"The conscience of the city" - that's how Victor Hugo described the sewers of Paris.
A century-and-a-half later, the wastewater running under the streets might also have become a lifesaver.
Indeed, it could turn out to be our greatest ally in the battle against Covid-19 and future pandemics.
Sewers have always contained vital information on viruses. But the persistent problem facing environmental virologists is that they have lacked the technology to unlock the most important data.
That is no longer the case. Drawing on an arsenal of sophisticated water analysis technologies, public health experts can now detect tiny strands of the disease in wastewater.
This means it is now possible to estimate the concentration of the virus among the population at a very localised level.
Secrets of the sewers
Examples of types of big data that can be collected from wastewater include the following:
• SARS-Cov-2, zika and other outbreaks
• Antibiotic resistant bacteria and healthy microbiome
• Hepatitis-C, poliovirus and other asymptomatic diseases
• Diet and nutrition
• Pharmaceuticals and drugs
• Nicotine, alcohol and environmental contaminants
(Source: Biobot Analytics)
Such information is vital. It can help governments track the progress of the disease, spot fresh waves in the very early stages and, ultimately, limit its spread through targeted measures such as lockdowns.
This could be revolutionary in containing pandemics.
Progress so far has been encouraging. Scientists have identified fragments of the virus's genetic material (RNA) in sewage and have proved that these are correlated with local infection rates.
The RNA can be detected within three days of infection. That is crucial when it comes to Covid-19. In humans, physical symptoms typically take five to 14 days to appear.
In fact, a large proportion of carriers (as high as 80%, according to some studies) show very mild symptoms or none at all and could therefore be inadvertent spreaders.
Data so far suggests that viral RNA in wastewater can be identified at least five to six days earlier than people experience physical symptoms.
That is a big window - research from Columbia University suggests that the death toll in US could have been lower by as much as 36,000 if the country had started imposing social distancing measures just one week earlier than it did.
In Switzerland, researchers from École Polytechnique Fédérale de Lausanne and the Swiss Federal Institute of Aquatic Science and Technology are testing samples from 12 wastewater treatment plants which together serve some 800,000 inhabitants.
The results are impressively granular - they have been able to spot the infection in February samples from Lugano and Zurich, when the cities respectively had only one and six known cases.
The eventual aim, the scientists say, is to develop an early warning system - both for any future waves of the coronavirus and for other viruses.
Researchers at Arizona State University's Human Health Observatory, who have been tracking health indicators in sewage for more than a decade, believe that techniques can be refined further to make it possible to identify a single infected individual among 2 million people.
To achieve this, it is necessary to establish an accurate estimate of how much RNA material is produced by an individual, as well as to factor in the degradation rates and to adjust for the profile of each community (such as variations in water consumption).
Hope for the future
The scale of the research - tests and analysis span not only the US and Switzerland, but also the UK, France, the Netherlands, New Zealand, Australia and beyond - gives hope that progress will be rapid.
For water companies developing such tech, the commercial opportunities could be considerable. Already, firms such as American Water Works are joining forces with scientists in the quest for an early warning system for the re-emergence of Covid-19.
Meanwhile, companies that manufacture molecular testing equipment such as Danaher and Thermo Fisher, also have a major role to play. US-based wastewater-testing startup Biobot Analytics, whose projects include Covid-19 tests, recently raised an additional $4.2m in seed funding.
Once scientists perfect analysis techniques, their use could broaden beyond virus spotting and tracking. Wastewater also holds vital information on the use of antibiotics and other legal and illegal drugs, hormones, pesticides and even X-rays.
Such data can be harnessed for early intervention in drug abuse, targeted promotion of healthier lifestyles, improving farming practices and more.
The generation and analysis of big data from water and sewage could become a defining feature of the industry in the coming years, offering another line of defence against health threats and giving rise to a new range of water investment opportunities.
Marc-Olivier Buffle is senior product specialist and Cédric Lecamp is senior investment manager at Pictet Asset Management