Comparison of COD and Toxicity Removal during Activated Sludge and MBBR Treatment of Kraft Pulp Mill Effluent

This study compared treatment of bleached eucalypt kraft pulp mill effluent in bench-scale activated sludge and moving bed biofilm reactors (MBBR) operated in parallel at hydraulic retention times (HRTs) of 12 and 24 hours and operated in series (activated sludge followed by MBBR) at HRTs of 12 hours each. COD, recalcitrant compound (lipophilic extractives and lignin), and toxicity removal efficiencies were compared in the reactors operated under different conditions. Results indicated high COD removal efficiency in both systems under the different configurations tested with no significant differences found between treatments, except for a greater proportion of high molar mass COD remaining after treatment at a 12 hour HRT. The MBBR presented higher extractives removal efficiency than the activated sludge system and greater solids retention under stress conditions. Treatment by either system reduced or eliminated chronic toxicity to Ceriodaphnia dubia after adequate adaptation of biosolids to the effluent.