My question is, has anyone actually tested the wash water OR any processes to filter out residual chemicals?
Back in the 1980s, when all the effluent regulations were hitting hard, there was a lot of research done on ways to deal with the effluent. An outfit called Pacex, as I recall, actually marketed a system to treat and reuse a substantial amount of the waste water. Using an ion-change column among other things. None of these systems make economic sense provided that you already have an adequate supply of water and a way to get rid of it.
The link that AgX supplied talks about a similar thing. (The user Photo Engineer, aka PE, spent much of his working career in the Kodak Research Labs, so when he says, "we did..." he is talking about Kodak.) The way that these things typically worked was that the manufacturers, such as Kodak, Fuji, Konica, Agfa, etc., wanting to continue selling their products, did the basic research to deal with various problems, published information about same, and then various companies would manufacture equipment to apply these technologies. But most of that infrastructure, or whatever you wanna call it, is long gone.
Regarding wash water, and the testing, etc., wash water is gonna get used fixer, including silver, in it. (Every time you move wet film from one tank to the next, some of the previous solution comes along with it.) There is no simple way to "filter" these things out. Your main concern is gonna be with the silver in both your fixer and the wash water. In the US there is a law called RCRA which defines what they call a "hazardous waste," which includes silver-bearing liquids with more than 5 milligrams per liter of silver in it. Now, it doesn't take much to get even your wash water over that limit so you should get familiar with those aspects. I'm not sure, but Ithink that RCRA has "small user exemptions" where a so-called small user might be ok with, for example, hundreds of gallons per month. But I am just making a wild guess on this - you are the one that is gonna be responsible for any problems. Personally I never dealt with that sort of thing, if you had a local sewage treatment plant then they could probably take care of your waste water (you have to know what the local regulations are), and the RCRA laws wouldn't even enter into this.