Okay, back to chem 101 eh? Here goes.

In order to electrolyze water into hydrogen and oxygen, you need to start with a pure source. If you use contaminated water, you will be electroyzing these molecular compounds as well, and your off gas production will be corrupted and unusable. It could also quickly ruin your electrolytic screens (fuel cells thus require pure water, or other pure polar hydogen molecular compound).

As for distillation, it is not whether another substance has the same boiling point as water, but the same condensation point as water, that will cause problems with purity of the process.

My suggestion is to pump all that sewage back into Lake Ponchatrain, add copious amounts of aerobic and anerobic bacteria, stir often, and let it all get digested over the next decade or so, then rebuild the ecosystem to deal with what's left. In a hundred years or so, we could then dredge the lake bottom to remove all the heavy metal precipitates and undissolved hydrocarbons, send these to the smelter, and things would be back to "normal"??? Nothing cleans up contamination so well as Mother Nature at work, it just doesn't go according to our relatively short timeline is all. Even DDT gets broken down in time.