Treatment of kraft evaporator condensate using a thermophilic submerged anaerobic membrane bioreactor

The feasibility of using a thermophilic submerged anaerobic membrane bioreactor (SAnMBR) for kraft evaporator condensate treatment was studied at 55 ± 1°C over 6.5 months. Under tested organic loading rate of 1–7 kg COD/m³ day, a soluble COD removal efficiency of 85–97% was obtained. The methane production rate was 0.35 ± 0.1 L methane/g COD and the produced biogas was of excellent fuel quality with 80–90% methane. A higher membrane fouling rate was related to the presence of a larger portion of fine colloidal particles (1–10 μm). The thermophilic SAnMBR was sensitive to the presence of toxic compounds in feed and unexpected pH probe failure (leading to a higher pH). Feed toxic shock caused sludge deflocculation and thus deteriorated membrane performance. Operating the reactor as a conventional anaerobic reactor to waste some of the fine flocs in treated effluent during the start-up process was an effective strategy to reduce membrane fouling. The experimental results from this study indicate that treatment of kraft evaporator condensate is feasible in terms of COD removal and biogas production using thermophilic SAnMBRs but pre-treatment may be needed to remove toxic sulfur compounds and membrane fouling caused by the large portion of fine particles may be a challenge.