such as S2- and chromium.
Challenges in treating tannery wastewater include effectively addressing the high fluctuations in water quality and quantity, ensuring efficient biodegradation of pollutants, handling the high concentration of suspended solids and their potential for putrefaction, and effectively removing toxic compounds such as sulfides and chromium.