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

Rania M Hafez; Ahmed Tawfik; Gamal K Hassan; Magdy Kandil Zahran; Ahmed A Younes; Aleksandra Ziembińska-Buczyńska; Filip Gamoń; Mahmoud Nasr

doi:10.1016/j.chemosphere.2024.142639

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

Chemosphere. 2024 Aug:362:142639. doi: 10.1016/j.chemosphere.2024.142639. Epub 2024 Jun 21.

Authors

Rania M Hafez¹, Ahmed Tawfik², Gamal K Hassan¹, Magdy Kandil Zahran³, Ahmed A Younes³, Aleksandra Ziembińska-Buczyńska⁴, Filip Gamoń⁵, Mahmoud Nasr⁶

Affiliations

¹ Water Pollution Research Department, National Research Centre, 33 El-Buhouth St., Dokki, Cairo, 12311, Egypt.
² Department of Environmental Sciences, College of Life Sciences, Kuwait University, P.O. Box 5969, Safat, 13060, Kuwait. Electronic address: ahmed.elmitwalli@ku.edu.kw.
³ Chemistry Department, Faculty of Science, Helwan University, Ain-Helwan, Cairo, 11795, Egypt.
⁴ Environmental Biotechnology Department, Silesian University of Technology, Akademicka 2, 44-100, Gliwice, Poland.
⁵ Gdansk University of Technology, Faculty of Civil and Environmental Engineering, Department of Sanitary Engineering, 11/12 Narutowicza St, Gdansk, 80-233, Poland.
⁶ Sanitary Engineering Department, Faculty of Engineering, Alexandria University, Alexandria, 21544, Egypt.

PMID: 38909865
DOI: 10.1016/j.chemosphere.2024.142639

Abstract

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 COD_removed 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.

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

MeSH terms

Anaerobiosis
Biofuels
Bioreactors
Charcoal* / chemistry
Fatty Acids, Volatile* / analysis
Methane*
Paper
Sewage* / chemistry
Waste Disposal, Fluid / methods

Substances

Sewage
Charcoal
biochar
Methane
Fatty Acids, Volatile
Biofuels