Is it time to change my RO units filters?

[Silver_Is_Money]

My softened well water currently measures 876 ppm TDS, and at the same time my under the sink RO units water is currently reading 45 ppm TDS. My RO filters are roughly 1 year old.

Have I reached filter change time yet?

Would a first ballpark guess that roughly half of the 45 ppm TDS is alkalinity (as CaCO3) likely get me into the alkalinity ballpark for my RO? (as in ~22.5 ppm alkalinity and ~27.5 ppm bicarb)

 

[ajdelange]

That's 100*(876 - 45)/876 = 94.9 % rejection which is pretty good. However, as you have 45 ppm TDS in the permeate you might want to consider replacing the cartridge(s) with new ones being sure you buy 98% rejection ones. That will get you down to 17 ppm TDS. But 45 isn't really that bad.

 

[mabrungard]

Membrane rejection rate is ion-specific. Since the input water is softened, there is probably a significant sodium or potassium content. Those monovalent ions pass through the membrane at higher rates than divalent ions. Its less likely that the product water will attain an overall 98% reduction in TDS.

My tap water has around 600 ppm TDS and that water is also softened prior to RO treatment. I notice that the TDS meter readings vary between about 15 and 25 ppm on any day. A TDS meter is not exactly a precision instrument.

 

[ajdelange]

Membrane rejection rate is ion-specific. Since the input water is softened, there is probably a significant sodium or potassium content. Those monovalent ions pass through the membrane at higher rates than divalent ions. Its less likely that the product water will attain an overall 98% reduction in TDS.

Here are the rejections for some Filmtec (Dow) membranes

SW30HR BW30 XLE NF270
Sodium chloride NaCl
99.7 99.4 98.6 80
Calcium chloride CaCl2
99.8 99.4 98.8 50

 

[Silver_Is_Money]

Is there a generality (ballpark) as to how much of RO water as measured by TDS alone can be presumed to be alkalinity and/or bicarbonate? If I presume ~22.5 ppm alkalinity and ~27.5 ppm bicarbonate, will this 'likely' (in a probability sense) get me to within say roughly 75% of the true figures (for the case where I ship off my RO water to Ward Labs and have these pinpointed)?

 

[ajdelange]

As I am sure you are aware "TDS" is estimated by measuring conductivity of the sample and comparing the conductivity to a table of conductivity vs concentration of something which is often NaCl. I also expect that you are aware that the mobility of ions, and hence the conductivity of solutions that contain them vary quite a bit between different species so that a meter whose scale reads ppm accurately for NaCl solution might not give an accurate result in a solution of Ca(HCO3)2. Recognizing this, at least one company (Myron L) has come up with a calibrating solution made up of 40% NaSO4, 40% NaHCO3 and 20% NaCl and claims that it is representative of natural waters. Thus seems a little strange to me as calcium is quite a bit more mobile than sodium and has an appreciably greater specific conductivity (12 vs 5). But we don't have much to work with here so let's suppose we put 400 mg NaSO4, 400 mg NaHCO3 and 200 mg NaCl into a liter of water. Actual TDS would then be 1000 ppm and the conductivity 1417 uS which, in a meter scaled to NaCl would read TDS 703.6. So we might suppose that if we used a NaCl based meter and it read 703.6 we could multiply that by 1000/703.6 and conclude that our sample was equivalent to 1000 ppm TDS of "442" (which is what the manufacturer calls its calibration solution based on this mix). Or we could multiply by 0.4*1000/703.6 and conclude that our sample contains the equivalent of 400/703.6 ppm NaHCO3 and finally, divide by the molecular weight of NaHCO3 to get (0.4*1000)/(703.6*84) = 0.00676791 as the multiplier to be applied to the TDS reading from a cheapie TDS meter to convert it reading to an estimate of the alkalinity (in mEq/L). Now you have to recognize that if you went out and bought a standard 1417 uS conductivity standard, and dunked your meter into it it would read 703.6 ppm which, multiplied by 0.0067691 would tell you the alkalinity of the standard is 4.76 mEq/L when in fact the alkalinity of this solution is practically 0. The use of the factor does, however, give one a path between TDS reading and an alkalinity estimate in natural waters depending on how well their alkalinity vs conductivity characteristics resemble those of 442.

Another approach would be to grab every water report that is posted on HBD and record the alkalinity and conductivity numbers. You could then fit a line through a scatterplot of alkalinity vs conductivity. You would then take a TDS reading from a NaCl based meter, convert it to us based on a table of conductivity vs NaCl concentration and the multiply the uS by the slope of the alkalinity vs. conductivity fit line.

 

[ajdelange]

Doing the math another way above we see that in the 442 conductivity of 1447 uS corresponds to 400 mg/L bicarbonate and, thus, 400/84 = 4.7619 mEq/L alkalinity. This makes the conversion factor from uS to alkalinity equal to 4.7619/1447 = 0.00329088 mEq/L/uS

The following picture shows the correlation between alkalinity and conductivity in 24 Ward Labs reports. As the graph shows alkalinity and conductivity are correlated but the correlation isn't vert tight (Pearson's r = 0.67) so that conductivity (measured directly or deduced from a TDS meter scaled to a known salt) isn't a very good predictor of alkalinity. OTOH the slope of 0.0056 is not that far off from what the 442 analysis predicted (0.0033).