Who was Joseph Bazalgette?
The engineer who built London's sewer system in the 1860s and 70s, ending the cholera epidemics that had killed tens of thousands — and who is a rare case of an infrastructure project deliberately over-engineered in a way that paid off for a century and a half.
The problem he solved. London's waste drained into the Thames, which was also a water source. Cholera outbreaks killed enormous numbers, and the prevailing miasma theory attributed disease to foul air rather than contaminated water. The political trigger was the Great Stink of 1858, when the smell from the river became so intolerable that Parliament, sitting beside it, finally funded a solution — having declined to do so through several epidemics.
What he built. Around 1,100 miles of street sewers feeding some 82 miles of main interceptor sewers running parallel to the river, carrying waste east of the city to be discharged on the outgoing tide rather than into the centre. The system used gravity where possible, with pumping stations where it could not, and incorporated the construction of the Thames Embankments, which narrowed and quickened the river while carrying the sewers, a road and later the Underground.
The decision that made him famous among engineers. Calculating the pipe diameter needed for the population, he then doubled it — reasoning that the work would be done once and that he could not predict future growth. That margin is why the system coped with a population several times larger than he designed for, and it is cited constantly as the case for building in capacity.
The irony. He was working from the wrong theory. Removing the smell was the goal; removing the contamination from the water supply was the effect. He solved the real problem for the wrong reason — an outcome that happens more often in public health than is comfortable.
His use of Portland cement at scale, then a relatively new material, required rigorous testing regimes that influenced construction practice.
The system is still in use, now supplemented by a major new deep tunnel built because population growth finally exceeded even his margin.