Reverse Osmosis Water. Why so aggressive and what to do about it? 1

[Swedishrigpig]

Gents

Our Ocean going vessel uses Reverse Osmosis to make drinking water out of Sea Water. Great machine, much less maintenance than the old Evaporator Distillers. However, although the water is very clean, it gets very aggressive and corrodes Steel Pipes, Aluminium coolers, Coffee machines, taps, valve seats, Expensive Variable Frequency Drives (fresh water cooled) even in systems like engine cooling water with corrosion inhibitor mixed into it, it eats up a aluminium radiator in a few months.
I have done some research, and it seems to be due to "lack of minerals or Ions" it will absorb that from wherever it can get it, pipes coolers etc.

The pH is 7.5.
Very low salts or chlorides.

1. How do I stop it?
2. Do I need to replace our entire Carbon Steel pipe system with Stainless Steel or Plastic?
3. Can a mineralizer make it "neutral" or mild again?
4. Can I save our Carbon Steel pipe work, even though it is corroded inside?
5. The water that comes out is always yellow/ brownish and looks terrible.

Regards,

RP

 

[bimr]

The water is oxygenated, has a low pH, and lacks minerals. The low pH is caused by dissolved carbon dioxide, the same chemical that provides the fizz in coca cola.

You best option is to use a neutralizing filter, which will add minerals and raise the pH. You can also install a chemical feeder but that may be difficult if you have a small system.

There is not much that you can do with the piping. Plastic pipe is also acceptable for potable water use.

The yellow/brownish color is probably iron from the piping and is an aesthetic issue.

 

[Swedishrigpig]

Thank you Bimr

We do use a mineralizing filter that uses 50/50 Calcite and Corosex.
The pH is higher than I first mentioned between 8-8.5.
We inject a corrosion inhibitor after the mineralizer, but it still not very friendly.

We have installed a carbon filter to clean up the water, but as you mentioned, this only cures the symptoms, not the root cause.

 

[bimr]

If your pH is already 8, then calcite will probably not work. Is there a process modification that will provide a lower pH out of the RO?

If the calcite will not work, then you will have to feed a food grade quality lime or soda ash. Here is an example.

http://www.nsf.org/certified/PwsChe...ction=&PlantState=&PlantCountry=&PlantRegion=

Mix the chemical into water and then feed it into the RO storage tank.

 

[zdas04]

Or you could stop shooting in the dark and get a detailed water analysis. With that pH, adding a base is probably not taking you in a good direction, but there is something in the water that is reacting. I'd find out what and focus my efforts on fixing that.

By the way, in spite of what chemical salesmen tell you "corrosion inhibitors" are not magical. They are specific chemicals intended to do a specific thing that has been shown to have a beneficial effect on certain modalities of corrosion in certain conditions. That is a lot of qualifiers. I've seen many situations where they had zero impact on the rate of corrosion occurances. I've seen other situations where they seemed to make the problem worse.

David

 

[bimr]

I think you are on the right track. I have to assume that the pH of 8-8.5 was taken after the neutralizing filter.

Corosex is probably too alkaline and aggressive for your application. You probably are better off using calcite alone. You might try the soda ash as well. Soda ash will add hardness without spiking the pH as much as corosex does.

The solution is adding some water hardness so that your water has slight scaling tendencies as indicated by the langelier index.

 

[dfmorvan]

Hi Swedishrigpig

Can you be a bit more specific with your analysis please?
As explained by birm, RO water is aggressive because the membrane removes the salts (including bicarbonates, HCO3- which acts as a buffer regarding Carbonic acid), but the gases (oxygen and carbonic acid goes thru it. Hence you get water with low mineral content and CO2. At the outlet of the RO you should get a pH of 6 to 6.5.
I do not understand why you still have corrosion with a pH of 8 after the neutralizing filter. What is the size of your system ?
Are you sure for the chloride level ?
Do you chlorinate your water to disinfect it ? At which level ?
What type of corrosion inhibitor do you use ?

regards
DFM

 

[Swedishrigpig]

Gents

I've come to the conclusion that the water is within the parameter and that we just have to live with the fact that RO water is aggressive. This seems to be the problem on many ships and offshore rigs.
If that is true, we need to replace our carbon steel pipe and install something else. I was thinking Stainless Steel, but what type?
FRP is too awkward to work withstand needs special fittings made. I prefer some metallic pipe that can be shaped to fit.
Is CuNiFer still used (for Potable water)?
Any other alloys suitable?

 

[zdas04]

Sweishrigpig,
Have you considered a spoolable composite product? They are about as chemically inert as they can get (HDPE liners), come in a spool so you spool out what you need and let it wind around pipes and other obstructions, it has outer layers of other stuff (Kevlar, fiberglass, steel, depends on the manufacturer) to provide good mechanical and hydraulic strength. This stuff is replacing steel fairly quickly. Price is in the neighborhood of steel (sometimes higher sometimes lower, depends on this week's steel demand), but it goes in a LOT cheaper (no welders).

Some of the brands are FlexPipe, FlexSteel, Peformance Pipe, and Fiberspar. They're all pretty good and end connections are installed mechanically (there are a couple of brands on the market that you have to build your end connections like you would build a boat, good to avoid).

David

 

[bimr]

You can use 304SS or 316SS pipe. A FRP pipe rated for this service would also work well.

 

[darai]

I'd have a talk with the designer/manufacturer of the RO system.

 

[QualityTime]

You might want to TRY sending the water to a holding tank that is full of say 1/4" limestone gravel. The tank will have to be sized to give you adequate residence time. This will add alkalinity to the water without adding turbidity to the water. You may need to add some chlorine to the water to keep any possible growth from occuring in the gravel. Make sure there is no light of any kind shining on the holding tank.

 

[bimr]

The original poster has already said that he had a neutralizing filter:

"We do use a mineralizing filter that uses 50/50 Calcite and Corosex. The pH is higher than I first mentioned between 8-8.5. We inject a corrosion inhibitor after the mineralizer, but it still not very friendly."

A neutralizing filter will only react with carbon dixoide that is present and will only raise the pH to about 7.2 to 7.3 pH units.

 

[QualityTime]

As one poster advised...yoou should get a water analysis done so that you have numbers to work with

The key is to get more alkalinity into the water without raising the pH beyond 8.5.

Municipal water treatment plants add lime to raise the pH to 10.5 to drop out hardness and then add CO2 to drop the pH back down to 8.5 before sending it out to the consumer. So let's try the reverse logic

Limestone powder is typically used to neutralize sulphuric acid. I wonder if neutralized sulphuric acid would have a high enough hardness that it can be added to your water without any calcium carbonate dropping out of solution. Hire a chemical engineering professor to figure it out. You could make a fortune out if there is a practical solution.

 

[stanier]

Common material for lower pressure piping applications is ABS. Veolia make permanent & portable RO plants and use it great quantities. Memcor (Seimens) use ABS. Gold Coast and Sydney Desaliantion plants (250 & 300ML/day) are full of the stuff.

15 to 850mm diameter. You can also get pre insulated version for chilled water.

ABS has advantages over GRP in cost, installation, vacuum rating and consistency between manufacturers. GRP is a basket case when it comes to procuring this pot pouri of glass, resin, sand fillers, tissues, roving angle, low strain tolerance, low impact resistance and other manufacturing vagaries.

http://www.tycowater.com.au/water/products/plastic_pipeline_systems/plastic_products#ABSDuraFree