On this unit the vlave should be closed to create backpressure. I have this same model. This is word for word from the Instruction Manual...
The dischacharge line has to have backpressure or the RO
will not work. Most vendors choose the cheap way to create this backpressure by putting a little plastic insert into the discharge line. We have chosen to spend the extra money and buy and mount a manual flushing flow restrictor. The NORMAL POSITION for this switch is at a 90-degree angle to the line.
If your RO
unit does not have this type of flow restrictor valve then I would say yes the waste line should be open but on this model with the flow restrictor valve it should be closed to cause backpressure and force the flow through the RO
membrane. My tap water has TDS
of around 300 in and I am now down to a reading of 9 out of my RO
unit with the valve in the closed position. I still have plenty of water coming from my waste line so it is not like closing the valve prevents the waste water from coming out...it still comes out.
Hope this helps....Mark