Stormwater Phosphorus Adsorption on Oxide Coated Media

As a minimum, mass transfer of phosphorus (P) species to and from filter media in stormwater systems requires a quantitative description of the equilibrium, kinetics and breakthrough phenomena. However such behavior can be extremely variable depending on the properties of media. Obtaining physically representative adsorption equilibria (as opposed to single-point isotherms or non-singular P-index guides) is crucial since the isotherms so derived generally are utilized as the basis for further adsorption design and engineering application. This study examined the favorable adsorption properties of aluminum oxide coated media (AOCM) and the unfavorable adsorption behavior of media such as perlite and sand for phosphorus. The adsorption of phosphate on AOCM has been found to be dependent on pH and maximum removal efficiencies have been obtained at slightly acidic conditions. Phosphorus adsorption was shown to increase with decreasing AOCM adsorbent size. Low desorption, which likely resulted from strong bonding between P and AOCM, was also obtained. Good adsorption capacities were obtained for the typical conditions of urban rainfall runoff. It is also found Freundlich isotherm could successfully represent data of P adsorption on AOCM. Upon treatments by AOCM, phosphorus concentrations could be reduced to a suitable level for discharge into natural surface waters, which suggests the promising application of AOCM. In comparison, perlite and sand had negligible capacity and phosphorus was easily desorbed from these media. AOCM was studied for breakthrough curve (BTC) characteristics and feasibility as a subsequent adsorptive filtration process after primary sedimentation of stormwater. This study provides insights on conditions pertinent to the design of engineered in-situ P treatment, such as how adsorption equilibrium and kinetics, bulk and intraparticle mass transfer, solution pH, P loading concentration, hydraulic retention time and surface loading affect the transport and BTC on porous AOCM. Smaller AOCM size leads to better BTC due to higher contacting area. Good BTC results favor low P concentration and low surface loading. The maximum adsorption capacities calculated from BTC are generally smaller than values from adsorption isotherm, indicating the presence of mass-transfer limitations and limited contact time in the flow through systems. Results of BTC of P adsorption on AOCM column indicate that AOCM are capable of efficient and effective treatment for urban rainfall runoff, while perlite and sand are wholly ineffective. BTC models, especially mechanistic models, provide valuable and cost effective information in guiding design and operation of stormwater treatment systems for P.