Mechanistic understanding and prediction of bioclogging in sand filters and subsurface flow constructed wetlands

INTRODUCTION Clogging of the pore-space is the primary cause of efficiency reduction and aging of wastewater treatment devices such as sand filters and subsurface flow constructed wetlands (US EPA 2000; Langergraber et al., 2003; Kadlec and Wallace, 2009). Significant efforts have been undertaken to study and understand the mechanisms leading to clogging and to design solutions to alleviate its adverse effects. Clogging is the plugging of pores in the filtering medium and results in a decrease of the hydraulic conductivity of the substrate. It is normally more pronounced near the inlet. Physical, chemical and biological mechanisms may contribute synergistically to pore blockage. Examples of such mechanisms are the filtration of fines and colloidal particles suspended in the pore water, growth of biofilms and precipitation of minerals, such as calcite and apatite. While mineral precipitation is sensitive to the chemical composition of the wastewater and filtering material, biomass growth and deposition of suspended solids are ubiquitous processes and affect nearly every treatment device, although to a varying extent (e.g., Kadlec and Wallace, 2009). Empirical design criteria based on areal mass loading rates are currently used to minimize the rate of pore blockage, although with limited success (e.g., Austin et al., 2007). Therefore, more fundamental, process-based approaches could help in understanding and predicting the development of clogging as a function of the design parameters, such as the medium grain size distribution and the mass loading rate (e.g., Austin et al., 2007).