*5.2. Aeration*

Depending on the type of CW system and the specific spot within a CW unit, aerobic or anoxic conditions may exist favoring certain pollutant removal processes. The removal of antibiotics, such as monensin, salinomycin and naracin, through microbial degradation was most active at the water/air interface or within the root zone under aerobic conditions [77]. Other works have also shown aerobic conditions to support the removal of veterinary pharmaceuticals from wastewaters [101]. On the contrary, the biodegradation of chloroacetanilide herbicides might be favored under anoxic conditions, as Elsayed et al. (2015) found that bacterial communities were most abundant and active at anoxic rhizosphere zone and anaerobic degradation accounted for the most dissipation of chloroacetanilides [102]. Besides the natural established aerobic/anaerobic conditions, some studies also introduced artificial aeration to promote the removal rates. For example, Chen et al. (2019) compared four different hybrid CW systems with/without the addition of aeration from an air blower and their results showed enhanced ARGs removal rates in both VSSF and HSSF with additional aeration [83]. Similar findings were also observed in a Feng et al. (2021) study, as they also noticed improved target ARGs removal with aerated treatments [58]. This indicated that for future applications, aeration units should be considered in the CW system design to improve the ARGs removal efficiencies. Alternatively, better designs to enhance aeration naturally in the CWs will likely enhance removal of ARGs.

#### *5.3. Types and Design of CWs*

Out of the 32 studies listed in Table 1 with identifiable CW type/design, 3 of them (around 9%) were surface flow (SF), 10 of them (around 31%) were subsurface horizontal flow (SSHF), 11 of them (around 35%) were subsurface vertical flow (SSVF), and 8 of them (around 25%) were hybrid systems containing more than one type (SF/SSHF/SSVF). In general, SSHF and SSVF are more widely applied in single CW type studies comparing to SF. This is due to how SSHF CW provides an anoxic system which promotes denitrification and other anoxic microbial processes; whereas SSVF CW provides an aerobic system which supports nitrification and other aerobic microbial processes [72,102]. In addition, SSVF CW can also remove organic compounds and suspended solids effectively [72]. In order to achieve better removal efficiency for various pollutants, a lot of studies applied hybrid system containing SSVF and SSHF [72,90,102]. For example, Huang et al. (2019a) showed that all three two-stage CW systems removed over 98% of the antibiotics without significant differences among treatments [63]. SSFV (down-flow) and SSVF (up-flow) had a better performance for ARGs and nutrients (especially for N) removal due to its establishment of anaerobic ammonium oxidation condition and limitation of bacterial growth [63].

Besides CW types, studies have also investigated the impacts of different flow directions (up-flow vs. down-flow) [85,87] as well as the water level (high-level vs. lowlevel) [89]. Their results showed that the configuration of down-flow SSVF followed by up-flow SSVF provided best pollutant removal performance, however, they also expressed concern about accumulation of ARGs in the surface soil for down-flow SSVFs [85,87]. Meanwhile, Liu et al. (2014) reported relatively higher removal efficiencies for SSVF with low water level [89] and this was supported by Lyu et al. (2018) as they found significantly higher tebuconazole removal in unsaturated CWs than saturated CWs [97]. For larger field scale studies consisting of multiple CW units, various configurations are utilized. The most common one was connecting CW units in series [40,41,90,91] but there were more complicated setups in some studies. For example, George et al. (2003) first had eight cells connected in parallel as the first stage and the remaining six cells connected in parallel as the second stage with series connection between stages [96]. Another study conducted in Japan had five stages connected in series and three treatments connected in parallel for each stage; and within each treatment there were three cells operating in rotational mode [72,102]. Such sophisticated design could not only provide better removal performance but also allow the avoidance of cross-contamination between different cells and provide the chances to perform operation and maintenance on a specific cell without disturbing the entire system.

#### *5.4. Hydraulic Parameters (HLR and HRT)*

Compared to wastewater treatment plants, CWs typically need lower HLR and longer HRT to achieve the similar level of removal performances. The hydraulic parameters are very important to consider during CW system design since lower HLR/longer HRT may provide better treatment but require much larger land area, while higher HLR/shorter HRT may occupy a smaller footprint but face low treatment efficiency and frequent clogging events and need more operation and maintenance inputs. Based on the study data listed in Table 1, Figures 3 and 4 were plotted to show the relationships between HLR/HRT and target contamination removal percentage. It is apparent from Figure 3 that removal efficiency had a positive correlation with HRT, meaning a greater removal rate with the longer retention time. After 7 days, an average removal efficiency of 90% was achieved, which is in agreemen<sup>t</sup> with the findings from previous research that a hydraulic retention time of 6–7 days was adequate for the removal of pollutants [93,103]. Meanwhile, Figure 4 showed that with the increase in HLR, removal efficiency would first increase, reach to a steady level (>90%), and later start to decrease. That is to say, the ideal HLR should be 10–30 cm/d for best pollutant removal performance, as greater or lesser HLR would both result in reduced removal efficiency. This was supported by findings from Lyu et al. (2017) as they reported decreasing removal rates over increased HLR [97]. Therefore, choosing the appropriate HRT/HLR for CW system has grea<sup>t</sup> impacts on the system performance. Furthermore, in some studies, hydraulic retention times were adjusted based on seasons, with them being longer in warmer seasons (8 days) and shorter in colder seasons (6 days) to address the water requirement variations due to evapotranspiration [92,93,98].

**Figure 3.** Relationship between HRT and removal efficiency in CWs.

**Figure 4.** Relationship between HLR and removal efficiency in CWs.

#### *5.5. Substrate Medium*

Since adsorption is one of the major mechanisms for pollutant removal in CW systems, the substrate medium's physical and chemical properties would have a huge impact on the removal performances. The Papaevangelou et al. study (2017) compared two substrates (fine gravel and cobbles) from the same riverbed with various sizes and found better removal performance of fine gravel for target pollutant-boscalid (fungicide) in the preliminary tests but no significant difference in performance of the substrates over longterm field study [98]. A lot of previous research have compared the removal efficiency of specific contaminants with various substrate medium. For example, Liu et al. (2013) showed that compared to volcanic rock, zeolite had a lower point of zero charge (PZC) indicating a higher affinity to cationic form of antibiotics at neutral pH levels and had smaller pore size indicating greater sorption sites; therefore, zeolite showed better removal efficiencies for selected antibiotics and ARGs [38]. Similar observations were made by Du et al. (2020) as zeolite medium had a better removal performance for both antibiotics and ARZs compared to quartz sand medium [85]. In the Huang et al. study (2017), brick-based substrate achieved better antibiotic removal performance compared to oyster shell-based substrate due to two major reasons: (1) greater porosity and average pore size that provided

more surface areas for sorption processes; (2) higher iron oxides contents in brick that provided better adsorption capacity [87]. Besides zeolite and brick material that were widely applied in CW substrates, medium with high organic matter content was also investigated since it could potentially increase pollutant removal through interactions with organic functional groups (such as phenolic and carboxyl groups), hydrogen bonding, and ion exchange [104]. For example, Feng et al. (2021) compared biochar and gravel based CWs and found that while treatment with only biochar-based substrate had no significant improvement in target contaminants removal, treatment with both biochar-based substrate and aeration showed much higher removal rates [58]. They also measured abundance of ARGs in the substrate indicating the accumulation of antibiotics in the substrate and proliferation of ARGs during the long-term operation [58]. That is to say, appropriate operation methods need to be taken to address the potential risks of ARGs development in such substrates. Therefore, to achieve better elimination of ARGs in practical approaches, the suitable selection of CW substrate medium is an important decision.
