# Methane Production Potential from Apple Pomace, Cabbage Leaves, Pumpkin Residue and Walnut Husks
## 4 Discussion
The decision to co-digest the FVR should be based on the identification of inhibitors which may be introduced to the biogas plant with this feedstock or may be released during its digestion. Degradation of proteins, nucleic acids and/or uric acid/urea during the AD process leads to the release of ammonium (NH\({}_{4}\)+) which is not degraded under anaerobic conditions [83; 84; 85; 86]. The NH\({}_{4}\)+ is in equilibrium with NH\({}_{3}\); however, the NH\({}_{3}\) concentration increases with temperature and pH. A high concentration of free NH\({}_{3}\) affects the community structure of archaea, which are responsible for CH\({}_{4}\) production [87]. This leads to an increase in volatile fatty acids (VFA) and consequently to a reduction in the pH value [88]. The threshold values for the NH\({}_{3}\) concentration range from 80 to 400 ppm. The AD of nitrogen-rich feedstock such as grass silage, poultry manure or slaughterhouse waste is most at risk of inhibition resulting from a high NH\({}_{3}\) concentration [89]. To prevent process failures due to NH\({}_{3}\) toxicity, different conventional strategies such as stripping, chemical precipitation, adjustment of C:N ratios, immobilization and adaption of microorganisms, bioaugmentation, dilution of substrates and/or co-digestion of nitrogen-rich wastes have been used [90]. In this study, the NH\({}_{3}\) concentration varied from 1.5 ppm up to 700 ppm and was much lower than ten, the threshold value, except for cabbage leaves. Therefore, all the studied residues can be used as co-substrates in the AD process; however, in the case of cabbage, there is a risk of process inhibition. The H\({}_{2}\)S is another inhibitor which causes a decrease in methane production and in addition negatively affects the installation and engine. The concentration of H\({}_{2}\)S in biogas depends on the feedstock and AD technology and ranges between 50 and 10,000 ppm [91]. The H\({}_{2}\)S in biogas is a product of the decomposition of sulphur-containing compounds such as amino acids, sulphoxides and sulphonic acids and a product of the biological reduction in sulphates in the feedstock. The presence of H\({}_{2}\)S in the reactor inhibits the AD process because H\({}_{2}\)S is capable of diffusing through cell membranes and causing the denaturation of proteins thus disturbing the metabolism of microorganisms [83]. This acidic and toxic gas together with water present in biogas creates the condensate which causes the corrosion of installation and negatively affects the condition of the engine [92,93]. A high concentration of H\({}_{2}\)S causes an increase in the cost of biogas production due to the necessity of H\({}_{2}\)S removal. The main technologies such as adsorption into liquid either water or caustic soda; adsorption on a solid or a biological conversion by sulphide oxidizing microorganisms with the addition of air or oxygen require capital, energy and media additional costs [91]. The H\({}_{2}\)S concentration in biogas was higher in the case of most studied materials, except walnut husk in comparison to the H\({}_{2}\)S concentration in biogas produced from mixed fruit and mixed fruit and vegetable waste which amounted from 120 to 250 ppm depending on the feedstock, organic load rate and temperature of digestion [94]. Much higher H\({}_{2}\)S concentration (up to 1700 ppm) was observed in biogas from the AD of food waste and the liquid fraction of dairy manure after solid-liquid separation [95]. In this case, high protein concentration in food might be the reason for such high H\({}_{2}\)S concentrations. The differences in protein content in the studied wastes affected the NH\({}_{3}\) and H\({}_{2}\)S concentration in the biogas. According to the literature, the highest protein content is in cabbage and varies from 15.6%\({}_{\text{DM}}\) to 24.8%\({}_{\text{DM}}\)[60,66]; however, in most studies, values of around 18-19.5%\({}_{\text{DM}}\) were found [96, 97, 98]. Lower protein contents were reported for pumpkin fruit.