Laboratory Processes and Back-Calculating Population Consumption
Thousands of hours of highly complex research has underpinned each of the advances made in WWA laboratory techniques. The disciplines involved mainly fall under the umbrella term 'analytical chemistry'. This has multiple applications in the fields of chemistry, biology, pharmacy, forensic science, environmental toxicology and engineering. A fundamental analytical chemistry technique is called chromatography. The basis of chromatography is the separation of the compounds present in complex mixtures - ranging from drugs in blood samples to hydrocarbons in crude oil to drugs in wastewater - which allows for unhindered measurement of a characteristic signal for the compounds in the mixture using some form of detector. The simplest use of chromatography is to infer the presence (or absence) of a particular compound or compounds, which is referred to as qualitative analysis. However, the size of the signal is related to the amount of compound present in the mixture and therefore (after appropriate calibration) the size of the signal can be used to measure the concentration of compounds in the mixture, which is a process called quantitative analysis.
The most common approach used in WWA is to combine a technique called liquid chromatography (LC) for separation of compounds with mass spectrometry (MS) as the means of qualitative and quantitative analysis. In effect, the signal from the mass spectrometer carries information about the size (mass) of the different molecules present in the mixture and some information about their molecular structure. As an example, if a mixture (such as wastewater) contains cocaine then a signal will be obtained that shows the presence of a mass of 304, which arises when the cocaine molecule acquires a hydrogen ion during analysis, as well as masses of 182 and 82 which arises when the cocaine molecule fragments. A signal comprising the masses 304,182 and 82 is characteristic for cocaine.
The benefit of analytical methods like LC-MS is that they provide very high levels of sensitivity that means they are capable of detecting residues in parts per trillion, usually expressed as nanograms per litre. By comparison, some drug monitoring systems engage commercial laboratories to routinely analyse urine provided by arrestees for traces of illicit drugs. The drug biomarkers in unadulterated urine samples occur in concentrations that are approximately 1,000 times higher than the concentrations found in WWA samples. Other techniques are being trialled that may provide greater efficiency in the laboratory and possibly even greater sensitivity in future WWA applications.
LC-MS and related methods have proven effective despite the presence of countless types of biological and synthetic substances that enter sewerage systems (EMCDDA 2016). Not surprisingly, the LC-MS instruments are not designed to contend with faeces, rubbish and the like, so solids are removed from samples using clean-up procedures before analyses commence.
Different laboratories have different equipment and have developed different, but equally valid, approaches to analysing biomarkers in waste- water (EMCDDA 2016). The potential variability that these differences may introduce into data presented a problem when planning large-scale projects involving multiple laboratories, such as those conducted across Europe. To remedy this, inter-laboratory validation studies are conducted, similar to those conducted in other research fields, so that the data produced by participating laboratories can be calibrated for reliable comparison (Castiglioni et al. 2013; Ort et al. 2014; EMCDDA, 2018). The congruence of data produced by different laboratories has also been enhanced by the development of the SCORE protocols (see further 2.2.2), which contain guidelines for analysing wastewater samples and reporting results.
After wastewater samples are analysed for biomarkers back- calculations are used to estimate community consumption of substances. These back-calculations are similar to those proposed by Daughton (2001) (see 2.1). Castiglioni, Bijlsma, Covaci et al. (2016,17) summarised the main steps taken in this process.
Many WWA publications present data based on this level of calculation - usually presenting these estimates over multiple days to show temporal trends.
4. For any given substance a fourth step may be to estimate how many doses were consumed. This involves dividing the estimated milligrams consumed (per day per 1,000 people) by the 'standard' dose in milligrams. For example, if WWA shows that consumption of a particular drug was 200 mg/day/1,000 people and the standard dose of that drug is 100 mg, then the estimate is that two doses of the drug were consumed per day per 1,000 people.
It is clear that WWA data measure consumption rates in large groups of people; they do not inform us about the use of drugs by individuals. This is the cardinal limitation of the WWA method that is explored further below in 2.4. It is also ironically WWA's main strength with respect to research ethics as discussed in 2.3.
Factors Relevant to Interpreting WWA Data
The WWA field is wary about communicating results without including caveats about the interpretation of its metrics. Perhaps this is partly due to the scale and nature of the media coverage of the first paper that reported back-calculation estimates of cocaine use (Zuccato et al. 2005, see 2.1)? The European Monitoring Centre for Drugs and Drug Addiction produced detailed reports in 2008 and in 2016, both of which were co-authored by key members of SCORE (see EMCDDA 2016). These reports catalogue key factors that are relevant in interpreting WWA data. It is important to review these factors to understand the utility of WWA data and its current limitations. Given the intensity of research that is focused upon overcoming these limitations, we agree with Mounteney et al. (2015) that WWA is likely to become a more important source of information on illicit drug use in the future.
It can be difficult to estimate population size in a WWTP catchment area. If researchers overestimate population size they will underestimate the amount of substance consumed per day per 1,000 people in the catchment. Underestimating population will have the opposite result.
Key terms used by statisticians on this topic are de facto and de jure populations. The first term refers to the actual number of people in an area at any given time. The second term refers to the number of people who reside in the area. De jure figures may be adequate for many national and local policy objectives. However, de jure figures are not ideal for WWA particularly because they do not account for population movement between catchments. Obviously populations can fluctuate very significantly in urban settings as people travel to work during the day. Nightclub districts, for example, are especially likely to have large changes in population size in the course of an evening.
Various techniques are being trialled to better approximate the size of populations (e.g. Been et al. 2014; Castiglioni et al. 2013; Gao et al. 2016; O'Brien et al. 2014; Thomas et al. 2012). The hypothesis driving these trials is that there could be a substance (or substances) present in wastewater that everyone (or at least an accurately known fraction of the population) deposits into the wastewater at an accurately known daily rate. If such substances could be quantified daily in wastewater then a reliable surrogate for population size would be achieved. Lai et al. (2011) demonstrated that it was possible to estimate population by focusing on levels of pharmaceuticals and commonly ingested substances such as artificial sweeteners in wastewater samples. The approach involved first estimating the consumption of these substances using WWA and triangulating the results with other sources of data, including prescriptions of pharmaceuticals and sales data for artificial sweeteners. Other studies have examined the utility of biomarkers that are regularly produced by humans as a proxy for population size (e.g. Thai et al. 2014b). These methods effectively use the information contained in the wastewater to calculate population size (see further Gracia-Lor et al. 2017). In Australia, the national census data can collect both de jure and de facto data on a specific date. Researchers are examining how this information may be used to develop better models for accurately estimating de facto population size for WWA data using wastewater samples themselves (see O'Brien et al. 2014; Lai et al. 2015). An alternative approach has also been trialled where population changes were monitored using mobile phone tower ping data in Oslo (Baz-Lomba et al. 2019).
Purity, Potency and Standard Doses
For many substances widely accepted (and static) metrics exist as to what constitutes a 'standard dose'. Examples include medications and legal products like alcohol. By contrast, a dose of illicit drugs can fluctuate for a wide variety of reasons (Brunt et al. 2015). The illicit market usually reacts to fluctuations in purity by changing prices. For instance, downward fluctuations can lead to lower prices and smaller standard doses; the reverse can occur if drug purity increases. Decreases or increases in purity can lead to consumers receiving respectively less or more of an illicit drug - at least for a period of time. These trends may be detected in WWA. It is therefore important for purity to be considered when interpreting WWA data (Bruno et al. 2014). Researchers can access relatively good information about standard doses in any given year. Forensic laboratories that support police agencies may be able to provide WWA researchers with up-to- date intelligence about purity. In Europe very accurate information can be accessed from the Trans-European Drug Information project, which analyses tens of thousands of illicit drug doses provided by drug users (Brunt et al. 2015).
While users have little control over what the market provides, users do have control over how much they consume per dose. It is known that experienced users can tolerate higher quantities of drugs than inexperienced users and therefore what is a single dose to one user may be multiple doses to another and it follows that each will contribute different amounts to the wastewater stream after a single dose.
Illicit drugs also differ in their potency. Potent drugs require a smaller dose to affect the human body than do less potent drugs. For example, in 2009 in Australia the standard dose of methamphetamine was 41 mg yet the standard dose for cocaine was 140 mg (Prichard et al. 2012). Load data conceal these differences because they only present the estimated mass consumed per day per 1,000 people. Estimating doses consumed per day per 1,000 people takes potency into account and facilitates comparisons of the relative strengths of drugs in different drug markets.
This was demonstrated in an early Australian study (Prichard et al. 2012). Analyses of samples collected in 2009 indicated that 158 mg of methamphetamine and 221 mg of cocaine were consumed daily per 1,000 people. However, this translated to 3.9 doses of methamphetamine and 1.6 doses of cocaine - indicating that methamphetamine was the stronger of the two markets during the time of sampling.
Dose-estimation can be used to assist in understanding the relative economic value of different illicit drug markets. This simply involves multiplying the number of doses estimated to be consumed daily per 1,000 people by information about the 'street value' of doses. Prices fluctuate, but good annual figures can be sourced from drug monitoring systems in different regions.