Temporal changes in hydraulic conductivity of sand porous media biofilters during wastewater infiltration due to biomat formation Deborah N.H. Beach a , John E. McCray b, * , Kathryn S. Lowe b , Robert L. Siegrist b a Department of Geological and Mining Engineering and Sciences, Michigan Technological University, Houghton, MI 49931, USA b Division of Environmental Engineering and Science, Colorado School of Mines, 1500 Illinois Street, Golden, CO 80401-1887, USA Received 27 March 2003; revised 11 January 2005; accepted 25 January 2005 Abstract 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 . q 2005 Elsevier B.V. All rights reserved. Keywords: Wastewater; Hydraulic conductivity; Infiltration; Porous media biofilters; Biomat 1. Introduction Porous media biofilters (PMBs), also known as sand filters or soil absorption systems, are commonly used in the United States to treat and manage domestic Journal of Hydrology 311 (2005) 230–243 www.elsevier.com/locate/jhydrol 0022-1694/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2005.01.024 * Corresponding author. Tel.: C1 303 273 3490; fax: C1 303 273 3413. E-mail addresses: dnbeach@mtu.edu (D.N.H. Beach), jmccray@mines.edu (J.E. McCray), klowe@mines.edu (K.S. Lowe), siegrist@mines.edu (R.L. Siegrist).