Watching the flooding on the news makes me wonder if a typical water filter used for backpacking can filter urban flood water. A guy on TV said the biggest risk of drinking the water is e-coli, but I worry about the industrial chemicals and petroleum in the water that may not be able to be filtered out.

Also, is Lake Pontchatrain freshwater or saltwater?

Wikipedia entry for Lake Pontchartrain.

Filtration is to remove particulates - and even a virus is a particulate (but really really small). However, contamination, in the form of a solution of water and some chemical, is not something that can be filtered out. Even distilling won't clean up dirty water, especially with polar solvents (ethyl alchohol and the like). The process of decontamination of water is a science that is complex, requires extensive equipment and monitoring, and is most certainly NOT the kind of thing your camping water filter can handle.
In the case of NOLA, I'd be using hot water heaters, toilet tanks, even indoor swimming pools if I could find them, as sources of water. If I were stuck in a building, I'd start improvising a rainwater collection system NOW, even if the sky is blue today - because there's thunderstorms in the forecast for tomorrow. A sheet of plastic and a bucket or trash barel can mean the difference between life and death.

Don't forget, the water in NO is all salt water. Not an easy way to purify it for consumption, filtered or not.

Distill it all. Set up a big honkin pot, put a fire under it, and a condenser for the steam. It won't get rid of every toxin, but it's better than nothing. Then you run it through a charcoal filter to be sure.