## 2 Common Substrates Used in Anaerobic Digestion
### Protein-Rich Substrates  
Proteins are also abundant in organic substrates, particularly in those derived from animal wastes. Slaughterhouse wastes, pig, cattle, chicken manure and any other type of manure from livestock farms are residues with a high protein content. When dealing with this material, ammonia accumulation may cause problems in the reactor performance if an equilibrating carbon source is not added to balance the C/N ratio of the feeding recipe. Another residue that has been studied recently as a suitable co-substrate is animal carcass. This waste is subject to strict regulations, but livestock farms must confront a significant risk associated with the transport of animal carcasses due to the possible cross-contamination that may take place because of the route the transport truck must follow during collection operations, with a risk of failure in decontamination when traveling from one farm to the other always existing. This risk could be reduced if alternatives are allowed in situ in compliance with Regulation (EC) 1069/2009 and (EU) 142/2011 for animal by-products, so these farms could safely pre-treat this material to make it a suitable co-substrate [99]. [PERSON] et al. [24] studied biogas production from animal carcasses, reporting a methane yield of 0.47 m\({}^{3}\) CH\({}_{4}\)/kg vs. from biochemical methane potential (BMP) tests. [PERSON] et al. [100] reported a doubling in gas production when studying the co-digestion of swine manure along with animal carcasses, and [PERSON] et al. [101] proposed the optimization of the hydrothermal pretreatment of animal carcasses for increasing biogas production, given the regulation requirement already established for category 2 material. Ammonium ions released from the degradation of proteins inhibit anaerobic activity at values close to 4.0 g/L [102]. However, several factors are relevant in the response of the microflora to ammonium. acclimation is crucial for tolerating high levels of this cation in the reactor liquor, along with temperature and pH. Co-digestion of substrates, with different C/N ratios, is a proper strategy for enhancing degradation performance [103] and avoiding toxic ammonia concentrations. Thus, the use of grass, straw and lignocellulosic biomass in general, or micro-algae biomass as co-substrates are suitable options that have been evaluated under small laboratory conditions in many cases. However, the availability of these materials should be carefully considered, with this not being always possible due to the limitations imposed by transport distances. The use of energy crops as co-substrates is currently a feasible option in some European countries, with maize silage being widely \begin{table}
\begin{tabular}{c c c} \hline \hline
**Organic Substrates** & **Methane Yield** & **References** \\ \hline Sewage sludge & 0.13–0.45 & [66; 67; 68; 69; 70; 71; 72] \\ Food wastes & 0.33–0.5 & [69; 72; 73] \\ Pig, swine manure & 0.3–0.5 & [19; 74; 75; 76; 77; 78] \\ Poultry manure & 0.03–0.11 & [19; 74; 75; 77; 79] \\ Chicken manure & 0.52 & [80] \\
0.053–0.75 & [19] \\ Cattle manure & 0.11–0.54 & [12; 81] \\ Slaughterhouse waste & 0.2–0.8 & [82; 83] \\ Brewery waste & 0.3–0.51 & [84; 85] \\ Residual glycerine & 0.56 & [86] \\ Corn stover & 0.3–0.4 & [75; 87] \\ Sunflower crop wastes & 0.2–0.4 & [88; 89] \\ Rapeseed crop wastes & 0.25 & [75] \\ Wheat straw (steam explosion pretreatment) & 0.25–0.35 & [90; 91] \\ Rice straw & 0.26 & [92] \\ Grass: Napier grass, Canary grass, King grass & 0.15–0.60 & [93; 94; 95] \\ Meadow grass & 0.39 & [96] \\ Microalgae _Chlorella_ sp. & 0.23–0.26 & [77; 97] \\ Microalgae _Nannochloropsis aculata_ & 0.3–0.35 & [98] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Methane yields reported by different authors when evaluating individual substrates. used as a co-substrate. However, this option is not adequate for many countries and may be undesirable due to the rise of market distortions. Digestion technology should be integrated into the economic cycle without requiring additional incentives.