The transition from a fossil-based economy to a circular bioeconomy is a critical challenge and opportunity in the face of global climate change. Sweden and Finland, with their abundant forest resources and strong commitment to sustainability, are well positioned to lead this transition. The WoodPro project exemplifies this effort by exploring innovative ways to valorize forest residues into high-value products such as 2,3-butanediol (2,3-BDO), biopolymers and hydrochar. This perspective outlines the project’s multidisciplinary approach, which integrates advanced bioprocessing technologies with dynamic system analysis to optimize the sustainability and economic feasibility of these biorefining pathways. We highlight the potential of these interconnected processes to reduce greenhouse gas emissions, close nutrient loops and stimulate rural development, while positioning the Nordic countries as global leaders in the circular bioeconomy. The insights gained from this project highlight the importance of holistic, systems-based approaches in achieving carbon neutrality and offer a model for similar transitions worldwide.

In a circular economy, the efficient utilization of all materials as valuable resources, with a focus on minimizing waste, is paramount. This study shows the possibilities of upgrading the lowest-valued residuals from the forest industry into a new product with both liming and fertilizing properties on forest soil. Hydrothermal carbonized sludge mixed with bark and ash in the proportions of 45:10:45 was densified into fertilizer pellets that meet the nutrient requirements of 120 kg N per hectare when 7 tons of pellets is spread in forests. The pellets met a high-quality result according to durability and density, which were above 95% and 900 kg/m3. However, pellets exposed to wet and cold conditions lost their hardness, making the pellets dissolve over time. Small amounts, <5‰, of nutrients, alkali ions, and heavy metals leached out from the pellets under all conditions, indicating good properties for forest soil amendment. The conclusion is that it is possible to close the circle of nutrients by using innovative thinking around forest industrial residual products.

At Biogasbolaget AB in Karlskoga in south-central Sweden, organic wastes like food waste, manure, and silage are digested anaerobically to yield biogas, which subsequently can be upgraded to biomethane, and used as a replacement for fossil-diesel in public transport. The digesters at the firm are currently operating below their maximum capacity. This chapter deals with the evaluation of the potential of hydrochar to augment biogas production in a batch process. Hydrochar produced from two sources – forestry sector and municipal organic wastes – were compared, and using the Automatic Methane Potential Testing System (AMPTS II) in the lab at Karlstad University, the optimal dosage was determined. Experiments were also conducted with hydrochar alone, to verify if the hydrochar was being anaerobically digested to yield biogas. The hydrochar sourced from municipal waste, when dosed at 8 g/l, produced 841 Nml of biogas /gram of VS (volatile solids) in the substrate, 93% greater than the reference case of no addition of hydrochar. The forestry-sector-sourced hydrochar on the other hand, at the same dosage, registered an increase of just 16.6%. A streamlined environmental life-cycle analysis showed that significant climate-benefits can be availed of, implying environmental sustainability, when the additional biogas is refined and used to replace fossil-diesel in public bus transport. Hydrochar-assisted anaerobic digestion of organic wastes may be posited as a technology which may entrench itself in the circular bio-economies of tomorrow, around the world, and bywhile doing so, contribute to a set of sustainable development goals. While these were batch-digestion experiments, this part of the two-part series recommends more-realistic continuous-digestion experiments which incidentally form the focus of Part 2.

In the Part I of this two-part series, the potential of hydrochar of two different provenances to augment biogas production in a batch process, was evaluated, using the Automatic Methane Potential Testing System (AMPTS II) in the lab at Karlstad University. In Part 2 of the two-part series, this part, single-stage anaerobic co-digestion in two continuously-fed reactors, replaced the batch process of Part 1. A The possibility of connecting an existing digester with a hydrothermal carbonization (HTC) reactor was investigated, and a life-cycle costing analysis was carried out to determine if, in addition to the environmental benefits written about in Part I, investing in an HTC system to produce hydrochar in-plant to augment biogas production will be economically feasible. Hydrochar addition resulted in a 59% rise in biogas yield (and 53.5% in methane yield). The pH remained stable around 7.6 throughout the digestion process. The study confirmed It will be the techno-economic feasibility for coupling an ally practical and feasible to interconnect an HTC plant with a digester supplying 25% of the digestate it produces, to the former, as the raw material for hydrochar production. The rest of the digestate (rich in carbon, nitrogen and phosphorus) can find use as fertiliser. Investing in an The LCC analysis showed that investing in an HTC plant contributing to a rise in methane production of 17% (or 53%), will result in a net profit of 363 million SEK (or 1237 million SEK) over a 20-year period. If the Karlskoga biogas plant decides to rely on purchasing hydrochar from the external market instead, the corresponding net profit will be 177 million SEK (or 1052 million SEK) over the same 20-year period, implying that a decision to integrate and interconnect is likely to be economically more feasible, in a circular bioeconomy in the future.

This chapter deals with the modeling and analysis of a proposed industrial symbiotic system in Karlstad (Sweden), involving algal cultivation, a combined heat and power (CHP) plant and a wastewater treatment plant (WWTP). It has been organized into two parts—the first one focusing on the modeling and the second one, on the application of the model and an energetic and environmental analysis performed using the model developed in the first part. The focus is on the actual symbiotic relationship, the associated flows of materials and energy, and the environmental performance of the symbiotic system, located in Karlstad. In the proposed symbiosis, exhaust gases from the CHP are used as a source of carbon dioxide for the photosynthetic algae (which in the process double up as carbon sinks) as well as a source of heat for drying the harvested algal biomass. Recovered waste heat is used to maintain an appropriate temperature in the cultivation pond and the nutritional needs (nitrogen and phosphorus) of the algae are fulfilled by circulating digestate water from an anaerobic digester at the WWTP. Two possible scenarios for bioenergy recovery (from the algae) were studied—anaerobic digestion (scenario A) and direct combustion in the CHP (scenario B). The outcome is encouraging and shows that algae can be cultivated at the location of the CHP plant in Karlstad, over a longer period of time, vis-à-vis a stand-alone system, and thereby the average daily output can be augmented from 14 to 18g/m2. From an environmental point of view, scenario B fares better than scenario A. The integrated system has a net energy ratio of 2.6 for scenario A; and 5.3 for scenario B. The model and the analysis based on it clearly show that there are excellent opportunities for cultivating algae for use as biofuel locally in Karlstad, which can be harnessed by resorting to industrial symbiosis. A full-scale, detailed life cycle environmental analysis focusing on a range of environmental impact categories can be carried out in the future.

The purpose of this study was to develop a numerical model to estimate the oxygen-transfer rate for a laboratory-scale bottom aeration system at a 1.28 L reactor volume and to contribute to fundamental knowledge regarding the oxygenation of surfactant solutions. The primary goal of the study has been to develop a computational fluid dynamics (CFD) model using Euler–Euler (EE) mixture model coupled with the advection-diffusion equation to predict the oxygen-transfer rate in bubble columns containing clean water. The secondary goal has been to apply the model to water-based solutions containing the surfactant lauric acid (DDA) to identify options for further development of the model to make it applicable to surfactant solution systems. The Sauter mean diameter (SMD) was calculated to represent the average bubble diameter, based on available experimental data for different combinations of superficial velocities rate and DDA concentration. The oxygen-transfer rate in clean water fit well with experimental data at lower superficial velocities, and the differences in volumetric mass-transfer coefficients were 0.7% and 3.3% for superficial velocities of 0.24 cm/s and 0.48 cm/s, respectively. For surfactant solutions, the model overestimates the oxygen-transfer rate due to surfactant adsorption at the bubble/water interface and the consequent decrease in the mass-transfer coefficient not being modeled sufficiently. A correction factor for the mass-transfer coefficient based on a larger sample size of experimental data may need to be calculated and applied to improve model predictability.

Sweden’s pulp and paper sector accounts for a significant proportion of national energy usage, and generates wastewater that causes eutrophication of nearby sinks. In this paper, the possibility of optimizing a biological purification process at the Stora Enso Skoghall mill south of the city of Karlstad in central Sweden, with respect to electricity usage and the addition of nutrients, has been investigated. A computational model of the treatment process was developed, based on process data obtained from the mill, and nine different scenarios compared subsequently, with energy use, environmental impacts and operational expenses, as criteria. These nine scenarios were compared with the reference case, which is the status quo of the biological treatment at the mill. The results depicted that the most energy-efficient and cost-effective alternative was a combination of measures such as lowering the oxygen level in the MBBR from 3 mg l-1 to 2 mg l-1 and using Hyperclassic in the aerated lagoon; an arrangement that yielded a 48.5% reduction in operational expenses, and a 60% decrease in the energy use, vis-à-vis the reference case, without affecting the efficiency of the purification process. This also uncovered an opportunity to mitigate the global warming impact and the eutrophication potential, by approximately 100 tons of CO2-eq. year-1 and 140 kg PO43--eq. year-1. All attempts to optimize the use of resources and decrease the anthropogenic environmental footprint ought to be made to come closer to the targets set by the United Nations’ sustainable development goals (SDGs). The authors’ conclusion predicated on the results of the modelling and analysis done in this study is that the potential of seemingly small process modifications, such as lowering the oxygen level in the MBBR, and applying a more optimal dosage of nutrient salts, must not be overlooked.

In this study, two different sample preparation methods to synthesize activated carbon from pine wood were compared. The pine wood activated carbon was prepared by mixing ZnCl2 by physical mixing, i.e., "dry mixing" and impregnation, i.e., "wet mixing" before high temperature carbonization. The influence of these methods on the physicochemical properties of activated carbons was examined. The activated carbon was analyzed using nitrogen sorption (surface area, pore volume and pore size distribution), XPS, density, Raman spectroscopy, and electrochemistry. Physical mixing led to a slightly higher density carbon (1.83 g/cm(3)) than wet impregnation (1.78 g/cm(3)). Raman spectroscopy analysis also showed that impregnation led to activated carbon with a much higher degree of defects than physical mixing, i.e., I-D/I-G = 0.86 and 0.89, respectively. The wet impregnated samples also had better overall textural properties. For example, for samples activated with 1:1 ratio, the total pore volume was 0.664 vs. 0.637 cm(3)/g and the surface area was 1191 vs. 1263 m(2)/g for dry and wet mixed samples, respectively. In the electrochemical application, specifically in supercapacitors, impregnated samples showed a much better capacitance at low current densities, i.e., 247 vs. 146 F/g at the current density of 0.1 A/g. However, the physically mixed samples were more stable after 5000 cycles: 97.8% versus 94.4% capacitance retention for the wet impregnated samples.

Hydrochar is produced through a process called hydrothermal carbonization (HTC) and constitutes a carbon-rich solid material with different remarkable applications. This study investigated the effects of wood ash on the physicochemical and morphological properties of biosludge-derived hydrochar in pelleted form relevant to the use of the pellets as a soil nutritional and liming agent and as a biofuel source. The hydrochar was mechanically compressed into uniformly-sized pellets under applied pressures of 4 and 8 kN after blending with varying percentages of wood ash in the order 0, 20 and 50%. The pure and blended pellets were characterized to determine the impact of wood ash on key properties, correlated to the two applications mentioned above. Results demonstrated a strong relationship between key features of the pellets and ash proportion. The wood ash-blended hydrochar pellets showed good hydrophobicity as a consequence of increased contents of alkali and alkaline earth metals, but were low in aromatic functional groups compared to the pure hydrochar pellet. Furthermore, the heating value of the pure hydrochar pellet was about 4% higher than that of its parent material and indicates that this pellet has the capacity to serve as a source of energy. The study generally reveals that blending hydrochar produced from biosludge under HTC conditions with up to 20%–50% of wood ash and mechanically compressing into homogeneous pellets has promising potential for a nutrient-rich material that can enhance soil fertility.