Once-through water finds a variety of applications ranging from municipal water supplies to the cooling of industrial processes. In once-through systems, cooling water passes through the heat transfer equipment only once before being discharged. Large volumes of water are used with most often only a small temperature increase across the exchanger. The mineral content of the water essentially remains the same. The water sources are usually drawn from wells, lakes, rivers and oceans. Once-through cooling systems, however, should be treated with a total program approach that must be designed to handle corrosion, scale, fouling and microbiological growth, both micro as well as macro contamination, since any or all of these problems might be found in once-through systems depending on the source of the water. The majority of once-through systems are found in electric utility power plants, pulp and paper mills, steel mills, food plants and municipal potable drinking water systems.

There are only two available sources of water: surface sources, which include lakes, rivers, streams, oceans and the polar ice caps, and ground sources, which include wells and springs. Actually, the two sources are often not distinct. What is surface water at one point may percolate down to supply an aquifer and/or the water table may rise to maintain a water level in a river.