2. From London to Paris, from sewers to subways, from Belgrand to Hénard

2.1 Sanitation

We know that it was insalubrity and its epidemic repercussions (cholera, etc.) that gave rise to the first sewer systems (hence the word sanitation). The first sewers were discharged into the Seine in the 1830s. Two contemporary engineers, Eugène Belgrand in Paris (recruited by Baron Haussmann) and Sir Joseph Bazalgette in London, designed and built underground wastewater collection systems for both cities (from 1854 in Paris, 1856 in London). They adopted the principle that everything unpleasant to the senses should be placed underground.

For the most part, these pipes will be built in trenches at modest depths. In Paris, 600 km of tunnels can be visited. In London, only the main axes, some 150 km long. The large diameter of the Paris sewers...