The correct answer is A. Sludge digestion tank.
Sludge digestion tanks are anaerobic, meaning that they do not require oxygen to function. They are used to treat wastewater sludge, which is a solid waste that is produced during the wastewater treatment process. Sludge digestion tanks break down the sludge into a more stable form that can be disposed of or recycled.
B. Sedimentation tanks are used to separate solids from wastewater. They are not anaerobic, as they require oxygen to function.
C. Activated sludge treatment is a process that uses aerobic bacteria to treat wastewater. It is not anaerobic, as it requires oxygen to function.
D. Trickling filters are a type of biological wastewater treatment system that uses a bed of rocks or other media to support a biofilm of microorganisms. They are not anaerobic, as they require oxygen to function.