Bazalgette

At the time, the Thames was little more than an open sewer, devoid of any fish or other wildlife, and an obvious health hazard to Londoners. Bazalgette's solution (similar to a proposal made by painter John Martin 25 years earlier) was to construct 83 miles (134 km) of underground brick main sewers to intercept sewage outflows, and 1,100 miles (1,800 km) of street sewers, to intercept the raw sewage which up until then flowed freely through the streets and thoroughfares of London. The outflows were diverted downstream where they were dumped, untreated, into the Thames. Extensive sewage treatment facilities were built only decades later.

The system was opened by Edward, Prince of Wales in 1865, although the whole project was not actually completed for another ten years.

Bazalgette's foresight may be seen in the diameter of the sewers. When planning the network he took the densest population, gave every person the most generous allowance of sewage production and came up with a diameter of pipe needed. He then said 'Well, we're only going to do this once and there's always the unforeseen.' and doubled the diameter to be used. Every Londoner should be grateful for this foresight as the then unforeseen was the tower block. If he had used his original, smaller pipe diameter the sewer would have overflowed in the 1960s. As it is they are still in use to this day.

http://en.wikipedia.org/wiki/Joseph_Bazalgette#Sewer_works