evoracer

It's the membrane that removes the TDS, not the prefilters. They protect the membrane from damage.

What's the PSI going to the membrane? And the water temperature?

The formula to determine how long DI resin lasts is: TDS divided by 17.1; a cartridge has a 260 grain capacity. So, 30 TDS / 17.1 = 1.76 ... 260 / 1.76 = 147g of pure water. At 20 TDS, you'd get 222g of pure water.

I would also like to help determine where your TDS is coming from. Are you on a well? What state and municipality are you in? If you are on a well, do you run a pre-filter after the well? 750+ is incredibly high no matter what your local.

Thanks for the replies.
@melev I don't know the PSI, don't have a gauge. Water temp I estimate in the 40's-50's. I am in a pretty temporate area. Temps here stay within 20 degrees of 65 year round, coldest maybe 40 in a winter night But the numbers I mentioned have been the same since day I, brand new membrane. My concern is with such high incoming I'm not really expecting the membrane to clean it up more than it is, wondering if I can chip away at that high TDS the membrane is seeing.
@midnight I live in a small beach town called Los Osos on California's central coast. Not on well, but in septic. We have been fighting each other over a sewer system since the 70s here, and our tap water is famously bad. I have a friend who works for the local water company who said 600+ is standard here, with older houses above 700 being normal. He said the limit they go by, not a written rule but a rule of thumb, was below 1000!!

Definitly get a pressure guage. I was running 60psi into my RO membrane, which I thought was pretty good, but my water is so cold, only 40F, that I was only getting 1-1/2 GPH out of a 100 GPD unit, and only getting 20ppm TDS out of the RO. I added a booster pump to get the pressure up to 100psi, and now I'm getting 2-1/2 gph at 2ppm TDS.

How many gallons of water do you use a month on average? What I am really asking is what is it costing you to make pure water. Adding a pressure gauge would help for sure at that water temperature.

Hmm. On average, probably 30-40 gallons for water changes, plus i'd say at least 10-15 gallons per week in top-off. And summer's coming, so a bit more (my tank is in the garage, so it does get a bit warmer in there). So DI will process probably 70 gallons in the winter and I would estimate 90 or so in the summer (the DI was just recently added, before I always bought water). We also use the RO for drinking water, so countless gallons there. Ok well I will order a pressure gauge first. Maybe all I need is to boost incoming pressure? That would be nice.

BTW what pressure am I looking for? Can I buy a hose bib pressure gauge for a quick test or do I have to order an inline gauge for the RO line?

Evo, I have an inline one that you can install prior to the membrane. You want to know what the pressure is across the membrane. Also, the water being colder affects production rates, so a booster pump will help immensely, and should drop the TDS as the membrane will be more effective.

Do you sell the boosters as well? I didn't see them on melevsreef.com.

Ok, ordered a booster pump and pressure gauge, should be here this week. Will update when they have been installed.

Wow. Installed the gauge just before the membrane, 32 psi after the (new) prefilters! Installed the booster pump and my TDS now drops to 12. Once I observed it drop to 5, not sure why the difference. However, the pressure with the pump running is only 58 at the same spot. It is adjustable, what should I shoot for pressure wise at the membrane? I think I read somewhere 80?

Forgot to mention, the water is cool enough to drink right out of the tap, I'd say 50-55 degrees. Even during the summer.

Measure the water temperature to know for sure. I had a friend do that recently and his measured 57 degrees.

If the water pressure is less than 40 at the membrane, it won't be very efficient. What is the pressure after the booster pump prior to the membrane? What is the pressure before the prefillters? Those filters may be a problem.

75 PSI at the membrane would be good. Some booster pumps have a governor to avoid excessive pressure on the housings. And booster pumps come in different sizes (flow rates). Perhaps you got one under-rated for your membrane?

Got the Aquatec 8800, which was the largest one I found in regular circulation. As the above post says, pressure at the membrane is 58 psi with the pump running. Pressure before the prefilters is roughly the same as after without the pump running. I will adjust the pump and see if I can achieve 75psi.

Adjusted the pump as far as it will go, got 68 psi at the membrane. Nearly a 30 psi boost. Tonight my incoming TDS was rather high (it seems to fluctuate from day to day) at 791. With the pump cranking the output is down to 17, which is a 98% rejection, right what the membrane is rated for. By my math that's 260 gallons of DI, up from 150. So a nice boost.
I am still going to try and remove some incoming solids by adding a large prefilter or two, but for now I am somewhat satisfied.

Forgot to mention Water temp is pretty low today, around 40. It is currently about 50 degrees outside, only got to maybe 60.