# Biogas Production Enhancement through Chicken Manure Co-Digestion with Pig Fat
## 2 Materials and Methods
### Analytical Objects  
The entire anaerobic digestion process consists of many different parameters [27, 32]. At first, the chemical composition and physical properties of raw material, feeding material, and drained digestate were analysed. Several essential parameters of biogas were used to indicate the biogas production process and to calculate the biogas production efficiency parameters. The chosen process indicators parameters for biogas were: volumetric raw  
Figure 1: Research diagram: 1—Biogas reactor; 2—Stirrer drive; 3—Stirrer; 4—Electric heating pad with insulation; 5—Heating controller; 6—Temperature sensor; 7—pH electrode; 8—Biogas output port, 9—Gas volume meter; 10—Monitor and computer; 11—Biogas valves; 12—Two gas bags; 13—Biogas analyser; 14—Feeding port; 15—Substrate drain valve; 16—Digestate sampling port; 17—Moister meter; 18—Manual operation (biogas and digestate convey to analysis). biogas production per day (Bdt), methane concentration in the raw biogas (Cm), and hydrogen sulphide concentration in the raw biogas (Cs). The methane yield (Mfm) obtained from the feeding material mass--L-kgFM-, the specific methane yield (Mts) obtained from the feeding material total solids--L-kgTS-1, and the specific methane yield (Mvs) obtained from the feeding material volatile solids--L-kg VS-1 were calculated for comparative analysis of the results by the modified equations [25]:  
\[\mathrm{M_{fm}=C_{m}\cdot 100^{-1}.Bdt}.m^{-1} \tag{1}\]  
\[\mathrm{M_{ts}=C_{m}\cdot 100^{-1}.Bdt}.m_{ts}^{-1} \tag{2}\]  
\[\mathrm{M_{vs}=C_{m}\cdot 100^{-1}.Bdt}.m_{vs}^{-1} \tag{3}\]  
where Bdt--the volume of biogas produced during the time interval (dt), in litres; m--the feeding material mass, kg; mts--the mass of total solids of the feeding material, in kilograms; mvs--the mass of volatile solids of the feeding material, in kilograms; Cm--the methane concentration in biogas, as a percentage. The methane yield Mfm is an important parameter for calculations of the efficiency of biogas plant production. The specific methane yield Mts obtained from total solids allows the comparison of the same material containing different amounts of water. The specific methane yield Mvs is the most important parameter for comparing any feeding materials because Mvs indicates the possible extent of biodegradable material and shows how much material will degrade, depending on specific material chemical composition and physical features. Basically, the biogas energy is determined by methane concentration in biogas. In our research, the methane energy value obtained from biomass biological conversion under anaerobic conditions EBG was calculated using ISO6976:2016 standard [33] to determine the parameters of a gas mix by formula:  
\[\mathrm{E_{BG}=\Sigma(E_{n}\cdot C_{n}\cdot 100^{-1})} \tag{4}\]  
where En--the energy value of single gas presented in the biogas under standard conditions, MJ\(\cdot\)(m\({}^{3}\))\({}^{-1}\); Cn--the concentration of single gas presented in the biogas, as a percentage. According to ISO6976:2016, the calculation of natural gas energy evaluates the influence of any single gas presented in the natural gas mixture (biogas belongs to the gas mix). Every single gas has different properties, and each single gas influences the value of gas mix energy. In commercial gas (natural gas, biomethane, and biogas, too), the important parameter is Wobbe Index, which is not in a linear expression, thus it significantly changes, depending on carbon dioxide concentration especially. However, the high and low energy values of gas mixture depend less on other gases of the mixture and biogas energy usually is calculated by a simplified formula with an expression of methane energy multiplied by methane concentration in biogas only. The difference of such energy calculation is low when compared to ISO6976:2016 standard (less than 0.15%), and it is possible to calculate by the simplified formula [25].