Originally Posted by Erik Prestmo

I used to do this for a living, in part of my professional life.
So did I!

Mason, P 204 gives the equation as:

dX/dT = K[(a - x) - w]

Where w = conc of hypo in the water adjacent to the emulsion, K = a constant based on conditions under test (thickness, hypo type, silver concentration, swell, etc), a= the initial concentration of the hypo, and x is the loss after time t.

This equation is most efficient with running water as you reach the minimum desired level of hypo. With changes of water you get concrete downward steps that approach this value, but in a less efficient manner.

If done correctly, they can both work, but usually the method of changing water is misused and Mason, who originally advocated this method, changed his opinion in later years (and in this text) to that of recommending running water with agitation.

Of course, re-presenting this argument here opens a years old debate on APUG here and on Photo Net on this subject.

I suggest you use what works for you but test for residual hypo and silver to insure proper lab procedure.

At the same time, there are other ways to improved hypo and silver removal if one is interested, but none of them are perfect either as they use additional chemicals to promote silver and hypo removal by a variety of methods.

There is no significant problem with the slight amount of hypo produced and the slight amount of silver produced in any method of wash. In fact, the multiple wash method produces, on average, exactly the same amount of hypo and silver in the waste water as any other method.

So the consumption of water may differ, but the effluent is "contaminated" by the same amount in both cases. Your film is probably better off using the continuous wash method however.

At present, I am working on ways to reduce the amount of wash needed for any film or paper. I find that the cost is quite considerable to the consumer but depending on water resources, some may be willing to pay that price.

PE