Welcome and thanks for the questions.
The water was nearly identical each time we ran the machine—with or without feet in the bath. I believe that you're right; (that the amount of discoloration is due to impurities in the water) but what I don't think most people know is why this is true.
Water in itself is not a conductor. If you had perfectly pure water in a large container, you could submerge a television set in the water and it would still work perfectly. I've actually seen this done. This works because the water does not conduct electricity by itself and therefore would not cause the electrical circuits in the television to short out (I don't encourage anyone to try this).
The impurities in the water cause it to conduct electricity. This is why the operator of a foot spa adds stuff to the water—often sea salts of some kind.
The increase in impurities cause more electricity to flow in the water which greatly enhances the efficiency of electrolysis and greatly speeds up the electrode corrosion and therefore discolors the water. There are other effects in play here as well, but I believe the effect of electrode corrosion via electrolysis is likely the most significant in relation to the discoloration of the water.
I've heard of some people who have had progressively clearer water with each session seeming to indicate that they are less toxic with each new session. This would—on its face—seem to indicate that less substance is coming out of the foot with each session, which would—on its face—seem to indicate that the person is less toxic with each session, which would—on its face—seem to indicate that people are successfully detoxing with each successive session.
My concern with this idea is this.
For enough of—let's say—some combination of heavy metals to come out of the body in such an amount that would cause this amount of discoloration to the water in the bath would be a whole lot of heavy metal, so much that it would be way beyond fatal to the person it just came out of.
I've seen, for example, people giving or receiving foot baths claim that the metal flecks that are found in the bottom of the bath after a session came out of the body. There are a couple of problems with this idea.
First, metal flecks of that size could not come out through the skin without rupturing the skin.
Second, if that much heavy metal came out of a person, they would have already been dead because of its toxic effects while the metal was still in the person.
Third, if that is not all composed of heavy metal but also consists of beneficial metals, we should not be happy to be loosing them during a foot bath.
Fourth, some have claimed that the metal comes out in small amounts and coagulates as an effect of the electricity. If that's true, we should be able to add minerals to water, and then by using the same electrolysis process with non-corrosive electrodes, be able to produce the same effect. Instead, we would likely simply transfer metal from one electrode to the other, which is, of course, the process used to coat metals in industry, which is called—among other things—electrolysis. I'd have to do more research to confirm this, however.
Finally, I would have to point out that, in
our experiments, we had very little change in the chemical makeup of the water when feet were in the bath as compared to sessions conducted when no feet were in the bath.
I can only speculate as to why some people experience a lightening of the bath water with each session, but I personally believe we can only know for sure with a series of independent experiments. I would be careful to be sure that all conditions in the session are the same with each experiment, for example, the same water source is used, the same amount of additives are put into the water, the same amount of voltage is applied to the water, etc.
We were going to manufacture our own version of the foot bath and market it, and, this is the reason we conducted these experiments—to determine that these devices actually did something useful. We were particularly interested in the removal of heavy metals as we believe this is a current "silent" epidemic.
Unfortunately, our experiments show no significant difference between water with feet and water without, so we cannot allow ourselves to move forward on a project, regardless of how profitable it would be, without knowing that it really does have a health benefit.
Now, I have seen a study conducted by a manufacturer of foot spas where the bath water actually showed a small increase of some sugars and urea, but of course, this is pretty useless in terms of detoxing.
I have seen another foot spa manufacturer display live cell analysis as being more favorable after a foot spa session. Unfortunately, I know very little about live cell analysis so I can't comment intelligently on this. Nevertheless, I would like to see an independent study check this matter out further.
Concerning people feeling better: Yes. I have personally heard testimonies about people feeling better after several foot spa session. In some cases, minor ailments go away. Of course, the rest of the story is that in each case I've personally heard, the ailments come back soon after the sessions finish.
We also have to consider that those receiving sessions often are given a very beneficial mineral supplement before each session. I wonder if this could be the real reason for the health benefits. It's well-known that most people are mineral deficient and there are decades of testimonies showing people who have recovered from minor ailments by simply adding a mineral supplements to their diet.
So...
I'm a skeptic about foot baths in their relation to heavy metals. They may have other beneficial effects, but I haven't see any good evidence of this yet.
I have to admit that I also am concerned about the long-terms effects of exposing the body to voltages that are unnaturally high. For the sake of those receiving baths, I hope there will not turn out to be a problem here at some point in the future.
Also, after looking at these devices internally, I am concerned about their safety. The ones I looked at had no ground fault protection (GFI) and if a single person ever gets hurt or dies from a malfunction, it would be 1 person too many. I'm not an expert in electrical safety, but I do have electronics training and was ISCET certified just after college.
In conclusion, I really hope we can find a safe, effective way to quickly detox people without stressing the liver or kidneys, and other than sweating and using saunas, I just don't personally believe we've found it yet.
Hope this helps.
http://herballure.com/Studies/IonicFootSpa.html 
