Which of the following is a type of secondary treatment system at municipal wastewater treatment facilities where large amounts of air under pressure is injected into aeration tanks?

The activated sludge process is a widely used method in secondary treatment systems at municipal wastewater treatment facilities that effectively removes organic matter through biological processes. In this system, a mixture of wastewater and microorganisms is aerated in aeration tanks. The large amounts of air injected under pressure provide the oxygen required for aerobic bacteria to thrive, allowing them to metabolize organic pollutants present in the wastewater.

This process involves creating a mixed liquor that enhances the growth of microorganisms, leading to the breakdown of contaminants. Following aeration, the mixture is typically allowed to settle in a secondary clarifier where the biomass, also known as sludge, separates from the treated water. The clarified water can then be discharged or further treated, while a portion of the sludge is often recycled back to the aeration tank to maintain the population of active microorganisms.

The importance of the activated sludge process lies in its efficiency at reducing biochemical oxygen demand (BOD) and eliminating pathogens, making it crucial in municipal wastewater treatment. Other methods, like biological contractors, sand filters, and trickling filters, utilize different mechanisms or media but do not directly involve the aeration and pressure dynamics characteristic of the activated sludge process.