Effect of hot acid hydrolysis and hot chlorine dioxide stage on bleaching effluent biodegradability

The hot acid hydrolysis followed by chlorine dioxide (A/D^*) and hot chlorine dioxide (D^*) technologies have proven very useful for bleaching of eucalyptus kraft pulp. Although the characteristics and biodegradability of effluents from conventional chlorine dioxide bleaching are well known, such information is not yet available for effluents derived from hot acid hydrolysis and hot chorine dioxide bleaching. This study discusses the characteristics and biodegradability of such effluents. Combined whole effluents from the complete sequences DEpD, D^*EpD, A/D^*EpD and ADEpD, and from the pre-bleaching sequences DEp, D^*Ep, A/D^*Ep and ADEp were characterized by quantifying their colour, AOX and organic load (BOD, COD, TOC). These effluents were also evaluated for their treatability by simulation of an activated sludge system. It was concluded that treatment in the laboratory sequencing batch reactor was efficient for removal of COD, BOD and TOC of all effluents. However, colour increased after biological treatment, with the greatest increase found for the effluent produced using the AD technology. Biological treatment was less efficient at removing AOX of effluents from the sequences with D^*, A/D^* and AD as the first stages, when compared to the reference D stage; there was evidence of the lower treatability of these organochlorine compounds from these sequences.