Well, the quality of raw ...

Well, the quality of raw water can and will vary, and one should select a closest fit to the target treated water quality to offset treatment costs. For drinking water as an example, the following extract from the World Health Organisation is a useful reference: 

"The presence of dissolved solids in water may affect its taste (1). The palatability of drinking- water has been rated by panels of tasters in relation to its TDS level as follows: excellent, less than 300 mg/litre; good, between 300 and 600 mg/litre; fair, between 600 and 900 mg/litre; poor, between 900 and 1200 mg/litre; and unacceptable, greater than 1200 mg/litre (1). Water with extremely low concentrations of TDS may also be unacceptable because of its flat, insipid taste."

So the closer the raw water quality is to this level, the better...

Trust this is of assistance