Anaerobic co-digestion of paper sludge: Feasibility of additional methane generation in mechanical-biological treatment plants

Synergism of floated paperboard sludge cake /sewage sludge for maximizing biomethane yield and biochar recovery from digestate: A step towards circular economy

Anaerobic digestion of floated paperboard sludge (PS) cake suffers from volatile fatty acids (VFAs) accumulation, nutrient unbalanced condition, and generation of digestate with a risk of secondary pollution. To overcome these drawbacks, sewage sludge (SS) was added to PS cake for biogas recovery improvement under a co-digestion process followed by the thermal treatment of solid fraction of digestate for biochar production. Batch experimental assays were conducted at different SS:PS mixing ratios of 70:30, 50:50, 30:70, and 20:80 (w/w), and their anaerobic co-digestion performances were compared to the mono-digestion systems at 35 ± 0.2 °C for 45 days. The highest methane yield (MY) of 241.68 ± 14.81 mL/g CODremoved was obtained at the optimum SS:PS ratio of 50:50 (w/w). This experimental condition was accompanied by protein, carbohydrate, and VFA conversion efficiencies of 47.3 ± 3.2%, 46.8 ± 3.2%, and 56.3 ± 3.8%, respectively. The synergistic effect of SS and PS cake encouraged the dominance of Bacteroidota (23.19%), Proteobacteria (49.65%), Patescibacteria (8.12%), and Acidovorax (12.60%) responsible for hydrolyzing the complex organic compounds and converting the VFAs into biomethane. Further, the solid fraction of digestate was subjected to thermal treatment at a temperature of 500 °C for 2.0 h, under an oxygen-limited condition. The obtained biochar had a yield of 0.48 g/g dry digestate, and its oxygen-to-carbon (O/C), carbon-to-nitrogen (C/N), and carbon-to-phosphorous (C/P) ratios were 0.55, 10.23, and 16.42, respectively. A combined anaerobic co-digestion/pyrolysis system (capacity 50 m3/d) was designed based on the COD mass balance experimental data and biogenic CO2 market price of 22 USD/ton. This project could earn profits from biogas (12,565 USD/yr), biochar (6641 USD/yr), carbon credit (8014 USD/yr), and COD shadow price (6932 USD/yr). The proposed project could maintain a payback period of 6.60 yr. However, further studies are required to determine the associated life cycle cost model that is useful to validate the batch experiment assumptions.

Environmental and economic analysis of the transformation of paper mill sludge treatment technologies in China

Paper mill sludge (PMS) is featured with a high content of cellulose and hemicellulose, and using its characteristics to make paperboard can achieve a high-value utilization of PMS, which has attracted growing interest. In this study, currently prevalent landfill, incineration technologies (generating heat and electricity by incineration), and three paperboard technologies (medium density fiberboard, pulp board, and corrugated paper) were evaluated and compared via life cycle assessment (LCA) and life cycle costing (LCC) methods. LCA results show that the PMS-to-pulp board outperforms others with an energy conservation and emission reduction (ECER) value of - 2.86 × 10-8, while the landfill exhibits the highest overall environmental impact with an ECER value of 4.80 × 10-9. LCC results reveal that the PMS-to-pulp board delivers the highest economic profit with $257.357, while the landfill is the lowest with $ - 35.63. The PMS paperboard technologies are more economically friendly than the incineration technologies due to additional electricity/steam consumption during the PMS pre-drying process in incineration. In addition, different scenarios were set up to explore national GHG emission reduction potential by increasing paperboard technologies application rate and reducing the proportion of landfill and incineration. The scenario analysis suggests that replacing 90% of landfill and incineration ratio with PMS paperboard technologies could tremendously improve the overall emission reduction performance with - 9.08 × 1010 kg CO2 eq. This result indicates that the PMS treatment technology transformation has a significant favorable impact on the achievement of the "carbon neutrality" target.

Pulp-paper industry sludge waste biorefinery for sustainable energy and value-added products development: A systematic valorization towards waste management

The pulp-paper industry is one of the main industrial sectors that produce massive amounts of residual sludge, constituting an enormous environmental burden for the industries. Traditional sludge management practices, such as landfilling and incineration, are restricted due to mounting environmental pressures, complex regulatory frameworks, land availability, high costs, and public opinion. Valorization of pulp-paper industry sludge (PPS) to produce high-value products is a promising substitute for traditional sludge management practices, promoting their reuse and recycling. Valorization of PPIS for biorefinery beneficiation includes biomethane, biohydrogen, bioethanol, biobutanol, and biodiesel production for renewable energy generation. Additionally, the various thermo-chemical technologies can be utilized to synthesize bio-oil, hydrochar, biochar, adsorbent, and activated carbon, signifying potential for value-added generation. Moreover, PPIS can be recycled as a byproduct by incorporating it into nanocomposites, cardboard, and construction materials development. This paper aims to deliver a comprehensive overview of PPIS management approaches and thermo-chemical technologies utilized for the development of platform chemicals in industry. Substitute uses of PPIS, such as making building materials, developing supercapacitors, and making cardboard, are also discussed. In addition, this article deeply discusses recent developments in biotechnologies for valorizing PPIS to yield an array of valuable products, such as biofuels, lactic acids, cellulose, nanocellulose, and so on. This review serves as a roadmap for future research endeavors in the effective handling of PPIS.