grease trap

I am looking at having an outdoor grease trap installed in a parking lot. The installation instructions specify a vent be installed at inlet and outlet, as well a recommend the vessel is vented. These vents would be 2". This seems like a lot of vents since this will be installed with a manhole...

My home is from the early 1900's and I have a large main drain that is cast iron. The drains that run to that main one are the kitchen sink, the basement shower, upstairs shower, laundry and upstairs bathroom. The picture is of the grease trap drain that the kitchen sink runs to. Now I moved...