Case Study of 2,3,7, 8-TCDF Feedback from Aerated Lagoon Benthal Deposits, 1996 Environmental Conference Proceedings

Canadian Federal pulp and paper mill regulations require treated wastewaters contain less than 50 ppq 2,3,7,8- tetrachlorodibenzofuran (referenced as TCDF). At a bleached pulp mill in Canada, process changes essentially eliminated generation of TCDF. However, aerated lagoon benthal sediment contained significant residuals from historic bleaching practices. In 1993 and the first four months of 1994, TCDF feedback from aerated lagoon benthal sediment caused elevated concentrations in treated wastewater. A sampling program was undertaken to quantify TCDF in wastewater, suspended solids, and benthal sediment. The objective was to identify factors affecting transfer from sediment to overlying wastewater and evaluate potential corrective actions. Water column TCDF concentrations significantly increased in the first half of the aerated lagoon first bay. Levels then declined as biological solids generated during soluble BOD removal settled out in the second and third bays. A simplified mass balance found benthal sediment TCDF was being redistributed from lagoon front to back. Water column TCDF level increased where BOD removal activity was at its peak and was independent of vertical or spatial differences in benthal TCDF level. Increases in first bay water column TCDF appeared to be declining with time. Since the July 1994 sediment sampling, treated wastewater TCDF concentrations have steadily declined and remained below the 50 ppq regulatory limit.