Structural extracellular polymeric substances (SEPS) as valuable biopolymers, can be extracted from waste activated sludge (WAS). However, the extraction yield is typically low, and detailed information on SEPS characterizations, as well as proper treatment of the sludge after SEPS extraction, remains limited. This study aimed to optimize the conditions of heating-Na₂CO₃ extraction process to increase the yield of SEPS extracted from WAS. Subsequently, SEPS were characterized, and, for the first time, insights into their protein composition were uncovered by using proteomics. A maximum SEPS yield of 209 mg g^-1 volatile solid (VS) was obtained under optimal conditions: temperature of 90 °C, heating time of 60 min, Na⁺ dosage of 8.0 mmol/g VS, and pH required to precipitation of 4.0, which was comparable to that from the aerobic granular sludge reported in literature. Proteomics analysis unveiled that the proteins in SEPS primarily originated from microorganisms involved in nitrogen fixation and organic matter degradation, including their intracellular and membrane-associated regions. These proteins exhibited various catalytic activities and played crucial roles in aggregation processes. Besides, the process of SEPS extraction significantly enhanced volatile fatty acid (VFA) production during the anaerobic fermentation of residual WAS after SEPS extraction. A maximum VFA yield of 420 ± 14 mg COD/g VS_added was observed in anaerobic fermentation of 10 d, which was 77.2 ± 0.1 % higher than that from raw sludge. Mechanism analysis revealed that SEPS extraction not only improved WAS disintegration and solubilization but also reduced the relative activity of methanogens during anaerobic fermentation. Moreover, SEPS extraction shifted the microbial population during anaerobic fermentation in the direction towards hydrolysis and acidification such as Fermentimonas sp. and Soehngenia sp. This study proposed a novel strategy based on SEPS extraction and VFA production for sludge treatment, offering potential benefits for resource recovery and improved process efficiency.

Persulfates ((peroxymonosulfate (PMS) and peroxydisulfate (PDS)) can disintegrate waste activated sludge (WAS), but their influence on volatile fatty acid (VFA) production during anaerobic fermentation is largely unclear. Particularly, it is unknown whether persulfates can improve fermentation without any preactivation. This study investigated how the direct addition of PMS and PDS into the fermenter influences VFA production from WAS, and uncovered possible mechanisms of improved VFA production. At the PMS dosage of 1.0 mM/gVS, maximum VFA yield (1025 ± 55 mg COD/L) was observed in 5 days of fermentation, which was 24% higher than that with an equimolar addition of PDS and 132% higher than that without persulfate addition. However, the cost-effectiveness of PMS in enhancing VFA production is lower than that of PDS. The in situ generated hydroxyl and sulfate radicals from persulfates in the fermenter enhanced not only WAS disintegration and solubilization but also the formation of biodegradable substances for acidification. Moreover, the direct addition of persulfates into the fermenter shifted the microbial community toward hydrolysis-acidification such as Clostridium_sensu_stricto_10 and Fonticella. Results of this study also suggest that preactivation is not necessary when persulfates are applied to improve WAS fermentation.

In recent years, volatile fatty acid (VFA) production through anaerobic fermentation of sewage sludge, instead of methane production, has been regarded as a high-value and promising roadmap for sludge stabilization and resource recovery. This review first presents the effects of some essential factors that influence VFA production and composition. In the second part, we present an extensive analysis of conventional pretreatment and co-fermentation strategies ultimately addressed to improving VFA production and composition. Also, the effectiveness of these approaches is summarized in terms of sludge degradation, hydrolysis rate, and VFA production and composition. According to published studies, it is concluded that some pretreatments such as alkaline and thermal pretreatment are the most effective ways to enhance VFA production from sewage sludge. The possible reasons for the improvement of VFA production by different methods are also discussed. Finally, this review also highlights several current technical challenges and opportunities in VFA production with spectrum control, and further related research is proposed.

The pyrolysis characteristics and kinetics of pre-dried dewatered sewage sludge under different heating rates (10, 20, and 30 °C/min) were investigated by thermogravimetric analyzer-fourier transform infrared analysis (TG-FTIR) and differential scanning calorimetry (DSC), and a heated tubular reactor. The thermal analysis results showed that four weight loss zones (<165, 165–400, 400–600, and >600 °C) were observed in TG curve, and a higher heating rate led to the increase of the maximum devolatilization rate and the higher temperature corresponding to the peak. The activation energy of different heating rate (10, 20, and 30 °C/min) was 70.0, 60.6 and 65.0 kJ/mol, respectively. The FTIR analysis of gaseous products indicated the concentrations of CO₂ and CO increased steadily with the increased heating rate, while the concentration of CH₄ first increased then decreased. Moreover, the yield of gaseous products and bio-oil increased with the increasing of the heating rate. Finally, the surface structure of chars and the chemical properties of bio-oil obtained were characterized using nitrogen adsorption, scanning electron microscopy, elemental analysis and gas chromatography–mass spectrometry. Overall, the results highlighted that pre-dried dewatered sewage sludge can be effectively converted into biofuel and porous material.