Laboratory development of permeable reactive mixtures for the removal of phosphorus from onsite wastewater disposal systems

Laboratory batch and column studies were conducted to develop permeable reactive mixtures to remove phosphorus from the affluent of onsite wastewater disposal systems. Mixtures can be placed in situ, as horizontal or vertical reactive barriers in sediments receiving wastewater discharge, or can be used in single pass, self-contained treatment modules in alternative treatment systems. Reactive mixtures composed of silica sand, high calcium crushed limestone, and readily available metal oxides were tested to evaluate phosphorus attenuation. Iron/calcium oxides, produced from steel manufacturing, and fine-grained activated aluminum oxide outperformed other oxides tested during batch experiments. These materials removed greater than 99% of PO4 from a 10 mg/L PO4-P solution within 1 h of contact. Long-term attenuation capacities of the mixtures were assessed by continually loading bench-scale columns with a 3.3 mg/L PO4-P solution, at representative groundwater flow rates. A column containing 50 wt % silica sand, 45 wt % limestone, and 5 wt % iron/calcium oxide averaged >90% reduction in phosphate over 4 years (approximate to 1450 pore volumes). X-ray and SEM microprobe analyses of the reacted solids showed phosphorus accumulations on the surfaces of iron oxide phases and discrete precipitates of microcrystalline hydroxyapatite [Ca-5(PO4)(3)-OH]. A second column containing 50 wt % silica sand, 40 wt % limestone, and 10 wt % activated aluminum oxide achieved >99% reduction in PO4 over a period of 2 years (approximate to 413 pore volumes). The treatment performance in this system can be attributed to the high adsorption capacity of the aluminum oxide.