Once operational, what water source is used to mix the chemical used in the ROWPU?

The correct answer in this context is raw water. In Reverse Osmosis Water Purification Units (ROWPUs), the source of water used for mixing chemicals typically is raw water. Raw water refers to untreated surface or groundwater that has not undergone any purification processes. It is necessary for diluting chemicals like coagulants or disinfectants that assist in the water treatment process.

Brine refers to a highly concentrated salt solution that is a byproduct of desalination or other processes and is not used as a mixing source for treatment chemicals. Product water is the treated water that results from the purification process and is not a source for mixing chemicals. Brackish water is a mixture of freshwater and saltwater, which is also not appropriate for the chemical mixing needed in ROWPU operations. Therefore, raw water is essential in this context as it is the untreated source that serves as the initial input for the treatment chemicals involved in the purification process.