Shut down of water mains or low main pressure. Air bubbles may be present in water after there has been a break in or draining of a water main. Most water mains are equipped with air relief valves, which permit air to enter the main when the pressure drops. These valves also permit air to escape when the main is refilled. However, if the main is filled too 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 a normal atmospheric pressure. A reduction in pressure (for example when water fills a glass) releases air bubbles and results in a milky appearance.
Temperature changes. Cold water can hold a greater amount of air in solution than warm. Water at normal atmospheric pressure and a temperature of 30 deqrees 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 or carbonated appearance.
Overheating of hot-water systems. Complaints of air in water have been traced to the overheating of hot water tanks. This usually occurs in homes where there are old-style, manually operated heaters. However, it occurs in newer homes where automatic hotwater tanks fail to operate properly or where the thermostat is set at an excessive temperature +above 140 degrees F.
Water releases air bubbles when it is heated. For this reason, hot water usually contains some air bubbles. This condition is most noticeable during the winter months when the water normally contains the most air in solution and in the first water drawn from a hotwater tank after the tank has been idle overnight.
Warming of cold-water lines. Water releases air bubbles when it is warmed. It is not unusual, therefore, for cold-water lines in basements, aboveground, or attached to the sides of building and exposed to the sun, to deliver milky water.