To provide a wastewater treatment method detecting the concentration of a toxic substance, especially persulfuric acid contained in exhaust gas desulfurization wastewater, and subjecting the toxic substance to a necessary treatment process corresponding to the detection result.

The exhaust gas desulfurization wastewater treatment method includes a process for detecting persulfuric acid in the exhaust gas desulfurization wastewater 1 by a biosensor 3 for measuring the concentration of dissolved oxygen changed in the presence of the toxic substance inhibiting the metabolism of nitrite bacteria, especially persulfuric acid, an activated carbon treatment process for removing persulfuric acid or organic matter contained in the wastewater by an activated carbon treatment device 6 and a nitrification/denitrification process for decomposing an ammonia nitrogen compound in the wastewater by a nitrificaticn/denitrification device 5 and the order of the activated carbon treatment process and the nitrification/denitrification process is changed corresponding to the detection result of the toxic substance due to the biosensor 3 to treat wastewater. The apparatus 8 for treating exhaust gas desulfurization wastewater using the exhaust gas desulfurization wastewater treatment method is also disclosed.