Anaerobic digestion (AD) is a sustainable technology that combines biogas production with proper waste management. However, the operation of an anaerobic reactor under the stress of nanoparticles (NPs) is unknown. Therefore, this study assessed the disadvantages and advantages of adding two NPs to 11 L semi-continuous stirred-biodigesters and examined their contributions to biogas generation. Iron oxide NPs (INPs) and titanium oxide NPs (TNPs) were mixed daily with manure from dairy cattle and added to the biodigesters daily at concentrations of 100 mg/L (D1) and 500 mg/L (D2), respectively. In addition, a control with no NP supplementation was studied (D0). The results revealed that the generation rates of biogas and CH₄ increased by 25.43 and 62.43%, respectively, for D1 during the first 7-day retention time, as compared with the D0. Conversely, TNP addition reduced the generation rates by 28.76 and 56.92%, respectively, during the same period. Overall, for D1, the biogas and CH₄ yields over the study period increased by 5.48 and 12.35%, respectively, whereas for D2, the rates were reduced by 11.36 and 18.58%, respectively. Therefore, the presence of INPs had a stimulatory influence while that of the TNPs had an inhibitory effect.