The effect of the pH level on the dissociative behavior of calcium ion from oyster shell was evaluated in batch experiments. The results indicated that when the initial pH changed from 3.0 to 10.0, equilibrium pH possibly maintained at about 8.0. Nitrification behaviors at various pH and NH4+-N loads were investigated simultaneously in two aerated biofilters packed with oyster shell and plastic balls, respectively. The results showed that when the influent NH4+-N concentrations varied from 15 to 60 mg l− 1, 63.6 to 97.5 percent of NH4+−N could be removed by the oyster shell biofilter, but only 13.3 to 29.0% could be removed by the plastic ball biofilter at the influent pH of 5.3∼ 5.7. Furthermore, due to the sufficient alkalinity supply in oyster shell biofilter, the effluent pH could be maintained above 6.5, indicating that oyster shell shows greater buffer capacity.

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