Depending on the different stages of leather tanning (preparation, tanning, and finishing), tanning wastewater sources can be broadly categorized into three types:
1. In the preparatory stage, wastewater mainly originates from washing, soaking, hair removal, liming, deliming, softening, and degreasing; the main pollutants include organic waste, inorganic waste, and organic compounds.
2. In the tanning stage, wastewater mainly comes from washing, pickling, and tanning processes; the main pollutants are inorganic salts and the heavy metal chromium.
3. In the finishing stage, wastewater mainly comes from washing, squeezing, dyeing, fatliquoring, and dust removal processes; pollutants include dyes, oils, and organic compounds.
Therefore, tanning wastewater is characterized by large volume, significant fluctuations in water quality and quantity, high pollution load, high alkalinity, high color, high suspended solids content, relatively good biodegradability, and a certain degree of toxicity.
