第 36 至 39 題為題組      Early civilizations often built drainage systems in urban areas to handle rainwater that ran down thestreet during a storm. The Romans constructed elaborate systems that also drained wastewater from thepublic baths. However, as the population of the cities grew, the old drainage systems became overloaded.During the Industrial Revolution, manufacturing waste was added to sewage, which increased the need formore efficient sewage treatment. In the mid-19th century, the first steps were taken to treat wastewater.      Sewage or wastewater treatment in modern times is the process of removing harmful physical,chemical, and biological elements from wastewater and house sewage. The whole process starts withscreening out large objects such as paper and wood, and removing heavy materials like dirt. The screenedwastewater is then ready to go through a series of concrete tanks for further treatment. In the second step,the sewage passes into the primary tanks. Here, human waste, called sludge, settles to the bottom while oilsand grease float to the top, where they are collected. At the same time, organic matter like eggshells orcoffee grounds in the sewage is broken down into smaller substances. The remaining sewage then entersthe secondary tanks for the third stage of treatment. The solids that were not treated in the primary tanksare removed here through decomposition, which digests the material. Then, the liquid sewage is filteredthrough sand. This filtering process gets rid of almost all bacteria, as well as other solid particles that remainin the water. Finally, the wastewater flows into the last tanks, where the chemical chlorine is added to killthe remaining bacteria. After the bacteria are destroyed, the chlorine is eliminated from the water, and thetreated clean water is discharged into a river or the ocean.
36. What is the passage mainly about?
(A) The elements of sewage.
(B) The treatment of wastewater.
(C) The history of draining systems.
(D) The ways to control manufacturing waste.