Milky Water

Have you ever experienced water that is milky or white in appearance? It could be air in the water. How might it happen?

Air bubbles may be present in water after there has been a break in or draining of a water main. If the main is filled rapidly or if there are no air valves in the line, air is trapped within the main. Water can absorb more air at higher water pressure. Water under a pressure of 40 psi is capable of absorbing about four times the amount of air it absorbs at normal atmospheric pressure. A reduction in pressure (for example when water fills a glass) releases air bubbles and results in a milky appearance.

Cold water can hold a greater amount of air in solution than warm water. Water at normal atmospheric pressure and a temperature of 30 degrees F can retain 14.6 mg/L of dissolved oxygen in solution; whereas, water at atmospheric pressure and a temperature of 80 degrees F holds only 8.14 mg/L. Therefore, air is released upon warming cold water saturated with air. The air is released in the form of small air bubbles, which give the water a milky appearance.

Sometimes air in water can be traced to the overheating of hot-water tanks. This can occur in homes where there are old-style, manually operated heaters. However, it can occur in newer homes where automatic hot-water tanks fail to operate properly or where the thermostat is set above 140 degrees F. Water releases air bubbles when it is heated. For this reason, hot water usually contains some air bubbles. Milky water is most noticeable in the first water drawn from the hot-water tank after the tank has been idle overnight.

In addition, it is not unusual for water to release air bubbles as the result of cold-water lines being warmed. For example, milky water might be delivered from cold-water lines in a warm basement.

(Information adapted from “Opflow”, April 2005, Vol.31, No.4. American Water Works Association.)