Hello,
I'm currently using yuoki's mods and i try to reduce pollution by using the fatmice engine. This engine produce "negative pollution" (so it reduces pollution. The problem is that i can't put those engines inside a factory to reduce pollution because the pollution translation doesn't seem to work with negative one... Is it "normal" or not ?

Your issue is that Factorio does allow entities to produce negative pollution, but does not allow negative pollution clouds. Instead of making negative pollution clouds and sending them out of the factory building to cancel out the positive pollution clouds, what's happening is that your engine is just constantly resetting your factory's total pollution to zero, because it can't go any lower than zero. Since there is zero pollution inside the factory, Factorissimo can't transfer anything, so the pollution outside remains entirely unimpressed.

You'll have to place your pollution scrubbers outside, otherwise you'll just have a very clean factory building.

By the way, it also won't reliably work if you place a polluting factory building inside a pollution-cleaning factory building. The pollution will be transferred outside faster than the pollution cleaners can clean it all up.

Thanks for the answer. I've just spent a few hours redesigning my base and was about to try putting polluting factory inside a pollution cleaning one... i guess i wont try then :/ So I wont win any more space for those pollution cleaning stuff ;) I'll have to redesign one more time ;D

Could you not slightly change the factories so that they have a fixed offset on pollution on the inside. And when you transfer pollution from/to the outside just add it or remove it depending if the value minus the offset is negative or positive.