Off taste from RO water

Hi , I've an ro unit and a whole house softener that feeds into it. I've been having ongoing softener problems for a long time which I have a guy working on at the moment. Basically still getting hardness iron and I believe manganese in my water. Anyway there's a slight off taste from my ro filtered water. As a test I boiled some down for about an hour last night and compared the taste of the boiled stuff to some straight from the ro unit. The boiled stuff tasted much better, kinda hard to describe but more pleasant to drink. It's like the straight ro stuff is slightly bitter. There was no residues in the pot after boiling to suggest minerals settling out so I'm wondering what it could be that boiling made it taste better? Someone suggested to me that bacteria can grow through an ro membrane? There is choliforms in my incoming water. Could that be it? The filters and membrane are a few months old and don't get heavy usage.

Thanks

L

Assuminng your RO unit is OK (what does the TDS meter, if you have one, say?) then the only thing that should be in the water that might flavor it to the point where you taste something would be dissolved CO2. The fact that you find the water more pleasant when boiled indicates that it may be the CO2 you are tasting.

Now I do know what you are talking about because I have sometimes had the same impression - i.e. that RO water tastes a little bitter. Sometimes I think this is entirely psychological or is some kind of adaptation of the sort one sees in those experiments when he stares at a yellow circle in a blue rectangle for a time and then looks at a white piece of paper where he sees a blue circle in a yellow rectangle. IOW tap water often tastes a bit sweet (because of the chloride I suppose) and when this is absent the tongue may signal bitter.

Bacteria are much too big to go through the membrane.

Thanks AJ, I don't have a TDS meter but that's interesting about co2. I assume this'll be ok for brewing because it tastes good when boiled? I've had this problem since it was put in several years ago to varying degrees, despite several filter changes. I was wondering could the unit be overwhelmed by poor quality incoming water which still has iron etc in it? Or would this matter?

Thanks

L

CO2 is not the only dissolved gas that can make through the membrane process. Another significant gas is hydrogen sulfide H2S (rotten egg). I wouldn't be too surprised that there is hydrogen sulfide since there is iron in the water. If the RO system does not have a activated carbon post-filter, then H2S could make it through the system into the house RO system. Adding that post filter could clean up the problem.

Can he aerate the water to drive off the gasses? Maybe that can help pinpoint the issue.

mabrungard said:

CO2 is not the only dissolved gas that can make through the membrane process. Another significant gas is hydrogen sulfide H2S (rotten egg). I wouldn't be too surprised that there is hydrogen sulfide since there is iron in the water. If the RO system does not have a activated carbon post-filter, then H2S could make it through the system into the house RO system. Adding that post filter could clean up the problem.

Click to expand...

Thanks for that Mabrungard, mines a standard 5 stage ro, does that mean it hasn't got an activated carbon post filter?
Thx

L

It probably has a carbon pre-filter as most systems do for protection of the membranes against chlorine/chloramine. This should snag the sulfide (if there is any). If the water were aerated as has been suggested, clearwater iron,Fe(II), would be oxidized to Fe2(OH)3, a gel which would be caught by the particulate filter in front of the carbon filter. H2S would be oxidized to colloidal sulfur which would also be caught by the particulate filter.

You should be able to identify the RO membrane cartridge housing(s) in your system. All the other filters have one inlet and one outlet port. The RO cartridge housings have one inlet and 2 outlet (permeate and concentrate) ports. If there is another filter in line after the RO cartridge it is a KDF or GAC (carbon) polish filter.

Does your pre-treatment (i.e. the water that goes into the RO) water smell of rotten eggs? If not then sulfide is not your problem. It's not likely to be your problem anyway as you'll have at least one stage of GAC in the line.

Hi,
This is the replacement filter kit I most recently used:

1 x Membrane 50gpd
1 x polypropolyne 10" 5 micron

1 x Polypropolyne 10" 1 micron

1 x carbon Block Filter

1 x activated carbon filter
At the moment the water entering my ro smells fine but I'm pretty sure that it does smell albeit infrequently, and I think it's a rotten egg type smell

Thx

L

Is the funny (bitter) taste there when the sulfur smell is absent? I suspect it would be and that, therefore, it is not sulfur.

You have two carbon filters, one of which is a polish filter which should remove most anything stinky which the carbon block and 2 stages of particulate filtration didn't.

You said in #1 that you had coliforms in your water. I suppose it's possible the membrane is bio-fouled so you might want to check the owners manual for procedures for removing biofilms or a new membrane cartridge. But I'm really thinking its accomodation - i.e. thinking its bitter because it is not sweet. Try tasting some distilled water and see if you that tastes bitter too.

ajdelange said:

Is the funny (bitter) taste there when the sulfur smell is absent? I suspect it would be and that, therefore, it is not sulfur.

You have two carbon filters, one of which is a polish filter which should remove most anything stinky which the carbon block and 2 stages of particulate filtration didn't.

You said in #1 that you had coliforms in your water. I suppose it's possible the membrane is bio-fouled so you might want to check the owners manual for procedures for removing biofilms or a new membrane cartridge. But I'm really thinking its accomodation - i.e. thinking its bitter because it is not sweet. Try tasting some distilled water and see if you that tastes bitter too.

Click to expand...

Thanks AJ, the taste that I can best describe as a slight bitterness, totally disappeared in any RO water that I boil and allow to cool. I think this adds weight to your assumption that it may be dissolved CO2 or the lack of perceived 'sweetness'. I think I'm just going to brew with it instead of bottled water that I used last time. My first filter, which I think is 5 micron is gone grey so I guess its fouled with hardness or manganese as these are both high in my water. I'm reluctant to start changing filters until I get my softener workign properly. Got the service guy comign around next tuesday because I'm still seeing Iron and hardness coming through in my house...

Thanks

L

Yes, your first filter would be the 5 micron (go after the big particles first) and if fouled with iron hydroxide it could be gray though it is usually a really disgusting brown. Manganese hydroxide tends to be the same ugly brown. How does the 1u filter look?

I cant see the 1u as the first ones in a transparent enclosure(5u) but the others are in white plastic. Can this 'fouling' actually affect the output of the RO unit. I dont have much of an understanding of the science behind them but it appears to me that they are filters based on physically blocking particulates so I dont understand how anything other than RO water could make it out the output? I would have thought a buildup in the 5u or 1u would just result in a reduced throughput?

Thanks

L

Yes, that's the main effect. As a filter fouls the pressure drop across it increases for a given flow. For a fixed source pressure the flow rate will decrease but there will still be more of a pressure drop across it i.e. of the total mains pressure proportionally more will appear across the fouled filter and proportionally less across the RO membrane. Membrane flow is thus decreased. As the pressure across the membrane approaches the osmotic pressure of the water the flow slows to a trickle.

The most common cause of off flavor/odor in RO water is a post-membrane "taste and odor" filter that is past its useful life and needs to be replaced. They are very inexpensive and easy to replace.

If CO2 really is your issue, rather than boiling the water, you ca remove the CO2 @ less cost by simply aerating the RO water.

Russ

Larso said:

I would have thought a buildup in the 5u or 1u would just result in a reduced throughput?

Thanks

L

Click to expand...

As prefilters clog and rob pressure from the RO membrane, the membrane performance (how well it cleans the water, a.k.a. rejection rate) drops. In residential systems, the drop is severe at the lower end of the pressure range (e.g., less than 40 psi).

Russ

I recently had a client that had to toss out several thousand gallons of beer that was too acidic. Their water supply has a significant dissolved CO2 content and that gas passes easily through a RO system into the product water. This brewery had recently made a change that prevented the treated RO water to vent off the CO2 and it stayed in the brewing liquor. The result was a bunch of tart beers. They installed an air stripping tower to pass the treated water through. The tower helps get that excess CO2 out of the treated water. This is a common step for water utilities that use RO treatment.

Something doesn't compute here. If the water came out of the RO system under pressure and were kept under pressure then the CO2 would stay in solution but as soon as it was splashed into the mash tun or piped into the mash tun, stirred and heated the CO2 would escape. I can see how this (stripping) would be important for a utility where the entrained CO2 has no opportunity to escape. The pH would stay low and the possibility of corrosion would be increased. But in a brewery? Must be something involved I don't understand.

The water is stored in a separate unpressurized tank. Treated water is transferred to the HLT for the brew day. They formerly had a vent in the HLT and they capped that vent. It was that point that they started to incur the low pH in their mash.

I'm wondering if your softener bed is fouled with irn and or manganese.

Did you get a water test done?

If you know the feedwater has coliform - get that addressed at the well - you don't want that in the equipment you've mentioned. I didn't read anything in the thread indicating you had equipment in place to disinfect the water.

And yes - bacteria absolutely can foul the membrane.

Russ