In a laboratory experiment, John uses a mesh to separate soil particles from water. Which technique of separation is he using?

If a mesh is being used to separate soil from water, it sounds to me like John  is employing a filtration technique.

Although a mesh would remove some of the larger impurities from the water, most filtration methods are more complex, as the above diagram demonstrates, where water is filtered through several layers of other substances in order to make it cleaner.

Other Methods of Separation

John could also have distilled the water, which can involve boiling the water to remove a harder, less visible impurity, such as removing salt from saltwater. The water could have been spun at high speed to separate it from a substance of differing density (centrifugal method) or allowed to settle, so the denser material sinks to the bottom of its container.

This short film looks at various separation techniques: