low water inlet pressure with RO/DI

so i just got my 75 gpd upgrade kit for my RO system from bulk reef supply.

i currently have a 75gpd and am going to add the 75gpd to make a 150gpd total. I dint notice till now that BRS recommends that you have 65 psi inlet pressure to RO system. i only have 45-50. I am assuming that this will only decrease my "GPD" not my "TDS". Am i right??? should i wait till i get a booster pump??? (which btw are kinda expensive)

 

Check with BRS and see what the minimum is, you're right it should only affect your GPD if it is in the effective range.

 

Increasing the pressure will increase the GPD and the rejection rate (lowering the TDS after the membrane).

Effect of Pressure

Feedwater pressure affects both the water flux and salt rejection of RO membranes. Osmosis is the flow of water across a membrane from the dilute side toward the concentrated solution side. RO technology involves application of pressure to the feedwater stream to overcome the natural osmotic pressure. Pressure in excess of the osmotic pressure is applied to the concentrated solution and the flow of water is reversed. A portion of the feedwater (concentrated solution) is forced through the membrane to emerge as purified product water of the dilute solution side.

Water flux across the membrane increases in direct relationship to increases in feedwater pressure. Increased feedwater pressure also results in increased salt rejection but the rejection is less direct than for water flux.

Because RO membranes are imperfect barriers to dissolved salts in feedwater, there is always some salt passage through the membrane. As feedwater pressure is increased, this salt passage is increasingly overcome as water is pushed through the membrane at a faster rate than salt can be transported.
However, there is an upper limit to the amount of salt that can be excluded via increasing feedwater pressure. As the plateau in the salt rejection curve exceeds a certain pressure level, salt rejection no longer increases and some salt flow remains coupled with water flowing through the membrane.

 

You need the extra pressure as the way you configure a two membrane system is to send the waste from the first membrane through the second membrane. The waste from the first is already at a reduced pressure after having passed through the membrane so will not have the energy required to pass through the second.