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Biolgical treatment of kraft condensates in feedback-controlled packed bed and sequencing batch reactors

University of British Columbia

In the pulp and paper industry there is an increasing amount of interest in the attainment of a closed cycle mill and in maintaining adequate air quality in and around the mill. As a result of these trends, kraft condensates will have to be treated and reused to a greater extent than is currently practiced. The treatment of a large volume of kraft condensates may be accomplished more effectively by biological oxidation than by the currently available technology of steam stripping. The self-cycling fermentation (SCF) technique was applied to the control of a laboratory-scale recirculating packed bed reactor (PBR) and a sequencing batch reactor (SBR) treating accumulator condensate and evaporator condensate from a kraft pulp mill. The SCF control strategy uses the level of dissolved oxygen (DO) to dictate the rate at which untreated wastewater is fed to, and treated wastewater is harvested from, a semi-continuous bioreactor. One PBR run and two SBR runs were performed, each SCF-controlled run lasting approximately one month. During these runs, methanol, which is responsible for the majority of the biological oxygen demand (BOD) of condensates, COD, and the effluent volatile suspended solids (VSS) concentration were routinely measured. Also measured in the SBR were the volatile suspended solids concentration in the reactor (MLVSS) and the influent and effluent concentrations of the odorous total reduced sulfur species (TRS), H2S, CH3SH, DMS, and DMDS. Finally, abiotic stripping experiments were performed with the SBR to evaluate the relative amount of TRS removal from the reactor that was due to stripping. The COD removal efficiency from the accumulator condensate was 88 + 5% from an influent COD of 3060 mg/L. The COD removal efficiency from the evaporator condensate was 64 ± 5% from an influent COD of 1740 mg/L. By triggering a new cycle only when all of the methanol was consumed, the SCF-control strategy ensured these consistent COD removal efficiencies despite significant fluctuations in operating conditions. Overall, the SBR exhibited a performance that was superior to the PBR. During a representative react phase of the SBR, the COD removal rate was 39 kg COD/m3-day. This removal rate compares very favorably with the removal rate of full-scale activated sludge reactors, and it was seven times greater than the removal rate during a representative cycle of the PBR. The PBR produced an effluent with less than 20 mg VSS/L, but the buildup of biomass in the bed caused some operational problems. Due to some upset conditions that can be avoided in the future, the effluent VSS concentration from the SBR was generally greater than 100 mg/L. However, during the stable operation of the SBR, the biomass exhibited good settleability, with as little as 63 mg VSS/L in the effluent from a cycle that had a MLVSS concentration of 5590 mg/L. Over 90% of the TRS was removed from the evaporator condensate during treatment in the SBR. The results of the stripping experiments indicate that most, i f not all, of the TRS removal during SCF treatment, was due to stripping. In this study, the SCF-controlled SBR has been shown to be a promising method for efficiently removing methanol, TRS, and COD from kraft condensates that are intended for reuse.

Thesis/Dissertation

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2009-04-28

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http://hdl.handle.net/2429/7639

Chemical and Biological Engineering

University of British Columbia

UBCV

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