Light assisted anaerobic digestion for the disposal of cadmium containing waste activated sludge

The accumulation of cadmium (Cd) from wastewater into sludge poses a significant challenge to the recovery of valuable resources and energy, thereby hindering its safe and sustainable management. Improving the efficiency of anaerobic digestion (AD) while mitigating the bioavailability of Cd within such complex systems is critical for the effective treatment of sludge. Recent advancements in light-assisted microbial-semiconductor biohybrid systems in simplified systems or isolated components have demonstrated promise in enhancing electron acquisition by non-photosynthetic microorganisms. However, the potential of cadmium sulfide (CdS), a prototypical semiconductor material, to facilitate electron transfer to methanogens and thereby enhance methane production in intricate AD systems remains largely unexplored. This study introduces a novel strategy for managing Cd-containing waste activated sludge (WAS) by employing a light-assisted AD process following alkali pretreatment. This approach achieved a increased methane yield , significantly outperforming conventional (dark) AD and light-assisted AD without alkali pretreatment. The alkali pretreatment disrupted the extracellular polymeric substances in the sludge, promoting direct contact between CdS particles and methanogens, thereby strengthening electron transfer pathways. Electrochemical analysis corroborated the enhanced electron transfer, further validated by metatranscriptomic evidence that revealed upregulated expression of extracellular electron transfer pathways in the microbial community. This study introduces an innovative framework for optimizing AD performance in Cd-containing WAS, offering valuable insights into the efficient treatment of heavy metal-laden sludge and advancing the sustainable utilization of sludge-derived resources.