It smells. Its dirty. No one wants it.
Early this year I toured 2 wastewater plants in Colorado that are using a unique type of mixing in their wastewater treatment. Wastewater treatment for the most part is controlling air and pH to provide the biology the right environment to thrive. Then once the bugs grow you slow the flow down so they settle and you remove some and some you return to the start of the process.
For more information on Enviromix check out their website.