Dual inline TDS locations?

[sandalscout]

Hey everyone, I know a few of you are running these on your RODI units, and I believe it's standard practice to place them after the RO and after the DI to show how effective each is, but I'm curious if that's where everyone runs their meters at. Seems to make the most sense. If you are running it that way, how do you know when to replace the carbon and sediment filters? How do you know when high readings after the RO is actually the result of the RO or before that (although, my mind is telling me that if the RO is shot, it's likely because the prefilters are letting too much through).

So, yeah, basically, how do you know when the carbon and sediment filters are bad if you are not measuring the TDS after them?

 

[andywe]

You could monitor the prefilters, but they really have little to do with dissolved solids as they do large suspended particles and chlorine. The main purpose of the carbon is to filter chlorine, chloramine, and other like chemicals. The membrane filters solids dissolved in the water like iron, and the DI takes care of phosphates and other remaining minute traces ionically.

You can add a tds to before the membrane and before the prefilter, but I personally don;t see much use for that. Typically you'll see your prefilter turning yellowish/brown, and carbon I change every 6 months unless I am making lots of water and then will do it every 3 months. The BRS refillable RO inserts are great for this even though carbon filters and pre filters are inexpensive. Just my 2 cents.

 

[jdieck]

If I only had one dual TDS meter I would measure after the RO and after the DI.
This way you can determine when to change the DI resin and also if the membrane is deteriorating by tracking if the RO is increasing.
Only if noticing increase in RO TDS I would provisionally measure the inlet to the membrane and the outlet from the membrane to calculate the rejection rate and determine if the membrane needs replacement (3 to 5 years) or if the increase was due to increased TDS at the inlet.
You can install a T with a plug for this purpose between the carbon and the membrane.
You can also use this T to sample the water comming out from the carbon filter and test it using common chlorine test tablet kit (From pool store). As soon as you detect any chlorine, it is time to replace the carbon filter.
If you inctall a preassure gauge at the inlet line to the RO/DI system before the sediment and compare the inlet pressure to the pressure inside the membrane (You shall habe a pressure gauge at the membrane inlet) the difference in pressures (if more than 15 psi) can tell you if you need to replace the sediment filter.

 

[sandalscout]

Andy, yeah, that makes alot more sense, I wasn't thinking about that correctly at all. How frequently do you have to change your sediment block? Mines is still bright white after a year, but maybe that's normal? Since they are so cheap, I think I'll probably just start changing my carbon and sediment filters every 6 months. Can't hurt.

jdieck, thanks, that definitely helps, and throwing an extra T in there is cheap enough, I'll likely go ahead and do that to allow me to measure anyway, especially since I already have a handheld TDS meter.

Thanks!
Matt

 

[andywe]

I do have a pressure gauge and although it is a poor indicator, it can indicate your pre filters clogging if pressure starts dropping off. I just change at 6 month intervals and call it a day. The reality is my water is not that bad for suspended solids. It's less of a worry.

 

[sandalscout]

I too have a pressure gauge, and I've noticed no drop at all. The prefilters are clean looking, and my tap water is only in the 140ppm range anyway (surprised me BIG time, with the limestone around here I really expected much higher) so perhaps I'm not stressing my RODI system too much.

 

[jdieck]

<a href=showthread.php?s=&postid=14758821#post14758821 target=_blank>Originally posted</a> by sandalscout
I too have a pressure gauge, and I've noticed no drop at all. The prefilters are clean looking, and my tap water is only in the 140ppm range anyway (surprised me BIG time, with the limestone around here I really expected much higher) so perhaps I'm not stressing my RODI system too much.

May be you need to switch to a lower rating sediment filter that will trap smaller particles. A 0.5 micron absolute rating might be the one for your source water that has little sediments.

 

[DMBillies]

<a href=showthread.php?s=&postid=14757322#post14757322 target=_blank>Originally posted</a> by sandalscout
Mines is still bright white after a year, but maybe that's normal?

Does your water come in bottles labeled "Fiji"?

At moderate use (which for me is probably about 20-30 gallons per week of filtered water), I feel like I'm changing out the white pre-filter about once every 3-4 months. I wait to change the carbons until I see a 1ppm uptick in my RO output. Usually by that point they are just starting to have a slightly discolored look. They are cheap, so I see no need to push them any longer.

I didn't change my RO for over two years (even with a system volume of almost 300 for the majority of that time). I just noticed that my RO was putting out 1-2 TDS even with a full pre-filter change, so I thought it was about time... plus I kind of figured I was starting to press my luck. I have iron pipes that knock some rust loose now, so the water is probably dirtier, but I'm using significantly less and I haven't noticed a change in how the filter is running in terms of my change out schedule.

 

[jdieck]

Brian:
A couple of clarification notes here:

a) The TDS out of the membrane does not really reflect the state of the carbon filters as they can not affect TDS. Carbon is there to remove volatile organic compounds and specially chlorine which can seriously damage the RO membrane so you can not rally know if the carbon has to be changed with a TDS meter. The only way to know is to monitor for chlorine after the carbon filter(s).

b) If you want to measure the deteriorating status of the membrane, you need to measure the rejection rate which is the amount of dissolved solids being rejected. It is calculated by the difference in TDS entering the membrane and leaving the membrane as a percent of the total TDS entering the membrane.
Rejection % = (TDSin - TDSout)/TDSin x 100
If you measure the rejection of the new membrane when new (Usually between 98% and 99% you can measure the deterioration as this number drops. If below 95%you may decide to replace the membrane. The lower the rejection the faster the DI filters will get used up.
RO TDS of 1 or 2 ppm out of the membrane before the DI filters is still a very good output from the membrane even if your inlet TDS is as low as 100 ppm. A membrane usually lasts between 3 and 5 years and in cases of relatively low TDS input water good carbon filter monitoring and back flushing after each use I have seen some last up to 7 years.

c) A pressure gauge serves not only to measure the drop in pressure due to plugging of the sediment filter but also will serve for adjusting the system. The lower the pressure input to the membrane the lower the production rate and the higher the water waste.

d) Rust from pipes can be in the form of solids which will be retained by the sediment filter or as iron ions in the water which are very effectively removed by the membrane so I would not worry about the state of the pipes.

e) DI cartridges, if you are using them can be replaced when the TDS in the output rise. Usually you should not measure any TDS out of the DI filters until they need to be replaced.

In general if you track these trends and use these methods you can insure your system performance is OK while at the same time getting the most out of your filters.

 

[sandalscout]

<a href=showthread.php?s=&postid=14776868#post14776868 target=_blank>Originally posted</a> by jdieck
May be you need to switch to a lower rating sediment filter that will trap smaller particles. A 0.5 micron absolute rating might be the one for your source water that has little sediments.

Good point. I'm currently using a 1 micron sediment, and BFS sells .25 and .35 absolutes, just stinks because they are 4 and 8 times as expensive as the 1 micron filters.

<a href=showthread.php?s=&postid=14783225#post14783225 target=_blank>Originally posted</a> by DMBillies
Does your water come in bottles labeled "Fiji"?

HAHAHA, yeah, that's it! I went down and looked at it all a bit more, and it's not quite as bright as I thought, so I might be mistaken. I'll have new filters and resin tomorrow, so it's all getting replaced, and I ordered 2 sets of filters, hope it'll last me a year.

 

[sandalscout]

Thanks again for all of your input. I got my new filters, no DI resin and the dual TDS meter tonight. I replaced the filters and resin, hooked up the TDS meter after the RO and after the DI, move my extra T between the filters and the RO, and fired it all back up. I'm getting 2PPM after the RO and 0 after the DI. sweet!

 

[jdieck]

Good! I would recommend calculating the rejection rate and the pressure now so you can compare them with future readings.