The water collection and treatment infrastructure systematised in the 19th century, starting with the work undertaken by Baron Haussmann as part of the renovation campaign entitled ‘Paris embellie, Paris agrandie, Paris assainie’ (‘Paris made beautiful, Paris made bigger, Paris made cleaner’), and then modernised over time, certainly met Napoleon’s hygienic ambitions, but also generalised the creation of conditions favourable to the emission of ‘sewer gases’. H2, CH4, NH3, CO, NH3, H2S, CO2 and N2O and volatile organic compounds (VOCs and CSV). These natural pollutants are not only harmful when their concentration exceeds a certain threshold, but some of them are very aggressive to equipment.
Resulting from the natural processes of biodegradation of organic matter, these gases, including H2S, not only make a significant contribution to the greenhouse effect (1/3 of CH4 emissions in the city of Paris come from the wastewater network), but also threaten people’s health and safety and cause odour nuisance, the impact of which is felt as far as the price of neighbouring property. Without counting the collateral damage to property values, the annual cost of rehabilitating infrastructure directly damaged by these gases is estimated at around 10% of the cost of wastewater collection and treatment, or €1.2 billion in France. These figures should be seen in the light of the number of such infrastructures throughout France. There are around 3 million sewage treatment works and 425,000 kilometres of sewage network managed by 12,392 dedicated services. This organisation alone represents an asset value equivalent to more than 6% of the national GDP, and a large proportion of this is made up of small installations such as pumping stations.