In a pathbreaking article in 1965, Abel Wolman, arguably among the most significant US water engineers of the 20^th century, coined the phrase “The Metabolism of Cities”, and set forth the theme that cities, like organisms, could be thought to have inputs, metabolic processes, and waste products.  Medical science has long known that by analyzing outputs from individuals, whether in exhaled breath or in wastes (feces, urine), the knowledge of individual health can be improved.

Since measurements of poliovirus in sewage in the 1940’s, it has been known that analysis of sewage from a population can give insight into the population prevalence of pathogens.  Antibiotics and their residues, and pharmaceutical and personal care products have been measured in wastewater for some time and reflect population prevalence in use. More recently, measurements of drugs of abuse in wastewater have given insight into regional and temporal variations in this practice.

With the COVID19 pandemic, the presence of SARS-CoV-2 RNA in wastewater has been detected (though at this writing, no viable SARS-CoV-2 virus has been reported) and shows relationships to the disease intensity in a population.  This has raised the profile of this approach to the general public.  The application of this approach raises a number of scientific and ethical challenges that need consideration.

The use of wastewater-based epidemiology (WBE) for COVID19 brings the challenge of processing difficult samples for analysis by sophisticated molecular biological techniques.  Those who have approached this from a medical background do not understand the substantial variability of the matrices that need to be sampled.  Methods that have been optimized for clinical and diagnostic purposes (in relatively consistent matrices) may not be the most fit for purpose in the environmental context.  We need better statistical and microbiological methods for assessing normalized loads and relation to health status.  We need better ways to predict the fate and transport of pathogens and their markers within the sewershed to properly account for dynamics in our complex systems.  How do we apply these techniques to unsewered populations, whether in higher income countries or in LMIC’s?  We are starting to see WBE of COVID19 take advantage of metagenomic sequencing approaches.  What are the ways to best interpret these data in a quantitative sense?

The application of WBE, and more broadly the concepts of “sewage sociology” can be a very useful and efficient way to assess the health status of broad populations.  We need to understand the breadth of measurements that can be taken which could lead to actionable improvements in public health.  It has become possible to contemplate taking measurements at the neighborhood or even building scale, to assess the health status of smaller numbers of people. This value has been shown by sampling of individual residence halls at universities, where a positive “hit” for the RNA of SARS-CoV-2 has been followed up with individual testing and identification of infected persons.

In some sense, this smaller scale measurement in the sewer system is no different than what has been used for decades to assess the occurrence of improper waste discharges from industrial sources.  However, in another sense it is profoundly different — with the focus on individual or small numbers of persons, versus of individual enterprises.

It is generally considered that there is not an ethical issue involved in sampling at a community wastewater treatment plant.  In many countries, depending on the ethical and legal settings, sampling at the individual or household setting, would require informed consent of the persons involved.  For small neighborhoods, dormitory buildings, etc., the potential for stigmatization may arise.  There needs to be more involvement between those practicing WBE at smaller scales and legal and ethical experts so that the promise of these monitoring techniques can occur without overriding concerns for rights, liberty and privacy.

In recent years, the values of materials and chemical energy embodied in wastewaters and residuals have received increasing attention as we proceed to achieve a more circular economy.  We should remember that these streams may also be valuable for the information that they contain which can better inform us about the health of communities being served.  Just as effort is being devoted to better valorize materials in waste, more effort is needed to develop approaches to valorize the information content that waste contains.  Perhaps some day, the next emerging pathogen, or chemical toxin, can be recognized from an initial diagnosis in wastewater.  In this way we may more fully operationalize the conceptualization of Abel Wolman as cities having a metabolism.