• Energy Research

Abstract The key requirements for accelerating the anaerobic digestion process and clean energy production involve the disintegration of the recalcitrant structure of waste activated sludge and transforming it into soluble organic components for microbial degradation. Herein, the visible light-active ZnO-ZnS/Ag2O-Ag2S nanocatalyst was used for the photocatalytic solubilization of sludge to improve methane production through anaerobic digestion. The photocatalysis released the soluble substances in sludge slurry, in which soluble chemical oxygen demand was increased from 410 to 3892 mg L−1 after 6 h. The volatile solids content was only decreased by 13.1%, which indicated that a short period of pretreatment could avoid the mineralization of organic matter. Methane production was improved by 54% in photocatalytic pretreated sludge, where the cumulative methane production was up to 0.6985 mL g−1 of VS compared with 0.4533 mL g−1 of VS in untreated sludge. Similarly, a higher organic matter removal efficiency of 54% was achieved due to the increased bioavailability of organic compounds under the influence of the pre-solubilization of sludge during photocatalysis. Moreover, photocatalysis reduced the start-up time for methanogenesis from 23 to 13 days. Overall, this study determined the significance of visible light-active ZnO-ZnS/Ag2O-Ag2S photocatalysis for sludge treatment and cleaner energy production, while providing a useful reference for its industrial applications.

Production of biogas from crop wastes via the biological route is one of the most demanding and promising technologies for energy generation for sustainable development. The incorporation of photocatalytic pretreat-ment as a robust catalytic technique has been reported to enhance the overall efficiency of the anaerobic process. Rice husk has been pretreated using Indium Vanadate decorated Titania nanocomposite to enhance its biode-gradability for biogas production. Scanning electron microscopy confirmed the deformities and crevices that may support the role of the photocatalytic pretreatment in the subsequent anaerobic degradation process. FT-IR spectroscopy investigated the structural characteristics of pretreated and photolyzed rice husk samples. Gas chromatography was used to quantify the CH4 content of biogas yield after different treatments of rice husk. Photocatalytic pretreatment of rice husk showed 2.8 times higher cumulative biogas production than raw or untreated rice husk. Statistical analysis showed a significant difference between natural and pretreated rice husk biogas yield with a p-value of < 0.01. The results suggest that visible light active photocatalysis is an efficient pretreatment method that can facilitate the microbes to degrade the rigid rice husk for enhanced biogas pro-duction via the anaerobic digestion process.

Landfill leachate (LFL) treatment is a severe challenge due to its highly viscous nature and various complex pollutants. Leachate comprises various toxic pollutants, including inorganic macro/nano components, xenobiotics, dissolved organic matter, heavy metals, and microorganisms responsible for severe environmental pollution. Various treatment procedures are available to achieve better effluent quality levels; however, most of these treatments are nondestructive, so pollutants are merely transported from one phase to another, resulting in secondary contamination. Anaerobic digestion is a promising bioconversion technology for treating leachate while producing renewable, cleaner energy. Because of its high toxicity and low biodegradability, biological approaches necessitate employing other techniques to complement and support the primary process. In this regard, pretreatment technologies have recently attracted researchers’ interest in addressing leachate treatment concerns through anaerobic digestion. This review summarizes various LFL pretreatment methods, such as electrochemical, ultrasonic, alkaline, coagulation, nanofiltration, air stripping, adsorption, and photocatalysis, before the anaerobic digestion of leachate. The pretreatment could assist in converting biogas (carbon dioxide to methane) and residual volatile fatty acids to valuable chemicals and fuels and even straight to power generation. However, the selection of pretreatment is a vital step. The techno-economic analysis also suggested the high economic feasibility of integrated-anaerobic digestion. Therefore, with the incorporation of pretreatment and anaerobic digestion, the process could have high economic viability attributed to bioenergy production and cost savings through sustainable leachate management options.