WHAT’S the largest source of mass moving in and out of a city every day? You think, if it’s a port city, it must be boats – or, you know, maybe if it’s a landlocked city, it’s trains or trucks or cars or planes. No, it’s water. It’s water. There’s so much more water moving in and out of a city any day than there is any kind of cargo. It’s basically pure water coming in. And then the water that leaves has some traces of almost every human activity that’s going on in the city.”
Once Eric Alm is in full flow, it is hard to stop him. But it isn’t hard to understand his enthusiasm. Alm, a biological engineer at the Massachusetts Institute of Technology, is one of a growing band of researchers turning their attention to the fluid coursing through our sewers. This waste water, as it is known, contains the whispered biochemical confessions of millions of people, and by listening to them, scientists can paint surprisingly detailed pictures of our health, wealth and environment, head off epidemics, track pandemics and even spot new “designer” drugs before their effects show up in the population.
The field, called waste water-based epidemiology, not only has the potential to revolutionise public health but also transform our view of sewage from disgusting waste to something incredibly valuable. “You can think of the city as one big organism,” says Rolf Halden at Arizona State University, another convert to the study of waste water. “It has one metabolism.” And that makes sewage the lifeblood of our communities.
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You might think that what happens in the smallest room of the house is strictly between you and the porcelain. But once you flush, the products of your privy quickly become public. They take the downpipe into the drain and then to the sewer, where they coil and coalesce with those of your neighbours, your neighbours’ neighbours and so on, before ending up at the local treatment plant as what is effectively a pee and poo sample for the whole sewage catchment area.
The idea that this might contain useful information was first suggested a couple of decades ago. In 2005, a took samples from the river Po in Italy, whose catchment includes waste water from 5 million people. Researchers tested the samples for cocaine and its principal metabolite, benzoylecgonine – the main compound the body breaks it down into. This revealed that the river was carrying the waste that would be produced by consumption of 4 kilograms of cocaine each day, which translates into around 40,000 separate uses of the drug. Previous estimates had put consumption at 15,000 hits. The work inspired scientists across Europe to of information about drugs in waste water and establish a continent-wide monitoring system.
Halden heard about these developments from colleagues in Europe and, in the early 2000s, began to apply the approach in the US. He and his team collected waste water samples from across the country and stored them as an archive, originally dubbed the National Sewage Sludge Center, but soon changed to the Human ÎçŇą¸ŁŔű1000ĽŻşĎ Observatory. It wasn’t well received. “There was just tremendous resistance to even accepting that there could be useful information in composited waste water,” he says. However, the approach was vindicated when the team showed that two antimicrobial compounds called triclocarban and triclosan – commonly added to soaps and other consumer products – persist in sewage for much longer than previously thought. They were known to be harmful to humans and the environment, and the finding ultimately in consumer products in the US. Bans further afield followed.
Tracking viruses
Despite such early successes, for years, many waste water-based epidemiologists struggled to get funding and recognition. That changed in late 2019, when reports of a sinister new respiratory illness emerged from China. Scientists realised that the virus responsible, SARS-CoV-2, could be spotted in sewage. They could detect low levels of the virus in waste water samples, days before people began showing up at hospitals, allowing health authorities to direct their resources accordingly. What’s more, Halden calculated that tracking the virus in this way was about 60 times cheaper than using clinical testing.
Public health authorities soon caught on. The US Centers for Disease Control launched a national waste water surveillance system in 2020, and has awarded grants totalling approximately $220 million to projects monitoring SARS-CoV-2 in this way. In the US and in other countries, this has been instrumental in helping authorities . “That’s really where the dam broke and a lot of people began to become believers and practitioners of the technology,” says Halden.
The power of this approach to track diseases became apparent in February 2022, when sewage monitoring in London, UK, before any clinical cases appeared. Based on these detections, booster vaccines will soon be offered to children in the city. The same waste water tools that detected covid-19 in sewage in the US are also being adapted this month to monitor the spread of monkeypox across some states.
But monitoring such as this only touches the surface of what waste water epidemiology could do. In principle, it can detect almost anything in sewage, from markers of diet and stress to prescription drugs and personal care products. It is even possible to determine whether a compound has been through a human body or not, because the liver to anything passing through it. “The sky’s the limit,” says Halden.
What is possible partly depends on where the samples are taken. All sewage ends up at a treatment plant, also called the sewage works. Sampling here allows you to look at a lot of people, which means that you can get a good overall picture of how the general population is exposed to a chemical or infectious agent of interest. On the other hand, because sewage has travelled a long way and taken a long time to get to the sewage works, many chemicals, especially those related to human metabolism, have broken down by the time they get there. But these ephemeral molecules can be captured further upstream. By sampling from water flowing beneath manholes, scientists can also gain a more detailed picture of what is happening in a particular neighbourhood or even individual buildings. During the pandemic, for example, they used this method to track SARS-CoV-2 infections in nursing homes, hospitals and other places where people crowd together.
Once they have their samples, the researchers separate the sludge from the liquid and the hunt begins. Each chemical of interest has particular qualities like mass and electrical charge, the chemical equivalent of a photofit that scientists can use to pick it out from the crowd with techniques such as mass spectrometry. Meanwhile, another technique, called PCR, can detect specific stretches of genetic material to identify particular viruses, bacteria or antibiotic resistance genes.
But detection is the easy part. “It’s not about going in and taking a measurement. It’s about developing a platform that can help you reach insights about what’s going on,” says Alm. One problem is that, along with toilet waste, sewage contains a cocktail of water from showers, washing machines, factory effluents and many other sources. Another is that the amount of a chemical present depends on where the sample was taken and where the people excreting it were located. Also important is how the human body absorbs, processes and excretes a chemical of interest, as well as how and when people consume it.
Alm has helped found a company called Biobot Analytics to get meaningful insights from waste water samples. It uses computer modelling and machine learning to bring together multiple sources of data, such as maps of housing and how many people in the city might be taking a particular illicit drug, to predict where and when in the sewage system it is best to sample. Halden, meanwhile, is CEO of a spin-off company, AquaVitas, which is using similar approaches to help health authorities monitor public health threats.
Illegal drugs
Such quantified, near-real-time analysis of waste water allows authorities to track things like activity in the illegal drug market. This is already happening in Australia. It can also , as Richard Bade at the University of South Australia and his colleagues showed in a study that examined waste water over New Year 2019-2020. Being able to quantify waste water data is also crucial when studying population exposures to environmental pollutants, to see whether safety thresholds have been crossed, says Barbara Kasprzyk-Hordern at the University of Bath, UK, “You really need to know the numbers.”
Take bisphenol A, one of a class of chemicals known as endocrine disruptors that mimic the effect of hormones in the body. There is increasing evidence that these might be partly responsible for the global rise in diabetes and obesity. Kasprzyk-Hordern’s team showed that most exposure to bisphenol A – previously believed to come mainly from food packaging – was instead . Waste water levels of this chemical were higher on weekdays and lower at the weekend, and higher in catchments with industrial areas than residential ones, probably reflecting occupational exposure.
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Waste water monitoring can also be used to check if public health policies are working, says Kevin Thomas at the University of Queensland, Australia. He and his colleagues studied samples to see what effect a minimum unit price policy had on alcohol consumption in the Northern Territory in Australia. They found immediately after the policy came into force in October 2018, but recovered to near pre-policy levels after 15 months, suggesting that additional measures were needed to maintain the reduction.
The team has undertaken similar studies for tobacco. Likewise, Kasprzyk-Hordern and her colleagues have been in the UK for traces of antibiotics and for bacterial genes conferring antibiotic resistance. This will provide a baseline to test the effectiveness of public health campaigns aimed at tackling the problem of antibiotic resistance by reducing antibiotic use.
Integrating such data with other sources of information can yield even deeper insights. Jake O’Brien at the University of Queensland led a team that correlated Australian census data with a range of biomarkers – chemicals produced by the human body – to investigate the relationship between socio-economics, diet and drug consumption. They found that , while poorer ones consumed more opioid painkillers, antidepressants and medication for high blood pressure. This suggests that waste water could be used to study the social, economic and demographic determinants of health. In long-term research, it could even track the prevalence of non-communicable conditions such as diabetes and mental health problems.
“There is so much that can be done,” says Kasprzyk-Hordern. But there are also challenges. Collecting samples on a large scale is one. Some researchers envision a future where at least some of the sampling and in a network of “smart sewers”. These might even be linked with each other and other environmental sensing systems via the “internet of things”. This remains a distant dream, but Alm’s team is currently developing while researchers in Europe have developed a for sniffing out illegal drug factories.
Right to privacy
Another challenge is protecting the rights of citizens (see “The ethics of your effluent”, below). Politicians objecting to waste water testing for SARS-CoV-2 in North Dakota recently that would ban or restrict such testing for genetic material or evidence of disease, citing concerns over privacy. The bill wasn’t passed, but it highlights the importance of public trust in the technology, says Halden. A recent survey of some 3000 adults across the US indicated that , but with some limitations: people supported its use for applications such as checking for diseases and environmental toxins, but were wary when it came to use related to lifestyle, diet and mental health.
Still, if waste water-based epidemiology can be developed and regulated, it could go a step beyond public acceptance to active public participation. Kasprzyk-Hordern envisages a future where citizens are directly involved, collecting information – about weather or other environmental factors – and adding it to the mix. Waste water epidemiologists are fond of quoting the author Victor Hugo, who described the Parisian sewers as , a place that revealed society’s hidden truths. Thanks to science, that conscience is being thoroughly examined.
The ethics of your effluent
Analysing samples of sewage may not seem like an ethical minefield. In flushing the toilet, don’t we abandon our waste in the same way we do when we dump our rubbish in landfill? Besides, it isn’t possible to identify an individual from the myriad chemical traces that turn up at the sewage works.
Not so fast, say Natalie Ram, a privacy law scholar at the University of Maryland. Waste water monitoring of large populations may not be too problematic, but when scientists sample smaller populations, thorny ethical issues can arise.
Highlighting high levels of illegal drug use or disease in a neighbourhood could lead to that community becoming stigmatised, says Ram, a particular problem in countries like the US where postcodes are heavily segregated along socioeconomic and racial lines.
It also raises the issue of personal privacy: as the size of the population being sampled shrinks, so the possibility of someone being identified from substances in waste water grows, especially if the sewerage data is cross-referenced with information from other sources, she says.
This certainly isn’t what the scientists who study waste water epidemiology have in mind. Many are acutely aware of the issues and have drawn up ethical guidelines, and undertake public consultations to ensure the technology is put to proper use.
Nevertheless, the lack of a legal framework is a real problem. “Most technology tools ultimately get turned to law enforcement purposes,” says Ram. “That’s why I think intervening at the front end and putting robust privacy protections in place is so important.” These could include shielding data from law enforcers and limiting what information is collected and the duration it can be held.
“When we have a technology that holds substantial promise to be put to public benefit, we should make sure that we regulate it appropriately and with public trust and privacy in mind,” she says.
