According to your last post, you still have not addressed the issue of all the sewage backing up into your basement. I did not get the idea you flushed all of this water down your waste lines into the basement and I don't think you have other units in your building (a house).
When I lived in a similar house up north, I installed a very good quality wing-nut (test) plug in the basement floor drain. Here is my reasoning: The floor drain is there to evacuate any water draining into the basement. Unless you have extensive ground water coming in, the only water you should have in the basement is from the clothes washer and/or a laundry tub. While this would be an inconvenience, this water is not a health hazard.
On the flip side, water backing up from the street into your basement is ..., well you experienced it. I only had to clean up the city's backwash once to come to this conclusion.
I recommend eliminating the sewage problem and taking the chance on 10 gallons of suds. There may be a real good reason why this is not solid reasoning; if not please leave a post.