What brand unit do you have ? many of the better ones have a capillary waste flow re-stricter that is temperature sensitive and will expand and or contract to control the amount of rejection water according to the temperature of the source water and GDP rating of the membrane . Some of the less expensive units may control their in other ways , Thats why in general I suggest that all adjust the waste water outlet so that you get about 4 to 5 gallons of waste water to every 1 gallon of good water . then if you add a auto cut off and a float in the sump or holding vessel for water the Auto cut off will cut off all the water when it reaches about 25% back pressure of the homes water supply pressure is reached . And if you run the waste at the proper amounts then it never any need to backwash the membrane as it keeps its self flushed of TDS all the time..