A reverse osmosis water purifier that monitors Total Dissolved Solids (TDS) at the onset of entering the water filtration system and downstream upon exiting the system. A comparison of the TDS levels is made to each other or predetermined levels, and action is taken regarding whether to bypass the RO filter, or continue filtering through the RO membrane, or combine the two fluid streams. A microbiological barrier filter is introduced in-line with the egress port of a reverse osmosis filter, and downstream of the bypass water circuit. The microbiological filter is utilized to remove microbiological contaminants from the output water, either directly from the RO filter output, or the bypass filter circuit, or both.