杏吧原创

WHAT鈥橲 the largest source of mass moving in and out of a city every day? You think, if it鈥檚 a port city, it must be boats聽鈥� or, you know, maybe if it鈥檚 a landlocked city, it鈥檚 trains or trucks or cars or planes. No, it鈥檚 water. It鈥檚 water. There鈥檚 so much more water moving in and out of a city any day than there is any kind of cargo. It鈥檚 basically pure water coming in. And then the water that leaves has some traces of almost every human activity that鈥檚 going on in the city.鈥�

Once Eric Alm is in full flow, it is hard to stop聽him. But it isn鈥檛 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 鈥渄esigner鈥� 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. 鈥淵ou can think of the city as one big organism,鈥� says Rolf Halden at Arizona State University, another convert to the study of waste water. 鈥淚t has one聽metabolism.鈥� And that makes sewage the聽lifeblood of our communities.

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聽then to the sewer, where they coil and聽coalesce with those of your neighbours, your neighbours鈥� 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聽million people. Researchers tested the samples for聽cocaine and its principal metabolite, benzoylecgonine聽鈥� the main compound the body breaks it down into. This revealed that the聽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聽 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 Health Observatory. It wasn鈥檛 well received. 鈥淭here was just tremendous resistance to even聽accepting that there could be useful information in composited waste water,鈥� he says. However, the approach was vindicated when the team showed that two antimicrobial compounds called triclocarban and triclosan聽鈥� commonly added to soaps and other consumer products 鈥� 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聽waste water-based epidemiologists struggled to get funding and recognition. That聽changed in late 2019, when reports of a聽sinister new respiratory illness emerged from聽China. 杏吧原创s 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聽at hospitals, allowing health authorities to聽direct their resources accordingly. What鈥檚 more, Halden calculated that tracking the virus聽in this way was about 60聽times cheaper than using clinical testing.

Public health authorities soon caught on. The US Centers for Disease Control launched a聽national waste water surveillance system in聽2020, and has awarded grants totalling approximately $220 million to projects monitoring SARS-CoV-2 in this way. In the聽US聽and in other countries, this has been聽instrumental in helping authorities . 鈥淭hat鈥檚聽really where the dam broke and a lot聽of聽people began to become believers and practitioners of聽the technology,鈥� 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. 鈥淭he sky鈥檚 the limit,鈥� says Halden.

What is possible partly depends on where the samples are taken. All sewage ends up at聽a聽treatment plant, also called the sewage works.聽Sampling here allows you to look at a聽lot聽of people, which means that you can get聽a聽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聽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聽and the hunt begins. Each chemical of interest has particular qualities like mass and electrical charge, the chemical equivalent of a聽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. 鈥淚t鈥檚 not about going in and taking a measurement. It鈥檚 about developing a platform that can help you reach insights about what鈥檚 going on,鈥� 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聽chemical present depends on where the sample was taken and where the people excreting it were located. Also important is聽how the human body absorbs, processes and聽excretes a chemical of interest, as well as聽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聽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, 鈥淵ou really need to know the numbers.鈥�

Take bisphenol A, one of a class of chemicals known as endocrine disruptors that mimic the聽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鈥檚 team showed that most exposure to bisphenol A聽鈥� previously believed to come mainly from food packaging聽鈥� 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聽reflecting occupational exposure.

Health check

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聽recovered to near pre-policy levels after 15聽months, suggesting that additional measures were needed to maintain the reduction.

The team has undertaken similar studies for聽tobacco. Likewise, Kasprzyk-Hordern and her colleagues have been in the UK for traces of antibiotics and聽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鈥橞rien at the University of Queensland led a team that correlated Australian census data with a range of biomarkers 鈥� chemicals produced by the human body 鈥� 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.

鈥淭here is so much that can be done,鈥� says聽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聽network of 鈥渟mart sewers鈥�. These might even聽be linked with each other and other environmental sensing systems via the 鈥渋nternet of things鈥�. This remains a distant dream, but Alm鈥檚 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 鈥淭he ethics of your effluent鈥�, 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鈥檛 passed, but it highlights the importance of public trust in the聽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聽be developed and regulated, it could go a step beyond public acceptance to active public participation. Kasprzyk-Hordern envisages a聽future where citizens are directly involved, collecting information聽鈥� about weather or other environmental factors聽鈥� and adding it to聽the mix. Waste water epidemiologists are fond of quoting the author Victor Hugo, who聽described the Parisian sewers as , a place that revealed society鈥檚 hidden truths. Thanks to science, that conscience is being thoroughly examined.