Wastewater analysis is a rapidly developing scientific discipline with the 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 different cities (Daughton, 2001; van Nuijs et al., 2011; Zuccato et al., 2008). It involves sampling a source of wastewater, such as a sewage influent to a wastewater treatment plant. This allows scientists to estimate the quantity of drugs consumed by a community by measuring the levels of illicit drugs and their metabolites excreted in urine (Zuccato et al., 2008) .

Wastewater testing in European cities

In 2010 a Europe-wide network (Sewage analysis CORe group — Europe (SCORE)) was established with the aim of standardising the approaches used for wastewater analysis and coordinating international studies through the establishment of a common protocol of action. The first activity of the SCORE group was a Europe-wide investigation, performed in 2011 in 19 European cities, which allowed the first ever wastewater study of regional differences in illicit drug use in Europe (Thomas et al., 2012). That study also included the first intercalibration exercise for the evaluation of the quality of the analytical data and allowed a comprehensive characterisation of the major uncertainties of the approach (Castiglioni et al., 2014). Following the success of this initial study, comparable studies were undertaken over the following years, covering 68 cities and 23 countries in Europe in 2019. A standard protocol and a common quality control exercise were used in all locations, which made it possible to directly compare illicit drug loads in Europe over a one-week period during nine consecutive years (van Nuijs et al., 2018). For the 2019 wastewater monitoring campaign, raw 24-hour composite samples were collected during a single week in March. These samples were analysed for the urinary biomarkers (i.e. measurable characteristics) of the parent drug (i.e. primary substance) for amphetamine, methamphetamine and MDMA. In addition, the samples were analysed for the main urinary metabolites (i.e. substances produced when the body breaks drugs down) of cocaine and cannabis, which are benzoylecgonine (BE) and THC-COOH (11-nor-9-carboxy-delta9-tetrahydrocannabinol).

This report is focused on illicit stimulants. No results for cannabis are reported because cannabis use is estimated by measuring its main metabolite (THC-COOH), which is the only suitable biomarker found so far, but is excreted in a low percentage. More research is needed to understand the excretion percentage of THC-COOH or find alternative biomarkers (Causanilles et al., 2017a).

The specific metabolite of heroin, 6-monoacetylmorphine, was found to be unstable in wastewater. Consequently, the only alternative is to use morphine, although it is not a specific biomarker and can also be excreted as a result of therapeutic use. This underlines the importance of collecting the most accurate figure for morphine use from prescription and/or sales reports.

Patterns of illicit drug use: geographical and temporal variation

2019 key findings

The project revealed a picture of distinct geographical and temporal patterns of drug use across European cities (see Interactive: explore the data from the study).

Overall, the loads of the different stimulant drugs detected in wastewater in 2019 have increased, compared to previous years.

The BE loads observed in wastewater indicate that cocaine use remains highest in western and southern European cities, in particular in cities in Belgium, the Netherlands, Spain and the United Kingdom. Very low levels were found in the majority of the eastern European cities, but the most recent data show signs of increase.

The loads of amphetamine detected in wastewater varied considerably across study locations, with the highest levels reported in cities in the north and east of Europe. Amphetamine was found at much lower levels in cities in the south of Europe.

In contrast, methamphetamine use, generally low and historically concentrated in Czechia and Slovakia, now appears to be present also in Cyprus, the east of Germany, Spain and northern Europe. The observed methamphetamine loads in the other locations were very low to negligible.

The highest mass loads of MDMA were found in the wastewater in cities in Belgium, Germany and the Netherlands.

Seventeen countries participating in the 2019 monitoring campaign included two or more study locations (Austria, Belgium, Cyprus, Czechia, Germany, Finland, France, Greece, Italy, Lithuania, Netherlands, Portugal, Spain, Slovakia, Slovenia, Sweden and Turkey). The study highlighted differences between these cities within the same country, which may be explained in part by the different social and demographic characteristics of the cities (universities, nightlife areas and age distribution of the population). In the large majority of countries with multiple study locations, loads were higher in large cities compared to smaller locations for all four substances.

In addition to geographical patterns, wastewater analysis can detect fluctuations in weekly patterns of illicit drug use. More than three-quarters of cities show higher loads of amphetamine, BE and MDMA in wastewater during the weekend (Friday to Monday) than during weekdays. Higher loads are found during the weekend (Friday to Monday) than during the week for BE and MDMA in three-quarters of the cities and for amphetamine in half, whereas methamphetamine loads are distributed more evenly across the week. The results point to a more recreational use of cocaine and MDMA, and to lesser extent amphetamine, contrasting with a more problematic profile of methamphetamine use.

Thirty-four cities have participated in at least five of the annual wastewater monitoring campaigns since 2011. This allows for time trend analysis of drug consumption based on wastewater testing.

Cocaine

The BE loads observed in wastewater indicate that cocaine use remains highest in western and southern European cities, in particular in cities in Belgium, the Netherlands, Spain and the United Kingdom. Very low levels were found in the majority of the eastern European cities, but the most recent data show signs of increase.

Figure 1



A relatively stable picture of cocaine use was observed between 2011 and 2015 in most cities. The general patterns detected were similar in the first five consecutive monitoring campaigns, with the highest and lowest BE loads found in the same cities and regions. Most cities showed either a decreasing or a stable trend between 2011 and 2015. In 2016, there were initial signs that this pattern was changing, with increases observed in the majority of cities each year since then. In 2019 the trend shows a further increase in use, with 27 of the 45 cities with data for 2019 and 2018 reported an increase. Increasing longer-term trends are reported for 13 out of the 14 cities with data for 2011 and 2019.

Figure 2: Aggregated trends in cocaine metabolites in 10 European cities, 2011 to 2019

Several sources indicate that cocaine use is increasing. A 2018 EMCDDA trendspotter study found that the greater availability of cocaine on Europe’s drug markets may be leading to an increasing trend of use in some countries and possibly expanding to eastern Europe, where the drug was previously little used. The picture emerging from general population surveys suggests stable or increasing levels of cocaine use. Analyses of wastewater reveals an increase in cocaine residues in most cities for which data were available for 2015 and 2019. There are three plausible explanations for an upward trend in BE in wastewater: more people are consuming cocaine; the same people are consuming more cocaine; and the purity of the drug has increased. With increased purity, a given quantity of cocaine will result in more BE entering the wastewater. This increase could also be explained by a combination of these three causes (EMCDDA, 2018).

In the majority of countries with multiple study locations, cocaine loads were higher in large cities compared to smaller locations. In addition to geographical patterns, wastewater analysis can detect fluctuations in weekly patterns of illicit drug use. More than three-quarters of cities show higher loads of BE in wastewater during the weekend (Friday to Monday) than during weekdays, which may reflect a pattern of more recreational use.

MDMA

The highest mass loads of MDMA were found in the wastewater in cities in Belgium, Germany and the Netherlands.

Figure 3



Until recently, general population surveys in many countries showed that MDMA prevalence was declining from peak levels attained in the early to mid-2000s. In recent years, however, the picture has remained mixed with no clear trends. Where prevalence is high, this may reflect MDMA no longer being a niche or subcultural drug limited to dance clubs and parties, but now being used by a broader range of young people in mainstream nightlife settings, including bars and house parties.

Looking at longer-term trends in wastewater analysis, for 11 out of the 12 cities with data for both 2011 and 2019, MDMA loads were higher in 2019 than in 2011. Sharp increases were observed in some cities, including Amsterdam, Eindhoven and Antwerp. In most cases the loads increased between 2011-16, decreased in 2017 and remained stable in 2018. However, 2019 data point to increases in most cities.

Figure 4: Aggregated trends in MDMA residues in 10 European cities, 2011 to 2019

In the large majority of countries, MDMA loads were higher in large cities compared to smaller locations. Also, more than three-quarters of cities showed higher loads of MDMA in wastewater during the weekend (Friday to Monday) than during weekdays reflecting the predominantly recreational use of ecstasy now seen in a range of social settings.

Amphetamine and methamphetamine

Amphetamine and methamphetamine, two closely related stimulants, are both consumed in Europe, although amphetamine is much more commonly used. Methamphetamine consumption has historically been restricted to Czechia and, more recently, Slovakia, although recent years have seen increases in use in other countries.

The loads of amphetamine detected in wastewater varied considerably across study locations, with the highest levels reported in cities in the north and east of Europe. Amphetamine was found at much lower levels in cities in the south of Europe.

Methamphetamine loads also varied across locations. The drug was present in wastewater in cities in Cyprus, the east of Germany, Spain, several northern European countries (Denmark, Finland, Lithuania and Norway) as well as Czechia and Slovakia. The observed methamphetamine loads in the other locations were very low to negligible.

Figure 5



Figure 6



Overall, the data related to amphetamine and methamphetamine from the nine monitoring campaigns showed no major changes in the general patterns of use observed. However, the most recent data show that 21 of the 41 cities with data for 2018 and 2019 reported an increase for amphetamine, with the loads found being higher during weekends.

Figure 7: Aggregated trends in amphetamine residues in 6 European cities, 2011 to 2019

Figure 8: Aggregated trends in methamphetamine residues in 7 European cities, 2011 to 2019

No differences could be detected for amphetamine and methamphetamine when comparing the loads found in large cities compared to smaller locations.

For amphetamine, since 2018, more cities show higher loads of amphetamine in wastewater during the weekend (Friday to Monday) than during weekdays, possibly indicating more use in recreational settings as compared to the past. In contrast, methamphetamine use was found to be distributed more evenly over the whole week, possibly reflecting methamphetamine being associated with more ongoing and high-risk consumption by a small cohort of users.

Comparison with findings from other monitoring tools

Because different types of information are provided by wastewater analysis (collective consumption of substances within a community) and by established monitoring tools, such as population surveys (prevalence in the last month or year), a direct comparison of the data is difficult. However, the patterns and trends being detected by wastewater analysis are largely, but not completely, in line with the analyses coming from other monitoring tools.

For example, both seizure and wastewater data present a picture of a geographically divergent stimulant market in Europe, where cocaine is more prevalent in the south and west, while amphetamines are more common in central and northern countries (EMCDDA, 2017). Similar results are also found in data coming from population surveys on drug use. While the general pattern detected in wastewater is in line with established monitoring tools, there are some exceptions: the amphetamine loads in wastewater in Paris have been below the level of quantification over the consecutive annual monitoring campaigns, contrary to indications from other monitoring tools.

Data from established indicators show that methamphetamine use has historically been restricted to Czechia, and more recently also Slovakia, although recent years have seen increased use in other countries (EMCDDA, 2016a). These findings have been confirmed by recent wastewater-based epidemiology, with the highest methamphetamine loads found in Czech, Slovak, German and Finnish cities.

Established indicators show that, until recently, MDMA prevalence was declining in many countries from peak levels in the early to mid-2000s. Data from wastewater and from established indicators show that this appears to be changing, with the large majority of cities reporting higher wastewater MDMA loads in 2016 or 2017 than in 2011.

Similarly, studies based on self-reported drug use and those using wastewater data both point towards the same weekly variations in use, with stimulants such as amphetamine and cocaine being primarily used at weekend music events and in celebratory contexts (Tossmann et al., 2001).

limited but steadily increasing number of studies have been published comparing drug use estimates obtained through wastewater analysis and estimates provided by epidemiological surveys (EMCDDA, 2016b; van Wel et al., 2015). While in 2012 only one reported study tried to evaluate sewage analysis alongside traditional epidemiological techniques (Reid et al., 2012), this number has now increased to over 20 published research articles that are focused on comparing information provided by wastewater analysis and information provided by other indicators.

A first study, performed in Oslo, Norway, and published in 2012, compared the results from three different datasets (a general population survey, a roadside survey and wastewater analysis) (Reid et al., 2012).

Other, more recent studies compare and correlate wastewater-based consumption estimates of illicit drugs with other data sources, including self-reported data (Been et al., 2015; Castiglioni et al., 2016; van Wel et al., 2016a), consumption offences (Been et al., 2016a), illicit drug seizures (Baz-Lomba et al., 2016; Kankaanpää et al., 2014, 2016), purity of drug seizures (Bruno et al., 2018), syringe distribution estimates (Been et al., 2015), toxicological data (Kankaanpää et al., 2014, 2016) and the number of drug users in treatment (Krizman et al., 2016).

The majority of comparative studies have been carried out within Europe, including in Belgium (van Wel et al., 2016a), Croatia (Krizman et al., 2016), Germany (Been et al., 2016a), Finland (Kankaanpää et al., 2014, 2016), Italy (Castiglioni et al., 2016), Spain (Bijlsma et al., 2018), Switzerland (Been et al., 2015; Been et al., 2016b), Turkey (Daglioglu, 2019) and across European countries (Baz-Lomba et al., 2016; Castrignanò et al., 2018; Löve et al., 2018). Outside Europe, in recent years studies have been published comparing wastewater-based estimates with other data sources in China (Du et al., 2015), Australia (Tscharke et al., 2015) and in countries where data on drug use are limited due to financial constraints or lack of monitoring tools (Archer et al., 2018; Moslah et al., 2018; Nguyen et al., 2018).

These examples confirm the promising future of wastewater-based epidemiology as a complementary approach to obtain a more accurate and balanced picture of substance use within different communities. Wastewater analysis can predict results from population surveys and can be used as a ‘first alert’ tool in the identification of new trends in drug consumption. In order to check the quality and accuracy of data, further comparisons between wastewater analysis and data obtained through other indicators are needed.

Limitations of this method

Wastewater analysis offers an interesting complementary data source for monitoring the quantities of illicit drugs used at the population level, but it cannot provide information on prevalence and frequency of use, main classes of users and purity of the drugs. Additional challenges arise from uncertainties associated with the behaviour of the selected biomarkers in the sewer, different back-calculation methods and different approaches to estimate the size of the population being tested (Castiglioni et al., 2013, 2016; Lai et al., 2014; EMCDDA, 2016b). The caveats in selecting the analytical targets for heroin, for example, make monitoring this drug in wastewater more complicated compared to other substances (Been et al., 2015). Also, the purity of street products fluctuates unpredictably over time and in different locations. Furthermore, translating the total consumed amounts into the corresponding number of average doses is complicated, as drugs can be taken by different routes and in amounts that vary widely (Zuccato et al., 2008).

Efforts are being made to enhance wastewater monitoring approaches. For example, work has been undertaken on overcoming a major source of uncertainty related to estimating the number of people present in a sewer catchment at the time of sample collection. This involved using data from mobile devices to better estimate the dynamic population size for wastewater-based epidemiology (Thomas et al., 2017).

New developments and the future

Wastewater-based epidemiology has established itself as an important tool for monitoring illicit drug use and future directions for wastewater research have been explored (EMCDDA, 2016b).

First, wastewater analysis has been proposed as a tool to address some of the challenges related to the dynamic new psychoactive substances (NPS) market. This includes the large number of individual NPS, the relatively low prevalence of use and the fact that many of the users are actually unaware of exactly which substances they are using. A technique has been established to identify NPS that involves the collection and analysis of pooled urine from stand-alone portable urinals from nightclubs, city centres and music festivals, thereby providing timely data on exactly which NPS are currently in use at a particular location (Archer et al., 2013a, 2013b, 2015; Causanilles et al., 2017b; Kinyua, et al., 2016; Mackulak et al., 2019; Mardal et al., 2017; Reid et al., 2014;). The European project ‘NPS euronet’ aimed to improve the capacity to identify and assess the NPS being used in Europe. The project applied innovative analytical chemical and epidemiological methods and a robust risk-assessment procedure to improve the identification of NPS, to assess risks, and to estimate the extent and patterns of use in specific groups (e.g. at music festivals) and among the general population (Bade et al., 2017; González-Mariño et al., 2016).

Second, in addition to estimating illicit drug use, wastewater-based epidemiology has been successfully applied in recent years to providing detailed information on the use and misuse of alcohol (Boogaerts et al., 2016; Mastroianni et al., 2017; Rodríguez-Álvarez et al., 2015), tobacco (Senta et al., 2015; van Wel et al., 2016b) and medicines in a specific population (Baz-Lomba et al., 2016, 2017; Been et al., 2015; Krizman-Matasic et al., 2018; Salvatore et al., 2016). Furthermore, wastewater analysis can potentially provide information on health and illness indicators within a community (Kasprzyk-Hordern et al., 2014; Thomaidis et al., 2016; Yang et al., 2015).

Third, the potential for wastewater-based epidemiology to be used as an outcome measurement tool, in particular in the evaluation of the effectiveness of interventions that target drug supply (e.g. law enforcement) or drug demand (e.g. public health campaigns) has not yet been fully explored. Close collaboration between the different stakeholders involved, including epidemiologists, wastewater experts and legal authorities, is highly recommended in order to start examining these potential wastewater-based epidemiology applications (EMCDDA, 2016b). The WATCH project included a 30-day synthetic drug production monitoring campaign in three cities in Belgium and the Netherlands. High levels of MDMA were recorded during the whole monitoring period in one city in the Netherlands, suggesting continuous discharges of unconsumed MDMA from sources within the wastewater catchment area, indicating drug production was taking place in this region.

Fourth, by back-calculating the daily sewer loads of target residues, wastewater analysis can provide total consumption estimates, and specific efforts are now being directed towards finding the best procedures for estimating annual averages. In 2016 the EMCDDA presented for the first time illicit drug retail market size estimates in terms of quantity and value for the main substances used (EMCDDA and Europol, 2016). It is envisaged that findings from wastewater analysis can help to further develop work in this area.

Finally, new methods such as enantiomeric profiling have been developed to determine if mass loads of drugs in wastewater originated from consumption or from the disposal of unused drugs or production waste. It is now important to assess the possible utility of wastewater analysis to report on drug supply dynamics, including synthetic drug production (Emke et al., 2014). For example, recent malfunctioning of a small wastewater treatment plant in the Netherlands was caused by direct discharges in the sewage system of chemical waste from a drug production site. Further analysis revealed the actual synthesis process used to manufacture the corresponding drugs. The study confirmed that the chemical waste from the illegal manufacturing of stimulants will result in a specific chemical fingerprint that can be tracked in wastewater and used for forensic purposes. Such profiles can be used to identify drug production or synthesis waste disposal in the wastewater catchment area (Emke et al., 2018).

Wastewater analysis has demonstrated its potential as a useful complement to established monitoring tools in the drugs area. It has some clear advantages over other approaches as it is not subject to response and non-response bias and can better 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 timeframes on geographical and temporal trends. In order to check the quality and accuracy of data, further comparisons between wastewater analysis and data obtained through other indicators are needed.

As a method, wastewater analysis has moved from being an experimental technique to being a new method in the epidemiological toolkit. Its rapid ability to detect new trends can help target public health programmes and policy initiatives at specific groups of people and the different drugs they are using.