I live in a place where there is relatively soft water in the municipal supply.

I found in a 'junk week' that someone had moved to town and then thrown out their reverse osmosis unit from when they had previously lived. It was crusted with iron and lime on the inlet filter, so it definitely did its job at the previous location.

I rehabbed it , and have never been happier. Now I no longer have the need to lug distilled jugs home in the winter from the grocery store.

when the water is of unknown qualtity I have used calgon, at a rate of 1g/l when processing E-6 away from home. (it was about 1/2 tsp as I recall).

I have taken e-6 concentrates in the camper when off on car camping trips.

After filtering and leaving lake water to settle, I boiled it on a camp stove, to drive off dissoved oxygen, then mixed the chems when the water cooled, after I had dissolved the calgon to keep anmy dissoved items in suspension.

To temper the whole affair I would immerse all the bottles and tank (loaded in a changing bag) in a large wash tub that held lake water and heated on the camp stove water, to bring the whole affair to 100F. Timing was by way of wrist watch. Drying was by way of stove alcohol mixed in 20% with the final rinse - the film was dry in under 5 minutes, inside a camper with only me standing very still for that period of time.