AQA GCSE Chemistry Paper 2 Practice Exam 2026 – All-in-One Guide to Master Your Chemistry Exam Success!

In the sewage treatment process, following sedimentation, aerobic biological treatment is a crucial step. During sedimentation, solids settle out from the sewage, leading to the formation of sludge at the bottom and clearer wastewater on top. The clearer wastewater then moves on to the aerobic biological treatment phase, where it is exposed to microorganisms that thrive in the presence of oxygen.

This biological treatment is essential for breaking down organic materials that were not removed during sedimentation. The microorganisms consume these organic pollutants, leading to further purification of the water before it is either released into the environment or undergoes additional treatment processes. This step helps ensure that the treated water meets safety and environmental standards.

Screening occurs at the very beginning of the sewage treatment process, removing larger debris before any other treatments take place. The disposal of sludge is a subsequent action that generally happens after biological treatment, once the sludge has been processed further. Anaerobic reactions are important as well, typically seen in the processing of sludge rather than the direct treatment of water after sedimentation. Therefore, aerobic biological treatment specifically follows sedimentation, making it the correct answer.