*4.2. Field-Scale*

For field-scale studies, it can be further divided into two groups: pilot-scale CW studies and full-scale in situ CW studies. The former one typically had a smaller dimension (within an average volume of 1 m<sup>3</sup> for each CW), while the latter took a greater surface area (typically over 100 m2) and served as a functional water treatment system for agricultural wastewater or farm runoff. The pilot-scale studies can be seen as a scaled-up version of

laboratory-scale studies, as they are in larger volumes and typically operated in greenhouses or open fields receiving more real-world weather conditions than lab studies, but they can still be modified timely during operation to achieve better performance since their scale is still manageable. Therefore, during the pilot-scale study time (ranging from 1 to 16 months), water samples were collected periodically to evaluate the CW performance over time [38,62,63,86–89]. While the full-scale studies were based on fully established CWs that have been operating for several years, therefore, the system typically already reached a steady state for removal performance requiring little manipulation during study time. Compared to the smaller scaled studies that typically collect water samples at the influent and effluent points with multiple and periodic sampling events, the larger scaled studies tend to have more sampling points throughout the system within only one or few sampling events to monitor the removal performance over the entire treatment system [40,41,66,90,91]. Another major difference between pilot-scale and full-scale CW studies was that pilot-scale studies often spiked up the target contamination concentrations even if they already existed in the influent water, but the full-scale field studies treated the existing concentrations and measured field concentrations. Therefore, full-scale studies might show relatively lower removal efficiencies since it is more challenging to achieve high removal performance at lower influent concentrations.
