Meh, let nature process the sewage back into drinking water, use sewage-derived clean water for other non-human-consumption purposes if it was that important...but it's not. If it's clean enough to drink then it's clean enough to just dump it anywhere.
People don't farm algae for fuel for a number of reasons. But the biggest reason is that algae need sunlight the same way solar cells do. It would take hundreds of acres of lakes or ocean to make enough algal oil to power a few cars.
Capitalism: The cream rises. Socialism: The scum rises.