Does anyone, who actually knows, what is considered too much dev/ss/fix to be sent down a muncipal collection system...
Well, yeah, I know something about this, but... (in the US) it depends strictly on your local regulations. As a single user, you don't have any chance of "overstressing" your local water treatment plant. But... you may be restricted by the regulations. So you have to read them to know.
Here's how it works, generally, in the US. Your local municipality has a set of laws regarding what, specifically, users can put down the sewer. If you are a commercial user you generally need to have a "permit" which specifies the pertinent details, including how your effluent will be monitored, and specifically what for. For a photofinisher this will typically give values for "BOD" and "COD" (these are "oxygen demand" values; your developing agents, preservatives, and fixers consume oxygen as they are degraded) as well as a silver limit. The silver limit is probably gonna be in the range of several milligrams per liter to as low as one or two tenths of a milligram. I know what it takes to get down to a couple tenths of a milligram, and you have basically no chance of getting that low (without dilution, which is not permissible).
However, almost all of the municipal regs have a "small user" exemption, perhaps a couple hundred gallons per month, or so, where they basically don't see it as worth their trouble. And probably individual (non-commercial) users are not regulated at all, beyond the standard restrictions of no flammable liquids, or things that could endanger sewer workers, etc.
Your local POTW (publicly owned treatment works) is strictly regulated by the EPA, and unless the POTW is approaching any sort of limit, it's doubtful that your municipality will change any of their regs. Unless your area is loaded with badly behaved photofinishers, I can't imagine that silver will ever be a concern to them. As a note, about the only person I recall seeing on this site who is knowledgeable about the silver situation is user Maris; however he is not from the US.
Anyway, bottom line is that, in the US, you should look at your local regs. Personally, I would suggest that anyone who does any significant amount of processing should try to get their hands on at least some of their waste silver. A good place to start would be to use a two-stage fixer (this helps concentrate the silver in the first stage, AND reduces the carryover to wash water), and use some sort of silver recovery system. One of the members here, John Nanian is a dealer for the "Silver Magnet," a small electrolytic unit, and is probably the most sensible thing to use for many people here. Even if it takes them upwards of a half-dozen years to collect enough to be worth sending it in.
Ps, the sort of effluent I'm talking about is the mainstream sort of photofinishing material, not various toners or off-wall-developing agents, etc.