Mechanical resistance properties of gravel used in subsurface flow constructed wetlands: implications for clogging

Abstract

Gravel constitutes the filter medium in subsurface flow constructed wetlands (SSF CWs) and its porosity and hydraulic conductivity decrease over time (clogging), limiting the lifespan of the systems. Using gravel of poor quality accelerates clogging in wetlands. In this study, gravel samples from six different wetland systems were compared with regards to their mineral composition and mechanical resistance properties. Results showed that both mineralogy and texture are related to mechanical resistance. Accordingly, gravel with high content of quartz (> 80%) showed a lower percentage of broken particles (0.18-1.03%) than those with lower content of quartz (2.42-4.56% media broken). Although granite is formed by high durability minerals, its non-uniform texture results in a lower resistance to abrasion (ca. 10% less resistance than calcareous gravel). Therefore, it is recommended to use gravels composed mainly of quartz or, when it is not available, limestone gravels (rounded and uniform) are recommended instead. The resistance to abrasion (LAA test) seems to be a good indicator to determine the mechanical properties of gravels used in CWs. It is recommended to use gravels with LAA below 30% in order to avoid a rapid clogging due to gravel crumbling and subsequent mineral solids accumulation.

Potential Application of Chilean Natural Zeolite as a Support Medium in Treatment Wetlands for Removing Ammonium and Phosphate from Wastewater

“…This research can be helpful considering the serious operational problem of SSF-TWS: clogging [64]. Clogging is the accumulation of solids (wastewater suspended solids) or the buildup of biofilm (chemical precipitates, plant detritus accumulation and biofilm growth) that results in porosity and hydraulic conductivity and water transport decrease over time [65]. This problem could affect the contact time between zeolite and wastewater; thus, reducing the removal capacity of ammonium and phosphate by the SSF-TWs during the treatment.…”

Hydraulic Conductivity Variability in Horizontal Subsurface Flow Constructed Wetlands

“…Porous medium obstruction may be associated with suspended solids in wastewater applied to the system, formed biomass, suspended mineral solid deposition, refractory organic material accumulation, and chemical precipitate formation (KNOWLES et al, 2011;PEDESCOLL et al, 2011b). Porous medium clogging of HSSF-CW by plant roots and rhizomes is still the subject of controversy since some authors consider that these plant organs promote a pumping or suspension of support material (BRASIL;MATOS, 2008) in order to create a porous volume equivalent or higher than that previously existing in the medium.…”