Comparison of a Mesophilic and Thermophilic Novel Sludge-Bed Anaerobic Membrane Bioreactor Treating Prehydrolysis Liquor from a Dissolving Pulp Mill

Abstract

Performance of the novel sludge-bed anaerobic membrane bioreactor (SB-AnMBR) at a mesophilic (35°C) and a thermophilic (55°C) temperature was compared for the treatment of the prehydrolysis liquor (PHL) at organic loading rates (OLRs) ranging from 0.8 to 10 kg-COD/m3-d. An average chemical oxygen demand (COD) removal of 91% and a specific yield of 0.36 m3-CH4/kg-CODremoved/day [% relative standard deviation (RSTD) =4.6] was observed at a mesophilic temperature irrespective of the increase in loading rate. The temperature of the mesophilic (35°C) SB-AnMBR reactor was increased in a single step to achieve a thermophilic temperature (55°C). The one-step increase in temperature exerted stress on the biomass during the first 60 days of the operation at thermophilic temperature leading to the presence of slow-degrading lignin in the effluent with an average removal efficiency of less than 56% and average total COD removals of less than 60%. Subsequently, the methane yield was found to increase to an average value of 0.38 m3/kg-CODremoved/day, indicating adaptation of biomass in the reactor. Flat-sheet membranes, used in both the mesophilic and thermophilic SB-AnMBRs, showed only two fouling events at a sustained flux of 0.1 m3/m2/d during combined continuous operation period of approximately 800 days.