Acceptable RO/DI TDS readings??

[bkd067]

I was wondering what is the highest TDS reading you are allowing before changing your filters on your RO/DI unit? I know 0 ppm is ideal but is it acceptable to allow it raise to slightly above 0 ppm without any adverse affects?

I have produced approximately 150 gallons with my current filters and the TDS has been at 0 ppm. I just made another 5 gallons and the TDS reading for the current filters is now reading 2 ppm.

Shouldn't I be getting more water production than 150 gallons before the TDS starts rising above 0 ppm? Could I be doing something wrong or is it the quality of the city water that I am filtering? Are there different qualities of filters on the market out there that I should be purchasing.

I purchased my 6 stage RO/DI unit from Filters Direct in Santa Ana and the replacement filters as well.

Any advice or help from experienced reefers would be greatly appreciated.

Thank you-Brian

 

[Toddrtrex]

For me, whenever my TDS gets above 2, I will order/replace my DI resin (( that is the one that gets used up first, though I will double check to make sure my RO membrane is still within its rated rejection rate )).

Was this unit bought new? 150 gallons seems way too little to start to have your TDS creeping up. What is your starting TDS, and what is the TDS right after your RO membrane. I am wondering if your RO membrane is seated all the way in -- 150 gallons would be about right to use up your DI resin if your RO membrane wasn't seated correctly.

 

[bkd067]

Hey Todd thanks for the response. The unit is a Water General RD-102 100gpd purchased from Filters Direct. The last filter change I replaced the qty 1-sediment filter qty 2-carbon filters and the DI resin in both DI resin chambers. I am only on my second set of filters and DI resin. I was told it would be a while before the membrane would need to be replaced.

The inlet water pressure(source) to the unit is operating anywhere between 60-80psig

I tested the tap water inlet to the unit and it was at 380 PPM TDS. I have seen it as high as 600-700 PPM on certain days.

The reading outlet a the membrane before the DI chambers was at 290 PPM TDS. The DI resin appears to be the dark brown/red color it has been. I does not look like it has turned yellow.

The final product is running about 2 PPM TDS

Is this unit acceptable for generating top off water and mixing my own salt solutions or should I purchase a better unit?? Any help/suggestions are greatly appreciated. Thanks guys and gals

 

[CSAreefer]

if i read this right your tap water tds is 380 and its only 290 after the ro before the di? if thats the case something is messed up your ro should be removing at least 90+% of the tds alone and the di picks up the rest that would be why you are creeping up so quickly your di resin is doing ALOT of extra work.

edit: i own this very same unit for over a year now and just put my second set of filters and di in it i change them about every 6 months when my tds gets to about 5-10

 

[Toddrtrex]

Yea, it sure sounds like your RO membrane is either cashed, or not fully seated. Most 100 GPD membranes have a 90% rejection rates. So a TDS of 380 should give you a post RO TDS of 38.

 

[eyesinthedrk]

on my ro unit after i initially open the valve post ro jumps as high as 120 tds, and by the end of the first gallon reduces to 7, the whole time post di is now reading 1 but was 0, so you may need to check post ro water after its been running open for 5 min or so,

because of this, to extend DI life i will install a bypass valve between the ro filter and the di chamber to bleed off all water that comes out of my ro higher than 7

 

[tmz]

Well your input water is pretty high in tds which could account for short life on your filters. The thing about post di tds is that even at 1ppm or 2ppm it may be mostly things like phosphate an silcate which attach more loosely to the resin and are thehfirst to be released when the resin exhausts. I use a tds meter in between my two di cansiters and when the first in line puts out 1ppm ., I change the resin and rotate the canisters in line.

 

[jjstecchino]

TDS after the RO membrane is way to high. It seems the membrane is doing very little. Either is not seated well and tap water is mixing with RO water inside the membrane housing or the membrane itself is shut.
If your unit has a bypassable restrictor (either with a valve or with a bypass circuit) if the membrane is seated well you can try to flush it by bypassing the restrictor.

 

[snorvich]

Zero. Or I change it. Problems can occur when it is not zero and those problems can not easily be identified.

 

[Buckeye Hydro]

This may help:

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 it is 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 â€"œ remember that all resins are not created equal!

Russ @
Buckeye Field Supply