For brewery usage, a 3 stage RO system is completely sufficient. 1-micron filter, carbon block filter, and then the RO membrane. If you run a pressure tank, remember that the available capacity is roughly one-half the total volume since half the volume is air. I have a 20 gal and 3 gal set of pressure tanks ganged together and all I can get out for immediate usage is about 10 gal. I have to plan ahead if my brewing efforts will require more than 10 gallons. If you can manage it, discharging your RO into a non-pressurized vessel can improve the efficiency of the treated water throughput by reducing the backpressure on the membrane.