The behavior of COD during press washing

Pulp washing is an important unit operation, affecting bleaching efficiency and economy. To establish the COD washing efficiency, pulps and filtrates are collected from relevant positions around the washer, in some cases during Dress wash evaluations we have experienced strange COD bohavior such as: Increased COD concentration in the press-washed pulp compared with the unwashed pulp COD "generation". i.e. more total COD is found in the washed pulp and the filtrate compared with the unwashed pulp and the wash liquor It was shown that the increase in COD concentration is a combined effect of the high outlet pulp consistency and the distribution of COD between fiber wail liquor and free liquor. Washing parameters that only consider COD concentrations, such as the DR value. are therefore most likely not control. The COD "generation" during press washing is caused by the analysis method. A high pulp consistency will in some situations result in an increase in the COD amount analyzed. COD "generation" affects the Norden E-10 value negatively.