The biological treatment techniques used can be classified into attached growth processes and suspended growth processes. Which of the following treatments is classified under attached growth processes?

Activated sludge process:

• This process is a type of suspended growth treatment method.
• It involves aerating the wastewater to encourage the growth of microorganisms that break down organic matter.
• The microorganisms remain suspended in the water, which is why it is classified as a suspended growth process.

Rotating biological contactor:

• This is a type of attached growth treatment method.
• It consists of a series of discs mounted on a rotating shaft. The discs are partially submerged in wastewater, allowing microorganisms to grow on the surface of the discs.
• As the discs rotate, the microorganisms contact the wastewater and break down organic matter.
• The microorganisms remain attached to the discs, which classifies this process as an attached growth process.

Sludge digestion system:

• This process is typically used for the treatment of sewage sludge.
• It involves the breakdown of organic matter by microorganisms under anaerobic or aerobic conditions.
• Although microorganisms are involved, they are not attached to a surface, thus it is not classified as an attached growth process.

Aerated lagoon:

• This is another type of suspended growth treatment method.
• It involves aerating a lagoon to promote the growth of microorganisms that degrade organic matter.
• Similar to the activated sludge process, the microorganisms are suspended in the water, classifying it as a suspended growth process.