Method used for the secondary treatment of wastewater

Conventional activated sludge is a common method used for the secondary treatment of wastewater that can remove both carbon and nitrogen from wastewater by switching between oxic (aerobic) and anoxic (anaerobic) conditions. Describe the metabolic processes that are used in conventional activated sludge including the electron donors and acceptors, and the groups of organisms involved.

Improving the sustainability (e.g. reducing energy usage, and recovering resources) of wastewater treatment systems is a current goal. Describe two ways in which the conventional activated sludge system could be adapted to increase its sustainability. Describe the difference between the conventional system and these other processes and explain how they make the process more sustainable, as well as any drawbacks that the alternate system might have.