Interesting question. I suppose one could install a separate trap under the sink for the dishwashers (both could discharge into the same one). That would eliminate any sink backup.

I'm not sure if simple check-valves (backflow preventers) are allowed in this type of DWV application...? Methinks not.

You probably just need longer and separate drains for each of the 3 sinks. Provide details of your home construction, and perhaps one of the pros here can tell you how much more $$$ you are looking at... probably a LOT. Something tells me your plumber LOVES keeping YOU kosher... for your own good. ;')

your thinking is about lowering the risk of cross-contamination to a certain threshold, deemed right. It is impossible to reduce the risk to Absolute Zero and any discussion of that leads to ludicrous scenarios, never likely, but still conceivable.

The threshold is one that you will call "good" or that a kashrut inspector will call good, if he (or she) gets into that level of the building process. There are many ways to achieve what you seek, all leaving some possibility open that water sullied with compound A will come up the drain of compound B sink. If Even if a separate P trap were to be installed on all five drains (2 DW drains, 3 sink drains) it is conceivable (but almost never going to happen).

Perhaps you'll consider a "proper for kashrut" preventative operation like closing the strainer on the appropriate sink, and cleaning it if you see the seal was not made. Since sink drains can be deemed to be dirty, perhaps you will decide to use grids on the sink bottom. Some sinks have grids suspended in space above the sink bottom (they rest on ledges on the side walls, e.g. Franke sinks like the Orca, and e.g.2. you can solder a ledge on a stainless steel sink), and this may be deemed one step closer to fine.

If your sole concern is about dirty water when it is hot warm or lukewarm, then lengthening the drain can resolve that single concern. How you do it will depend on your licensed and insured Master Plumber and his professional decisions given your space and your DWV constraints (your plumbing's venting, your DFU's, and your pipe diameters downstream of the five drains). Only he (or she) knows for sure. Not a repair plumber who may often be called upon to work on P traps and drains.

HTH
-david

I've seen commercial grade, 3-compartment, sinks that drain separately without p-traps into a single funnel that is p-trapped. There is no chance of cross-contamination as the funnel is open at the top and would overflow. I believe this configuration is standard for commercial installations.
The problem I would see for you is knowing when there is a drain stoppage before it backs up. A 2" drain line might be an adequate way to address this.

Are there procedures that allow you to "recover" after an accidental contamination? You will have to put down the seal and inspect before every use. Or you will have to perform your contamination procedures before every use. I know your are not the first to experience this. You are best to consult with an appropriate individual with experience and knowledge of generally accepted methods.

Thank you all so much for these thoughtful and impressive responses! I will forward them to my kashrut expert and see what his thoughts are. I so appreciate your time and expertise.

Fixizin: What sort of details would be needed to get a better idea of cost/possible solutions? (and if you think kosher plumbing is expensive, you should try pricing a kosher rib roast! ;-) )

David: It is mainly hot water (~115 or over) that causes a problem. The grids in the bottom of the sink are a great idea.

Tim45z10: Yes, you can rekasher a sink as long as it's metal or stone. It's a pain though!

Again, many thanks to all who responded.

My mother in law kept a glatt kosher kitchen in Europe. She thought you are just making yourself crazy. Kosher kitchens were kept for centuries in the most primitive conditions. Are you sending your waste to milkadick and flieshedick sewers?

A kosher kitchen can enhance your families life. It's a beautiful thing. These modern contrivances are not necessary. You can keep a kosher home with NO indoor plumbing at all. Don't let these people "fadri you a kop".

I suggest you fill you home with loving family and friends, delicious healthy kosher foods and lots of JOY!!! Keep it simple, and real.

I'm sorry, I should translate. "fadri you a kop", means to twist your head around. "milkadick and flieshedick", Means dairy and meat.

So freedee, if hot water from a meaty DW bubbled up into your MIL's dairy sink, she wouldn't consider that a problem?

If dirty water bubbled up into the sink, that would be a problem for anyone. In the more than 20 years since I've had a dishwasher, I haven't had that happen. If it did, one day in the next twenty years, I would do what I had to to clean it up. Once it's down the drain, it's waste. That would be like separating your trash. I better not give those rabbis any new ideas. Those men who come up with these ideas to make young women, like yourself, crazy, have no idea how much work it is to keep a kosher kitchen!

Your drain and your trash doesn't have to be kosher.