Experimental Analysis and Modeling of a Stormwater Perlite Filter Jorge Girona ´s 1 *, Jose ´ M. Adriasola 2 , Bonifacio Ferna ´ ndez 2 ABSTRACT: This paper presents the study of a mixed porous media composed of expanded perlite and a nonwoven needle-punched geotextile used to reduce the suspended solids load and concentration in urban runoff. Laboratory procedures were designed to quantify the suspended solids removal efficiency and variation in time of filtration rate. Different grain-size distributions of expanded perlite, diverse suspended solids concentrations, and different hydraulic and geometric conditions were tested to determine the most effective filter media. A dimensionless parameter, termed Global Performance Index (GPI), was developed to reach this objective. Measured data were also used to build a dimensional and a regression model to represent the performance of the filter media mathematically. The theory, derivation, and performance of both models are presented and compared with an existent empirical model. The dimensional model better reproduces the observations, becoming a useful tool for the design, operation, and evaluation of commercial porous media filters. Water Environ. Res., 80, 524 (2008). KEYWORDS: expanded perlite, stormwater filter, suspended solids, stormwater runoff, water quality, best management practices. doi:10.2175/193864708X267432 Introduction Stormwater infiltration is a common technique used in best management practices for urban stormwater drainage and control. This technique reduces runoff discharges and volumes and pro- motes groundwater recharge. However, there are also some negative effects associated with infiltration, such as the risk of groundwater contamination and the reduction of the infiltration rate through time, as a result of infiltration surface clogging (Dechesne et al., 2002; Raimbault et al., 2000; Urbonas, 1999). One of the alternatives to control these problems is to use stormwater filters, which reduce the suspended solids load and concentration before stormwater reaches the infiltration areas (Urbonas, 1999; Urbonas and Stahre, 1993). It is expected that any solution should be easy to implement, operate, and maintain. In the specific case of a modular stormwater filter device, it must be easy to install, clean, and renew; its size must be reduced; and it has to be built based on standardized elements, which facilitate the achievement of different design criteria by minimum changes or additions. To accomplish these objectives, the filter media must be carefully selected. This article presents a preliminary investigation of expanded perlite as an alternative porous media to be used in stormwater filter devices, which help to reduce suspended solids loads and concen- trations in urban stormwater. The paper is organized as follows. The first section describes the filter media used in this study, the main characteristics of the filter media, and the reasons supporting this selection. The next section presents the experimental measurements, depiction of the main variables that characterize the performance of the filter media, analysis of the experiments results, and a compar- ison of different design alternatives. These results are used to de- velop different models, which are discussed and compared in the next section. Concluding remarks are offered in the final section of the paper. Filter Media Selection Several materials have been reported as filter media in the literature. Clark and Pitt (1999) summarize the most current and widely used media, including sand, activated carbon, and peat moss. Each has advantages and limitations, and the selection depends on the desired pollutant removal performance and associated con- ditions, such as land use (Clark and Pitt, 1999). Most of these filters must be built in situ because of the amount of material needed to reach good performance and because of the large concrete structures involved in the construction. An alternative approach can be the design of small, easy-to-install filter devices that do not require a complicated building process and are used to treat smaller draining areas. Additionally, a filter device should be designed to achieve high filtration rates and removal efficiency. From that point-of view, it is very important to select a filter media that meets the following properties: (1) high specific surface area; (2) low mass density, allowing an easy installation and transportation of the filter and/or the filter media; and (3) structural resistance to handle typical installation and operation loads. A material that satisfies these characteristics is expanded perlite. Perlite is a natural siliceous rock that, when heated to a suitable point in its softening range (760 to 11008C), expands 4 to 20 times its original volume, reaching an extremely light weight and a high specific surface area (Purchas, 1997). This expansion is the result of the presence of a significant percentage of combined water in the crude rock. Figure 1 presents the different states for the perlite, while Figure 2 shows the very complex porous microstructure of expanded perlite. Some of its typical applications are in the construction, agriculture, food, beverage, medical, and chemical industries (Uluatam, 1991). Expanded perlite has already been used and studied as a filter media to treat residential and industrial wastewater (Demirbas et al., 2002; Dogan and Alkan, 2003; Dogan et al., 2004; Joseph and Rodier, 1994; Uluatam, 1991). Additionally, perlite and expanded perlite have also been used as stormwater filter media or as a component with other materials (Adriasola, 2003; CALTRANS, 2004; CONTECH Stormwater Solutions Inc., 2002a, 2002b; Milesi 1 Department of Civil and Environmental Engineering, Colorado State University, Fort Collins, Colorado. 2 Departamento de Ingenierı ´a Hidra ´ulica y Ambiental, Pontificia Universi- dad Cato ´lica de Chile, Santiago, Chile. * Department of Civil and Environmental Engineering, Colorado State Uni- versity, Fort Collins Co. 80523, USA; e-mail: jorge.gironas@colostate.edu. 524 Water Environment Research, Volume 80, Number 6