13The stability of five illicit drug markers in wastewater was tested under different sewer 14 conditions using laboratory-scale sewer reactors. Wastewater was spiked with deuterium 15 labelled isotopes of cocaine, benzoyl ecgonine, methamphetamine, MDMA and 6-acetyl 16 morphine to avoid interference from the native isotopes already present in the wastewater 17 matrix. The sewer reactors were operated at 20 o C and pH 7.5, and wastewater was sampled at 18 0, 0.25, 0.5, 1, 2, 3, 6, 9 and 12 hours to measure the transformation/degradation of these 19 marker compounds. The results showed that while methamphetamine, MDMA and benzoyl 20 ecgonine were stable in the sewer reactors, cocaine and 6-acetyl morphine degraded quickly. 21Their degradation rates are significantly higher than the values reportedly measured in 22 wastewater alone (without biofilms). All the degradation processes followed first order 23 kinetics. Benzoyl ecgonine and morphine were also formed from the degradation of cocaine 24 and 6-acetyl morphine, respectively, with stable formation rates throughout the test. These 25 findings suggest that, in sewage epidemiology, it is essential to have relevant information of 26 the sewer system (i.e. type of sewer, hydraulic retention time) in order to accurately back-27 estimate the consumption of illicit drugs. More research is required to look into detailed 28 sewer conditions (e.g. temperature, pH and ratio of biofilm area to wastewater volume among 29 others) to identify their effects on the fate of illicit drug markers in sewer systems.
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