## 4 Agro-Food Substrates for Anaerobic Digestion
### Livestock Manure
\begin{table}
\begin{tabular}{l l l l} \hline \hline \multicolumn{1}{c}{**Parameter**} & \multicolumn{1}{c}{**Unit**} & \multicolumn{2}{c}{**Description, Range and Measurement Method**} \\ \hline \hline \multicolumn{4}{c}{**Process stability**} \\ \hline \multirow{4}{*}{Temperature T} & \multirow{4}{*}{\({}^{\circ}\)C} & \(\bullet\) & 25–40 \({}^{\circ}\)C (mesophilic systems—optimal 38–40 \({}^{\circ}\)C) \\  & & \(\bullet\) & 45–65 \({}^{\circ}\)C (thermophilic systems—optimal 55 \({}^{\circ}\)C) \\  & & \(\bullet\) & \(<\)25 \({}^{\circ}\)C (psychophilic or cold-rare systems) \\ \cline{3-3}  & & Variations of only 2-3 \({}^{\circ}\)C within the optimal range can affect performance. Measurement during process by i.e., thermocouples \\ \hline pH & - & Neutral environment, between 6.5–7.5, acceptability 6–8. \\ \hline \multirow{4}{*}{Volatile Fatty Acids VFAs} & \multirow{4}{*}{mgAc/L} & A significant increase highlights a malfunction; it can lead to a drop in pH (acidosis), which also leads to an irreversible block of the process. Typical range \\  & & 200–2000 mgAc/L. Measured by UV/FTIR-IR \\ \cline{3-3}  & & spectrometer, gas chromatography or indirect by COD. \\ \hline \multirow{4}{*}{Alkalinity TA} & \multirow{4}{*}{mg CaCO\({}^{3}\)/L} & Buffer capacity of the system which contributes to guarantee neutral pH. A stable system has values in the range of 2500–5000 mg CaCO3/L \\  & & Direct measure with NIRS or titration, indirect through redox potential sensor and electrical conductivity \\ \hline VFA/TA & - & Ratio value \(<\) 0.3 indicates the stable operation of the digester. \\ \hline \multicolumn{4}{c}{**Reactor management**} \\ \hline Operational pressure P & mbar & Measurement during process \\ \hline Capacity & \(\mathrm{m}^{3}/\mathrm{d}\) or t/d & Measurement of mass or volume per day \\ \hline Reactor volume Vr & \(\mathrm{m}^{3}\) & Determined by design/construction \\ \hline Hydraulic retention time & \multirow{2}{*}{d} & Ratio between the volume of the reactor considered and the flow rate to the reactor. Calculation from process \\ HRT & & data, indirect by measuring the incoming flow. \\ \hline Sludge Retention Time & \multirow{2}{*}{d} & Ratio between the total mass of volatile solids in the reactor and the flow of solids extracted from the reactor. \\ SRT & & Indirect by measuring concentration and flow rate SV. \\ \hline \multirow{4}{*}{Organic Load rate OLR} & \multirow{4}{*}{Kg or TS/m\({}^{3}\) d} & Quantity of input substrate (influent flow rate), calculated by concentration of organic substance and \\  & & referred to the volume unit of the reactor. Indirect \\ \cline{1-1}  & & measurement by flow measure and organic substance \\ \cline{1-1}  & & concentration (sample weight for ST, SV, or COD and \\ \cline{1-1}  & & BOD measurement) \\ \hline Substrate removal efficiency & \multirow{2}{*}{\%} & Function of the SV concentration in the influent flow rate (kg/m\({}^{3}\)) \\ \cline{3-3}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & Output biogas flow. If connected to the flow and \\ \cline{1-1} Biogas flow rate & \multirow{2}{*}{Nm\({}^{3}/\mathrm{d}\) or Nm\({}^{3}/\mathrm{y}\)} & concentration of the incoming substrate, it is called a \\ \cline{1-1}  & & specific production (m\({}^{3}\)/m\({}^{3}\)). Measurement during \\ \hline Methane concentration & \multirow{2}{*}{\%} & \% CH4 - \%CO2 - \%H2S - O2 – other Measurement during \\ CH4 and biogas composition & & operation by electrochemical or IR spectroscopic method \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Main parameters used to check the efficiency of anaerobic digestion plants [40]. The concentration of substrate is a significant operating factor affecting the stability and methane yield of the AD process of manure. Nevertheless, an excessive substrate concentration could slow biogas process due to accumulation of inhibiting elements (i.e., total ammonia, free ammonia and volatile fatty acids).