## 3 Metallic Nanoparticles Used for the Enhancement of Bio Gas Production
### Concentration of Nanoparticles  
Nanoparticles have been acquired from both anthropogenic and natural resources. In waste sludge, a very high concentration of NPs could have accumulated. However, the toxicity and the impact of NPs on the sludge treatment stream is still an area that requires a great deal of research [55]. [PERSON] determined the effects of ZnO NPs and CeO\({}_{2}\) nanoparticles on the sludge AD process, toxic potential of sludge to plants and bacteria and dewatering process of the sludge. The concentration of nanoparticles is very important in determining their role for the process of methane and biogas production (Table 1). Not all nanoparticles stimulate the anaerobic digestion system, rather some nanoparticles inhibit the production rate considerably when compared with a controlled sample. Types and concentration of nanoparticles play a vital role in the production rate of the anaerobic digestion system. In comparison with a control sample, the exposure concentration of ZnO at 1000 mg/L resulted in inhibition to 65.3% biogas volume and 47.7% methane composition. At an endurable exposure concentration of zinc oxide, the inhibition effect could be overcome after an incubation of 14 days [56]. [PERSON] reported the AD process of cattle manure and effects of metal oxide on methane and biogas productivity [56]. During the experiment, the influence of CuO nanoparticles was higher than the other test compounds. At day 14, out of the total biogas produced in the control, there was a reduction of 30% caused by 15 mg/L concentration of Cu. CuO in bulk, with concentrations of 120 and 240 mg/L, caused a reduction by 19% and 60%, respectively, whereas in the presence of Cu microparticles, biogas production was less inhibited. Differences between the bulk particles and nanoparticles of CuO (p \(<\) 0.05) were validated by statistical analysis. [PERSON] [62], [PERSON] [63] and [PERSON] et al. [64], reported that a toxicity of nanoparticles to bacteria, causing damage to the cell membrane and inhibiting biogas production by the release of metal ions. Whereas, ZnO bulk particles were compared with test samples containing ZnO nanoparticles. An inhibition of 74% and 43% of the highest yield resulted from ZnO nanoparticles concentrations of 120 and 240 mg/L, respectively. Test bottles containing bulk ZnO showed a decline of 18% and 72% of the total biogas yield at day 14. Both bulk and nanoparticles of ZnO were unable to produce a prominent difference in biogas inhibition [56]. From the above mentioned observations, it can be concluded that biogas yield is affected by CuO and ZnO nano-sized concentration and particle size. CeO\({}_{2}\) can enhance biogas production by 11%, even at the low concentration of 10 mg. A bacterial toxicity test also confirmed a positive effect of CeO\({}_{2}\) at a similar concentration. However, the toxicity levels of both the nanoparticles decreased when they were applied to the sludge, rather than naturally occurring. Bacterial toxicity also reduced when the AD process ended. In addition, the exposure concentration of NPs was directly proportional to the time required for dewatering the digested sludge. After the end of the AD processes, toxicity of the sludge also reduced notably. After exposure of the sludge to a concentration of 1000 mg/L of CeO\({}_{2}\) nanoparticles, there was an inhibition of 47.5% for bacterial viability, but after the AD process, the sludge had an inhibition of 30.4% for bacterial viability. Similarly, 1000 mg/L of ZnO nanoparticles before digestion had an inhibition of 92.3%, while this value was just 34.8% after the AD process [55].