Anaerobic digestion of wastewater from the production of bleached chemical thermo‐mechanical pulp – higher methane production for hardwood than softwood

Abstract

BACKGROUND: Chemical thermo-mechanical pulp (CTMP) mills holds a large biomethane potential in their wastewater. Their broadened market has involved increased bleaching and utilization of different raw materials. Therefore, the main aim of this study was to obtain and maintain a well-functioning anaerobic digestion (AD) process, with a high methane yield and total organic carbon (TOC) reduction, when digesting CTMP wastewater, from different production protocols including shifts in raw material and bleaching. A lab-scale upflow anaerobic sludge bed (UASB) reactor was used for the tests. RESULTS:The variations in raw material (spruce, birch and aspen) and consequently in TOC-loading (3.6-6.6 kg TOC m −3 and day −1 ) did not affect the UASB process negatively. Methane production values from 360 to 500 NmL g −1 TOC were obtained, with the highest yield for wastewater from the production of birch-followed by aspen-and spruce pulp. The acetic acid and filtered-TOC (fTOC) reduction ranged from 90 to 95% and 61 to 73%, respectively. CONCLUSIONS: A well-functioning process maintained during shifts in raw material for pulp production which shows that AD is feasible for CTMP mills with a diversified product portfolio. Furthermore, the increased use of hardwood and bleaching will most likely increase their potential as a biomethane producer.

Anaerobic digestion in the kraft pulp and paper industry : Challenges and possibilities for implementation

“…Similarly, the composite wastewater stream from the CTMP pulping and bleaching at mill B gave higher COD content (11 100 mg/L compared to 8300 mg/L; Table 4) and higher methane potential (180 compared to 90 ml CH4/g COD) when hardwood was processed. This is in line with previous studies on pulping effluents and dewatering effluents from a sulphite mill, where hardwood streams were more degradable (Yang et al, 2010), and was confirmed in follow-up continuous experiments on kraft alkaline bleaching effluents (Larsson, Madeleine et al, 2015) and CTMP wastewater (Larsson et al, 2017). One reason may be the higher concentration of lignin and the presence of resin acids in softwood, as these compounds are recalcitrant to AD and can have inhibiting effects on the microbial degradation (Sierra-Alvarez et al, 1990).…”

“…One reason may be the higher concentration of lignin and the presence of resin acids in softwood, as these compounds are recalcitrant to AD and can have inhibiting effects on the microbial degradation (Sierra-Alvarez et al, 1990). In addition, more COD was released during CTMP pulping of hardwood, and there were higher concentrations of acetate in the wastewaters (Larsson et al, 2017).…”