Oil and grease present in an emulsified state in wastewater discharged from industries can be removed by settling out using chemical reagents. This method is commonly known as chemical coagulation and flocculation.

Coagulation is the process of destabilizing the emulsion by adding chemicals called coagulants. These coagulants neutralize the charges on the suspended particles, allowing them to come closer together. Flocculation is the process of forming larger, heavier particles called flocs by gently stirring the wastewater after coagulation.

The settling out of oil and grease in wastewater can be explained by the following steps:

1. Coagulation: Chemical coagulants such as aluminum sulfate (alum) or ferric chloride are added to the wastewater. These chemicals react with the oil and grease particles, neutralizing their charges and causing them to aggregate.

2. Flocculation: After coagulation, gentle stirring or mixing is applied to the wastewater to promote the formation of flocs. The stirring helps the small oil and grease particles to come together and form larger, heavier flocs.

3. Settling: The flocs formed during flocculation are denser than water and tend to settle down under the influence of gravity. This settling process is facilitated by the addition of settling aids such as polyelectrolytes. These settling aids help in the formation of larger and denser flocs, which settle more rapidly.

4. Clarification: Once the flocs have settled, the clarified water can be separated from the settled solids using various methods such as sedimentation tanks or clarifiers. The clarified water can then be further treated or discharged.

Chemical coagulation and flocculation are effective in removing oil and grease from wastewater as they facilitate the separation of these substances from water through the formation of larger, settleable flocs. However, it is important to note that this method may not be sufficient to remove all traces of oil and grease, especially if the wastewater contains high concentrations. In such cases, additional treatment processes like filtration or adsorption may be required.