Adding a Second Reverse-Osmosis Membrane
March 6, 2010
My current 150 gallon-per-day RO/DI setup with two RO membranes.
After tiring of watching so much waste-water go down the drain with my old RO/DI setup, I decided to install a second RO membrane
to increase it's efficiency. As you may know, most RO membranes produce waste-water in a 3:1 ratio to filtered water. This ratio only
worsens over time as your sediment and carbon filters begin to clog and the pressure of the water reaching the membrane begins to
I had been thinking about adding a second RO membrane in-line after the first for quite some time. I saw this first at a local
fish store that had actually piggy-backed three RO membranes. I never found out what kind of waste-water ratio they were getting, but
the idea seemed intriguing. Most vendors claim that adding an additional RO membrane can cut waste-water in half as long as the tap
water pressure is sufficient. The benefit seems great, but I couldn't find many individuals who had personally done it.
Sufficient tap water pressure is the major caveat of this approach. Most vendors recommend that the first RO membrane should be
receiving at least 65 PSI for a dual membrane setup to work properly. After purchasing and installing two pressure
The left pressure gauge measures tap pressure, and the right measures pressure reaching the first membrane.
I found that my tap pressure was only 60 PSI and only 58 PSI was reaching the RO membrane. That is to say, brand new sediment
and carbon filters were dropping the pressure by 2 PSI. Although not quite 65 PSI, I decided I was close enough and purchased
an additional membrane upgrade kit from
www.bulkreefsupply.com. I went ahead and purchased a replacement for my current RO
membrane as well, as it was reaching the three year mark.
The logic behind the installation is simple enough but I found that a lot of pressure was needed to open my RO canister and
pull out the old RO membrane. That is until I realized that heavy pliers were required, as described in this
Melev's Reef article.
The two membrane setup has proven a success for me. Currently, with relatively new sediment and carbon filters, I am observing
a nearly 1:1 waste-water ratio. In this picture below, the container on the left has been filled with waste-water and the container
on the right with filtered water.
My nearly 1:1 waste-water to filtered water ratio.
One thing I have noticed is that my effective rejection rate has been slightly reduced, which could exhaust the DI filter
faster. However, I think the difference is basically negligible. Previously, my single RO membrane was taking in tap water at
179 TDS and producing water at 6 TDS (a 97% rejection rate). Currently, my two RO filters are together taking in water at 179 TDS and
producing water at 11 TDS (a 94% rejection rate). Interestingly, my waste water is at a whopping 315 TDS. I don't have any
measurements on the waste-water from my single membrane setup, but I'm sure that they were not this high.
I really wasn't looking for an increased rate of water production but this new setup also affords me that. Whereas it used to take
me 1 hour 20 minutes to produce 5 gallons of filtered water, it now takes approximately 50 minutes. Keep in mind that I inadvertantly
replaced my old 100 GPD RO membrane with two 75 GPD membranes, so I was not expecting time to he halved.
Copyright © 2010-2018 Hamza Muhammad Arain.
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