Here, we introduce a means of utilizing waste oyster shells which were obtained from temporary storage near coastal workplaces as CO2 adsorbents. The calcined CaO can be easily dissociated to Ca2+ cation and CO3 2？ anion by hydrolysis and gas-liquid carbonation reaction and converted to precipitated calcium carbonate (PCC) in algae-containing water. The calcium hydroxide and carbonation combination in algae-containing water significantly contributed to improving water quality which is very dependent on the addition amount of calcined powders.