## 4 Taking Advantage of Process Synergies
[156] when studying co-digestion at different C/N ratios, indicating that the modification of the C/N ratio favors microbial activity and improves solid removal. The benefit associated with the addition of the co-substrate is considered as a priming effect by [PERSON] and [PERSON] [157] in resemblance to the enhanced organic matter decomposition that takes place in other habitats such as soils and sediments. This effect would explain the greater removal of volatiles frequently reported by several authors. However, the addition of a readily degradable substrate can affect the outcome of the digestion system; not always getting benefits. [PERSON] et al. [49] indicated that anaerobic digestion proceeds through preferential degradation, accumulating complex structures in the remaining solids. Therefore, the co-digestion may lead to a degradation of carbohydrates contained in the co-substrate, hardly modifying the other components of the mixture [23]. In addition, not all BMP results can be directly extrapolated to a continuously operating system, as demonstrated by [PERSON] et al. [86]. These authors indicated the successful performance of batch co-digestion tests under different co-digestion ratios but failures when attempting the semi-continuous operation of these same mixtures. [PERSON] and co-workers [158] set a mathematical connection between BMP and continuous operation using Monod kinetics. These authors indicated that although these tests give no clue regarding chronic toxicity, there is an evident link associated with microbial kinetics for both modes of operation which set the optimum operating conditions regarding the organic loading rate (OLR), HRT and methane production. Therefore, the different constituents of the feeding mixture may not be fully degraded if the OLR and HRT of the reactors are not in accordance with the microbial dynamics, although BMP tests predicted successful results. Co-digestion allows the treatment of substrates that otherwise would not be possible or would lead to extremely low methane yields. This is the case in the study performed by [PERSON] et al. [159] when co-digesting a mixture composed of chicken manure, sewage sludge and wine distillery wastewater. The results reported by [PERSON] et al. [115] and [PERSON] et al. [160, 161] are similar, who studied the digestion of slaughterhouse wastes. The high lipid content in the first case and the high protein content in the second, where residual blood from slaughterhouses was studied, prevented the correct development of the fermentation when attempting the mono-digestion of these substrates. The benefits of co-digestion are undeniable, but high installation costs along with the need for high-skill personnel for the operation and maintenance tasks are two important barriers to be overcome. Policies should focus on solving these issues, proposing solutions for small- and mid-size treatment waste systems which present serious difficulties in attaining proper waste valorization at a reasonable cost. Partial decentralization may become a practical solution if small digestion units are dedicated to treating local wastes, whereas raw biogas may be transported and upgraded at a centralized treatment plant. Figure 2 shows a schematization of different substrates commonly used in anaerobic co-digestion focusing on expected inhibitory effects.