Compare different aeration systems used in activated sludge process.

Diffused aeration uses compressed air through diffusers (fine bubble: 1-3mm, 5-8% transfer efficiency; coarse bubble: >6mm, 3-4% efficiency). Mechanical aerators (surface or submerged) use impellers to transfer oxygen. Jet aerators combine liquid pumping with air. Selection factors: oxygen transfer efficiency, power consumption (1.5-2.5 kWh/kg O2), maintenance requirements, and tank geometry. Fine bubble diffusers are most efficient but require more maintenance. Deep tanks favor diffused air.