Biodegradability of Different Size Classes of Bleached Kraft Pulp Mill Effluent Organic Halogens During Wastewater Treatment and in Lake Environments
Less than 5% of the organic halogen emissions from bleaching of pulp are molecules of known structures. The major part consists of mainly hydrophilic molecules of unknown structure, chlorohumus. This paper focuses on the impact of size of the halogenated molecules on the environmental fate. The tetrahydrofuran-soluble fraction of the wastewaters was studied because >90% of the wastewater contained organic halogen (AOX) dissolves in this solvent. We show that halogen emissions of modern kraft pulp mills range from 100 to 1000 g mol−1 in size. The molecular size had little limitation on biodegradation during secondary wastewater treatment. Thirty to 70% of the AOX was removed during wastewater treatment (full scale). Molecules of <500 g mol−1 were slightly more degraded than those >500. Fifty to 80% of the remaining halogens were biodegraded in the lake ecosystem, studied in 2 m3 outdoor mesocosms over four seasons. The molecular size distribution of the tetrahydrofuran-soluble fraction of wastewater showed little change in clear lake water mesocosms, although over 50% of the AOX was removed. Sedimentation explained only a minor fraction of AOX removal from the water column. The tetrahydrofuran-soluble halogenated material which accumulated in the sediments showed a molecular weight distribution completely different from that in the water column or in the wastewater, indicating extensive metabolism.