The main function of pretreatment is to remove SS, oil, chromium ion and sulfide as much as possible, and reduce the concentration of organic matter and toxic substances to ensure the efficient and stable operation of subsequent biological treatment. Coagulation sedimentation and air flotation are common pretreatment methods for leather wastewater. Coagulation and sedimentation: chemical agents such as NaOH, ferrous sulfate and PAC are mainly added to the wastewater to precipitate and remove sulfide and chromium ions in the water; Air flotation, mainly by adding demulsifier and flocculant to water, effectively removes oil and SS from water through the floating and attachment of tiny bubbles.

For the pretreatment process, it needs to be combined with the subsequent biological treatment process selection. Wei Jiatai [2] believes that the low-load operation process (such as oxidation ditch process) has strong impact load resistance and has low requirements for pretreatment. High-load processes (such as contact oxidation) need to improve pretreatment efficiency accordingly. Therefore, when contact oxidation is used as a biological treatment process, the requirements for pretreatment are strict. If the pretreatment fails to reach the expected goal, it will affect the treatment effect of the subsequent contact oxidation method, thus affecting the operation stability of the whole system.