Computer-Implemented method of monitoring the performance of a reverse osmosis membrane in a drinking water supply system, the method carried out by a computer program, the method comprising the steps of: initiating (step 74) a reverse osmosis membrane recalibration; determining (step 76) the flow amount of a predetermined amount of water through the system; measuring (step 80) the initial permeate conductivity downstream of the reverse osmosis membrane after the predetermined amount of water has flown through the system; calculating (step 82), using the measured initial permeate conductivity (step 80), a predetermined total dissolved solids rejection ratio, and calculating therefrom a threshold trip point which corresponds to said predetermined total dissolved solids rejection ratio, wherein calculating said threshold trip point includes: measuring said initial permeate conductivity values; equating an average of said initial permeate conductivity values to said total dissolved solids rejection ratio, and defining said average at ninety percent (90%) rejection; and calculating said threshold trip point at seventy-five percent (75%) of said average; storing (step 84) the threshold trip point in memory of the system; routinely measuring (step 86) the permeate conductivity downstream of the reverse osmosis membrane after said flow of said predetermined amount of water has been reached; determining (step 90) whether the permeate conductivity, as routinely measured in the prior step, is below the threshold trip point; and providing (step 92) an indication upon determining when the permeate conductivity is below the threshold trip point. initiating (step 74) a reverse osmosis membrane recalibration; determining (step 76) the flow amount of a predetermined amount of water through the system; measuring (step 80) the initial permeate conductivity downstream of the reverse osmosis membrane after the predetermined amount of water has flown through the system; calculating (step 82), using the measured initial permeate conductivity (step 80), a predetermined total dissolved solids rejection ratio, and calculating therefrom a threshold trip point which corresponds to said predetermined total dissolved solids rejection ratio, wherein calculating said threshold trip point includes: measuring said initial permeate conductivity values; equating an average of said initial permeate conductivity values to said total dissolved solids rejection ratio, and defining said average at ninety percent (90%) rejection; and calculating said threshold trip point at seventy-five percent (75%) of said average; measuring said initial permeate conductivity values; equating an average of said initial permeate conductivity values to said total dissolved solids rejection ratio, and defining said average at ninety percent (90%) rejection; and calculating said threshold trip point at seventy-five percent (75%) of said average; storing (step 84) the threshold trip point in memory of the system; routinely measuring (step 86) the permeate conductivity downstream of the reverse osmosis membrane after said flow of said predetermined amount of water has been reached; determining (step 90) whether the permeate conductivity, as routinely measured in the prior step, is below the threshold trip point; and providing (step 92) an indication upon determining when the permeate conductivity is below the threshold trip point.