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

London (very hard water) user here - I use RO for everything (mostly for coffee). I have built my own system. Works nice for photographic chemistry, especially final rinse doesn't leave any marks at all. I use tap water for washing.
If you want RO for this and also maybe for drinking look at Ecosoft products - they offer much better value than Everpure. I do install a lot of their RObust PRO/Coffee systems in various places (professional coffee machines engineer/technician). Uses non-proprietary filters and membranes.

I have a whole house water softener in addition to the ro systems. Soft water is required for American dish washers as no built in softener, also for laundry and bathing. The softened water has so much sodium carbonate it's useless for m8xing certain developers. I use hard water for all outdoor sill cocks.

Troop and Anchell seem pretty insistent that distilled/filtered water is necessary to prevent early failure in developers like XTOL. Is that just speculation on their part?

farpointer said:

Troop and Anchell seem pretty insistent that distilled/filtered water is necessary to prevent early failure in developers like XTOL. Is that just speculation on their part?

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This relates to the presence of esp. iron ions in the water that will trigger a fenton reaction that progressively destroys the ascorbate in the developer. This is well documented. For factory-made XTOL, this is likely not really an issue (as long as the manufacturer's storage lifetimes are observed) due to the inclusion of effective chelating agents that bind the iron so it becomes harmless. For DIY derivatives of XTOL that leave out these chelating agents, this can and indeed will be a problem, but it can be avoided by using the developer immediately after mixing it and discarding it thereafter. Given the low cost of the ingredients and the low environmental load posed by them, this is a feasible solution.

One thing. RO water uses a lot of water to do it's job. If you live in a place with natural soft water, like much of the northeast US you really don't need anything from a household and photography standpoint. In areas where water is scarce RO systems may be illegal.

When I first moved to the country and was on community water I was shocked. So much crud in the water couldn't dissolve developer with a lot of sulfite.

I once bought a RO unit for home use. We used it for water pitcher purposes, tea and coffee. It lasted a long time but eventually burst a second filter casing and I decided to retire it. We used Britta now in the house.

I used RO only for the gallon of water to mix chems from powder and the final rinse. Used plain tap for the 1:1 mixes, stop and wash.