Mass Transfer Behavior of Media for Treatment of Stormwater Phosphorus

Mass transfer of phosphorus (P) onto filter media in stormwater systems is characterized by adsorption kinetics, adsorption equilibria (isotherms) and breakthrough behavior. Such a behavior can be extremely variable depending on the selection of the filter media. This study examined the favorable adsorption properties of media engineered for treatment of P, aluminum oxide coated media (AOCM), in contrast to the unfavorable adsorption and desorption behavior of media such as perlite. The use of AOCM for aqueous P adsorption has a prominent advantage because very porous substrate facilitates fully contact between P and AOCM. High adsorption capacities were obtained for the typical conditions of urban stormwater. Media specifically engineered for selected solute adsorption, even under the competitive conditions of stormwater, such as AOCM, can demonstrate significant advantages of media such as perlite or sand. Results also demonstrated that a competitive Freundlich isotherm could successfully represent data of P adsorption on AOCM. Upon treatment by AOCM, P concentrations could be reduced to a suitable level for discharge into natural surface waters without the need for hydrodynamically unstable co-subtrates such as filtered TSS to provide the adsorption capacity for media with little capacity such as perlite or sand. Without such unstable co-substrates media such as perlite or sand have negligible capacity for P. Additionally, this study demonstrated that P is easily desorbed from perlite or sand, in comparison to AOCM. This result can be explained because of the strong chemical adsorption generated by AOCM in comparison to weak physical bonds onto perlite, sand or TSS.