Didn't think of that! See, thats why I come here, real smart bunch of folks!
Should that anti back flow valve also be stopping air though?
There's two types of back water valves. One that's normally open and one that's normally closed. In western Canada we are only allowed normally open.
Normally open - closes when water tries to go backwards into the house. Best one. The reason this one is better is if the drains need to be snaked the auger bit won't get stuck on the flapper. Also, this allows the sewer gas from the street to escape through the top of your house and protects the city mains from building methane gas. Google Chinese sewer explosions.
Normally closed -opens when water and sewage leaves the house. Worst one. Auger bit can get stuck and you'll pay dearly to have it unstuck. Doesn't allow city sewer gases to vent through house hold venting system.
Air isn't a problem with either one as there's always a vent to the roof from the main drainage system -National plumbing code.
Best way to describe a vent in layman's terms is simple. If you've ever played with a straw in liquid, hold your thumb over the straw after its full of liquid, the liquid stays in the straw. Lift your thumb and it flows out.
In any case. Most sewer systems in bigger cities are not tied to the storm drains. Instead, the storm drains have their own drainage system that goes to a body of water. There's some debate about this due to oils and other things getting into river systems and lakes, however, sewerage plants have a tough time dealing with huge storms and the result is dumping untreated sewage into the lakes and rivers anyway because they can't process it fast enough.
If you look at manhole covers and there's a fish stamped on it, or a painted fish near the drain or cover you know this goes to a river, creek, or lake. Meaning, don't dump or throw garbage in them.