What is a common feature of aerobic treatment units?

Aerobic treatment units are designed to treat wastewater by using aerobic microorganisms, which require oxygen to function efficiently. The process involves introducing air into the treatment system, allowing these microorganisms to metabolize organic matter and nutrients present in the wastewater. As a result, aerobic systems can effectively reduce biochemical oxygen demand (BOD) and other pollutants through the oxidation of organic materials. This characteristic of utilizing oxygen distinguishes aerobic treatment units from anaerobic systems, which do not require oxygen and rely on different processes for breakdown.

The other options presented fall short of accurately describing the nature of aerobic treatment units. Gravity discharge is not a defining characteristic, as aerobic units can utilize pumps for effluent removal. Additionally, aerobic systems may vary in size but are not necessarily larger than anaerobic systems, which can also be compact. Lastly, aerobic treatment units typically do require a power source to operate blowers or fans for aeration, making the claim that they do not require power inaccurate.