Feedback of 2,3,7,8-TCDF (tetrachlorodibenzofuran) from aerated lagoon sediments

Process changes at a bleached softwood pulp mill in Canada essentially eliminated generation of 2,3,7,8-TCDF (tetrachlorodibenzofuran). However, aerated lagoon sediment deposits contained significant TCDF residuals from historic bleaching practices. The aerated lagoon discharge TCDF concentration was elevated and believed to result from TCDF feedback from lagoon sediment. A sampling program was undertaken to quantify TCDF in wastewater, suspended solids, and sediment. Measurements found water column TCDF concentrations significantly increased in the lagoon first bay front half and then declined as generated biological solids settled out in downstream lagoon segments. A simplified mass balance found sediment TCDF was being redistributed from lagoon front to back. Wastewater TCDF level increased where soluble BOD removal activity was greatest and was independent of vertical or spatial differences in sediment TCDF concentration. Increases in first bay wastewater TCDF appeared to be declining with time concurrent with reduction in final discharge TCDF concentration below the regulatory limit. Net feedback from first bay sediment was 19.46 mg/day, but net export in the lagoon wastewater discharge was only 1.40 mg/day. It was estimated first bay historic deposits of TCDF would be depleted in 1 to 4 years. Additional wastewater samples were collected 14 months later to confirm feedback decline at internal lagoon locations. Redistribution of sediment TCDF was still occurring from lagoon fron to back, but lagoon bay 1 gross TCDF feedback from sediment had declined by 88%.