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1. Introduction
Since 2021, the Home Office Wastewater Analysis Programme (WWAP) has taken samples from wastewater treatment plants (WWTPs) across England and Scotland. These have been used to calculate the quantity consumed of several key illicit drug types.
The WWAP utilises wastewater analysis, which is a method used internationally, to provide robust and timely estimates of drug consumption in England and Scotland.
Wastewater analysis Phase 1 ran from May 2021 to May 2023 and was delivered by scientists and epidemiologists from the Environmental Research Group (ERG) at Imperial College London, through Imperial Projects. Sampling gradually expanded, reaching 18 WWTPs in England in early 2023, covering 25% of England’s residential population. Phase 2 ran from November 2023 until September 2024, and sampling expanded to 30 WWTPs across England and Scotland, covering 25% of England’s residential population, and 46% of Scotland’s.
Compared to international WWAPs, the Home Office WWAP is still in its infancy and the insight gained from it will develop with time. Prior to the WWAP, insights into levels of drugs misuse have largely been reliant on the Crime Survey for England and Wales (CSEW). However, the CSEW is likely to be subject to some uncertainty due to its reliance on self-report methods, where respondents may not feel comfortable reporting their illicit drug use. In addition, the CSEW is based on a sample of the population living in private households in England and Wales. This means it omits people who are homeless or living in institutions, such as prisons, and other settings. Given that there is an increased risk of heroin and crack cocaine use among some of these populations (See ‘Drug-related harms in homeless populations’), it is likely that the CSEW underestimates consumption of these drugs.
2. Methodology
2.1 Wastewater analysis Phase 1
From May 2021 to May 2023, the first phase of the WWAP monitored 20 substances, prioritised by the Home Office. The 20 substances monitored were:
- 5-aminoisotonitazene
- 6-acetylcodeine
- amphetamine
- benzocaine
- benzoylecgonine
- cocaethylene
- cocaine (monitored in wastewater itself and its metabolite, benzoylecgonine)
- diacetylmorphine (heroin, monitored as its metabolite 6-monoacetylmorphine (6-MAM))
- EDDP (ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine, metabolite of methadone)
- isotonitazene
- ketamine
- lidocaine
- methadone
- methamphetamine
- MDMA (ecstasy)
- morphine
- norketamine
- phenacetin
- procaine
- tetramisole (levamisole)
These substances were monitored across 18 WWTPs in England, covering 25% of England’s population. Between June 2021 and May 2023, 3,192 samples were analysed, with consumption estimates calculated for[footnote 1]:
- cocaine
- heroin
- ketamine
- MDMA
- amphetamine
- methamphetamine
Each wastewater sample was taken over a 24-hour period to ensure it was representative of the entire day. This reduces the possibility of missing the presence of drugs at certain times.
2.2 Wastewater analysis Phase 2
From November 2023 to September 2024, the second phase of the WWAP monitored all 20 drugs included in Phase 1, as well as:
- xylazine
- 12 fentanyl analogues
- 10 nitazene analogues
In Phase 2 of the programme, 691 samples were collected across 30 WWTPs in England and Scotland. This covered 25% of the population of England and 46% of Scotland.
2.3 Producing consumption estimates
To compare drug consumption in 2023 and 2024, samples chosen for comparison were taken from the same WWTPs at similar times of year. This is due to drug consumption estimates varying across WWTPs and throughout the year.
The most suitable data to use was based on samples taken from 16 WWTPs (covering 18% of England’s population) between January and April 2023 and between January and April 2024. An average of all samples within these periods was calculated for each WWTP individually. 95% confidence intervals for each WWTP in each period were also calculated. This resulted in a consumption estimate for each WWTP between January and April 2023 and between January and April 2024, with upper and lower bounds for each figure. An average of the 16 estimates for 2023 was then calculated, and the same was done for 2024. This was repeated for the upper and lower bounds, resulting in an overall consumption estimate for the 16 WWTPs, with confidence intervals to display the range in which it is 95% likely the true figure lies.
As wastewater analysis estimates how much of a pure substance is consumed, consumption estimates were adjusted by retail purity estimates sourced from the National Crime Agency and UK Focal Point. Overall consumption estimates from these 2 periods have been compared to estimate the change in consumption of each drug.
The independent opinion of the academics from the Environmental Research Group at Imperial College London was that the WWTPs selected by the Home Office for monitoring in the first 2 phases of the WWAP were not representative of England as a whole. This is partly because these WWTPs were chosen to monitor drug consumption in the areas involved in Project ADDER, and to capture a high proportion of the population. This resulted in the catchment areas being predominantly urban, with higher prevalence of drug-related harms (for example, drug related deaths) and levels of deprivation than the national average. Therefore, overall consumption estimates in this report only represent the areas covered by the 16 WWTPs sampled from (which account for 18% of England’s population) and cannot be generalised to the whole of England.
2.4 Limitations
Correction factors are used to adjust the wastewater data for known processes that impact sample results. For example, to convert the amount of a drug found in wastewater into the amount consumed, it must be adjusted by how much the drug is estimated to break down in the human body (that means metabolised) and while travelling to the WWTP where sampling takes place. However, as these are estimated, a level of uncertainty will always remain. Other factors are more difficult to control for, such as in-sewer wastewater losses (for example, by combined sewer overflows), and the way people administer drugs (for example, smoking or injecting) which can impact how much of the drugs are excreted. It is due to these factors, and other limitations outlined below, that wastewater analysis should be used as part of a wider set of metrics to measure drug consumption and related harms, rather than in isolation. These metrics include drug seizures in England and Wales, drug misuse statistics from the Crime Survey for England and Wales, and the National Crime Agency’s annual National Strategic Assessment.
It is not possible to assign consumption to an individual person, and so it is impossible to use wastewater analysis data to determine the number of people consuming a particular substance. In addition to this, wastewater estimates only consider the targeted substance and therefore cannot be used to estimate purity. Although wastewater estimates have been adjusted by retail purity estimates, limited purity data exists for amphetamines, methamphetamines, MDMA, and ketamine so purity has been assumed to have remained consistent for these drugs from 2023 to 2024.
For some substances (for example, MDMA), it is not possible to distinguish drugs directly disposed to the sewer network from those consumed by a person, so wastewater estimates are likely to be an overestimate of total drug consumption.
Some substances have medicinal applications (for example, amphetamine and heroin/diamorphine) and therefore wastewater estimates include use for medical purposes. Comparison to NHS prescription/dispensing data may help distinguish this in the future, but further research is required to assure robustness of this approach.
Estimation of population size is one of the largest uncertainties of wastewater-based epidemiology. Population is usually based on census data. However, this does not consider people moving in and out of the area for work or travel. Therefore, drug consumption may be overestimated in those areas which have large tourist or commuting populations. Additionally, the quality of the flow data, which shows the amount of wastewater and rainfall passing through the WWTP each day, directly influences the consumption estimates and therefore the quality of the data.
The programme does not currently test for cannabis due to complexities of the sampling method. The Crime Survey for England and Wales indicates cannabis is the most widely used drug in England and Wales, with 6.8% of people aged 16 to 59 years reporting use in year ending March 2024. Testing for cannabis in the WWAP will be considered as the science evolves.
There were no positive detections during the second phase of the WWAP of nitazenes. These substances are more difficult to detect in wastewater due to (a) their stability, (b) the fact they are not consumed in large quantities, and (c) testing methods not being as developed as they are for other drugs. The testing for synthetic opioids is being developed as part of the WWAP, including the development of new methods and expansion of the substances tested for.
3. Findings
Wastewater has been sampled from 16 WWTPs (covering 18% of England’s population) between January and April 2023 and between January and April 2024. Overall consumption estimates (measured in milligrams per 1,000 people per day) from these 2 periods in these areas have been compared to produce the following trends:
- cocaine consumption is estimated to have increased by 7%; his figure includes both powder and crack cocaine as it is currently not possible to produce individual estimates for these drugs; it is currently being explored whether this can be achieved in future outputs
- heroin consumption is estimated to have decreased by 11%
- ketamine consumption is estimated to have increased by 85%
- MDMA consumption is estimated to have increased by 52%
- amphetamine consumption is estimated to have decreased by 18%
- methamphetamine consumption is estimated to have decreased by 30%
Table 1: Wastewater analysis consumption estimates in January to April 2023 and January to April 2024, with confidence intervals, measured in milligrams consumed per 1,000 people per day
| Drug | January to April 2023 | January to April 2024 |
|---|---|---|
| Cocaine | 4,336 +/- 799 | 4,642 +/- 1,112 |
| Heroin | 1,301 +/- 360 | 1,153 +/- 816 |
| Ketamine | 545 +/- 128 | 1,008 +/- 305 |
| MDMA | 140 +/- 64 | 214 +/- 133 |
| Methamphetamine | 119 +/- 37 | 83 +/- 23 |
| Amphetamine | 4,605 +/- 2,240 | 3,783 +/- 3,439 |
Notes:
- These estimates are based on data from the independent analysis provided by the team from the ERG [at Imperial] between January and April 2023 and between January and April 2024 from 16 WWTPs covering 18% of England’s population.
- These figures do not reflect national-scale estimates. Estimates were adjusted by retail purity by the Home Office. However, purity figures for 2023 and 2024 individually were only available for cocaine and heroin from the National Crime Agency. All other drugs have the same purity estimate applied for both 2023 and 2024.
4. Next steps and future developments
Wastewater Analysis Phase 3 is being delivered by a consortium of Aqua Clear, Eurofins, and Gingko Biosecurity. This phase is running from September 2024 until April 2025. Phase 3 has increased the number of sites sampled from to 50 sites, covering 29% of England’s population and 46% of Scotland’s population.
The selection of synthetic opioids and prescribed medications tested for will also expand to include more nitazenes, benzodiazepines, as well as gabapentin and pregabalin. Results from Phase 3 will be available later in 2025.
The development of England-level consumption estimates will be explored as part of the third phase of the WWAP.
- Consumption estimates were produced for the highest priority drug types, which are thought to be more commonly consumed in the UK or are associated with evidence gaps. This also depended on correction factors for these drugs being robust enough to produce accurate consumption estimates (see Limitations for further information on correction factors). ↩
