In its broadest sense, aeration is the process by which the area of contact between water and air is increased, either by natural methods or by mechanical devices. Ordinary usage in water works practice has however, been given the term in the more limited sense referring specifically to use of mechanical devices or procedures. In this limited sense aeration clearly defines itself as a method of treatment rather than merely a modification of natural conditions at the source of supply. The terms 'natural aeration` or 'reaeration` are used to represent nonmechanical procedures or slower aeration of large bodies of water under natural conditions. In the progress of water from source to consumer, aeration is one of the most elemental techniques frequently employed in the improvement of the physical and chemical characterisitcs of water.