In _________ process, 90% of the impurities are removed from the water.

In the sludge activation process, 90% of the impurities are removed from the water. 

This process is carried out in the sludge activation tank by increasing the activity of microorganisms in sludge, which helps to break down organic matter and stabilize the sludge. During the sludge activation process, the sludge is aerated to provide oxygen to the microorganisms, which can increase their metabolic activity and help them break down organic matter more efficiently.