A question regarding TDS numbers on an RO system

[Geordan]

Hey all,

Haven't been around a lot lately, as I've picked up and moved from my tiny condo in the city to a big country home on the river, leaving me little time for brewing.

However, I haven't stopped thinking about it, so when it came time to have a water treatment system installed for our well water last month (water was incredibly hard with wicked amounts of sulpher), I had an RO unit installed as well. I'd been brewing with RO bought from the store in the old condo, so the convenience of getting it right from the sink at home appeals to me.

My only question is about an "acceptable" level of TDS -- I located a thread stating that below 10ppm is what's "needed" for brewing, but a response stating that 85% rejection or so is typical. In my system, I'm getting between 25-30 on the TDS reading, which represents 87% rejection.

What are the feelings with a number hovering around 30? Is it too high, or should I stop worrying?

 

[ajdelange]

95 - 99% is more typical for an RO system so if you are only getting 87% that should be looked into. The exception would be if this is a large (whole house) system that uses feedback to increase recovery in which case 87% rejection with a TDS of 30 is quite reasonable. Supposing this to come from 6 mg/L each of calcium, magnesium, chloride, sulfate and bicarbonate (it wouldn't exactly as the 'TDS' displayed by the meter is not the actual TDS) I think you'll agree that these are not excessive levels. If, assuming I'm right about the feedback, you wanted water with less mineral content for brewing you could cut the feedback off while you collected the brewing water and turn it back on for normal use.

 

[Geordan]

ajdelange,

Thanks for the response. It's not a whole house system -- at the moment it only services a tap on my counter, and soon the new fridge. I asked the gentleman who installed it today about the TDS this morning (he was here checking a different issue, but it's what jogged my memory to post this) and he said that the 25-30ppm number was "totally normal" and "pretty much as low as it gets."

In your experience, is this a setting I can adjust on my own, or a system limitation? (Will be able to tell better when I've got lights in the basement and am not working with flashlights down there...)

Followup question: you posit that I'm looking at approximately 6ppm of each calcium, magnesisum, chloride, sulfate and bicarbonate -- is this a "safe guess" that all five are coming through at the same amounts, or is it common for an RO to strip far more of one than the other?

 

[ajdelange]

I asked the gentleman who installed it today about the TDS this morning (he was here checking a different issue, but it's what jogged my memory to post this) and he said that the 25-30ppm number was "totally normal" and "pretty much as low as it gets."

It would be normal if the TDS going into the system were 500 - 600 ppm (95% rejection) but if its only 125 - 150 then it's not normal.

In your experience, is this a setting I can adjust on my own, or a system limitation? (Will be able to tell better when I've got lights in the basement and am not working with flashlights down there...)

In a system such as the one you are describing it is not likely. Systems in which recovery/rejection are adjustable are usually larger systems equipped with a pump and operating at pressure of over 100 psig. In these systems the concentrate (waste) line is fitted with a valve. The tighter you close this valve the higher the pressure on the concentrate/feed side and the more permeate flow you get. Both lines are fitted with flow meters. By lowering the pressure (opening the valve) you lower the pressure across the membrane and increase the concentrate flow thus lowering recovery but rejection still stays pretty much the same. In an adjustable system the concentrate line is tapped before the concentrate valve and the side arm of the 'T' is routed back, through a valve and flow meter, to the pump inlet. By opening the feedback valve you reduce concentrate flow but are sending concentrate back to the input so that ion concentrations on the feed side are higher and system rejection goes down.

Followup question: you posit that I'm looking at approximately 6ppm of each calcium, magnesisum, chloride, sulfate and bicarbonate -- is this a "safe guess" that all five are coming through at the same amounts,

No, you can't assume that at all. Rejections vary depending on the ion in question. Some systems come with a sheet that tells you what the rejection is for each ion. Working from memory here sodium is highly rejected (99% ?) whereas bicarbonate is less so (95% ?). The ratio in the output is

feed_Na_to_ bicarb_ratio* (1 - rej_Na)/(1-rej_HCO3) )

Thus with 99% sodium rejection and 95% bicarbonate rejection your output ratio would be 5 times the input ratio and the absolute levels would depend on the concentrations of these ions in the feed.