Sludge Blanket Anaerobic membrane bioreactor (SB-AnMBR) Treatment of Prehydrolysis Liquor from the Dissolving Pulp Industry
Performance of innovative sludge-blanket anaerobic membrane bioreactors (SB-AnMBRs) was evaluated at mesophilic (35oC for 400 days) followed by thermophilic (55oC for 330 days) temperatures for the treatment of the pre-hydrolysis liquor (PHL) waste stream from a dissolving pulp production plant. PHL with an average total chemical oxygen demand (COD) of 100 g/L was generated from the steam hydrolysis of wood and mainly consisted of sugars (hemicelluloses), acetic acid, furfural and lignin. The bioreactors were fed with PHL at organic loading rates (OLR) ranging from 0.8 to 7.5 kg-COD/m3-d to study the performance with respect to the COD removal, methane (bio-energy) production, effluent characteristics, and membrane fouling. Average COD removal of 91% and specific methane yield of 0.36 m3/kg-CODremoved/day were achieved during the pseudo-steady period of the continuous mesophilic operation at each loading rate. The effluent lignin concentration showed an increasing trend (0.2 g/l to 2.7 g/l) with an increase in the OLR. At thermophilic condition, the AnMBR exhibited accumulation of volatile fatty acid (VFAs), the presence of slow-degrading lignin in the effluent and lower COD removals (less than 60%) possibly due to the one-step increase in the temperature (from 35 to 55oC) exerting temperature stress on biomass during the first sixty days of the operation. Subsequently, it was observed that the methane yield slowly increased to 0.38 m3/kg-CODremoved/day. This phenomenon can be ascribed to the shift in the population increase of thermophilic anaerobic bacteria. There was no sugar and furfural found in the effluent of the SB-AnMBR at both temperatures during the stable period. This suggested good potential of using the AnMBRs for the treatment of PHL as well as production of bio-energy. Flat-sheet membranes were used in the SB-AnMBRs. Membranes did not show significant fouling based on monitoring of temporal variations in the trans-membrane pressure at a sustained flux of 0.1 m3/m2/d during the aprox. 700 days of the continuous operation.