A separating device. The separating device is used in a tank, for separating solid, gas, and liquid of polluted water, including a first panel, and a plurality of first dividers. The first panel is disposed in the tank, includes a first body and a plurality of first tubes. The first body has a plurality of first openings. The first tubes, connected with the first openings, are disposed under the first body. The first dividers, corresponding to the first tubes, are horizontally disposed under the first tubes. A gap is formed between each first divider and each first tube. The first tube projects to the first divider, forming a projecting perimeter within the first divider.