Recovery and Fractionation of Phosphorus Retained by Lightweight Expanded Shale and Masonry Sand Used as Media in Subsurface Flow Treatment Wetlands

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Abstract
Most subsurface flow treatment wetlands, also known as reed bed or root zone systems, use sand or gravel substrates to reduce organics, solids, and nutrients in septic tank effluents. Phosphorus (P) retention in these systems is highly variable and few studies have identified the fate of retained P. In this study, two substrates, expanded shale and masonry sand, were used as filter media in five subsurface flow pilot-scale wetlands (2.7 m3). After 1 year of operation, we estimated the annual rate of P sorption by taking the difference between total P (TP) of substrate in the pilot cells and TP of substrate not exposed to wastewater (control). Means and standard deviations of TP retained by expanded shale were 349 ± 171 mg kg-1, respectively. For a substrate depth of 0.9 m, aerial P retention by shale was 201 ± 98.6 g of P m-2 year-1, respectively. Masonry sand retained an insignificant quantity of wastewater P (11.9 ± 21.8 mg kg-1) and on occasion exported P. Substrate samples were also sequentially fractionated into labile P, microbial P, (Fe + Al) P, humic P, (Ca + Mg) P, and residual P. In expanded shale samples, the greatest increase in P was in the relatively permanent form of (Fe + Al) P (108 mg kg-1), followed by labile P (46.7 mg kg-1) and humic P (39.8 mg kg-1). In masonry sand, there was an increase in labile P (9.71 mg kg-1). Results suggest that sand is a poor candidate for long-term P storage, but its efficiency is similar to that reported for many sand, gravel, and rock systems. By contrast, expanded shale and similar products with high hydraulic conductivity and P sorption capacity could greatly improve performance of P retention in constructed wetlands.