Complex sentence: They improved the sanitation of the city by building new sewers and water mains under the streets, and built a new reservoir and aqueduct to increase the supply of fresh water, and installed tens of thousands of gaslights to illuminate the streets and monuments.
Split into simpler sentences: They improved the sanitation of the city by building new sewers and water mains under the streets and built a new reservoir and aqueduct to increase the supply of fresh water. In addition, they installed tens of thousands of gaslights to illuminate the streets and monuments.