Assessment of stability of drug biomarkers in municipal wastewater as a factor influencing the estimation of drug consumption using sewage epidemiology.

Stability of the selected urinary biomarkers of six illicit drugs and two therapeutic opioids in municipal wastewater was studied in order to determine errors associated with their possible transformation in the sewer. The stability was assessed in experiments conducted at 10°C and 20°C in order to simulate typical winter and summer temperature conditions in the sewer system. Among fourteen substances tested, the most unstable compounds were morphine-3-β-D glucuronide (MG), 6-acetyl morphine (6-AM), cocaine (COC) and 6-acetyl codeine (6-AC), while all other investigated compounds appeared to be relatively stable over a period of 72 h. The transformation of all degradable compounds followed pseudo-first order kinetics with significantly longer half-times (t1/2) at winter conditions. At 20°C, t1/2 of MG, 6-AM, COC and 6-AC was 7h, 87 h, 35 h and 58 h, respectively, while the corresponding t1/2 values at 10°C were 18 h, 139 h, 173 h and 87 h. The main transformation mechanism of MG, 6-AM and 6-AC was most probably their enzymatic hydrolysis to morphine (MOR) and codeine (COD), while COC transformation to benzoylecgonine (BE) was primarily governed by chemical hydrolysis. The results indicate that the effect of the observed transformation of urinary biomarkers of COC and 6-AM on the estimates of COC and heroin consumption are relatively small (<10%) if the in-sewer hydraulic retention time is lower than 12h. Acidification of the wastewater samples proved to be the good way to stabilise the wastewater samples for the analysis of all selected compounds, except for 11-nor-9-carboxy-∆9-tetrahydrocannabinol (THC-COOH). This finding should be taken into account when selecting the preservation technique for multiresidual analyses of different groups of illicit drugs.