Normally, a drinking water unit comes with a 50 GPD membrane. This is good if there is no more than 3-4 people in the house. The 75 GPD membrane is used if you have 5-7 people in the house. The 100 is used for 7+ people in the house. Most people do not need to use the 150 GPD membrane for drinking water. You can, but it's not necessary.