Key factors in the clogging process of horizontal subsurface flow constructed wetlands receiving anaerobically treated sewage

•The clogging solids are predominantly inorganic.•The greater source of clogging solids is the wear of the filter material.•The organic solids influence most the pore spaces, due to their greater volume.•Near the outlet, the solids characteristics are closer to those of the substrate.

The influent solids load, biofilm formation and occupation of void spaces by precipitates have been identified as major contributors to clogging the pores of horizontal subsurface flow constructed wetlands (HSSF-CWs). Several publications suggest that most of the accumulated material is inorganic, and this indicates that wear of the substrate may be an important constituent of clogging. The objective of this study was to characterize, in mineralogical, physical (specific mass) and chemical terms (neutralization power and volatile and fixed solids) the clogging material from the pores and the substrate medium (blast furnace slag) in two small full-scale HSSF-CWs, one planted with cattail (Typha latifolia) and another kept unplanted. The system received urban wastewater pre-treated in an upflow anaerobic sludge blanket reactor, was operating for seven years and showed signs of heavy clogging leading to overland flow. Samples were collected at intermediate points along the two units in order to identify the origin of the accumulated material and thereby enable the proposition of a conceptual model for the clogging process of these systems. The results indicated that most of the mass of clogging solids were inorganic and originated from wear of the substrate. However, the volume occupied by the organic solids was larger, and those were the major contributors to the clogging process.