TDS Meter Readings

[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.