View Full Version : RODI Filter Help

09/18/2015, 05:23 PM
I am buying a used 90gdp 4 stage Spectrapure RODI System. Where can I get good quality filters for that in cheaper price.

Do I have to change all the filters? 1st and 2nd stage are less than a year.

Can I refill the DI cartridge? where can I get the RO membrane ? all RO membrane are same or I have to buy particularly from spectra pure.

09/18/2015, 05:27 PM
Buckeye hydro, bulk reef supply sells quality filters at a good price.

The carbon and sediment filters should be changed out every 6 months regardless of how much its used.

The membrane only needs to be changed out when rejection rate starts to fail. You want to keep the same gpd membrane, if you change the gpd membrane then you have to change the flow restrictor also. Their 90 gpd membrans can be used with 75gpd membranes with no issues. I have been useing the maxr 75gpd membrane from buckeye hydro and love it, more spendy but worth it imo

The di stage can be refilled and i change mine out when tds reads 1.

09/18/2015, 05:29 PM
Also like to inform in my are TDS is 66 on tap water. The ro di I will be using for 30 gal tank for 3gal water change once every two week.

09/18/2015, 05:30 PM
BRS filter will fit on the spectra pure 90 gdp RODI system?

09/18/2015, 05:31 PM
Yes, they are all interchangeable as long as you get the 10" ones

09/18/2015, 06:06 PM
It might be cheaper to buy a new unit with all the filters than to buy replacement filters for a used unit.


09/18/2015, 07:01 PM
. . . The membrane only needs to be changed out when rejection rate starts to fail. You want to keep the same gpd membrane, if you change the gpd membrane then you have to change the flow restrictor also.

How do you know when rejection rate starts to fail?

What is a "flow restrictor"? I have 6 stage BRS RO/DI filter and have never seen a reference to this?

Much appreciated,


BTW, I agree with seamonster 124. OP may be better off buying new unit with filters included. May not cost much more than old unit + purchasing new filters. Plus lots of new things available with new RO/DI units that OP may want to check out.

09/18/2015, 07:06 PM
You can figure it out by knowing your tap waters tds and your tds of ro water before the di. If your ro waters tds starts to climb then your membrane is getting plugged.

All units have a flow restrictor on them they allow you to get the proper 4:1 waste to good water which most membranes need so they dont get clogged. The less waste you have the faster your membrane will get plugged up.

09/18/2015, 07:08 PM

Yours will have one of these 2 on your unit

Buckeye Hydro
09/19/2015, 03:07 PM
Rejection rate calculation
(Tap water TDS - RO only TDS) divided by Tap water TDS x 100 = % rejection.

09/22/2015, 03:09 PM
Thanks for the info. Upon examination, I see that I have a Membrane Flush/Flow restrictor valve, which BRS describes as "an all in one flow restrictor equipped with a flush valve."


For the past 3 1/2 years, I have been using it exclusively as flush valve, which I open for 10-15 minutes to flush RO before I start drawing water for use in tanks.

Stupid question: why do I want to make sure the rejection rate is 4 to 1? I'm guessing that if it's more than 4 to 1 I'm wasting water? Or is it to improve efficiency of the RO/DI unit?

If the latter, I'm not sure I can improve efficiency of my unit. (Hubris alert!) In early 2002, I bought a 5 stage 75 gpd RO/DI unit with sediment filter, 5 micron carbon unit, 1 micron carbon unit and DI filter. When I added a second (larger DT) about 30 months ago, I added a second 75 gdp RO unit plus a booster plump. (At that time I also changed out the original RO membrane.) I also change sediment filter every 3 months and the carbon filters every 6-9 months.

FWIW, I live in NYC and my tap water appears to be excellent. My DI filter lasts more than 18 months (I just checked mine after 16 months of use (producing about 60 gals per week of product water) and only about 33% of the filter resin had been used (i.e., changed from blue to light brown).

So, again, is there any other reason why I care about maintaining 4 to 1 ratio, other than to reduce water bill, which admittedly is a significant factor in NYC?

Thanks again and I truly appreciate your advice, guys.


Buckeye Hydro
09/22/2015, 03:25 PM
The ratio of waste water to ppurified water doesn't really have anything to do with the REJECTION RATE. REjection has to do with the TDS of the RO water relative to the TDS of the feedwater.

What you're talking about is RECOVERY. That is, what % of the feedwater ends up as purified water. People in this hobby prefer for some reason to talk about this as a ratio of waste water to purified. So if you don't have enough waste water, you can shorten the life of your membrane. At the other end of the spectrum if you have too much waste water you are needlessly sending water down the drain.