Anaerobic Digestion of Wastewaters from Pulp and Paper Mills : A Substantial Source for Biomethane Production in Sweden

Abstract

The Swedish pulp and paper industry is the third largest exporter of pulp and paper products worldwide. It is a highly energy-demanding and water-utilising industry, which generates large volumes of wastewater rich in organic material. These organic materials are to different extents suitable for anaerobic digestion (AD) and production of energy-rich biomethane. The implementation of an AD process within the wastewater treatment plant of a mill would increase the treatment capacity and decrease the overall energy consumption due to less aeration and lower sludge production and in addition produce biomethane. Despite the many benefits of AD it is only applied at two mills in Sweden today. The reason for the low implementation over the years may be due to problems encountered linked to the complexity and varying composition of the wastewaters. Due to changes in market demands many mills have broadened their product portfolios and turned towards more refined products. This has increased both the complexity and the variations of the wastewaters´ composition even further, as the above changes can imply an increased pulp bleaching and utilisation of more diverse raw materials within the mills.The main aim of this thesis was therefore to generate knowledge needed for an expansion of the biomethane production within the pulp and paper industry. As a first step to achieve this an evaluation of the biomethane potential and the suitability for AD of wastewaters within a range of Swedish pulp and paper mills was performed.Thus, around 70 wastewater streams from 11 different processes at eight mills were screened for their biomethane potential. In a second step, the impact of shifts in wood raw material and bleaching on the AD process and the biomethane production was investigated and further evaluated in upflow anaerobic sludge bed (UASB) reactors.ii The screening showed that the biomethane potential within the Swedish pulp and paper industry could be estimated to 700 GWh, which corresponds to 40% of the Swedish biomethane production during 2014. However, depending on the conditions at each specific mill the strategy for the establishment of AD needs to differ. For mills producing kraft pulp the potential is mainly found in wastewaters rich in fibres, alkaline kraft bleaching wastewaters and methanol-rich condensates. The biomethane potential within thermo-mechanical pulp-(TMP) and chemical thermo-mechanical pulp (CTMP) mills is mainly present in the total effluents after pre-sedimentation and in the bleaching effluents as these holds high concentrations of dissolved organic material. The screening further showed that the raw material used for pulp production is an important factor for the biomethane potential of a specific wastewater stream, i.e.hardwood (HW) wastewaters have higher potentials than those from softwood (SW) pulp production. This was confirmed in the lab-scale UASB reactor experiments, in which an alkaline kraft bleaching wastewater and a composite pulping and bleaching CTMP wastewater were used as s...

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

“…One way to avoid this cost is to co-digest the alkaline stream with an acidic bleaching stream at the same mill. This was demonstrated in a lab-scale study at a volumetric ratio of up to 3:1 for alkaline and acidic bleaching wastewater (Larsson, 2015). Despite the previous inhibition observed in BMP tests on acidic ECF bleaching streams (Paper I), the same reduction of the organic matter was achieved during co-digestion as when treating the alkaline stream alone.…”

“…As the volumetric flows of wastewater in pulp and paper mills are very large, preferably streams with high methane potential and low inhibiting effects are selected and treated. One option at kraft mills is the alkaline ECF bleaching stream, as it at one of the mills studied represented 20%-25% of the dissolved organic matter released to the WWT in just 20% of the flow (Larsson, 2015). For mill C, implementing AD on the alkaline bleaching stream would render about 0.5 MNm 3 CH4/year, as based on the continuous study performed at the mill by Larsson, Madeleine et al (2015), and assuming that 40% of the total wastewater flow from the bleaching step is from the alkaline treatment and that the mill processes hardwood and softwood at 50:50 of the time.…”

“…However, there was a decrease in specific methane production when the acidic bleaching stream was included, likely due to competition for substrate by sulphate-reducing bacteria caused by a high sulphate content in the acidic bleaching stream (Larsson, 2015). This means that there is a trade-off regarding costs for pH adjustment versus decreased methane production, where from a mill perspective it is likely better to avoid the cost of chemicals.…”

Enhancement of methane production from the anaerobic digestion of chemical pulp and paper mill effluents

“…The evaporator condensates (EC), rich in degradable acetic acid; and fiber circulation effluent (FC), rich in lignin which poses and inherent restraint to anaerobic digestion treatment. EC are substrates that have been proven to have great potential for methane production (Larsson 2015), however both of the effluents need to be treated before discharging them to the nearest river. Thus, an efficient alternative to treat both of them together would be beneficial.…”