There are "food-grade" trashcans? Anyway, during primary fermentation there shouldn't be a problem as long as the room it's in is fairly clean and the can is well sanitized. This is called open fermentation, and plenty of commercial breweries do it. In the case of a trashcan I would say just put the lid on and don't worry about it, that should be enough to keep infection out, which by the way would need to come from flies or some other source because the C02 being produced is going to stop microbes and the dust particles they ride on from getting in. But once primary fermentation is done you need to rack to something. I'm assuming you are kegging given that volume of beer, so some of your keg's might need to become the equivalent of 'bright tanks'.
