Why is COD test result for wastewater is false when treating the wastewater by H2O2 first then measuring COD?

COD is the test for waste water by add H2SO4,HgSO4,Ag2SO4lkg H2SO4,K2Cr207 to water sample & after reflux for 6 hour titrate excess K2Cr2O7 with ferrous ammonium sulfate