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"Raw water" is untreated water. It enters a Reverse Osmosis (RO) plant where (after pre-treatment) it is pumped through RO membranes. Some water passes through the membranes this is the "treated water" and, because the membranes hold back salt, this water has reduced levels of "TDS" (total dissolved solids - or salt). Those dissolved solids don't disappear, they need to go somewhere. They mainly remain before the membrane, where the water gets increasingly salty. This water is therefore rejected hence called "reject water" or brine.

This whole process requires massive pressure, hence is very expensive in terms of energy requirements. It also leaves you with a much more saline (higher TDS) reject stream which you need to somehow deal with. As such RO and other desalination processes are normally only used where there is no other sustainable alternative.

"Raw water" is untreated water. It enters a Reverse Osmosis (RO) plant where (after pre-treatment) it is pumped through RO membranes. Some water passes through the membranes this is the "treated water" and, because the membranes hold back salt, this water has reduced levels of "TDS" (total dissolved solids - or salt). Those dissolved solids don't disappear, they need to go somewhere. They mainly remain before the membrane, where the water gets increasingly salty. This water is therefore rejected hence called "reject water" or brine.

This whole process requires massive pressure, hence is very expensive in terms of energy requirements. It also leaves you with a much more saline (higher TDS) reject stream which you need to somehow deal with. As such RO and other desalination processes are normally only used where there is no other sustainable alternative.