## 3 Metallic Nanoparticles Used for the Enhancement of Bio Gas Production
### Concentration of Nanoparticles
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**NPs Type** & **NP size** & **Concentration** & **Feedstock** & **Temperature** & **Incubation Time** & **Effect** \\ \hline \multirow{3}{*}{CeO\({}_{2}\)} & \multirow{3}{*}{192 nm} & \multicolumn{3}{c}{Sludge from} & \multirow{3}{*}{30 °C} & \multirow{3}{*}{40} & \multirow{3}{*}{11\% Increase in biogas production [55]} \\  & & & UASB & & & & \\  & & & reactor & & & \\ \hline \multirow{2}{*}{Fe/SiO\({}_{4}\)} & \multirow{2}{*}{7 nm} & \multirow{2}{*}{100 ppm} & Waste water & \multirow{2}{*}{37 °C} & \multirow{2}{*}{60} & \multirow{2}{*}{180\% Increase in biogas production and 234\% increase in methane [46]} \\  & & & & Sludge & & & 234\% increase in methane [46] \\ \hline Fe/SiO\({}_{2}\) & & 105 mol/L & – & 55 °C & – & 7\% Increase in methane production [57] \\ \hline Co/SiO\({}_{2}\) & – & 105 mol/L & – & 55 °C & – & 48\% Increase in methane production [57] \\ \hline Ni/SiO\({}_{2}\) & – & 105 mol/L & – & 55 °C & – & 27\% Increase in methane production [57] \\ \hline \multirow{2}{*}{Co} & \multirow{2}{*}{28 nm} & \multirow{2}{*}{1 mg/L} & fresh raw & \multirow{2}{*}{37 °C} & \multirow{2}{*}{40} & \multirow{2}{*}{71\% increase in biogas production [58]} \\  & & & manure & & & & \\  & & & & & & & \\ \hline \multirow{2}{*}{Ni} & \multirow{2}{*}{17 nm} & \multirow{2}{*}{2 mg/L} & fresh raw & \multirow{2}{*}{37 °C} & \multirow{2}{*}{40} & \multirow{2}{*}{116.76\% increase in methane production [58]} \\  & & & manure & & & & \\ \hline \multirow{2}{*}{Fe} & \multirow{2}{*}{9 nm} & \multirow{2}{*}{20 mg/L} & fresh raw & \multirow{2}{*}{37 °C} & \multirow{2}{*}{40} & \multirow{2}{*}{47.7\% increase in biogas production [58]} \\  & & & manure & & & & 67\% increase in methane production [58] \\ \hline \multirow{2}{*}{Fe\({}_{3}\)O\({}_{4}\)} & \multirow{2}{*}{7 nm} & \multirow{2}{*}{20 mg/L} & fresh raw & \multirow{2}{*}{37 °C} & \multirow{2}{*}{40} & \multirow{2}{*}{73\% increase in biogas production [58]} \\  & & & manure & & & & 115.66\% increase in methane production [58] \\ \hline \multirow{2}{*}{ZnO} & \multirow{2}{*}{140 nm} & \multirow{2}{*}{1 mg/g-TSS} & \multirow{2}{*}{WAS} & \multirow{2}{*}{35 °C} & \multirow{2}{*}{40} & \multirow{2}{*}{No effect [59]} \\  & & & 50 mg/g-TSS & AGS & & & 105 & No effect [60] \\ \hline nZVI & \textless{}50 nm & \(10\) mg/g-TSS & WAS & \multirow{2}{*}{37 °C} & \multirow{2}{*}{30} & \multirow{2}{*}{120\% increase in methane production [61]} \\ \hline Fe\({}_{2}\)O\({}_{3}\) & \textless{}30 nm & \(100\) mg/g-TSS & WAS & \multirow{2}{*}{37 °C} & \multirow{2}{*}{30} & \multirow{2}{*}{117\% increase in methane production [61]} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Nano additives concentration and their impact on the biogas and methane production rate. Due to the presence of Fe\({}_{3}\)C and non-toxic Fe\({}_{3}\)C ions, methane production can be enhanced by a nano iron oxide (Fe\({}_{3}\)O\({}_{4}\) NPs). Nanoparticles of Fe\({}_{3}\)O\({}_{4}\) at 7 nm were added to an anaerobic waste digester with a concentration of 100 ppm at 37 \({}^{\circ}\)C for 60 days. There was an increase of 234% in methane production and 180% in biogas production, which could be considered the highest and most remarkable increase in biogas production using nanoparticles [46]. Due to an enhancement in AD, higher organic matter processing and methane production can be achieved by utilizing a Fe\({}_{3}\)O\({}_{4}\) (magnetite) nanoparticles delivery system. This improvement in performance is due to the presence of FeC\({}_{2}\)/FeC\({}_{3}\) ions in the reactor. These ions are introduced in the form of nanoparticles, just like controlled drug delivery systems. As Fe plays a vital role in electron transport, methane and hydrogen production rates increase and bacterial growth is stimulated by promoting enzymatic actions [65]. Due to better biocompatibility and low toxicity, Fe\({}_{3}\)O\({}_{4}\) nanoparticles are the most prevalent materials [66]. [PERSON] et al.