It seems like the intended behavior should be for the dupe to get germy from being in a germy environment. They breathe in slime lung and pick up slime ore they get slimy. I would imagine that cleaning germ infested water and getting sopping wet in it should do the same. However, dupes can enter liquids host to millions of germs and pick up no hint of disease.
This behavior enabled me to grossly mismanage my sewage situation (i put my sewage tank with no way to drain it right over all the critical parts of my base and water supply) and then when the system finally overpressurized and failed it should have absolutely flooded my base with food poisoning germs. I was kinda trying to do this on purpose, sorta.
Well, nothing. bad happened. I had to inconveniently move some stuff around but i am just going to deconstruct it all later. I even got some high pressure high germ poop water in the water supply and that impacted nothing. No germs anywhere and then the food poisoning all died out.
no one gets germy interacting with germy liquid, except when picking up germy bottled liquid to carry to a unbottler, which gets them and the bottler germy. Just thinking out loud here, it seems like the bottling should be much more disasterous than the unbottling!
If the mechanic is supposed to put fear of germs in us and teach us not to poop where we eat, then it should be consistent in polluting us when we do horrible things like build your pressurized septic system above your kitchen.
without changing this parameter sewage treatment from a germ sense won't matter. I feel like when you make the irresponsable decision to pump your sewage outside the colony and into, say, a slime biome, practically speaking, this should kinda cause some sort of inconvenience for you someday if you ever try to move into or process that water. Instead, the disease dies out, rather than spreads. Wouldnt it be cool if new diseases could emerge from mixing food poisoning germs with diseases in the wild? Hm!
build a pressurized sewage system over your colony kitchen and water supply and marvel when a total failure is really easy to clean up and noncontaminating.