Frequently asked questions (FAQ):
wastewater-based epidemiology and drugs
Wastewater analysis is a rapidly developing scientific discipline with potential for monitoring real-time data on geographical and temporal trends in illicit drug use. On this page you can find answers to some common questions about the method, including background information, use, application and interpretation of the findings. The EMCDDA would like to thank the Sewage Analysis Core Group Europe (SCORE) for their help in drafting this publication.
Pick a question below to jump straight to its answer on the page.
- What are the challenges in measuring the use of the main illicit drugs?
- Can it be used to monitor the use of new psychoactive substances?
- Can it be used to monitor alcohol use?
- Can it be used to monitor the misuse of medicines?
- Can it be used at events such as music festivals?
- Can it be used to monitor drug production?
- Can it be used to evaluate drug policy and law enforcement interventions?
- Can results from a few cities be extrapolated to an entire country?
- How often is sampling needed to achieve reliable results for a year?
- Is it recommended to collect more samples from fewer locations or fewer samples from more locations?
- How can we be sure that the drugs found are due to use and not dumping?
What is wastewater-based epidemiology (wastewater-based epidemiology) and how does it contribute to our understanding of the drug situation?
Wastewater analysis is a rapidly developing scientific discipline with potential for monitoring real-time data on geographical and temporal trends in illicit drug use. Originally used in the 1990s to monitor the environmental impact of liquid household waste, the method has since been used to estimate illicit drug consumption in various cities. It involves sampling a source of wastewater, such as a sewage influent to a wastewater treatment plant. This allows scientists to measure the levels of illicit drugs and their metabolites excreted in urine, which can then be used to estimate the quantity of drugs consumed by a community. This provides a non-invasive, near-real-time analysis of drug use within the area served by the sewer network.
Wastewater-based epidemiology consists of several steps. First, representative composite samples of raw wastewater are collected. Second, the wastewater is analysed for selected substances and the amounts measured. Third, the concentrations of the measured target residues are multiplied by the daily flow rates of sewage to obtain the daily sewer loads of the target residues. Fourth, the total daily consumption of a drug is estimated by applying a specific correction factor to the daily sewer loads. The correction factor considers the average excretion rate of a given drug residue and the molecular mass ratio of the parent drug to its metabolite. Finally, daily values are divided by the number of people served by the treatment plant to obtain a per capita value to facilitate comparison among cities. This value can then be expressed in daily amounts (or daily doses) per thousand population.
The findings of such an analysis are subject to uncertainties at various points in the process:
- a suitable sampling method must be applied at the treatment plant and documented;
- appropriate biomarkers are required to identify the drugs in the wastewater (i.e. biomarkers that are human specific, stable in wastewater and excreted at measurable levels in wastewater);
- the tools used for analysis vary in their robustness;
- standard average excretion rates are necessary to establish the quantities consumed; a reasonable estimate of the population served by the treatment plant is required.
Each of these issues has been carefully considered by wastewater professionals, and full or partial solutions adopted. Information on some of these is outlined in the answers below.
Wastewater analysis has demonstrated its potential as a useful complement to established monitoring tools in the illicit drugs area. The various sources of information on the drug situation have different uses and strengths. In comparison with survey methods, wastewater analysis is not subject to response and non-response bias. By formally testing, it can identify the true spectrum of drugs being consumed rather than relying on individual recollection or belief. It also has the potential to provide timely information, within short time frames, on geographical and temporal trends. However, it cannot provide information on prevalence and frequency of use, numbers of users, types of user and purity of the drugs. Comparing data from wastewater analysis with data obtained through other indicators is an important area of continuing work that will help establish the merits and validity of both.
What are the ethical considerations when undertaking wastewater analysis?
Although primarily used to study trends in illicit drug consumption in the general population, wastewater analysis has also been applied to small communities, including workplaces, schools, music festivals, prisons and specific neighbourhoods. Using this method in small communities can involve ethical risks, such as potentially identifying a particular group within the community. In 2016 the Sewage Analysis Core group Europe (SCORE) published ethical guidelines for undertaking wastewater-based epidemiology and related techniques. The objective of these guidelines is to outline the main potential ethical risks in wastewater research and to propose strategies to mitigate those risks. Mitigating risks means reducing the likelihood of negative events and/or minimising the consequences of negative events.
Will wastewater-based epidemiology replace the ‘established’ methods of measuring drug use (e.g. drug use surveys)?
Wastewater-based epidemiology has established itself as a useful complement to established monitoring tools in the illicit drugs area. It has some clear advantages over other approaches, as it is not subject to response and non-response bias, and can more accurately identify the true spectrum of drugs being consumed, as users are often unaware of the actual mix of substances they take. This tool also has the potential to provide timely information, in short time frames, on geographical and temporal trends. To check the quality and accuracy of data, further comparisons between data from wastewater analysis and data obtained through other indicators are needed.
What are the challenges of using wastewater-based epidemiology to measure the use of the main illicit drugs (cannabis, cocaine, amphetamines, ecstasy/MDMA, heroin/opioids)?
The challenges of using wastewater-based epidemiology for this purpose relate to various factors, such as the methods of sample collection, the tools used for analysis, the choice of suitable biomarkers (i.e. biomarkers that are human specific, stable in wastewater and excreted at measurable levels in wastewater) and normalising the results to the population size. The uncertainties related to all of these factors have been characterised and a best practice protocol developed to keep them as low as possible.
Nowadays, applying wastewater-based epidemiology for estimating cocaine, amphetamine, methamphetamine and MDMA use is well established and the related uncertainties are well known. However, when applying it for estimating the use of cannabis and heroin, several specific challenges need to be carefully considered.
Cannabis use is estimated by measuring its main metabolite (THC-COOH), which is the only suitable biomarker found so far, but its excretion percentage is low. More research is therefore needed to improve our understanding of the excretion percentage of THC-COOH or to find alternative biomarkers.
The specific metabolite of heroin, 6-monoacetylmorphine, is unstable in wastewater; thus, the only alternative is to use morphine, although it is not a specific biomarker and is also being excreted as a result of its therapeutic use. It therefore requires particular care to derive accurate figures for therapeutic morphine use from prescriptions and/or sales reports.
Can wastewater-based epidemiology be used to monitor the use of new psychoactive substances (NPS)?
Wastewater-based epidemiology has been recently explored for monitoring the use of NPS, as it can be very useful as an early warning system for identifying new substances and can provide information on the substances used. For monitoring NPS, there are some specific challenges in using wastewater-based epidemiology related to detecting a biomarker, such as the low prevalence of use, and hence the low level of the drugs in the wastewater; their use being limited to specific sub-populations; the wide range of substances available on the market; and the lack of information on their metabolism, biotransformation and stability in wastewater. Despite these limitations, the few studies performed so far are very promising, and further research will allow the routine application of wastewater-based epidemiology to monitor the use of NPS in the population.
Can wastewater-based epidemiology be used to monitor alcohol use?
Research groups in Europe and Australia have optimised and validated bioanalytical assays to detect and quantify a minor metabolite of alcohol, ethyl sulphate, as a biomarker of alcohol use in wastewater. Measured concentrations of this biomarker in wastewater can be used to calculate population-normalised mass loads of ethyl sulphate to evaluate temporal and spatial trends in alcohol consumption. The research on alcohol use using wastewater analysis also reported transforming ethyl sulphate mass loads into amounts of alcohol consumed using a correction factor based on the metabolism of alcohol and the excretion of ethyl sulphate. Because ethyl sulphate is only a minor metabolite of alcohol, this correction factor is high and has considerable uncertainty.
Can wastewater-based epidemiology be used to monitor the misuse of medicines?
The use and sales of medicines that can be obtained by prescription only are registered in most EU countries. From the registration data it is possible to calculate what concentrations and loads of these pharmaceuticals, or their excretion products formed in the body as a result of their consumption, can be expected in the influent of a wastewater treatment plant. If the actual concentrations or loads of a certain pharmaceutical exceed the expected levels, then non-prescribed use is likely to have occurred, and this may point to unregistered sales through potentially illegal channels such as online pharmacies. For example, wastewater-based epidemiology has been successfully used to identify the misuse of sildenafil (active ingredient of Viagra) and weight loss agents, such as 2,4-dinitrophenol (DNP) and norephedrine.
Can wastewater-based epidemiology be used at events such as music festivals?
Wastewater-based epidemiology can be used at events in several ways. If the collective urine from the event is flushed into the domestic sewer system, measurements of loads in the influent of a wastewater treatment plant during, before or after the event can provide relative ratios of drug loads that can reveal the contribution from the event. Urine can also be collected separately (e.g. in portable toilets), sampled and analysed. This will provide an indication only of what kind of substances have been used, because the levels measured cannot be normalised to the amount of urine excreted and to the number of people in order to apply the standard wastewater-based epidemiology procedure.
Can wastewater-based epidemiology be used to monitor drug production?
In some Dutch cities the monitoring of illicit drugs in wastewater over a one-week period has shown incidental dumps of drugs into the sewer systems. For example, monitoring over longer periods of time (typically more than 30 days) in Eindhoven demonstrated that amphetamine and MDMA were being discharged frequently, which points to drug production in this region. Incidents at small wastewater treatment plants in the south of the Netherlands, where influents were analysed and synthetic drugs were encountered, have revealed chemical profiles typical of chemical waste from specific synthesis routes used to manufacture these drugs. Such profiles can be used to identify disposal of waste from drug production or synthesis in the investigated area.
Can wastewater-based epidemiology be used to evaluate drug policy and law enforcement interventions?
Wastewater-based epidemiology can be used to obtain objective, real-time estimates of the size of the drug market, as well as spatial and temporal trends in drug use, including sudden changes in consumption habits as a potential result of (reduced) availability. Wastewater-based epidemiology can also identify new consumption patterns (e.g. use of NPS).
Wastewater-based epidemiology can signal both incidental direct dumps and more frequently occurring discharges of synthesis waste from production sites. Wastewater-based epidemiology can also be used to relate chemical profiles encountered in wastewater to synthesis routes used in manufacturing.
Wastewater-based epidemiology cannot identify the location of dumps or that of manufacturing sites.
How do you set up a wastewater monitoring campaign?
A wastewater monitoring campaign has four main activities: (i) planning of sampling, (ii) sample collection, (iii) sample analysis, and (iv) interpretation/reporting. Please watch the video below for a detailed description.
The EMCDDA is in regular contact with a network of wastewater-based epidemiology experts around Europe and across the globe. These experts can and will guide you in setting up wastewater monitoring in your city/region/country. Please contact the EMCDDA for further details.
How do you establish the population covered by a wastewater monitoring campaign?
The size of population contributing to the sampled wastewater is essential for normalising and comparing results from different locations, but it is not an easy parameter to obtain. Two different approaches to estimating population have been proposed and used for wastewater-based epidemiology during recent years: de jure, the most straightforward and based on the residence census data for the area served by the wastewater treatment plant, and de facto, based on the actual contributors to the system.
The first approach uses a static number of inhabitants, comprising all of the resident population, but this information is not often available and does not take into account population changes due to commuting, holidays and recreational events. The second approach uses a population that comprises all those who happen to be present, regardless of the location of their formal or usual residence, and is normally based on a group of indirect population indicators, including various hydrochemical parameters, such as biological oxygen demand, chemical oxygen demand, nitrogen, phosphorus and ammonium, and several substances with known consumption/excretion, such as caffeine, nicotine, pharmaceuticals and endogenous metabolic products. However, these indicators may not consider wastewater loss or infiltration and are more vulnerable to additional limitations such as confounding factors (i.e. industrial discharges).
To date, the use of mobile device-based population activity patterns has been found to be the best option for assessing the de facto population, providing real-time measures of the population within the catchment area. However, these resources are not always available and moreover incur a high cost. Introducing a population biomarker as a de facto proxy could significantly diminish uncertainty during population normalisation. Drinking water production, electricity consumption and mass loads of ammonium have been recently assessed as very promising de facto markers for dynamic population estimates.
What are the costs associated with applying wastewater-based epidemiology?
Overall costs will depend on the size and design of the study. In general, the largest cost associated with wastewater-based studies is due to the time taken to make contact with the local sewage treatment plant and laboratory and thereafter to designing an appropriate campaign.
The costs associated with sample collection depend on the facilities and resources available at your local wastewater treatment plant. Sample collection should not be expensive, and the largest component is normally the staffing costs associated with the people actually collecting the samples.
The costs of analysis vary from country to country, but a fair estimate is approximately EUR 100-200 per sample. The costs will, however, depend on which drugs you intend to monitor and the number of samples requiring analysis.
Do weather conditions affect wastewater sampling?
During dry weather conditions, all wastewater from connected houses is expected to be collected at a wastewater treatment plant. In a separate sewer system, this should also be the case during wet weather. In a combined sewer system, surface runoff (rain or snow melt) and wastewater are collected in the same pipes (combined sewage). As a result of the limited hydraulic capacity locally in the sewer system and at the wastewater treatment plant, combined sewage may be discharged to surface waters during rain events. Therefore, a limited fraction of the wastewater with drug target residues may also be discharged. These volumes discharge upstream in the sewer system and are not considered in the flow measurements at the wastewater treatment plants. The event-specific, daily discharge volumes can vary depending on the sewer system (storage volume), the hydraulic capacity of the wastewater treatment plant, the time of day, and rain intensity and volume. The estimated annual average wastewater not arriving at the wastewater treatment plant during wet weather in combined systems is less than 5 %. Despite this relatively small percentage, we recommend sampling during dry weather.
Can results from a few cities be extrapolated to an entire country?
Wastewater-based epidemiology consists of several consecutive steps that allow researchers to identify and quantify target metabolic residues of illicit drugs in raw wastewater and back-calculate the amount of the corresponding illicit drugs consumed by the population served by the wastewater treatment plant. First, representative composite samples of raw wastewater are collected and analysed for selected substances. Second, the back-calculation of drug consumption is performed by calculating the daily sewer loads of target residues; this is done by multiplying the concentrations of the measured target residues by the daily flow rates of sewage. From this value, the total consumption of a drug is estimated by applying a specific correction factor, which considers the average excretion rate of a given drug residue and the molecular mass ratio of the parent drug to its metabolite. Third, daily values are divided by the number of people served by the treatment plant to facilitate comparison among cities. This value can be expressed in daily amounts (or daily doses) per thousand population.
It is difficult to estimate the reliability of an extrapolation from a few wastewater treatment plants to the entire country. It is recommended (i) to cover a sufficiently high percentage of the population, (ii) to rely on a good geographical distribution of locations, and (iii) to rely on different sizes/types of towns. To date, there are only subjective suggestions regarding how to take account of these three factors. Recently thresholds of at least 10 % of the population and/or a minimum of five wastewater treatment plants were applied to estimate a national average. A correlation between consumption and size of wastewater treatment plant catchment (population) was found for only a few substances and countries.
How often is wastewater sampling needed to achieve reliable results for a year?
Samples can, for example, be collected every 15 minutes over a period of one week for a quick snapshot of the drug market.
The frequency of sampling depends on the daily and seasonal variation in consumption, which in turn is site and compound specific. From the few long-term data sets available, temporal variability of most compounds at most locations does not exceed a coefficient of variation of 0.5.
Is it recommended to collect more samples from fewer locations or fewer samples from more locations?
This depends on the question to be answered. With a limited budget and the aim of achieving a first snapshot, fewer samples from more locations should be favoured. This is because spatial differences typically exceed temporal variability. Based on this, the monitoring set-up can be optimised.
How can we be sure that the drugs found are due to use and not dumping?
There are several ways to identify direct discharges of drugs in sewers:
- Ratio of parent drug to transformation product. The metabolic products that are excreted in the sewer after consumption of a drug are for the most part known, and so are their excretion rates (the percentage of each transformation product excreted). Their ratios in wastewater, though slightly variable, can be used for tracking direct discharges. For example, after cocaine consumption, both cocaine itself and its major transformation product are excreted in a ratio of about 1:5 (0.2). If ratios of greater than 0.5 are encountered, direct disposal of unconsumed cocaine is likely.
- Sudden changes in drug ratios. For a given city or sewer system, the mutual ratio between illicit drugs (e.g. amphetamine to benzoylecgonine) or between a drug and a compound commonly present in urine (e.g. caffeine, sweeteners) has been found to be relatively constant. A sudden change in this ratio may reflect direct dumps of one of the drugs monitored.
- Changes in weekly patterns. A peak load of MDMA appearing on a weekday, instead of on a weekend day, that cannot be attributed to a certain event is most likely caused by a dump of unconsumed MDMA.
- Enantiomer ratio. Many drugs, such as amphetamines, MDMA and cannabinoids, are chiral molecules, resulting in two or more enantiomers, which are optical isomers of the same compound. Although enantiomers have the same physicochemical properties, their biological properties are different because of the frequent preference of biological systems for one enantiomer over another. This selectivity results in differences in metabolism and excretion and in biodegradation in the environment or during wastewater treatment. Therefore, the enantiomeric composition of an originally racemic mixture of consumed drug (meaning equal amounts of two enantiomers) is changed once it ends up in the sewer. This change, called the enantiomeric fraction, is a useful indicator of whether direct disposal of a drug has taken place (racemic, equal amounts of two enantiomers) or the drug has been consumed (non-racemic, the amounts of the two enantiomers are not equal).
- Profile of drugs and precursors. Synthesis of drugs generates waste, consisting of solvents, raw materials, synthesis intermediates and byproducts. Direct discharges of this chemical waste into sewers or surface waters can be revealed by the presence of precursors or pre-precursors , intermediates and by-products that together constitute a profile typical of the synthesis route used.