Illicit drugs in wastewater
Concentrations of 12 illicit drugs in both influents and effluents from 8 WWTPs were summarized in Table 2. METH, COC and KET were the most frequently observed compounds, which were found in all influent samples. The detection frequencies of AMP, MC, COD, HR, MET, MDMA, and MDA in influents were greater than 75%, whereas BE was only detected in the WWTP TNQ-2 (Table2). METH concentrations ranged from 1.28 to 51.6 ng/L and from <LOD to 22.4 ng/L in the influents and effluents, respectively. The lowest and highest average concentrations of METH were observed at WJQ-1 (1.20 ± 0.5 ng/L) and XBQ-1 (51.62 ± 3.5 ng/L), respectively. In general, METH concentrations in the present study were similar to the influent levels from WWTPs at Nanjing, Shanghai and Nanning cities in China [15], slightly higher than in eastern Canada and Dutch sewage water [16], slightly lower than in UK WWTPs [17], and much lower than in WWTPs in Yinchuan, Xiamen and Shenzhen in China [14].
AMP concentrations were lower than LOD at XBQ-1, and the highest AMP concentration was detected at JTQ-1 (10.29 ± 1.2 ng/L). Positive correlations were discovered between METH and AMP concentrations (r=0.88). AMP is the main metabolite of METH and also the ingredient of Selegiline, the medicine for treating Parkinson's disease [18]. Studies have shown that the conversion ratio of METH to AMP after metabolism is between 4% and 7% [14]. It is logical to draw that when the concentration rate of AMP to METH was between 0.04-0.1, AMP was mostly derived from the transformation of METH. The ratio of concentration of AMP to METH was greater than 0.1 at most WWTPs in this study, indicating that the emergence of AMP was more likely to be related to the use of prescription drug Selegiline. This contrasted sharply with the status in European countries where more AMP relative to METH use was detected [19-21].
KET was observed in eight influent samples, and its concentrations were ranging from 0.78 - 2.50 ng/L, while NK was not detected in the influents (Table 2 and Fig. 2). Although KET was widely present in all WWTPs, its levels were as low as not able to generate detectable degradation product NK. Compared to the cities in those countries, NK was consistently observed in WWTPs in the southern cities of Shenzhen and Guangzhou, which was higher than in Beijing and Shanghai [22], implying the different consumption patterns of KET in different areas of China.
MDMA was observed in 75% of the samples. And the concentrations of MDMA ranged from 0.18-0.55 ng/L in influents and 0.18 - 0.67 ng/L in effluents. These concentrations were lower than many reported levels in countries with high consumption of MDMA, such as the US (70 ng/L) [23], UK (13.9 ng/L) [17] and Spain (180 ng/L) [24]. Compared with those cities, Changzhou is a relatively small area with less entertainment places, and the detected MDMA concentrations were nearly 100 times lower than in European countries. Similar to MDMA, low concentration of MDA was observed for 0.24-4.85 ng/L. The concentration range of COC is 0.18-0.75 ng/L, which is far lower than the influent concentration of UK WWTPs (5.1-208.9 ng/L) [25] and Canada WWTPs (289-823 ng/L) [26]. A comparative study was conducted in 9 WWTPs in Beijing, Guangzhou, Shenzhen and Shanghai in China [27], indicating that the illicit drug use pattern in China was different from European countries. In China, the use of METH and KET was the main concern, while the most popular ones in European countries were COC and MDMA.
MET was observed in more than 50% of both influents and effluents wastewater samples. Concentrations of MET in the influent were in the range of ≤LOD-1.24 ng/L and in the effluent for ≤LOD-0.18 ng/L. In this study, the concentrations of MET were lower than the reported levels in other countries, such as the US (62 ng/L) [18], Croatia (94 ng/L) [15] and Belgium (16 ng/L) [22].
Removal of illicit drugs from WWTPs
The removal efficiency of target analytes depends strongly on the wastewater treatment techniques. A summary of the removal efficiency of each WWTP was presented in Figure 3. In this study, the treatment techniques of the eight WWTPs included anaerobic anoxic oxic (A2/O) process, anoxic oxic (A/O) process and sequencing batch reactor activated sludge (SBR) process. SBR process was able to effectively remove illicit drugs from water, with the highest removal rate for the 12 target drugs of 79%, followed by the A2/O process (73%). In the SBR process, illicit drugs were firstly eliminated in the primary sedimentation tank. By mixing and aerating the wastewater with SBR process, the organic pollutants in the wastewater were decomposed, and the removal efficiency could reach more than 70%.
The negative removal of some illicit drugs was also observed in the WWTPs, with the highest negative removal rates of MC and MDMA up to -155.9% and -60.7%, respectively. Negative removal was also reported for KET [16, 20, 28]and other target drugs [18], which may be bound up with the increased transformation of precursor compounds or parent compounds, hydraulic residence time, and/or desorption from suspended solids in the wastewater treatment processes [19, 29].
BE was detected only in the influent of TNQ-2. BE is the major metabolite of COC. COC concentrations were relatively low in all 8 wastewater treatment plants, and the removal rate of COC in the sewage treatment process was basically below 50% [24]. This result was inconsistent with the previous study that COC was easily metabolized into BE and after human metabolism, only 1% of them was excreted from urine in the form of COC, and 25% -45% was excreted in the form of BE [4, 30]. METH, MET and MDA were effectively removed in different WWTPs, with removal rates ranging from 51.3% to 100%. Conversely, MDMA displayed negative removal in four WWTPs in this study. This phenomenon was possibly related to its highly recalcitrant property in wastewater and longer half-life of MDMA compared with other target drugs, which may elevate both the persistence of MDMA in wastewater and resistance to biological processes in the secondary treatment procedures [31].
Mean loads of illicit drugs
Among the 8 WWTPs, LYS-1 receives domestic sewage, XBQ-3 and JTQ-1 receive both domestic sewage and industrial sewage, and the rest 5 WWTPs receive industrial sewage only.
The mass loads of illicit drugs were ranged from 0.01 mg/d/1000 inh (MDMA in LYS-1) to 20.65 mg/d/1000 inh (METH in XBQ-1) (Fig. 4). Average loads of METH, AMP, KET, COD, MDA and MC in the eight WWTPs were 5.89±8.74, 0.84±0.62, 0.34±0.41, 0.48±0.76, 0.42±0.39 and 0.41±0.36 mg/d/1000 inh, respectively (Table 3). No obvious trend was observed regarding the average loads in the WWTPs relative to loads in 24 h.
High METH loads were found in XBQ-1 (20.65 mg/d/1000 inh) and XBQ-3 (18.99 mg/d/1000 inh). These loads were lower than those of wastewater samples (1500-1800 mg/d/1000 inh) in Australia [6] and other cities in China such as Haerbin (181.20±6.50mg/d/1000 inh) [19], Yinchuan (148.00±145.20mg/d/1000 inh) [15], and the mean METH load in WWTPs from18 cities in China was 67.80±45.2 mg/d/1000 inh, which was 3-fold higher than the mean load in this study [15].
COC and MDMA consumption was much lower than METH in this study, ranging from 0.02 to 0.15 mg/d/1000 inh for COC and 0.02-0.16 mg/d/1000 inh for MDMA. The consumption of COC and MDMA estimated in the present study was lower than 20.00-200.00 mg/d/1000 inh and 5.00-50.00 mg/d/1000 inh, respectively, in WWTPs of South-East Queensland in Australia. The average loading of MC was < LOD -0.92 mg/d/1000 inh, which was lower than 0.50 mg/d/1000 inh in Great Britain and 1.0 mg/d/1000 inh in Italy [32]. The average loading of MET was 0.09±0.06 mg/d/1000 inh, far lower than Sweden (0.50-29.00 mg/d/1000 inh) [33] and Finland (1.2-9.5 mg/d/1000 inh) [20].
The average daily load of MDA and MDMA was < LOD - 1.10 and < LOD - 0.16 mg/d/1000 inh, respectively. The measured level was comparable with other Chinese cities for Shenzhen and Guangzhou [22], but was much lower than the MDMA load of 5-41 mg/d/1000 inh in the working days of the 25 WWTPs in France [6]. The maximum load of HR was 0.98 mg/d/1000 inh, which was much lower than the average HR load of 61 mg/d/1000 inh in the Ebro River Basin [18]. HR load in Guangzhou and Shenzhen in China was less than 4.0 mg/d/1000 inh [15], while the HR load in western cities of China was higher. The daily load of COD was < LOD - 2.01 mg/d/1000 inh, which was basically consistent with the average COD load of 5.7 mg/d/1000 inh in South Korean [25], far low than the UK [24], with corresponding load of 565 mg/d/1000 inh.
Although COD and MET were detected in most WWTPs in this study, the detection concentrations were significantly lower than European cities with the estimated COD load of 2-1998 mg/d/1000 inh [3]. MET and COD are used as controlled medicines in China [34] (In 2018, the State Drug Administration listed COD as the prohibited substance for adolescents and children). In Europe however, MET is used as an alternative medicine for methadone oral solutions. In most EU countries, COD can be used legally. Drugs containing codeine are approved by national procedures and sold as prescription or over-the-counter drugs in different prescriptions [34].