A Framework for Public Health Authorities to Evaluate Health Determinants for Wastewater-Based Epidemiology

Abstract

Background:

Wastewater-based epidemiology (WBE) is rapidly developing as a powerful public health tool. It can provide information about a wide range of health determinants (HDs), including community exposure to environmental hazards, trends in consumption of licit and illicit substances, spread of infectious diseases, and general community health. As such, the list of possible candidate HDs for WBE is almost limitless. Consequently, a means to evaluate and prioritize suitable candidates for WBE is useful, particularly for public health authorities, who often face resource constraints.

Objectives:

We have developed a framework to assist public health authorities to decide what HDs may be appropriate for WBE and what biomarkers could be used. This commentary reflects the experience of the authors, who work at the interface of research and public health implementation.

Discussion:

To be suitable for WBE, a candidate HD should address a public health or scientific issue that would benefit from better understanding at the population level. For HDs where information on individual exposures or stratification by population subgroups is required, WBE is less suitable. Where other methodologies are already used to monitor the candidate HD, consideration must be given to whether WBE could provide better or complementary information to the current approach. An essential requirement of WBE is a biomarker specific for the candidate HD. A biomarker in this context refers to any human-excreted chemical or biological that could act as an indicator of consumption or exposure to an environmental hazard or of the human health state. Suitable biomarkers should meet several criteria outlined in this commentary, which requires background knowledge for both the biomarker and the HD. An evaluation tree summarizing key considerations for public health authorities when assessing the suitability of candidate HDs for WBE and an example evaluation are presented. https://doi.org/10.1289/EHP11115

Introduction

Wastewater-based epidemiology (WBE) is a rapidly developing approach to monitoring the health and well-being of people, via sampling within a wastewater catchment. In WBE, wastewater is analyzed for the presence of chemicals, metabolites or microorganisms based on the principles that any molecule ingested or absorbed by the human body will be excreted in urine or feces (either unchanged or as metabolites of the parent compound) and that many microorganisms are shed in urine or feces.¹ The concentrations of residues or microorganisms in urban sewage can therefore reflect the extent to which the population serviced by the sewerage network is exposed to a hazard.^1,2

The most well-developed application for WBE is the analysis of various illicit drugs consumed by a population.¹ However, the potential applications for WBE are rapidly expanding: WBE is being used to study consumption of substances such as nicotine,³ alcohol,⁴ caffeine,⁵ and new psychoactive substances (NPS)⁶; pharmaceutical use⁷; exposure to environmental contaminants (e.g., pesticides,⁸ phthalates,⁹ mycotoxins¹⁰); and in surveillance for microbial pathogens (e.g., influenza,¹¹ poliovirus,¹² hepatitis A,¹³ norovirus,¹⁴ SARS-CoV-2¹⁵) and for antimicrobial resistance (AMR).^16,17 There is also growing interest in monitoring endogenous biomarkers of disease (e.g., cancer, diabetes, cardiovascular disease),^18,19 or other measures of overall well-being such as nutritional status²⁰ or oxidative stress.^21,22 Recently, the COVID-19 pandemic has generated considerable interest in using WBE to inform public health responses, leading to a significant proliferation of WBE activities internationally.²³ This widespread investment in WBE infrastructure provides opportunities to extend analysis to biomarkers of other aspects of public health.

There is clear potential for WBE to support multiple aspects of public health research and practice, and the list of health-associated characteristics, hereafter referred to as health determinants (HDs), that could potentially be examined by WBE is almost limitless. Currently, however, much of the information obtained from WBE is either not directly actionable or can be obtained from other sources.²³ In addition, many applications are still in the academic or development stage.²⁴ As such, before any public health WBE program is implemented, a thorough evaluation of the suitability of the chosen HD for WBE is required. During this evaluation, it is important to consider the overall objectives and constraints of the program because context, priorities, and resource availability are likely to differ between the applied use of WBE by public health authorities and academic, curiosity-driven research. For example, will the data generated by WBE be used to inform immediate public health responses,²⁵ evaluate the effect of interventions,²⁶ guide policy making,²⁷ or support long-term monitoring of trends,²⁸ or will it help to expand academic knowledge? WBE is most useful in supporting public health outcomes if the data produced relates to a relevant public health issue, can be clearly and correctly interpreted, and is actionable in some way.²³ Public health authorities are often working with finite resources, necessitating pragmatic decision-making regarding their allocation. Given that undertaking WBE will divert some of those resources away from other public health activities, consideration must be given to the suitability of a HD for WBE, the quality and strength of the data that would be obtained, and how it would be used to inform a public health response to ensure that the implementation of WBE is warranted and appropriate.²⁹

The aim of this commentary is to present a framework that can be used to assist public health authorities in deciding whether WBE is appropriate to assess or monitor a public health issue of concern within their jurisdiction, and subsequently, what biomarkers could be used. The developed framework is intended as a logical progression through a series of evaluation criteria. These criteria were informed by the guidelines used in the selection of the New Zealand Environmental Health Indicators,³⁰ which are themselves based on known or plausible cause-and-effect relationships between environmental factors and a person’s health; surveillance of published literature to identify key requirements for successful WBE; and the authors’ own expertise and experience in WBE, both in research and development of new methodologies and in supporting the implementation of current WBE initiatives, including the New Zealand Drugs in Wastewater and COVID-19 in Wastewater programs. Finally, use of the framework is demonstrated using the evaluation of community-level alcohol consumption as an example.

Discussion

WBE has the potential to become a powerful public health tool, as evidenced by the integral role it has played in the recent COVID-19 pandemic.²³ However, as highlighted by both our own conversations with New Zealand public health authorities and in several recent commentaries in other jurisdictions,^23,29 public health authorities often need guidance when designing a WBE program to ensure that a) WBE is a suitable option for their chosen HD with regard to alternative monitoring options and the actionability of the data it will generate and b) they select a suitable biomarker for monitoring. In this commentary we present a framework to be used by public health authorities, or indeed other organizations seeking guidance on WBE, for evaluating the suitability of HDs for WBE and selection of appropriate biomarkers for monitoring. Although we expect that the key elements of this framework will be transferrable across public health authorities, the specific considerations within each element may be more nuanced, and the outcomes of using the framework may differ depending on the context of unique jurisdictions (e.g., reflecting public health priorities of their population, resourcing, and existing infrastructure and expertise). Not every criterion within the framework will apply to every HD, and failure to satisfy one criterion will not necessarily be means for excluding a WBE approach. However, there are some critical stop/go criteria that must be met for the HD to be considered further. An evaluation tree developed to support this framework is presented in Figure 1.

Figure 1.

Evaluation tree for the proposed wastewater-based epidemiology (WBE) framework highlighting key considerations for public health authorities when choosing a health determinant (HD) for WBE. Criteria specifically associated with assessment of the HD are indicated in shaded boxes with white text, and criteria specifically associated with assessment of potential biomarkers for the chosen HD are indicated in clear boxes with black text. Dashed decision points do not necessarily preclude a WBE approach, but the potential limitations should be carefully considered. Further details on the considerations associated with these criteria are provided in the main text. Note: B, biomarker.

Although WBE for illicit drugs is most commonly associated with assessment of legal compliance, it has the potential to inform health education policies and monitor their effectiveness³¹ and to direct health program efforts (e.g., drug rehabilitation services) to areas of the most need. Illicit substance abuse poses serious health risks, including for mental health, drug-related diseases, overdose, and suicide,³¹ with $>\mathrm{750,000}$ deaths per year worldwide due to illicit drug use.³² As such, we have included the consumption of illicit substances as example HDs in this commentary.

Evaluation of Candidate HDs

We have identified four key criteria for evaluating the suitability of HDs for WBE, as indicated in Figure 1. These include

• the requirement for individual- or population-level information (HD1)

• alternative methodologies for monitoring the HD (HD2)

• availability of a suitable biomarker (HD3)

• the requirement for changes in biomarker levels to be reflective of changes in the prevalence of the HD (HD4).

Although not included as a criterion of the framework, the candidate HD should relate to a relevant public health issue within the jurisdiction of the authority. We envisage that given both the mandate and pragmatism required of public health authorities as discussed above, authorities will be using the framework to assess HDs that they have already identified are of concern within their jurisdiction.

HD1.

Is population-, subpopulation-, or individual-level information required? Given that WBE is based on aggregate data and cannot identify individuals, applications generally focus on HDs where either a large proportion of the population is affected or where responses to those HDs will be at a population level (e.g., infectious diseases, where a few individuals have the potential to infect many others). Some demographic segmentation for a HD may be possible through sampling that focuses on specific suburbs, neighborhoods, or individual buildings,^33,34 although careful consideration should be given to the ethical implications of sampling small groups.^35,36 However, where it is essential to have information broken down by certain specific subpopulations (e.g., children, pregnant women) or data on individual-level exposure is required, a WBE approach may be not suitable. For example, WBE could be used to estimate the average exposure of a community to environmental hazards, such as pesticides,³⁷ but could not ascertain whether, how many, or to what extent individuals may be exposed at levels exceeding accepted risk levels or tolerable intakes. In addition, the inability to identify individuals means biomarkers of exposure or health status cannot be correlated with information from questionnaires or interviews—an often-important component of other epidemiological tools, such as biomonitoring surveys.

HD2.

Consideration should be given to other methodologies currently used to monitor the given HD. The presence of alternative methodologies does not necessarily preclude a WBE approach; rather, alternative methodologies may be useful for validating and calibrating WBE approaches. For example, results from WBE for nicotine consumption have been shown to be in good agreement with data from self-report surveys and sales data, validating this approach.^3,38 However, where alternative methodologies do exist, WBE should either provide a valuable complementary approach or be better than the current approach (i.e., faster, more reliable, or cheaper). For example, deaths due to heart attacks are generally accurately identified and counted, so WBE for this HD would add little value. Conversely, WBE has proven particularly useful as a complementary approach for evaluating illicit drug usage given that the illegal nature of these substances means reliable data are often difficult to obtain from other sources, such as self-report surveys.^39,40

HD3.

A measurable biomarker specific for a given HD must be identified. A variety of different chemicals or biologicals can act as biomarkers for WBE, and there may be more than one potential biomarker for a given HD. For example, AMR in a community can be evaluated using a range of different types of biomarkers, including antimicrobial chemicals,^41,42 antimicrobial-resistant bacteria,¹⁶ and AMR genes.¹⁷ For infectious diseases, the disease-causing organism (e.g., poliovirus^12,43) or its genomic material (e.g., DNA of Mycobacterium spp. for tuberculosis,⁴⁴ RNA of the measles virus⁴⁵) may be the biomarker. For illicit drugs, the drug itself [e.g., methylenedioxymethamphetamine (MDMA)] or human-specific metabolites (e.g., benzoylecgonine and ecgonine methyl ester for cocaine consumption) can act as the biomarker.⁴⁶ Similarly, for exposure to environmental hazards, the hazard itself (e.g., mycotoxins¹⁰) or a human-specific metabolite (e.g., phthalate metabolites⁹) can act as the biomarker. It is helpful to choose a biomarker for which there is considerable preexisting scientific knowledge, particularly regarding its metabolism. For example, a drug such as heroin that is almost completely broken down by the body into nonspecific metabolites is a less ideal biomarker. Morphine, the major metabolite of heroin, can also enter the wastewater network via therapeutic or illicit consumption of both morphine and codeine,⁴⁷ making it a nonspecific biomarker. Although it is possible to monitor for the minor, exclusive heroin metabolite 6-monoacetylmorphine (6-MAM), only 1.3% of consumed heroin is excreted as 6-MAM,⁴⁷ and its low stability means heroin consumption would likely be underestimated.⁴⁸ It is possible to use WBE to monitor for biomarkers common to several different HDs, such as isoprostanes, which are biomarkers of systemic oxidative stress produced in response to various HDs including cancer, diabetes, and alcohol consumption.²² However, where a chosen biomarker relates to multiple HDs, careful consideration must be given to interpretation of the data.^21,22

It has been proposed that a WBE approach could be useful for monitoring disease-associated biomarkers.^18,19 Indeed, a WBE approach has been used to assess the prevalence of the rheumatic arthritis disease gout.⁴⁹ However, many chronic diseases currently have no known specific biomarker that could be measured and thus are presently unsuitable for WBE. It has also recently been suggested that WBE could be used to track progress toward meeting the United Nations Sustainable Development Goals, with several potential biomarkers for monitoring the various goals proposed (e.g., the hunger hormone ghrelin to assess the prevalence of undernourishment).⁵⁰

When choosing a biomarker, consideration should also be given to WBE programs occurring elsewhere. If the HD is already the target of monitoring, we recommend the same biomarker ideally be employed to allow study comparisons.

HD4.

Changes in prevalence of the candidate HD must be reflected in changes in levels of the associated biomarker. A good example of this is WBE for tobacco consumption using the human-specific nicotine metabolites cotinine and trans-3′-hydroxycotinine as biomarkers of consumption.^3,51 Changes in tobacco consumption result in changes in cotinine and trans-3′-hydroxycotinine levels in wastewater, with several studies showing that estimates of tobacco consumption based on WBE were comparable to data from surveys.^3,38,51 In contrast, exposure to the organochlorine pesticide dichlorodiphenyltrichloroethane (DDT) is a poor candidate for WBE given that although DDT is excreted in urine, albeit at low levels ($<10\mathrm{\mu }\mathrm{g}/\mathrm{L}$ in nonoccupationally exposed individuals), the majority is stored in body tissues and slowly eliminated at a rate of $\sim 1\%$ of stored levels per day.⁵² As such, levels of excreted DDT are unlikely to directly reflect changes in exposure levels.

Evaluation of Candidate Biomarkers

According to the framework we developed, the next step in determining the suitability of a given HD for WBE is to evaluate candidate biomarkers. We have identified five key criteria for the evaluation of a biomarker for WBE (Figure 1), which include that the biomarker (B)

• be excreted and enter the wastewater network, ideally in urine or feces (B1)

• be accurately and reliably detectable in wastewater (B2)

• be present only in wastewater owing to human excretion (B3)

• be stable in wastewater (B4)

• that the biomarker–HD relationship is consistent over time (B5).

B1.

The most important consideration when selecting a biomarker for WBE is that the biomarker is shed or excreted in bodily secretions. Biomarkers excreted in urine may be preferred because of the frequency of urination; however, biomarkers predominantly excreted in feces are also readily detectable in wastewater (e.g., RNA from human enteric viruses⁵³). Biomarkers may also enter the wastewater network via oral, nasal, or dermal secretions (e.g., during showering/bathing, handwashing, spitting, mouth washing, nasal irrigation). For example, a recent SARS-CoV-2 study identified viral RNA in washbasin and shower siphons of households containing known COVID-19 cases,⁵⁴ and modeling has suggested that sputum can be a major source of SARS-CoV-2 in wastewater.⁵⁵ Although the ability of WBE to detect biomarkers predominantly present in nonurinary/fecal sources is an area requiring more attention, it would seem likely that the detection of such biomarkers will be influenced by the scale of the monitoring, with community-level monitoring less likely to detect these biomarkers compared with neighborhood- or building-level monitoring. Indeed, modeling for SARS-CoV-2 has suggested that at the building scale, contributions from either feces, urine, saliva, or sputum could dominate with respect to viral contribution to the wastewater network.⁵⁶ As such, when selecting a biomarker, consideration should be given to the scale of the monitoring and to the potential source contribution (e.g., contribution from showering is less likely when sampling from schools or workplaces).

If a candidate biomarker is not shed or excreted, or is excreted in insufficient quantities to be detected, it is unsuitable for WBE. Preexisting knowledge of the rate or timing of excretion of the chosen biomarker is also crucial. Depending on the biomarker, this may vary between individuals or across population demographics (e.g., age, gender, health status), so it is beneficial to have extensive quantitative data across different groups. In addition, in the case of infectious diseases, not every infected individual may shed detectable amounts of the biomarker. For example, some studies have found that only $\sim 40\%–60\%$ of people infected with SARS-CoV-2 shed viral RNA at detectable levels in their feces.^57,58

A good biomarker with respect to excretion is the anesthetic drug ketamine. Ketamine is excreted in urine, and the excretion factors for ketamine and its major metabolite norketamine were recently revised, resulting in proposed excretion factors of 20% for ketamine and $\sim 4\%$ for norketamine.⁵⁹ In contrast, the brain-specific intermediate filament protein glial fibrillary acid protein (GFAP), a blood-based biomarker of acute ischemic stroke,⁶⁰ is a poor candidate biomarker for WBE given that it does not appear to be excreted in urine or feces.

The accuracy of results from WBE will depend on the excretion profiles of the contributing population. For biomarkers where rates of excretion vary considerably between individuals, the lower the number of people within the population excreting the biomarker, the greater the impact of this variability in excretion rates. As the number of people excreting the biomarker increases, the variation in excretion rates will average out to give more consistent and reliable measurements. For any HD, lag periods may need to be considered owing to the length of incubation periods for infectious diseases or differences in metabolism rates.

B2.

Can the biomarker be accurately and reliably detected in wastewater? Excretion of a biomarker in urine, feces, or other bodily excretions that enter the wastewater network does not necessarily guarantee its detection in wastewater. Wastewater is a complex matrix, composed of solids, dissolved particles, heavy metals, nutrients, microbes, and other micropollutants.⁶¹ As such, multiple refining steps are often required to remove inhibitory substances (e.g., fats, proteins, detergents)³⁵ and concentrate the sample before detection of a biomarker is possible. These steps vary depending on the biomarker and the detection method.

Prevalence of the HD within the population will also impact its ability to be assessed using WBE. For example, acetaminophen (paracetamol) is generally an ideal biomarker with regard to being detectable in wastewater given that it is one of the most used analgesics around the world, with a maximum dose of up to $4\mathrm{g}$ per 24 h.⁶² As such, numerous WBE studies have detected acetaminophen at very high concentrations ($>100\mathrm{\mu }\mathrm{g}/\mathrm{L}$) in untreated wastewater.^63–66 In contrast, the illicit hallucinogenic drug lysergic acid diethylamide (LSD) and its metabolites are less ideal candidates for WBE. LSD has a very low active dose, with effects seen from a light dose of $\sim 15\mathrm{\mu }\mathrm{g}$ and a heavy dose being only $300\mathrm{\mu }\mathrm{g}$,⁶⁷ and it is generally only consumed by a small number of people, meaning levels in wastewater will likely be below the level of detection or quantification.^68,69

Depending on the nature of the WBE program, quantitative or qualitative data may be required. Many WBE studies employ a quantitative approach, which allows for long-term comparisons of consumption or exposure over time. For example, a recently published Australian study quantitively assessed temporal trends in artificial sweetener consumption over a 7-y period.⁷⁰ Similarly, the Sewage analysis CORe group–Europe (SCORE) network performs quantitative annual monitoring for consumption of a range of different illicit substances across Europe, allowing both temporal and geographical patterns to be assessed.⁷¹ In contrast, in surveillance for infectious disease (e.g., WBE for COVID-19 during the early stages of the pandemic,^72,73 monitoring for new variants of concern⁷⁴) or suspect screening for NPS,⁷⁵ qualitative data may be sufficient. In addition, for some biomarkers it may not be possible to obtain accurate quantitative information. For example, the phytocannabinoid delta-9-tetrahydrocannabinol (THC) and its metabolite carboxy THC (THC-COOH) are highly lipophilic, meaning they can preferentially bind to sewer pipes and solid materials rather than being transported to the inlet of the wastewater treatment plant without losses.⁷⁶

The range of WBE biomarkers that can be accurately and reliably detected, and quantified, in wastewater will likely expand in the future with the advent of new technologies. For example, WBE methods are being combined with proteomics approaches, opening the possibility of monitoring for protein-based biomarkers.^19,77 Future incorporation of biosensors into WBE monitoring may also allow for more real-time detection of target biomarkers, particularly infectious disease agents.⁷⁸

B3.

To be suitable for WBE, candidate biomarkers should ideally be present only in wastewater due to human excretion in response to exposure to the candidate HD.²² This can be particularly difficult given that wastewater often contains not only excreted matter (black water) but also water from showers/baths, sinks, dishwashers, and washing machines (gray water), as well as industrial and environmental inputs. Many biomarkers are present in a wide range of plant or animal tissues or can be produced (or consumed) by microbes.²⁰ The best approach to overcome the potential problem of exogenous sources is to monitor for a human-specific metabolite of the biomarker of interest. For example, cocaine can be present in wastewater due to human consumption or due to direct deposition into the wastewater system (e.g., drugs flushed down the toilet to avoid detection by law enforcement). To distinguish these possibilities, instead of monitoring for cocaine directly, its metabolites benzoylecgonine and ecgonine methyl ester can be measured.⁴⁶ Back calculations can be performed using correction factors incorporating the percentage of the parent drug excreted as that metabolite and the molecular mass ratio of metabolite to parent drug.⁷⁹ Similarly, caffeine consumption can be distinguished from leftover coffee poured down the drain by monitoring for its metabolite 1,7-dimethyluric acid.⁵ In contrast, bisphenol A (BPA) is a less ideal biomarker with regard to this step of the framework. Although BPA is detectable in wastewater, and has been assessed using a WBE approach, urinary BPA was noted to account for $<1\%$ of the load found in wastewater, with nonhuman sources, such as leaching from plastic products, domestic cleaning products, or industrial inputs, likely accounting for the rest.^80,81 Bisphenol sulfate has been used as a urinary biomarker of human BPA metabolism⁸²; however, sulfated bisphenol analogs have not been extensively investigated and further work is recommended to assess their occurrence and potential sources in wastewater.⁸⁰

For biomarkers where no suitable human-specific metabolite is available, it may be possible to compare levels in wastewater with data on average human excretion levels,²⁰ and in cases where levels in wastewater exceed that which would be expected based on excretion levels in urine/feces and given the population size, it can be assumed that there are nonhuman sources contributing to biomarker presence. For example, Choi et al. found riboflavin (vitamin B2) levels in Australian wastewater more than twice the expected level (based on average human excretion rates), indicating significant nonhuman sources.²⁰ Where sufficient information is available on background (i.e., exogenous) levels of the biomarker in wastewater, it may be possible to subtract these background levels from detected levels and still obtain useful information.

B4.

The stability of candidate biomarkers is another key consideration for WBE studies. This includes stability in-sewer, during collection, transport and storage, and during analysis.⁸³ Amphetamine-type drugs (e.g., amphetamine, methamphetamine, MDMA) are ideal examples of stable biomarkers, with little to no degradation after 26 h in wastewater (pH 7.5, 20°C),⁸⁴ and estimated degradation half-lives of $>200\mathrm{h}$.⁸⁵ In contrast, the urinary biomarkers of meat consumption anserine and carnosine are less-suitable biomarkers for WBE studies because they are rapidly degraded in wastewater, with 10% degraded within $\sim 5\text{\hspace{0.17em}min}$ in a laboratory sewer reactor.²⁰ It has been proposed that for best practice, $<10\%$ of the chosen biomarker should degrade within the mean residence time of the wastewater network.⁸⁶ However, many biomarkers can still provide useful information even if they degrade at a faster rate than this, particularly for qualitative studies.²⁰ A lack of information on biomarker stability does not necessarily preclude the possibility for WBE, but it may necessitate pilot studies to determine stability. However, it is important to acknowledge that many studies have focused on biomarker degradation in laboratory sewer reactors, rather than directly in-sewer, and that that these studies may overestimate degradation rates.⁸⁷

B5.

To facilitate ongoing monitoring, the relationship between a HD and its chosen biomarker should be consistent. One aspect of this is changes in the biomarker being measured. For example, NPS are constantly developed and modified to avoid detection or achieve new outcomes; in 2018, there were almost 900 different NPS monitored by the United Nations Office on Drugs and Crime.⁸⁸ Given that new NPS are constantly entering the market, the turnover of preferred NPS is rapid and long-term analysis is less meaningful. Despite these limitations, useful information can still be obtained from WBE studies of NPS consumption, including unbiased insight into the rapidly changing NPS market, differences in usage preferences between countries,^6,89,90 and short-term changes in usage associated with special events, such as New Year’s Eve celebrations.⁶ However, WBE programs for NPS need to be flexible to adapt to the rapidly changing NPS landscape. For example, using a qualitative suspect screening approach, target NPS do not need to be preselected prior to running the mass spectrometry analysis but, rather, an accurate-mass full spectrum is generated, which can then be post-screened for tentative identification of any suspect NPS based on its accurate-mass.⁷⁵ Further confirmation can subsequently be achieved by comparison with an analytical standard.

Special consideration must also be given to infectious diseases because excretion levels may change significantly over the course of an infection and new variants may change features of the biomarker, affecting its detection. For example, mutation in the SARS-CoV-2 spike (S) gene in a region targeted by diagnostic reverse transcription-polymerase chain reaction (RT-PCR) leads to S-gene dropout, therefore the PCR fails, leading to a false-negative result.⁹¹

Example Framework Evaluation for Alcohol Consumption

To demonstrate the use of our framework in evaluating a candidate HD, we have assessed the suitability of alcohol consumption for monitoring using WBE. Alcohol consumption is a major risk factor for health, economic, and social harms to individuals and communities^92–94; it therefore represents a relevant public health issue, with further understanding of consumption patterns and drivers being valuable in guiding education and harm-reduction initiatives, and policy decision-making.⁹²

HD1: Is population-level information useful or is information on individual-level consumption required?

Given that WBE cannot identify individuals,³⁵ it cannot provide insight into specific aspects of alcohol consumption, such as the prevalence of heavy drinkers or underage consumption. However, estimates of per capita consumption can be used to investigate general patterns of consumption, including spatiotemporal variation (e.g., weekly and long-term trends,^93,95 geographic variation or hotspots^95,96) and behaviors (e.g., during festivals or holidays^93,97) and to assess the effectiveness of interventions.²⁶

HD2: Are alternative methods available to estimate population-level alcohol consumption?

Conventional methods to assess alcohol consumption include surveys, sales data, and information from law enforcement and hospitalizations. These methods may provide insight into patterns of consumption (e.g., heavy or underage consumption); however, they are also time consuming, costly, and prone to bias (e.g., inaccurate self-reporting, inadequate population coverage) and do not account for informal or illicit production.^38,92,95 Wastewater-based methods can provide timely, cost-effective, and objective complementary data on total consumption within a community.^93,96

HD3: Is there a specific biomarker available to assess alcohol consumption?

Ethyl sulfate (EtS), a minor metabolite of ethanol, has been widely used as a WBE biomarker for alcohol consumption,⁴ and it is also an established biomarker used in clinical and forensic settings.^97–99

HD4: Are changes in alcohol consumption reflected in changes in EtS levels?

Excretion of EtS was shown to be dose dependent, based on the quantity of ethanol ingested.¹⁰⁰

B1: Is EtS excreted?.

EtS is excreted in urine and quantitative data on excretion is available.^100,101

B2: Can EtS be accurately and reliably extracted from wastewater?

Established methods exist to extract and quantify EtS in wastewater, and these have been widely applied around the globe,⁴ with several studies showing comparable results to survey data.^4,38,93

B3: Is EtS only in wastewater due to human consumption (or can exogenous sources be accounted for)?

EtS is a human metabolite of ethanol,¹⁰⁰ meaning it can be used to distinguish consumed alcohol from exogenous sources (e.g., disposal of alcoholic beverages, industrial sources). Further, one study suggests that in-sewer formation of EtS from unconsumed alcohol is unlikely.¹⁰²

B4: Is EtS stable in wastewater?

EtS has been reported to be stable in wastewater at room temperature for at least 1 wk and more than a month at $-20°\mathrm{C}$.¹⁰³ However, it was suggested that degradation of EtS may occur in-sewer (based on results using sewer reactors), and this should be accounted for when estimating alcohol consumption.¹⁰⁴

B5: Is the relationship between alcohol consumption and EtS levels in wastewater consistent over time?

The relationship between alcohol consumption and EtS production is consistent over time, with several long-term WBE studies reported.^69,93,95 This consistent relationship allows for both temporal and spatial comparisons of consumption,⁹⁵ as well as assessment of public health interventions.²⁶

Further Considerations

Having ascertained that a HD is appropriate for monitoring by public health authorities using a WBE approach, there are several other considerations that must be worked through when designing a monitoring program and confirming practicalities of implementation, as discussed by Safford et al.²³ and O’Keeffe.²⁹ Although these sit outside of the evaluation framework presented, they are important in ensuring that the program is fit for purpose and appropriate. For example, what level of detail is required to inform decision-making (e.g., detection limits, frequency of sampling, granularity of subcatchments); what are the resource needs (e.g., is there existing sampling in place that can be leveraged, what are the costs, is there sufficient expertise and capacity); how will the data be managed (e.g., who will have access, how will it be stored); how will the data be used (e.g., how will it be analyzed, do action limits needs to be set); what stakeholders need to be involved; and how will the results be communicated or reported? Further, full and careful consideration must be given to the ethics of data collection, analysis, availability, and use (e.g., sampling at subcatchment level in a way that minimizes the risk of stigmatization or harm to the community).^29,105 Some of these questions may guide prioritization of HDs for WBE where there are several suitable candidates (or competing non-WBE priorities). For example, if other wastewater-based monitoring programs are in place, is it possible to leverage these and add the chosen HD to the existing workflow (e.g., sample collection and/or laboratory analysis)? This can significantly reduce costs and resource or logistical constraints: Analysis of wastewater for illicit drugs typically involves the same extraction and detection protocol for a range of substances and metabolites, allowing them to be analyzed simultaneously.⁴⁶

Going forward, as both public health priorities and the technologies supporting WBE continue to evolve, it is imperative that WBE practitioners and public health officials engage in regular communication to ensure that the data collected through WBE is suitable for supporting public health research and policy and, further, that programs can be adapted to respond to changing public health needs.²³

Conclusions

The evaluation framework described in this commentary can be used by public health authorities to assess a wide variety of HDs for their suitability for WBE. It outlines key criteria to assess both the candidate HDs and their associated biomarkers, supported by suitable and non/less-suitable examples for each step. However, it is important to note that this framework must be flexible with regard to changes in public health priorities and new technological developments. It is anticipated that use of this framework will provide considerable support to public health authorities and other public organizations considering establishing or expanding their own WBE program.