Temporal changes in hydraulic conductivity of sand porous media biofilters during wastewater infiltration due to biomat formation

Porous media biofilters (PMBs) are commonly used to treat domestic wastewater. Biomats develop at the infiltrative surface of PMBs due to continued wastewater application and create an impedance to flow. The goal of this research is to quantify the temporal evolution of normalized biomat hydraulic conductivity (K_bm/b_bm) and effective hydraulic conductivity (K_e). K_e is the overall hydraulic conductivity of the infiltrative zone, including biomat and unsaturated media below the biomat. Research was conducted using eight one-dimensional (1D) sand columns with gravel-free and gravel-laden infiltrative surfaces. The columns were loaded at design rates of 100-200 cm/d for 20 weeks of column operation. The K_e values for these continuously loaded columns were determined from analyses of bromide-tracer tests, falling-head permeability tests, and volumetric water content measurements during biomat development. The reduction in the K_e due to biomat formation is due to two factors: reduced hydraulic conductivity of the thin biomat, and a reduced hydraulic conductivity of the subsoil due to development of a biomat-induced unsaturated flow regime. Unsaturated hydraulic conductivities of the subsoil below the biomat (K _ss) were estimated from capillary curves and water content measurements. For observed final biomat thicknesses (less than 1 cm), the biomat hydraulic conductivity, K_bm, is three orders of magnitude smaller than the unsaturated hydraulic conductivity (K_ss). However, the relatively large thickness of the vadose zone causes the K_ss to be an important contributor to the overall K_e value. For these columns, the final K_e values were approximately two orders of magnitude smaller than the original value. Because the exact thickness of the biomat (b_bm) is unknown during the flow experiments, the hydraulic conductance of the biomat zone is presented using a normalized hydraulic conductivity function (K_bm/b_bm). A similar K _bm/b_bm is reached regardless of wastewater loading rate. An exponential relationship exists between the volume of wastewater applied to the column and both K_e and K_bm/b_bm.