<a href=showthread.php?s=&postid=9780832#post9780832 target=_blank>Originally posted</a> by pyrrhus
Carlso, your membrane is rejecting less than 94% of the impurities in the water it will cost you extra money for the DI because of that alone.
you might be able to get around 140g of 0 TDS water at best before the DI is shot. With DI once the resin is used up and the reading after DI is greater than 0 the DI starts to release po4 and no3 and other impurities that it has previously absorbed, and can actually be of worse quality than the RO water feeding the DI.
Actually, after drilling out my neoprene water line to increase incoming pressure, and actually running the unit more than 5 - 10 minutes total before taking numbers... I rechecked the TDS levels with the increased pressure and after letting the filters "break in" for 30 minutes, with the output line going to my sink drain.
Now, I get consistent numbers that show the following:
TDS in (from tap water supply) = 221 ppm
TDS out (after RO only) = 9 ppm
TDS out (after RO and DI - I moved the meter 'downstream') = 0 ppm
BTW, after filling my 125g tank, 30g sump, 10g refugium (all of which took 2 and a half
days to accomplish), the numbers haven't fluctuated much; and still 9 after RO and 0 after DI...
This equates to a 96% rejection rate; not quite the 98% that the "premium" units advertise, but satisfactory to me...
I do question the idea that I will get around "140g of 0 TDS water before the Di resin is shot" simply because I
already have run through 165g to fill my setup, and 25g in 'reserve' tank for water top off, and have filled a 33g trash can to use for water changes... that's a total of 223g already run, and doesn't count the 5 to 10g I have let run down the drain while "breaking in" the unit over the first few days it was installed...and I am still at 0 post-DI...
And , I recieved a second set of all filters (except a 2nd membrane) included with my order, so
when the DI does run out I already have a backup waiting. Plus, with my system the DI canister is refillable, so when the time comes I will simply buy a good quality resin in bulk to refill the DI canister. And when time comes, a year or 2 down the line, to replace the membrane I will "upgrade" to a 50gpd Dow unit anyway...
I totally understand the equation being thrown around that "for every 2% less rejection rate ar the RO, you will shorten your DI life by half"... so, in my case, I will be going through DI resin about twice as fast as a guy with a $300, 98% rejection, "premium" unit.
However, that means I will have to buy (and consume) an additional $400 worth of DI resin before I spend as much total cash out as the guy with the premium unit. How?
Me - $100 machine +400 resin = $500 total
"Premium" guy - $300 machine + $200 resin = $500 total
(remember, I will burn through DI resin about twice as fast as the "premium" unit does because my rejection rate is 2% less than the premium unit...)
So then, the question is, how much time goes by before I burn through $400 worth of DI resin?
If my memory is accurate, if I buy 'nuclear grade" bulk resin it will cost me around $9 a pop to refill my canister... that means around 45 refills... I am going to say that (in spite of my experience so far) my DI will be "shot" at the 300g mark on my unit... that's 45 more refills at 300g / refill...
45 X 300g = 13, 500g of 0ppm water before I spend more total $$$ than the guy who bought the "premium" unit for $300...
If I change 25% of my water volume monthly (around 40g) and consume 60g month in top off, that's an average usuage of about 100g of RO / DI water a month...
Sooooo, 13,500g / 100g per month means I will pass the "break even point" vs. the "premium" unit user in approx 135 months ( or, 11.25
years from now ...)
Heck, in more 11 years I could just buy another "cheap" unit, couldn't I

???