THE USE AND BEHAVIOR OF SORPTION MEDIA IN MITIGATING EXCESSIVE DISSOLVED PHOSPHORUS IN SURFACE WATERS

Excessive phosphorus (P) is a threat to water quality and aquatic life, and one of the governing causes of eutrophication in water systems. It has been the object of much research that led to the implementation of P best management practices, aimed at curbing P export from agricultural and urban landscapes. However, these efforts are somewhat insufficient to mitigate and control dissolved P transport, a P pool 100% bioavailable for aquatic biota. Recent developments in nutrient management research highlight the ability of P removal structures to sequester dissolved P from flowing water, e.g., runoff and subsurface drainage, before it reaches water bodies. Phosphorus removal is accomplished through the use of reactive filter media, which are either manufactured, mined, or industrial by-products. These media, also referred to as P sorption materials (PSMs), vary in P removal ability, due to their origin, chemical and physical properties, or the conditions under which they operate. Consequently, there is a need to fully distinguish the characteristics of PSMs and their behavior in P removal structures that result in a superior P removal performance. In this study, six different types of PSMs were characterized according to their chemical and physical nature, and PSM-P interactions. To evaluate the variability of P removal capacity of steel slag, a series of flow-through experiments were conducted, using 18 different samples from different origins and generation processes. Phosphorus removal was evaluated on uncoated and aluminum(Al)-coated steel slag samples under two residence times. After chemically characterizing the samples, we found that, for the uncoated steel slags, electrical conductivity (EC), bulk density, particle density and magnesium (Mg) content could explain around 70% of the variability of P removal. Steel slags showed a high variability in their P removal ability, but such variability could be considerably decreased when coating the slags with Aluminum (Al). The Al-coating also allowed a significantly better P removal performance under shorter residence times. Flow-through experiments were also conducted to evaluate the ability to regenerate the P removal capacity of iron(Fe)- and Al-rich PSMs across two cycles of sorption-desorption with potassium hydroxide (KOH). This study found an average P recovery of 81%, 79% and 7% for Alcan, Biomax and PhosRedeem, Fe/Al-rich PSMs commercialized for contaminant removal. The most effective regeneration treatment was characterized by the largest KOH volume (20 pore volumes) and no recirculation, with up to 100\% reported P recovery, although a more economical/feasible use of 5 pore volumes of 1M KOH with recirculation was also found to perform well. The results suggested that the use of Al/Fe-dominated PSMs in P removal structures can be extended through the demonstrated regeneration technique. Iron-rich PSMs were further evaluated in regards to their behavior under anoxic conditions, a scenario that can be found in P removal structures with bottom-upward flow regimes. To evaluate the interference of redox-induced changes on P removal, PSM samples were incubated in a biogeochemical reactor in the presence of tile drainage water. Measurements of Eh throughout the incubation period indicated that PSMs, similar to soils, developed anoxic conditions. After incubation, the dissolved P concentrations in P-loaded and original PSMs were equally low, demonstrating the stability of P retention of PSMs under anoxic conditions. Additionally, the P removal ability of the original PSMs was not significantly altered by undergoing anoxic conditions, as determined from flow-through experiments following incubation. Anoxic-induced changes did not result in any limitations to the implementation of P removal structures with bottom-upward flow. These studies demonstrated the variability in P removal capacity of PSMs as a function of chemical and physical properties, the dominant P removal mechanism, and the operational characteristics of the P removal structure. The experimental data suggests that P removal structures are an effective and environmentally safe best management practice (BMP) that, in conjunction with traditional BMPs, are critical for the mitigation of dissolved P export to water systems.