Customers usually notice turbidity readings in excess of how many nephelometric turbidity units?

Turbidity is a key indicator of water quality and is typically measured in nephelometric turbidity units (NTU). When turbidity readings exceed a certain level, customers may start to notice a difference in the clarity and overall appearance of their water. A reading of 5 NTU is generally recognized as the threshold at which people begin to perceive turbidity visually.

At this level, the water may appear cloudy or murky, which can lead to customer complaints and concerns regarding the safety and quality of their drinking water. This perception is significant in water treatment and distribution, prompting operators to take necessary actions to address any underlying issues that might be causing elevated turbidity levels.

Levels above 5 NTU, such as 10 or 15 NTU, are more clearly visible and may indicate even more substantial quality issues, but the initial point at which customers typically notice turbidity begins at 5 NTU. Thus, that is why it is identified as the correct answer in this context.