Final answer:
Noah was happy to learn that the sewage tank was rusty and empty because it indicated that the waste had been properly disposed of and was not contaminating the surrounding environment.
Step-by-step explanation:
Noah was happy to learn that the sewage tank was rusty and empty because it indicated that the waste had been properly disposed of and was not contaminating the surrounding environment. In the past, inadequate waste disposal systems led to major issues like polluted rivers and foul odors in cities. However, advancements in sanitation and the development of sewage systems, as exemplified by the rebuilding of London's sewer system by Joseph Bazalgette, helped address these problems and improve public health.