Reverse Osmosis (RO) Water vs Distilled Water for Film Processing

Hi everyone,

I was wondering if anyone here has experience with reverse osmosis (RO) water filtration systems, particularly the Everpure MRS-600HE-II. Is it worth investing in one when processing high volumes of film daily? How does RO water compare to distilled water?

If so, which parts of the process is RO water typically used for, and how often are the filters replaced in practice? I understand they’re generally changed about every six months, but I’d love to hear how this works in real-world use.

Thank you very much!

Virtually all photochemistry is formulated to work with tap water. The water you draw from the tap in Paris is perfect for this.

The only position you might consider demineralized water for is the final rinse. Whether it's worthwhile to install a RO or other purification system for this depends on the local cost of demineralized water delivered in bulk vs. the CapEx and OpEx of the water treatment system of your choice.

Btw, I'll move this to a more appropriate subforum as it's not specific to color processing.

koraks said:

Virtually all photochemistry is formulated to work with tap water. The water you draw from the tap in Paris is perfect for this.

The only position you might consider demineralized water for is the final rinse. Whether it's worthwhile to install a RO or other purification system for this depends on the local cost of demineralized water delivered in bulk vs. the CapEx and OpEx of the water treatment system of your choice.

Btw, I'll move this to a more appropriate subforum as it's not specific to color processing.

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The simple water jug deionisers are more than sufficient for darkroom use, I have to use one in the house to stop Calcium build up in the kettle as our water supply is very hard.

Ian

I'm on a private well with high iron content and use a salt softening system, I use tap for most processes and distilled for mixng chems to stock solution and final rinse of film, I dilute stock to working with tap water.

koraks said:

Virtually all photochemistry is formulated to work with tap water. The water you draw from the tap in Paris is perfect for this.

The only position you might consider demineralized water for is the final rinse. Whether it's worthwhile to install a RO or other purification system for this depends on the local cost of demineralized water delivered in bulk vs. the CapEx and OpEx of the water treatment system of your choice.

Btw, I'll move this to a more appropriate subforum as it's not specific to color processing.

Click to expand...

Okay, thank you!

And I appreciate you moving the post to a more suitable subforum!

Ian Grant said:

The simple water jug deionisers are more than sufficient for darkroom use, I have to use one in the house to stop Calcium build up in the kettle as our water supply is very hard.

Ian

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Thanks, that makes sense! It’s good to know that even simple deionizers can handle hard water for darkroom use. I’m mainly curious about what purified water actually does in practice - like how much of a difference it makes for the final rinse, and how often it needs to be replaced in a busy workflow.

Rick A said:

I'm on a private well with high iron content and use a salt softening system, I use tap for most processes and distilled for mixng chems to stock solution and final rinse of film, I dilute stock to working with tap water.

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Do you use D&D machine for processing as well?

vicb said:

I’m mainly curious about what purified water actually does in practice

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Not a whole lot, really.
The main causes for problems in untreated tap water are:
1 Calcium salts that leave marks on dried film
2 Iron salts that negatively affect developer lifetime
3 Potentially chloride ions that affect developer activity

No 3 is generally not really a concern on most municipal water systems as the chloride content is too low to have a significant impact.
No 2 is generally not really a concern because the photochemistry and replenishment rates are formulated to account for the presence of some iron.
No 1 is in practice the most problematic one, and IMO the only sensible rationale to use demineralized or deionized water in your application, even though there are strategies for managing this issue even if tap water is used; e.g. a proper finishing regime and use of a final rinse formulated to work well for the purpose. In your other thread, you're in the process of addressing this issue, and I think that also emphasizes that deionized water is not a panacea for such problems to begin with.

Of course, if you have a cheap and easily available source (internally or externally) for demineralized/deionized water, nothing bars you from using it also to mix other chemistry and not just for a final rinse. That's up to you to decide.

That's my take on it at least; I'll happily stand corrected on any inaccuracies or omissions on my behalf.

I use distilled water to mix all chemistry that will be stored for later use and as a final rinse of my film. RO water is used for all general processing and tap water for washing both film and prints. No specific reasons for RO vs tap water, other than the RO water seems "cleaner" to me.

I use reverse osmosis water from my kitchen tap for basically all my photochemistry. Our water has pretty high TDS and when I'm mixing stuff from scratch I can omit the sequestering agents (which i tend to not have anyway)

I also do my final rinse with just reverse osmosis water and that seems to be sufficient to avoid water spotting.

It depends on your tap water quality, specially the hardeness. There are zones with very hard water (typical in Spanish Mediterranean coast) and the photo chemistry is your least concern. It is unpleasant to drink and causes problems in taps and shower heads. In those cases RO, deionized and/or bottled water is the only way to live.

Commercial developers try to address unsuitable traces in your water with additives (chelatin agents mostly). Besides that, there are the stains after the final rinse. If you don't see any, you are good to go with whatever water you are using.

I am fortunate enough to have a good quality tap water that I use almost for everything, including color final rinses. I only use (cheap) deionized water for the BW final rinse, the product requires to not squeeze at all after the bath (Rollei RWA SC).

By the way, Kodak published some guidelines of water quality in the famous Z-131 document. The published analysis of my tap water complies with it.

Something else worth adding is that wetting agent concentration is specified for use with in-spec tap water (as per the Kodak documentation posted above by @halfaman ), not distilled water - which is something I've run up against in the past.

Outside of emulsion making and certain scratch-mixing (if you don't have certain sequestrants, and if those sequestrants aren't essential to the process - e.g. some B&W reversal or ascorbate developers) of process/ toning chemistry, distilled water should not be needed apart from for things like (possibly) print spotting etc.

vicb said:

Do you use D&D machine for processing as well?

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What's D&D machine? I rotary process my LF film in Unicolor drums on a self-reversing motor base, and small single reel SS tanks for roll film.

Rick A said:

What's D&D machine?

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Dip & dunk. It might be relevant to keep in mind that @vicb is running a commercial lab processing up to hundreds of rolls of film per day. Their requirements may be a little different from home users.

koraks said:

Not a whole lot, really.
The main causes for problems in untreated tap water are:
1 Calcium salts that leave marks on dried film
2 Iron salts that negatively affect developer lifetime
3 Potentially chloride ions that affect developer activity

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I would add dissolved solids/particulates to this list.

I have to use RO water for everything here on the farm because the water is unusable as it comes out of the well: so much iron that it gradually ruins everything it touches.
As others have said, when making chemistry for film processing, it depends on your water quality whether buy can use tap water or not.