A special variant of vertical flow constructed wetlands (VFCWs) has been developed in France over the last two decades. The so-called French system is an alternative arrangement of intermittent loading VFCW beds. The main difference of this system is the absence of a fine-grained material top layer in the bed and the placement of an extended layer with coarse-grained gravel. A typical configuration of the French system includes a primary and a secondary treatment stage. A coarse bar screen is usually installed before the main treatment stages to remove large particles. Usually, the first stage includes three parallel beds. The main advantage of the French system is that raw wastewater is fed directly onto the beds of the first stage, which means that a primary settling tank is avoided and each bed receives the full organic load and the SS content, resulting in a decreased total surface area needed.
The feeding strategy is the same with the common VFCW type with intermittent loading and comprises feeding and resting periods. This high load application represents, however, a risk for fast clogging of the bed, taking into account that the major part of SS is retained in the first stage beds and a sludge layer is gradually created. This risk is minimized by the application of proper resting periods which allow for aerobic conditions restoration. Moreover, sludge accumulation on the bed surface means that management and handling cost for primary sludge are reduced. French systems have been used in series with horizontal flow CWs.