# Methane Production Potential from Apple Pomace, Cabbage Leaves, Pumpkin Residue and Walnut Husks
## 3 Results
### Production of Methane
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{Total solids (TS),\%} & \multirow{2}{*}{**100**} & \multirow{2}{*}{**Apple Pomace**} & \multicolumn{2}{c}{**Pumpkin**} & \multirow{2}{*}{**Walnut Husks**} & \multirow{2}{*}{**Cabbage Leaves**} \\  & & & & **Residue** & & \\ Total solids (TS),\% & 4.52 \(\pm\) 0.03 a\({}^{*}\) & 19.41 \(\pm\) 0.01 b & 6.13 \(\pm\) 1.33 ac & 14.54 \(\pm\) 1.09 d & 7.45 \(\pm\) 0.85 c \\ Volatile solids (VS), \%Ts & 74.53 \(\pm\) 0.01 a & 98.75 \(\pm\) 0.36 b & 85.74 \(\pm\) 2.18 c & 89.11 \(\pm\) 1.09 d & 90.31 \(\pm\) 2.40 d \\ Total Kjedalhi nitrogen (TKN), g kgcm\({}^{-1}\) & 29.45 \(\pm\) 4.84 a & 9.16 \(\pm\) 3.82 b & 31.00 \(\pm\) 6.43 a & 8.26 \(\pm\) 0.04 b & 26.05 \(\pm\) 0.88 a \\ Total phosphorus (TP), g kgcm\({}^{-1}\) & 12.37 \(\pm\) 0.18 a & 0.92 \(\pm\) 0.01 b & 4.76 \(\pm\) 0.95 c & 0.62 \(\pm\) 0.05 b & 4.12 \(\pm\) 0.36 c \\ Total potassium (K), g kgcm\({}^{-1}\) & 60.54 \(\pm\) 188 a & 6.69 \(\pm\) 0.07 b & 57.12 \(\pm\) 4.27 a & 41.36 \(\pm\) 5.03 c & 27.51 \(\pm\) 0.26 d \\ Total organic carbon (TOC), g kgcm\({}^{-1}\) & 410.78 \(\pm\) 2.19 a & 432.77 \(\pm\) 5.24 b & 425.26 \(\pm\) 1.28 ba & 442.15 \(\pm\) 0.97 b & 426.27 \(\pm\) 21.44 ba \\ CN & 14 & 47 & 14 & 54 & 16 \\ NLP & 2 & 10 & 7 & 13 & 6 \\ \hline \hline \end{tabular}  
* Lowercase letters—statistical differences at \(p<\) 0.05 among residues and inoculum.  
\end{table} 
Table 1: Chemical composition of inoculum and residues. on day 10 reaching 13 NL kgvs\({}^{-1}\) d\({}^{-1}\). Then, a short decline in daily methane production was observed in the next two days, followed by an increase lasting for 5 days followed by a slow decrease until day 30. Even though, the cumulative methane production from anaerobic digestion of walnut husks was the lowest since the daily methane production was lower compared to other studied wastes. From a practical point of view, a very important parameter indicating the substrate's suitability for biogas production is the rate of its decomposition, which affects the hydraulic retention time. This time can be calculated from cumulative methane curves as the time needed to obtain 50% (T50) and 95% (T95) of potential methane production. The time when analysed substrate decomposed in 50% (T50) does not differ among the studied residues. The T50 ranged from 8 days for pumpkin residue to 13 days for walnut husks (Figure 3). The time needed to reach 95% of potential methane production depended on the substrate and was the shortest for pumpkin residue (16 days) and apple pomace (20 days), while walnut husks and cabbage leaves required 28 and 27 days, respectively. The AD can be inhibited by a failure of process parameters, by substances supplied with the substrate, as well as by compounds formed in the digestion process. The level of inhibitors is a very important parameter which determines the conditions of biogas production. FVR are perishable but often are fed into the biogas plant in an unpreserved state. Therefore, it is very important to determine the level of substances in such unpreserved substrate that can inhibit the biogas production process. This is mainly addressed to the level of ammonia (NH\({}_{3}\)) and hydrogen sulphide (H\({}_{2}\)S) in biogas. In this study, for most of the substrates, the NH\({}_{3}\) and H\({}_{2}\)S concentrations were low and did not affect the AD process (Figures 4 and 5). The NH\({}_{3}\) concentration in biogas produced from walnut husks and apple pomace was the lowest, followed by biogas produced from pumpkin residue. The highest NH\({}_{3}\) concentration was observed in biogas from cabbage leaves. The AD of apple pomace and walnut husks produced biogas with a similar NH\({}_{3}\) concentration throughout the whole period of the experiment. The NH\({}_{3}\) concentration ranged from 1.5 ppm to 26.0 ppm and between 10.0 ppm and 49.5 ppm in biogas from walnut husk and apple pomace, respectively. The AD of pumpkin residue resulted in increasing NH\({}_{3}\) concentration to 145.0 ppm on day 5.