RODI tds limit
- Thread starter deuce
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Most TDS meters do not read in decimals so what you have is 2 TDS. What appears to be a decimal point is really a power on indicator in the LCD screen.
Since you have no idea what the TDS is made up of your limit should be anything over 0 TDS in the finished water. That 2 TDS could be phosphates, silicates, nitrates or even copper so it is best to replace the DI cartridge or resin as soon as you start seeing anything other than 0 TDS after a minute or two or use. You may see a quick TDS spike when you first start making water from what is called "TDS Creep", which is normal but it should drop back to 0 quickly once the creep has passed.
You also need to know your tap water TDS and RO only TDS before the DI so you can monitor your rejection rate or removal efficiency of the RO membrane itself which is the workhorse of the system and does 909-98% of the work, the DI is just a final polishing. If the membrane is not working properly you wil go through DI resin quickly and drive your cost of ownership up.
Both of my step kids were born in Bucyrus and we still have family around that area including Mansfield, Ashland, Shelby and Bailey Lakes! My wife grew up in Tiro.
If you didn't already know it one of the best RO/DI vendors around is there in Ohio at www.buckeyefieldsupply.com , Russ is a great guy and very knowledgable when it comes to RO/DI and water treatment.
Since you have no idea what the TDS is made up of your limit should be anything over 0 TDS in the finished water. That 2 TDS could be phosphates, silicates, nitrates or even copper so it is best to replace the DI cartridge or resin as soon as you start seeing anything other than 0 TDS after a minute or two or use. You may see a quick TDS spike when you first start making water from what is called "TDS Creep", which is normal but it should drop back to 0 quickly once the creep has passed.
You also need to know your tap water TDS and RO only TDS before the DI so you can monitor your rejection rate or removal efficiency of the RO membrane itself which is the workhorse of the system and does 909-98% of the work, the DI is just a final polishing. If the membrane is not working properly you wil go through DI resin quickly and drive your cost of ownership up.
Both of my step kids were born in Bucyrus and we still have family around that area including Mansfield, Ashland, Shelby and Bailey Lakes! My wife grew up in Tiro.
If you didn't already know it one of the best RO/DI vendors around is there in Ohio at www.buckeyefieldsupply.com , Russ is a great guy and very knowledgable when it comes to RO/DI and water treatment.
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Opus
2500 Club Member
Depends on which filters you are talking about. If you mean the carbon and sediment filters then the rule of thumb seems to be 6 months. I personally usually go 9 to 12 months because I don't make a lot of water. They don't do anything really to the TDS so you can't base replacement of them on your TDS reading. The RO membrane is where most of the TDS is removed and the remaining small amount is handled by the DI if you are using it. Your RO membrane should last a few years. DI is dependent on how much TDS is left after your RO membrane has done its job. My RO membrane is a couple of years old and when I measured last week I was getting a reading of 16 with my tap water close to 300. If I remember correctly my membrane was advertised as 95% rejection rate so it is still doing its job as advertised.
16 is pretty high with a tap TDS of 300. Thats 94.6% rejection rate. A good Dow Filmtec membrane should be at 96-98% if working properly. This is important since for every 2% you drop you cut your DI resin life in half and the cost of operation rises, it does not take many DI replacements to pay for a new membrane. My tap TDS is 630 and my RO only TDS is between 2 and 3 so my rejection rate has been averaging 99.4% and the membrane and system are over 5 years old.
A Opus pointed out, the sediment and carbon block filters have little to absolutely noting to do with TDS, they are there to protect the membrane from suspended solids, big stuff, and chlorine so it in turn can protect the DI resin. The three TDS readings i mentioned above are what you need to monitor membrane and DI condition and the 6 month rule is the easiest way to remember and keep up with sediment and carbon changes. yo ucan stretch the 6 months but it is advised to have a pressure gauge to monitor headloss or filter plugging and a low range chlorine test kit to monitor for chlorine breakthru. Its usually easier to stick to the 6 months for most people. This is important since membranes function better at higher pressures, when the filters get dirty and pressure drops the rejection rate goes down dramatically and again the DI replacement cost goes up.
A Opus pointed out, the sediment and carbon block filters have little to absolutely noting to do with TDS, they are there to protect the membrane from suspended solids, big stuff, and chlorine so it in turn can protect the DI resin. The three TDS readings i mentioned above are what you need to monitor membrane and DI condition and the 6 month rule is the easiest way to remember and keep up with sediment and carbon changes. yo ucan stretch the 6 months but it is advised to have a pressure gauge to monitor headloss or filter plugging and a low range chlorine test kit to monitor for chlorine breakthru. Its usually easier to stick to the 6 months for most people. This is important since membranes function better at higher pressures, when the filters get dirty and pressure drops the rejection rate goes down dramatically and again the DI replacement cost goes up.
Opus
2500 Club Member
Compared to the advertised rejection rate, it is where it should be. I just replaced my DI this past week and it had been probably 9 or more months since it was last replaced. Though, like I said I don't make a ton of water so my stuff probably last longer than others. Of course this will probably change over the summer months. We are at stage 3 water restrictions already and as the lake gets lower the TDS rises. Last time this happened the tap water got up to close to 500 TDS.
When I had a AWI Typhoon III RO/DI I could never get my TDS lower than 13-17 from the RO only, that was down from the 630 to 850 tap water TDS and DI resin lasted me 150 measured gallons per fresh 20 oz refill no matter what I tried. Having high TDS and hard water really sucks. If you have access to softened water, all membrane manufacturers recommend using it to feed the RO, the membrane lasts much longer and works better that way.
