melev

I use the 150gpd RO/DI system for all my water needs, both drinking water and RO/DI for my reef. Using it for maybe five years or more, you get used to how it operates. However, I recently had a customer with very high TDS reach out to me after buying one of my systems looking for a more efficient option because DI resin wasn't lasting long at all. After talking with my resource, I decided I'd try the suggestion with my own system to get some personal experience. This of course ended up with some math...

First thing I did was measure the current situation untouched so I could gauge the results. For the past few months, I've noticed the TDS coming out of the membrane is around 20 or more, which has caused DI resin to be depleted rapidly. I've made a lot of water (about 1200g), but it seemed like I needed a new DI cartridge just about monthly. I just accepted it, but that didn't mean I had to like it.

Current source TDS: 121
Water temperature: 72°F
PSI: 75
Membrane rated for 150gpd / 93% rejection rate
TDS of RO water: 22
Waste ratio: 53oz/128oz or 2.4:1
Production rate average: 6.25gph

I don't know how old that membrane was, but I'd say at least two years. Maybe more. It has a 93% rejection rate, so TDS should have been measuring 9 coming out of the system before the DI stage. Usually when a membrane is due for replacement, one or more of these things happen: TDS rises significantly, no water comes out, or waste water increases dramatically. In the above data, TDS was up somewhat, but the waste ratio was close to correct and I didn't notice any slowdown in production. A new membrane would have a waste ratio of 2.25:1 (2.25g waste to 1g good)

I removed that old membrane and installed a 100gpd membrane. I didn't change the flow restrictor, and let the system run for one hour to remove the food grade preservative, then tested everything.

Current source TDS: 121
Water temperature: 72°F
PSI: 78
Membrane rated for 100gpd / 98% rejection rate
TDS of RO water: 2
Waste ratio: 39oz/128oz or 3:1
Production rate average: 4gph

The significant drop in TDS after the membrane is excellent. Water production is slower of course, and the waste ratio is a little bit higher than before but is still better than the 4:1 ratio that is considered industry standard in water filtration.

With the TDS coming off the membrane so much lower, DI resin will last much longer. The formula to know how long DI resin lasts is on my website:

Each DI Cartridge has a 260 grain capacity:
Step 1) RO water TDS divided by 17.1 = Grain Usage
Step 2) 260 grains divided by Grain Usage = Estimated RO/DI Gallons until resin is depleted

If TDS measures 2 going into the DI stage, it will produce 2363 gallons before I have to change it. In comparison, before (with the previous membrane) a cartridge lasted about 200g before I started seeing a slow rise in TDS.

What about the cost of water? I took my latest water bill and subtracted the trash fee, leaving me with a total cost of $42.48. I used 4.4 CCF of water. That's 3291g, according to the water department. Some quick math translates that into $0.01 per gallon of water used. For each gallon of pure water I make, three gallons are wasted. That means one gallon of RO/DI water costs $0.04, not factoring the cost of replacement filters over time.

It seems like the RO/DI system runs a touch louder, which may be due to the slight increase in water pressure. That isn't a deal breaker for me, as hearing the system reminds me to shut if off when it isn't needed.

I must point out that water quality varies from county to county, and results will vary based on water temperature, pressure and TDS levels. Fort Worth's water measuring TDS of 121 out of the tap is pretty nice; historically it is more often in the 170 range.