The sewage system
Sewage system was first built in the Roman empire. These stood of channels dug in the earth, which directed the water from the houses and baths through the streets, out of the city. As these channels were uncovered yet, they could have been awfully smelly. Romans had also water pipe systems in some of their cities. These were blacksmithed of plumbum, and led the water from the mountains to the city with a minimal drop. In that time, they didn't know about the poisonous effect of the plumbum and its complexes, and boldly drank the poison melted in the water, which affected them through years. In the Roman empire the death rate was high anyway, because of the constant conqueror wars.
In the XXI. century, the domestic (sink, toilet, bathpools, etc) and industrial sewage water is directed in plastic - poly-vinyl-chloride - and concrete pipes. The sewage system uses gravity or pressure. Where no sewage system is built, in better case a sewage truck tranfers the polluted water to the water treatment plant, or in worse case, people simply pour the polluted water into the near river or lake. Awful ecological crime and primitiveness!!! To join the EU, 100% covering of sewage system is a reqirement - this is ready in only a few cities.
Did you know? In Budapest, only 60% of the area is connected to the sewage system!