In contrast to the surface flow wetland, in the horizontal sub-surface flow wetland, the waste water always remains underground and therefore does not come into direct contact with the outside air. This has the important advantages that the system is much less sensitive to low temperatures and does not attract mosquitoes. In addition, it needs less than half the area, compared to a surface flow wetland. Horizontal sub-surface flow systems, can be filled with sand or gravel and give good purification results. However, the system is prone to clogging, especially on the inflow side. Clogging at the inlet can therefore limit the life of this system. This system is currently still frequently used as part of manure processing wetlands.