It's been tried. No success. The Ph and temperature fluctuates too much. In any event, nearly all (Perhaps all.) sewage treatment plants require vast amounts of oxygen pumped into the system. This creates the ideal bacteria to ingest the waste matter. The resultant sludge is pumped from the tank into another, separate tank. There a whole other process takes place.
The environment is not suitable for what you suggest.
Related link: http://vasatwiki.icrisat.org/index.php/Algaculture