TDS Meter Readings

[Blue Ram]

I was thinking it is probably time to replace the filters in my RO/DI system. I tested the water from several sources and here are the readings:
- DI (used for reef system) 011 ppm
- RO (used for drinking water) 022 ppm
- freshwater invert tank - 199 ppm
- discus tank - 202 ppm

I have not tested the RO membrane as the system has been used for only 11 months and the product info indicates replacement every 2yrs.

Is 011 ppm acceptable.

[lastlight]

I'm pretty sure most replace the resin when measuring anything greater than zero. I look into replacing things upstream of the resin if the water going into the resin is higher than 10 so it doesn't burn too quickly. My resin lasts me ages.

22 out of your membrane is going to use resin fairly quickly. A few thorough backflushes may get your membrane readins down lower. I think it's a safe bet that your prefilters need replacing if they've been in use longer than 6 months.

The key to longevity for me has been religiously backflushing the membrane before and after use for 2 or 3 minutes.

[Chase31]

I woukd say its acceptable, but i would replace them as tds out of my ro is 2 and out of my di is 0

[lastlight]

I guess I should say it's far better than using tapwater considering you're in Calgary =)

[Myka]

- DI (used for reef system) 011 ppm
- RO (used for drinking water) 022 ppm

I have not tested the RO membrane as the system has been used for only 11 months and the product info indicates replacement every 2yrs.

Is 011 ppm acceptable.

Have you calibrated your TDS meter?

You said you didn't test the RO membrane, but you say RO for drinking water is 22 ppm. How is this different? If there is indeed 22 ppm coming out of the RO membrane it needs to be changed. DI resin will take out up to 20 ppm, but will burn out very quickly with this amount going in.

RO membranes generally last a few years, but this depends on the amount of TDS going in. The membrane will last a lot longer if there is only 60 ppm in the tap water versus 800 ppm in the tap water. Have you tested your tap water? What is it at? With the RO membrane already putting out 22 ppm in such a short period of time your tap water must be fairly high which would indicate a need to use dual RO membranes.

What is the flow rate for your RO? Does it happen to be 100 gpd? I ask because the 100 gpd membrane is less efficient than other membranes at only 96% rejection where most other membranes are 98% rejection.

I change the RO membrane when it gets to 12 ppm in the effluent, and I changed DI resin as soon at it gets to 1 ppm in the effluent.

[Blue Ram]

Tap water is measuring 173 ppm. It is a 75 gpd system.

[mike31154]

You need to establish some sort of baseline to find out if your system is in need of maintenance. It's a good idea to do this when it's new, or after the main components such as RO membrane are replaced. As Mindy mentioned, not all membranes provide the same rejection rate. Generally 75 & 50 gpd membranes give a better rejection rate than a 100. The drawback is slower production. Most of us can live with that because a higher rejection rate from the membrane means better RO water & longer lasting DI resin. In any case, 95% or better rejection rate is the goal. I'm getting better than 99% out of my 75gpd.

First, measure your tap water TDS, maybe a couple of times to get an average number. Next, determine what membrane you have & what the rated rejection rate is at the specified water temperature & operating pressure. Now run the system for at least 5 minutes & check the RO output TDS. Using the two values of tap water TDS & RO output TDS you should be able to calculate the rejection rate %. If you have a typical 75 gpd membrane which should give 98% when new & the result you calculate from your two TDS readings doesn't come close to meeting that, the membrane is not performing properly & should probably be replaced. Keep in mind that your water pressure & temperature will have an effect on these numbers, probably slight, but worth noting.

% Rejection = (Feed TDS - Product TDS/Feed TDS) x 100

My tap TDS is 210 to 220, my system has been running since Apr 09 with a throughput of over 12,000 gals to date & all I've needed to do maintenance wise is change the first stage 5 micron poly prefilter. The 75 gpd membrane gets the TDS down to 1 after about 5 minutes of running and 0 after another 10-15 minutes. DI resin is still the same as when I purchased the system. Colour is changing but DI output TDS has been 0 since day 1. Rejection seems to be better with colder water during the winter, sometimes in summer I only get 1 TDS out of the membrane. So in my experience, colder water is actually more efficient at producing pure water, but production is slower. My water pressure into the RODI system is usually 85 to 90 psi. Rejection rate has been in the 99% range for over two years.

So I guess to answer your questions, not easy to do since you don't provide details on membrane specs, tap water (feed water) TDS, temp, pressure. But to me, 11 TDS out of the DI would be unacceptable. To have 22 TDS out of RO with a properly working membrane, your tap TDS would have to be in the 400 range or higher? That would be some nasty tap water.

EDIT: looks like you posted before I finished my saga here! With 173 feed TDS, a 75gpd membrane & 22 TDS from RO, you have issues.

[mike31154]

Tap water is measuring 173 ppm. It is a 75 gpd system.

Going with those numbers (173 TDS feed & 22 TDS RO) your membrane rejection rate is 87%, well below the rating I reckon.