Requires drain water! DO NOT block off the drain:

All RO systems require draining of water when it is making water. It is part of the separation process. Tap water enters the RO membrane and it gets separated, pure water goes to the tank, and all the minerals are flushed to the drain. So for 1 gallon of water you use, it will drain 3 gallons out. The system has a automatic shut off valve, so when the tank is full, the drain will stop.