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

• Published in: Chemosphere (Oxford) Vol. 362; p. 142639
• Main Authors: Hafez, Rania M., Tawfik, Ahmed, Hassan, Gamal K., Zahran, Magdy Kandil, Younes, Ahmed A., Ziembińska-Buczyńska, Aleksandra, Gamoń, Filip, Nasr, Mahmoud
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 01.08.2024
• Subjects: Anaerobiosis; Biofuels; Bioreactors; Carbohydrate degradation; Charcoal - chemistry; Dual biogas/Biochar; Fatty Acids, Volatile - analysis; Methane; Microbial community; Nutrient-rich sludge; Sewage - chemistry; Techno-economic analysis; Waste Disposal, Fluid - methods; Techno-economic analysis; Dual biogas/Biochar; Nutrient-rich sludge; Carbohydrate degradation; Microbial community
• ISSN: 0045-6535; 1879-1298; 1879-1298
• DOI: 10.1016/j.chemosphere.2024.142639

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. Image created with Biorender.com under Agreement number: VB26T4VGHU. [Display omitted] •Paperboard sludge cake and sewage sludge improved methanogenic community.•Optimum PS:SS ratio was 50:50 (w/w), giving methane yield of 242 mL/g CODremoved.•Proteiniphilum, Brevundimonas, Methanobacterium were dominant genera.•Digestate was pyrolyzed into biochar with O/C, C/N, C/P ratios of 0.55, 10.23, 16.42•Combined anaerobic co-digestion/pyrolysis is feasible with 6.60 yr payback period.