In many cities, wastewater travels through sewer lines to be processed at a
wastewater-treatment plant. The treatment plant uses many methods to
break down materials in wastewater and make sure materials are safe before
release. Which of the methods used during treatment is meant to prevent
pollution that cannot be broken down by treatment from re-entering the
environment?
A. Heating of liquid waste
B. Settling out of solid waste
C. Aeration of liquid waste
D. Burying of solid waste