RO/DI Where to install 2nd TDS meter? Which filter to change + when?

[nanolutionary]

I've had my ro/di unit running for about two years now, I have been creating roughly 5 gallons per week.

 

I noticed that my tds meter - hooked up to the inlet and outlet - went from 0ppm at the outlet to a reading of 1-2ppm for the first time about a year ago. I took action and replaced the DI resin which brought it back down to 0 again. Then the reading went back up to approximately 2ppm so I decided to switch out all the other stages apart from the membrane stage itself few months ago, my readings went down to zero but are now creeping up and are around 3ppm at the moment, this fluctuates though, sometimes it isn't as high as 3ppm and even goes down to 1ppm or zero after it's been running for a while.

 

I may calibrate the TDS meter I currently have installed before purchasing a new membrane as the sales person at Osmotics where I got my RO/DI from was advising me to do this and against getting a new membrane if the TDS reading was as low as 3ppm.

 

I bought a second TDS meter a while back as well and was wondering where I should install this so that I can differentiate between which stage needs replacing and when, can anyone be a guide on what I should be doing here?

 

 

[WibblyPig]

You can install it between the RO and the DI so you know what you're getting out of your RO. It should always be 0 coming out of the DI

[nanolutionary]

If it's zero coming out of the DI then what would be the point of putting it through the final RO membrane?

[markalot]

It sounds like your RO membrane may have a problem. This is why it's important to meter the TDS coming out of RO to see how much work the DI filter needs to do. If it's 2 or 3 our of RO then the DI filter can handle the load and last for 6 months to a year. If the RO membrane is bad and you are getting 10+ or worse TDS out of RO then the DI filter has to do a lot of work and will exhaust quickly.

 

Because you replaced everything except the RO I believe the RO is the issue.

If it's zero coming out of the DI then what would be the point of putting it through the final RO membrane?

 

Install the meter between the two.

[nanolutionary]

Can I just confirm that I understand the stages correctly.

 

I have an osmotic r four stage RO/DI unit (hyperlinked in original post) the final stage is the RO membrane and the penultimate stage is the Deionisation stage where I pack in the resin?

[AZDesertRat]

The order of filters or stages should be

1. Sediment or particulate filter

2. Extruded carbon block, some systems use two but I prefer a single carbon as long as it is well protected by a low micron, absolute or near absolute rated sediment filter.

3. RO membrane

4. DI filter

 

If your DI is before the RO then it is plumbed incorrectly. Don't be confused by the way canisters are installed on the bracket, it may appear the DI is before the RO but if you look at the plumbing you should see the tap water enters the sediment filter, through the carbon then out the opposite side and up to the RO housing on top then back down through the DI. If this is not the case you need to do a little replumbing.

 

Photos of your actual system would help.

 

As far as inline TDS meters, I own two but do not use them, a good handheld is so much more accurate andcan be used anywhere so you don't need inlines which are not truly temperature compensated so havd built in inaccuracy.

If you have one dual probe meter place the IN probe after the RO membrane and the OUT probe after the DI. If you have two meters place one IN probe on the incoming tap water, the OUT after the RO, and one of the other probes after the final DI stage. The extra probe does nothing for you since they cannot be used portable and the TDS after the sediment and carbon will be the same as the tap TDS since they are coarse filters and do not remove TDS, other than 1-2 mg/L chlorine that is.

[nanolutionary]

I've had a look at the plumbing and you're right it is indeed plumbed as you guys say, the DI is the final stage.

 

I'll try repacking my DI resin and see if that makes any difference, so if my TDS reads anything above 0ppm after the ro membrane I know it's time to change the sediment filter/change the ro membrane if changing the sediment filters doesn't result in 0tds after the ro again?

[AZDesertRat]

No.

The RO only TDS reading should be 96-98% lower than the tap water TDS reading. As an example, the TDS in the USA averages 250 nationwide so if the tap TDS is 250 you would expect to see a RO only TDS of between 5 and 10 and still be within specs for a new Dow Filmtec or GE Osmotics RO membrane. More than 2-4% of the tap TDS and you need to start looking at replacing the membrane.

 

The final RO/DI TDS should be 0 with a good RO membrane and fresh DI resin. As it starts to creep up above 0 it means your DI resin is exhausting or losing its cation and anion electrical properties and starting to release weakly ionized substances back into the treated water.

 

TDS readings tell you absolutely nothing about the sediment and carbon filter conditions, remember they are normally in the 1 to 10 micron ranges, big floaty stuff. TDS or Dissolved Solids are in the 0.0001 micron range, down to the cellular or atom level, so thousands of times smaller than the sediment and carbon are able to filter out so pass right through them and are removed mostly by the RO membrane then polished off by the DI resin.

The sole purpose of the sediment and carbon block filters is to protect the RO membrane where 90-98% of the work is done and in turn the RO membrane protects the DI where the remaining treatment is accomplished. You change the sediment filter and carbon block every 6 months like clockwork. Another method is to monitor headloss through the sediment and carbon filters using a pressure gauge or preferrably two gauges, one before the sediment filter and one after the carbon block. When you start seeing a pressure drop across them its an indicator the filters are plugging or fouling and your membrane is losing efficiency due to lower entrance pressure so time to replace. Along with this you need a low range chlorine test kit to monitor chlorine breakthru from exhausted carbon.

You will probably find it is easier to replace them at 6 month intervals. At the same time it is wise to do a disinfection of the housings.

[nanolutionary]

Thank you for your advice guys, I have just fitted the TDS meters where advised and also repacked the DI resin. My TDS readings are now at zero at the DI outlet and also at the RO outlet!

 

I have also just ordered 3 sets of sediment and carbon filters which should then last me the next year and a half, according to osmotics the ro membrane if properly cared for by replacing the pre filters could last around 5 years.

 

Thanks again for your advice will be replacing the pre filters when they arrive and doing so every six months from now on too.