With the increase of leather production, the amount of pollution discharged from leather industry is also increasing, and the environmental pollution of leather industry is becoming increasingly prominent. In the tannery process, only 31.5% of the raw leather is converted into leather, while the rest is converted into waste or by-products. A large number of chemical materials such as acid, alkali, sodium sulfide, lime, surfactant, chrome tanning agent, fatliquoring agent, dyestuff and other organic additives are used in leather making process. Some of these chemical materials are absorbed and utilized by products, while others are polluted by wastewater.
Current tannery technology will eventually have wastewater discharge, which can only be discharged after treatment, otherwise it will pollute the environment. Tannery wastewater treatment technology develops with the development of environmental protection technology and equipment. After treatment, tannery wastewater can be reused in the preparation section of tannery production, and the reuse amount can reach 50%-60%. Dehairing wastewater can recover protein, alkali and sodium sulfide; degreasing wastewater can recover grease; sludge can be treated as agricultural fertilizer, building materials and so on.
Leather production can be divided into wet operation and dry operation. Wet operation includes preparation section and tanning section; dry operation is finishing section. Tannery wastewater mainly comes from wet operation preparation section and tanning section. The purpose of the preparation section is to restore the raw skin to a fresh state and remove the impurities on the skin, then remove the hair, epidermis, fat and fibrous stroma, and disperse the collagen fibers properly, so as to create a suitable condition for tanning. Tanning section is the key operation of leather making. The combination of tanning agent and leather denatures the protein, thus transforming the leather into leather.
Tannery wastewater mainly comes from various processes of wet operation. According to the process, it can be divided into five sort of wastewater. Alkaline, oil-rich soaking water; sodium sulfide-rich dehairing water; acid chrome tanning water; acid dyeing and fatliquoring water containing dyes; and other drip-leak washing wastewater. The content of BOD and COD in sewage is very high. It is a kind of high concentration organic wastewater with color, odor and toxicity.
The main treatment methods of tannery wastewater are physicochemical and biochemical methods. Chamber filter press or Automatic Membrane/Diaphragm filter Press can be used in the treatment of activated sludge process, biofilm process, oxidation ditch process and anaerobic process. Whatever the method, its pretreatment and pretreatment are necessary. Pretreatment usually enters the primary sedimentation tank and the regulation tank in the back section after the grille, removes sediment and sediment, regulates the quality and quantity of wastewater, and ensures the stable and continuous operation of the treatment system in the back section. After chemical coagulation and precipitation of wastewater, the slurry can be separated from the bottom of the tank by screw pump through a chamber filter press or an automatic membrane filter press. Generally, the removal rates of COD, BOD5, Suspension solid, total chromium and sulfur ions are 70% – 85%, 50% – 80%, 85% – 95%, 98% and 95%, respectively. The filtrate can be treated in the back stage. The filter cake after solid-liquid separation can be dried and put into the production of agricultural fertilizer or building materials after discharging from the chember filter press or automatic membrane filter press.