A common technique for separating substances in two physical states. For example, a mixture of solid and liquid can be separated into its components by passing the mixture through a filter paper. Filtration has many environmental applications. In water purification systems, impure water is often passed through a charcoal filter to remove the solid and gaseous contaminants that give water a disagreeable odor, color, or taste. Trickling filters are used to remove solid wastes in plants. Solid and liquid contaminants in waste industrial gases can be removed by passing them through a filter prior to discharge in a smokestack.