Phosphorus adsorption characteristics of oyster shells and alum sludge and their application for nutrient control in constructed wetland system.

This research aimed to evaluate the feasibility of employing some locally available oyster shells (OS) and alum sludge (AS) as the P adsorption media of constructed wetland beds. The batch and column tests were conducted with different sizes of OS and AS to observe the P adsorption characteristics and breakthrough, while P adsorption rates were determined from jar test. From the batch test, the ability to adsorb the amount of added P was different considerably between OS and AS media and sizes. At the same size, the P adsorption capacities of OS were higher than those of alum sludge. The P adsorption capacities of AS with 0.3-0.6 mm and > 0.6 mm sizes were similar, but those of 0.3-0.6 mm OS was higher than > 0.6 mm OS. Jar test results revealed the first-order P removal rates (k) to be 11.4, 4.5, 1.7 and 1.0 d(- 1) for the 0.3-0.6 mm AS, > 0.6 mm AS, 0.3-0.6 mm OS, and > 0.6 mm OS, respectively. From the column tests, the adsorption capacities were found to be more than 26 and 24.5 g/kg for the OS media sizes of 0.3-0.6 and > 0.6 mm, respectively, and 12 g/kg for the AS media. The column tests revealed that AS was able to adsorb P faster than the oyster shells, but due to its low adsorption capacity, the periods of P breakthrough in the effluent of the alum sludge columns were shorter than those of the oyster shell columns. Experiments with a pilot-scale vertical-flow subsurface constructed wetland unit packed with > 0.6 mm OS media demonstrated the high P removal efficiencies of more than 96.2% during the 210 days of operation.