Horizontal Subsurface Flow Constructed Wetlands in Arid and Semi-Arid Areas—A Review

Horizontal subsurface flow constructed wetlands (HSSFCWs) were first developed by Seidel in the early 1960s and upgraded by Reinhold Kickuth as the Root Zone Method in the late 1960s and early 1970s. These constructed wetlands (CWs) are used in wastewater treatment and replicate the natural processes of pollution removal. HSSFCWs employ substrate bed planted with wetland plants, and the wastewater maintained below the bed surface flows horizontally from the inlet of the system to its outlet. They are proposed as a sustainable solution instead of other systems such as membrane bioreactors or sequencing batch reactors mainly because of their minimal energy consumption and relatively low construction and operational costs. Indeed, they are very practical for wastewater treatment in decentralized, rural, and remote areas, where conventional treatment is not possible and limited. Nevertheless, they are very effective in removing organic and microbial pollutants. However, for the proper functioning of these filters, some parameters may affect their processing efficiency such as type of vegetations, composition of the substrate, retention time, or temperature. This review focuses on the influence of arid and semi-arid climatic conditions on the effectiveness of the removal of different contaminants in HSSFCWs. Various studies of wastewater treatment using HSSFCWs under these specific climatic conditions, particularly in Morocco, Algeria, Tunisia, and Egypt, will be discussed.