## 3 Results and Discussion  
### The Abundance and Diversity of ARGs  
A total of 171 ARG subtypes were identified from 17 ARG types in HX WWTP with 48 multidrug resistance genes representing the most diverse ARG subtype (Figure S1A). This may be related to the widespread use of various antibiotics in hospitals, communities, and animal husbandry [34]. ARG diversity and abundance was the highest in the influent (144 ARG subtypes, 540.2 ppm), followed by activated sludge (45 ARG subtypes, 50.7 ppm), and effluent (11 ARG subtypes, 7.6 ppm) (Figures 2A and S1B). This is unsurprising since the influent contained large amounts of ARBs and ARGs from urban wastewater [16; 35]. The dominant influent ARG types were multidrug resistance genes, tetracycline, macrolide-lincosamide-streptomycin (MLS), and aminoglycoside, which is consistent with the findings of Shatin WWTP [4]. ARG distribution in the influent may reflect the average abundance and diversity of intestinal ARGs of residents in this area [36]. Sulfonamide is the most abundant ARG type in activated sludge, followed by multidrug resistance genes, tetracycline, and aminoglycoside. Changes in ARGs are mainly driven by MGEs in activated sludge [37]. The main ARG types in effluent after ultraviolet disinfection were multidrug resistance genes, bactracin, and beta-lactam, which may be due to the resistance to common disinfection strategies. A recent study confirmed that bactracin, multidrug, and beta-lactam resistance genes were the main ARG types in large drinking water treatment plants after the ozone and chlorine disinfection [11]. Multidrug resistance genes dominated in influent, activated sludge, and effluent because multidrug resistance is the natural state for most microorganisms to overcome environmental pressure [38].  
Figure 1: Analysis pipeline of this study. Sequences with the same color may be derived from the same DNA fragment. ORF: open reading frame, ARG: antibiotic resistance gene, MGE: mobile genetic element.  
PARGs are the most worrying ARG category because they cannot be removed by disinfection [11; 26]. Moreover, PARGs entering the downstream environment present serious challenges [4; 39] and may lead to the widespread occurrence of ARGs in the natural environment [40; 41]. Eleven ARG subtypes from seven ARG types were PARGs including an aminoglycos resistance gene (_aad_A), a bactracin resistance gene (_bac_A), a beta-lactamase resistance gene (_class A beta-lactamase_), 2 MLS resistance genes (_mac_B and _mph_A), 2 multidrug resistance genes (_mex_D and _aq_Edelta1), 2 sulfonamide resistance genes (_sul_1 and _sul_2) and a tetracycline resistance gene (_tet_A) in the HX WWTP (Figure 2B). The abundance of PARGs gradually decreased from influent to activated sludge and effluent, but the proportion of total abundance of ARGs significantly increased from 16.9% to 40.9% and 98.4%, respectively (Figure S2). The persistence of _mac_B may be related to human activities and horizontal gene transfer between pathogens and local bacteria [42]. Furthermore, _tet_A is a recognized PARG in three WWTPs of Hong Kong, while _aad_A, _bac_A, _sul_1, and _sul_2 are often detected as PARGs in drinking water [11], lake water, and seawater [43]. Among them, _sul_1 and _sul_2 stably exist in hospital wastewater [44] and lakes [45; 46] of Xinjiang. The corresponding samples (such as HX-INF and CJ-INF) had no significant difference in ARG type abundance (Kruskal-Wallis, _p_> 0.05) and six PARGs (_bac_A, _class A beta-lactamase_, _mex_D, _qac_Edelta1, _sul_1, and _sul_2) were shared in both locations (Figures S1-S3).