Oil separation tank [1] is a wastewater pre-treatment structure that utilizes the density difference between oil droplets and water to generate upward flotation and remove floatable oil substances from oily wastewater. The structure of the oil separation tank is mostly of the horizontal flow type. The oily wastewater enters the rectangular oil separation tank through the water distribution tank, flows slowly along the horizontal direction, and the oil rises to the surface during the flow. It is pushed into the dehydration tank by the oil collection pipe or the oil scraper set on the tank surface. The heavy oil and other impurities that settle in the oil separation tank accumulate in the sludge hopper at the bottom of the tank and enter the sludge pipe through the sludge discharge pipe. The wastewater treated with oil separation overflows into the discharge tank of the drainage channel for subsequent treatment to remove emulsified oil and other pollutants。