[Note that this article is a transcript of the video embedded above.]

When the COVID-19 pandemic was just getting started in early 2020, every major city, state health department, and federal agency involved built out data dashboards you could access online to check case counts and trends. Public health officials could constantly be heard asking everyone to “flatten the curve,” that curve being a graph of infection rates over time. But how do you get such a graph? By and large, our measure of the pandemic came through individual case counts confirmed with laboratory testing and reported to a data clearinghouse like the local public health department or the CDC. There was a lot of confusion about testing, positivity rates, how that information applied to the greater population, and how it could be used to implement measures to slow the spread of disease. The limitations of individual testing data - including test shortages, reporting delays, and unequal access to healthcare - made public health decisions extremely challenging. Much of the controversy surrounding mask mandates and stay-at-home orders was provoked by the disconnect between what we could reliably measure and the reality of the pandemic on the ground. Public health officials were constantly on the lookout for more indicators that could help inform decisions and manage the spread of disease.

One of these measures didn’t really show up in the online data dashboards, but it was and continues to be, used as a broad measure of infection rates in cities. It’s a topic that combines public health, epidemiology, and infrastructure that didn’t get much coverage in the news. And there are both some interesting privacy implications and some really fascinating applications on the horizon. I’m Grady and this is Practical Engineering. In today’s episode, we’re talking about wastewater surveillance for public health.

If you are unfamiliar with the inner workings of a modern municipal wastewater collection system, boy do I have the playlist for you. But, if you don’t want to watch 5 of my other videos before you watch this one, I can give you a one-sentence rundown: Wastewater flows in sewers, primarily via gravity, combining and concentrating as it continues to a treatment plant where a number of processes are used to rid it of concomitant contaminants so it can be reused or discharged back into the environment. Just like a watershed is the area of land that drains to a specific part of a river or stream, a “sewershed'' isn’t an outhouse but an area of a city that drains to a specific wastewater treatment plant. The largest sewersheds can include hundreds of thousands, or even millions of people, all of whose waste flows to a single facility designed to clean it up.

Wastewater treatment plants regularly collect samples of incoming sewage to characterize various constituents and their strengths. After all, you have to know what’s in the wastewater to track whether or not it’s been sufficiently removed at the other end of the plant. In the early days of sewage treatment, sampling consisted only of measuring the basic contaminants such as nutrients and suspended solids. But, as our testing capabilities increased, it slowly became easier and less expensive to measure other impurities, sometimes known as contaminants of emerging concern. These included pharmaceuticals, pesticides, personal care products, and even illicit drugs. It didn’t take too long to realize that tracking these contaminants was not only a tool for wastewater treatment but also a source of information about the community within the sewershed, the gathering of which is a notoriously difficult challenge in the field of public health.

Rather than coordinating expensive and arduous survey campaigns where many people aren’t always truthful anyway or going through the hoops of privacy laws to gather information from healthcare providers, we can just take a sample of sludge from the bottom of a clarifier, send it off to a lab, and roughly characterize in hours or days, the dietary habits, pharmaceutical use, and even cocaine consumption of a specific population of people. If you’re a public health researcher or public official, that is a remarkable capability. To quote one of the research papers I read, “Wastewater is a treasure trove of biological and chemical information.”

Think about all the stuff that gets washed down the drain and all the things you consume that might create unique metabolites that find their way out the other side of your excretory system. Although wastewater surveillance is a relatively new field of study, we’re already able to measure licit and illicit drugs, cleaning and personal care products, and even markers of stress, mental health, and diet. That’s a lot of useful information that can be used to monitor public health, but one particular wastewater constituent took center stage starting in early 2020. Of course, for decades, we’ve tracked pathogens in wastewater to make sure they aren’t released into the environment in treatment plant effluent, but the COVID19 pandemic created a vacuum of information on virus concentrations that had never been experienced before.

We realized early in the pandemic that the SARS-CoV-2 virus is shed in the feces of most infected people. Even before widespread tests for the virus were available, many public health agencies were sampling the wastewater in their communities as a way to track the changes in infection rates over time. Realizing the importance of coordinating all these separate efforts, many countries created national tracking systems to standardize the collection and reporting of virus concentrations in sewage. In the US, the CDC launched the National Wastewater Surveillance System in September of 2020, complete with its own logo and trademarked title. Let me know if I should try to license this design for my merch store.

Individual communities can collect and test wastewater for SARS-CoV-2, and then submit the data to the CDC for a process called normalization. Virus concentrations go up and down with infections, but they also go up and down with dilution from non-sewage flows and changes in population (for example, in sewersheds with large event venues or seasonal tourism). Normalization helps correct for these factors so that comparisons of virus loads between and among communities is more meaningful.

There are some serious benefits from tracking COVID-19 infections using wastewater surveillance. It’s a non-intrusive way to monitor health that’s relatively impartial to differences in access to healthcare or even whether infections are symptomatic or not. Next, it is orders of magnitude less expensive than testing individuals. Nearly 80% of US households are served by a municipal wastewater collection system, so you can get a much more comprehensive picture of a population for just the cost of a laboratory test. It can also provide an earlier indicator of changes in community-wide infection rates. Individual tests can have delays and miss asymptomatic infections, and hospitalization counts come well after the onset of infection, so wastewater surveillance can provide the first clue of a COVID-19 spike, sometimes by several days. Finally, now that vaccination programs are widespread and there is significantly less testing being carried out, wastewater surveillance is a great tool to keep an eye out for a resurgence in COVID-19 infections, and it can even be used to monitor for new variants.

Of course, wastewater surveillance has some limitations too, the biggest one being accuracy. The science is still relatively new, and there are lots of confounding variables to keep in mind. In addition to changes in dilution from other wastewater flows and sewershed population mentioned before, the quantity of viruses shed varies significantly between individuals and even over the course of any one infection. Right now, wastewater surveillance just isn’t accurate enough to provide a count of infected individuals within a population, so it’s mostly useful in tracking whether infections are increasing or decreasing and by what magnitude.

There are also some ethical considerations to keep in mind. That term “surveillance” should at least prick up your ears a little bit. Monitoring the constituents in wastewater at the treatment plant averages the conditions for a large population, but what if samples were taken from a lift station that serves a single apartment complex, school, or office building? What if a sample was taken from a manhole on the street right outside your house? Could the police department use the data to deploy more officers to neighborhoods where illicit drugs are found in the sewage? Could a city or utility provider sell wastewater data to private companies for use in research or advertising? That’s a lot of hypotheticals, but I wouldn’t be surprised to see a Black Mirror episode where some tech company provides free trash and sewer service just to collect and sell the data from each household. If you wanted to open a new coffee shop, how much would you pay to learn which parts of town have the highest concentrations of caffeine in the sewage? Maybe it would be called Brown Mirror.

The truth is that public health professionals have put a tremendous amount of thought into the ethics and privacy concerns of wastewater surveillance, but (as with any new field of science), there are still a lot of questions to be answered. One of those questions is, “What comes next in this burgeoning field of wastewater surveillance,” where public health researchers have access to a literal stream of data. There are many measures of public health that can be valuable to policy makers and health officials, including stress levels, changes in mental health, and the prevalence of antimicrobial-resistant bacteria (one of the greatest human health challenges of our time). Of course, all the work that went into standardizing and building out capabilities of tracking infections will certainly give us a leg up on resurgences of COVID-19 or any future new virus, heaven forbid. My weather report already has a lot more information than it did 20 years ago, including pollen counts of various allergy-inducing tree species, air pollution levels, and UV-ray strength. We might soon see infection rates of the various diseases that spread through community populations to help individuals, planners, and public officials make better-informed decisions about our health. Sewers were one of the earliest and most impactful advents of public health in urban areas, and it’s exciting that we’re still finding new ways to use them to that end.