Unraveling the effects of cerium oxide nanoparticles on the metabolism of anaerobic digestion of waste activated sludge

To reduce the residual solids and increase energy recovery in wastewater treatment plants, the anaerobic digestion (AD) process needs to be optimized to generate more methane from waste activated sludge (WAS). Nanomaterials (NMs) have successfully been used in anaerobic digestion to increase methane production. Focusing on NMs with high redox activity, the biochemical route for methane production can be enhanced. Here, the influence of cerium oxide nanoparticles (CeO₂ NPs) on the AD of waste sludge was evaluated in terms of metabolite production and assimilation, key enzymes activity, and organic matter transformation. The fate of CeO₂ NPs in the anaerobic reactors was also determined via single particle ICP-MS and TEM imaging. Results indicated that 10, 50 and 100 mg of CeO₂ NPs per g of volatile suspended solids (VSS) acted as nano-catalyst during the anaerobic digestion of WAS, increasing the methane yield production up to 14.2%. CeO₂ NPs induced a decrease in the activity of two key enzymes involved in AD, such as protease and F420. Thus, biogas production was enhanced via the redox capability of the NPs. This includes the ability to perform the extracellular electron transfer (EET) to hydrolyze long-chain substrates, e.g. proteins into amino acids, and short chain organic acids such as maleic acid to shorter molecules and finally to methane. At the end of the nano-enhanced AD process, the CeO₂ NPs remained in the bisolids. Therefore, potential effects of nanoceria on soil microorganisms and plants should be studied further.