It sounds awful, doens't it ? I'm sure the tap water at Tony's place is just fine to drink though. I think if you ask some people who are out of town and are on a well, you'll hear about numbers higher than that.

Is it possible that he is taking a reading in the wrong place, like somehwere in the waste line? Just can't wrap my mind around that high of a number coming out of a reasonably serviceable RO membrane.

It's the TDS creep, it's legit. It is only that high for a few seconds and it is because the membrane is idle for a period of time. It's not the source water.

Made worse in my case because I double up 2 75gpd membranes to get 150, so I have to read one value that is the sum of both inline.

The TDS creep is why you should flush the membranes. If you don't then they won't last as long as otherwise.

Well in that case it makes sense. Had no idea you had two membranes in series.

I've heard that, too, but not so sure if that is true. I say that because I run an RO/DI system, and haven't changed out my DI media in over 2 years, and it is still good. Not only do I feed my RO/DI directly to my sump with my own DIY ATO dual float fail/safe system, but I also have 2 pressure storage tanks before the DI (but after the RO) in my system. So I always have about 10g or so of ready RO water, which quickly filters through the DI on demand. We use the RO side for house water in the kitchen too, and those storage tanks always ensure a ready supply for that, too.

My RO membrane is also more than 4 years old, and still working good. I have dual TDS gauges, one after RO (but before DI) and the other measure after RO/DI. However, I have a flush bypass valve (and DI bypass, too), and whenever my TDS from the RO gets over about 2 or 3, I do a flush (about every couple months). After DI, the water tds is still always 0.

Our TDS is pretty low compared to Alberta's. If you're like me in Coquitlam. I get about 5-10 TDS right out of the tap.

Our RO membrane will filter most out and I even have zero going into my DI resin at the last stage. This is after 2 years and about 400-500 gallons.

I do flush often and I also change my carbon and per filter every 5 months or so. I still haven't changed my DI.

We're just lucky.

Yep, most of you folks on the coast could probably forget about the RO membrane & run your water straight through DI after a couple of pre filters + carbon, your tap TDS is so low to begin with. All you might need is something to slow down the flow through your DI so it has time to do its thing. Still have to do the math though, cost of DI & how long it lasts without RO stage etc. This way you would not have any waste water as with the RO plumbed in there.

While my tds after DI is always 0, the tds after RO (before DI) will climb to 10 or more if I don't open the bypass valve and flush the membrane. Like I said, I usually do it when I see it climb to a few ppm, and then it is back down to 0 after the flush. Normally, my gauges show 0:0 or 1:0.

The water out of my tap in Langley is about 50 tds normally. However, it must be higher at times (although haven't caught it in a test), because my sediment and carbon filters still need to be changed about every 4 months or so. I can tell when they need changing because my flow slows down, and it takes longer to fill the pressure storage tanks. And I am always amazed at the red/brown sediment in the pre-filters, despite the seemingly clear tap water.

I still don't think fast flow through the DI is a problem though, as I have that with my RO pressure storage tanks. I do agree though, you should have an RO membrane.

With TDS 50 at source, DI only would probably not be economical. I've heard some of you on the west coast have TDS 10 or lower though, so it might be an option for those lucky few.

Just checked my tap water and it is 64 at the moment. Have never seen it at 10 in Langley. I understand they also supplement here with well water (not sure why as we are tied in with the Metro Vancouver water supply).

Back to the OP question, here is some info on DI replacement from the manufacturer of my RO/DI unit.
http://www.airwaterice.com/category/z.2/

I have this unit, but with the 75 gpd membrane. I bought it used and still going strong.
http://www.airwaterice.com/product/1...TDS-Meter.html