RODI and TDS question

[plc001]

All,

I have a 5 stage RODI system that the TDS before the DI is coming at 5-6ppm and after the DI is 0ppm.

I wonder how often does your DI is change? I have been with the same resin for a while.

I plan to change the DI once the TDS gets to be more than 1ppm, but it seems that it should have gone up but it has not.

I tested with a different meter and my numbers seem to be the same.

Should I be worried?

[dannable]

Depends how high the TDS is going into the unit. My tap water is 250+ tds and the filter guys said it should last for a few hundred gallons. I guess it averages about 6 months for most people. As long as your tds is 0 you should be ok IMO.

[a2fire2i]

How long have you had the resin? I have had mine about 7 months now, and the DI water is starting to read 1. The RO water is around 8. I wouldnt worry if you are getting the same test results with another test..

[plc001]

Hmm,

For some reason I was thinking that I had to replace the DI more often, but that could have been with my old RODI system. I used to change the DI every 4-6 weeks. But with the new system I have only change it once in 6 months. I only use about 25 gal per week.

Thanks!

[a2fire2i]

Yeah, you will only need the replace the DI when the readings of the water start to get above 1. The lower the TDS before it hits the DI, the longer it will last..

[BillyFSU]

Personally I'd change it at soon as it stops reading zero. The DI is close to exhausted at that point, and the inline tds meters arent 100% accurate so as soon as it starts flicking between zero and one its time for a change.

[mflamb]

Mine has been reading 4 into the DI and 0 out of the DI since I hooked it up. I've made almost 800 gallons so far. I'm not sure how long the chloramine filters last.

[rogergolf66]

yes your resin will last a long time if your RO is taking your DST to 5-6. that is normal if your RO was only taking the water to 25DST then the resin would need to be changed often

[plc001]

Thanks all.

[BuckeyeFS]

A good rule of thumb is to replace your sediment filter and carbon block after six months. A more precise way to maximize the useable life of these two filters is to use a pressure gauge to identify when pressure reaching the membrane starts to decline. This is your indication one or both of the filters is beginning to clog.

Also be cognizant of the chlorine capacity of the carbon block. The Matrikx+1 (“Chlorine Guzzler”) for example will remove 99% of chlorine from 20,000 gallons of tap water presented at 1 gpm. Original equipment suppliers commonly provide carbon cartridges rated at 2,000 to 6,000 gallons.

Regarding your RO membrane and DI resin, use your TDS meter to measure, record, and track the tds (expressed in parts per million) in three places:
1. Tap water
2. After the RO but before the DI
3. After the DI.

The TDS in your tap water will likely range from about 50 ppm to upwards of 1000 parts per million (ppm). Common readings are 100 to 400 ppm. So for sake of discussion, let's say your tap water reads 400 ppm. That means that for every million parts of water, you have 400 parts of dissolved solids. How do we go about getting that TDS reading down to somewhere near zero?

If you do some experimenting with your TDS meter, you'll note that your sediment filter and carbon block filter (collectively called prefilters) do very little to remove dissolved solids. So with your tap water at 400 ppm, you can measure the water at the “in” port on your RO housing and you'll see its still approximately 400 ppm.

The RO membrane is really the workhorse of the system. It removes most of the TDS, some membranes to a greater extent than others. For instance, 100 gpd Filmtec membranes have a rejection rate of 90% (i.e., they reject 90% of the dissolved solids in feed water). So the purified water coming from your 100 gpd membrane would be about 40 ppm (a 90% reduction). Filmtec 75 gpd (and below) membranes produce less purified water (aka “permeate”), but have a higher rejection rate (96 to 98%). The life span of a RO membrane is dependant upon how much water you run through it, and how dirty the water is. Membranes can function well for a year, two years, or more. To test the membrane, measure the total dissolved solids (TDS) in the water coming in to the membrane, and in the purified water (permeate) produced by the membrane. Compare that to the membrane’s advertised rejection rate, and to the same reading you recorded when the membrane was new. Membranes also commonly produce less water as their function declines.

After the RO membrane, water will flow to your DI housing. DI resin in good condition will reduce the 40 ppm water down to 0 or 1 ppm. When the DI output starts creeping up from 0 or 1 ppm to 3 ppm, 5 ppm, and higher, you know that your resin needs to be replaced. Sometimes people complain that their DI resin didn't last very long. Often the culprit is a malfunctioning RO membrane sending the DI resin “dirty” water. This will exhaust the resin quicker then would otherwise have been the case. Sometimes the problem is poor quality resin – remember that all resins are not created equal!

Russ @ BFS

[plc001]

Thank you Russ for the explenation. Very helpfull, copied, pasted, printed and saved;-)