Early civilizations often built drainage systems in urban areas to handlerainwater that ran down the street during a storm. The Romans constructedelaborate systems that also drained wastewater from the public baths. However,as the population of the cities grew, the old drainage systems becameoverloaded. During the Industrial Revolution, manufacturing waste was added tosewage, which increased the need for more efficient sewage treatment. In themid-19th century, the first steps were taken to treat wastewater.
         Sewage or wastewater treatment in modern times is the process of removingharmful physical, chemical, and biological elements from wastewater and housesewage. The whole process starts with screening out large objects such as paperand wood, and removing heavy materials like dirt. The screened wastewater isthen ready to go through a series of concrete tanks for further treatment. In thesecond step, the sewage passes into the primary tanks. Here, human waste,called sludge, settles to the bottom while oils and grease float to the top, wherethey are collected. At the same time, organic matter like eggshells or coffeegrounds in the sewage is broken down into smaller substances. The remainingsewage then enters the secondary tanks for the third stage of treatment. Thesolids that were not treated in the primary tanks are removed here throughdecomposition, which digests the material. Then, the liquid sewage is filteredthrough sand. This filtering process gets rid of almost all bacteria, as well as othersolid particles that remain in the water. Finally, the wastewater flows into the lasttanks, where the chemical chlorine is added to kill the remaining bacteria. Afterthe bacteria are destroyed, the chlorine is eliminated from the water, and thetreated clean water is discharged into a river or the ocean.
【題組】30.Which of the following is NOT a reason for developing more effective systemsto deal with wastewater?
(A) Population growth.
(B) Increasing manufacturing waste.
(C) Overloaded drainage systems.
(D) Heavy rainfall due to climate change.