Municipal sewage sludge energetic conversion as a tool for environmental sustainability: production of innovative biofue

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RESEARCH ARTICLE

Municipal sewage sludge energetic conversion as a tool for environmental sustainability: production of innovative biofuels and biochar Aïda Ben Hassen Trabelsi 1

&

Kaouther Zaafouri 2 & Athar Friaa 1 & Samira Abidi 1 & Slim Naoui 1 & Faycel Jamaaoui 1

Received: 25 June 2020 / Accepted: 23 October 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract In this study, municipal sewage sludge (MSS) is converted simultaneously into renewable biofuels (bio-oil, syngas) and high value-added products (biochar) using a fixed bed pyrolyzer. This work examines the combined effect of two factors: final pyrolysis temperature (°C) and MSS moisture content (%) on pyrogenic product yields and characteristics. A centered composite experimental design (CCD) is established for pyrolysis process optimization by adopting the response surface methodology (RSM). The statistical results indicate that the optimal conditions considering all studied factors and responses are 550 °C as final pyrolysis temperature and 15% as MSS moisture content. In these optimal conditions, biofuels yield is around 48 wt%, whereas biochar yield is about 52 wt%. The pyrolysis products characterizations reveal that (i) pyrolytic oil has a complex molecular composition rich with n-alkanes, n-alkenes, carboxylic acids, and aromatic compounds; (ii) bio-oil presents a high-energy content (high heating value HHV around 30.6 MJ/kg); (iii) syngas mixture has a good calorific value (HHV up to 8 MJ/kg), which could be used as renewable energy vector or for pyrolysis reactor heating; and (iv) biochar residue has good aliphatic and oxygenated group contents favoring its application as biofertilizer. These findings suggest that MSS conversion into biofuels and biochar is an appropriate approach for MSS treatment. MSS-to-energy could be proposed as an element for circular economy concept due to its effectiveness in producing high value-added and sustainable products and reducing environmental problems linked to MSS disposal. Keywords Pyrolysis . Biosolids . Response surface methodology . Bio-oil . Pyrochar . Circular economy

Introduction As a residual product of urban wastewater treatment, municipal sewage sludge (MSS) is a heterogeneous biomass with a relatively high organic matter content. MSS thermochemical conversion, particularly via pyrolysis, could contribute significantly to produce green energy sources (biofuels) and Responsible Editor: Ta Yeong Wu * Aïda Ben Hassen Trabelsi [email protected] 1

Laboratory of Wind Energy Management and Waste Energy Recovery (LMEEVED), Research and Technology Center of Energy (CRTEn), B.P. 95, 2050 Hammam-Lif, Tunisia

2

Laboratory of Microbial Ecology and Technology (LETMi), National Institute of Applied Sciences and Technology (INSAT), Carthage University, 2 Boulevard de la terre, BP 676, 1080 Tunis, Tunisia

sustainable biofertilizers (biochar) and, thus, reduce environmental problems related to conventional fuel consumption and chemical fertilizer import. Tunisia is generating yea