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: Carbohydrate degradation; Dual biogas/Biochar; Microbial community; Nutrient-rich sludge; Techno-economic analysis; Techno-economic analysis; Dual biogas/Biochar; Nutrient-rich sludge; Carbohydrate degradation; Microbial community; Dual biogas/biochar
• 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.
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•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.