LABORATORY DEVELOPMENT OF PERMEABLE REACTIVE MIXTURES FOR THE REMOVALOF PHOSPHORUS FROM ONSITE WASTE-WATER DISPOSAL SYSTEMS

Laboratory batch and column studies were conducted to develop permeabl e reactive mixtures to remove phosphorus from the affluent of onsite w astewater disposal systems. Mixtures can be placed in situ, as horizon tal or vertical reactive barriers in sediments receiving wastewater di scharge, or can be used in single pass, self-contained treatment modul es in alternative treatment systems. Reactive mixtures composed of sil ica sand, high calcium crushed limestone, and readily available metal oxides were tested to evaluate phosphorus attenuation. Iron/calcium ox ides, produced from steel manufacturing, and fine-grained activated al uminum 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 t he mixtures were assessed by continually loading bench-scale columns w ith a 3.3 mg/L PO4-P solution, at representative groundwater flow rate s. A column containing 50 wt % silica sand, 45 wt % limestone, and 5 w t % iron/calcium oxide averaged >90% reduction in phosphate over 4 yea rs (approximate to 1450 pore volumes). X-ray and SEM microprobe analys es of the reacted solids showed phosphorus accumulations on the surfac es of iron oxide phases and discrete precipitates of microcrystalline hydroxyapatite [Ca-5(PO4)(3)-OH]. A second column containing 50 wt % s ilica sand, 40 wt % limestone, and 10 wt % activated aluminum oxide ac hieved >99% reduction in PO4 over a period of 2 years (approximate to 413 pore volumes). The treatment performance in this system can be att ributed to the high adsorption capacity of the aluminum oxide.