tub for fermenting?

[farmer]

I'm using a 7.5 gal tub for mash and want to go to a bigger batch. Was thinking about a 30 gal trash can. I'm sure there is something better I can use. Any suggestions? Also why do we use an air lock? I notice that when I have went through breweries that they have their mash in open vats and I don't even have any birds flying around in my house.