The milky white appearance is caused by dissolved air bubbles.
When tap water is transported under pressure through the water network, as water flows under pressure, more air dissolves into it, causing a significant amount of gas to dissolve into the water.
As the water passes through the reverse osmosis (RO) membrane, it is further pressurized by the pump to maintain flow. This additional pressure increases the solubility of the dissolved gases even more. When the water passes through the RO membrane, these gases are broken into even smaller bubbles.
Because the system is continuously pressurized, the gases cannot escape. When the water exits the system, the sudden drop in pressure causes the dissolved gases to expand into visible bubbles. When these bubbles accumulate, the water looks milky white. These bubbles will gradually disappear if the water is left standing.
This phenomenon can occur in both purified water and concentrated water. It is a normal physical occurrence and does not affect water safety or taste.