## 5 Improving Reactor Performance
### Addition of Adsorbents, Conductive Materials and Nanoparticles  
The addition of certain compounds to attenuate the negative effects associated with acid intermediaries and microbial products may aid in obtaining a better performance of high-solid and solid-phase digestion systems. Carbon conductive materials, bio-electrodes and adsorbents may provide alternative routes or protective sites for microorganisms, thus allowing anaerobic degradation to proceed under highly inhibitory conditions. [PERSON] et al. [209] reported an outstanding capacity for thermophilic reactors working at high OLR when biochar was added. These authors attributed this excellent performance to the ability of char particles to favor VFA syntrophic oxidation thanks to the electron-accepting capacity of the carbon particles. [PERSON] et al. [210] also studied high-solid digestion of food waste and cow manure. To prevent negative effects, natural zeolites were added to the reactor, thus obtaining a biogas yield in the range of 680 and 920 mL/g VS. The use of adsorbents in digestion reactors temporarily reduces the level of ammonia and total VFA allowing for the process to proceed steadily [211, 212]. The experiments carried out by [PERSON] et al. [12] clearly demonstrated this fact. These authors studied the digestion of poultry blood as a single substrate. In this study, the addition of activated carbon allowed the digestion to be completed, whereas in the control reactor with no adsorbent addition total inhibition took place. Adsorption is not the only mechanism justifying the better performance of the biological process, as the presence of the adsorbent creates protecting sites to the microflora and mass transfer limitations, thus microorganisms attached to these particles experience a lower concentration of inhibitory compounds. Several authors have also proposed the mechanism of direct interspecies electron transfer (DIET) as a phenomenon responsible for explaining the greater capacity of these anaerobic systems to degrade short-chain fatty acids and enhanced biogas production [213, 214, 215, 216]. The addition of biochar particles and the presence of bioanodes in anaerobic reactors have similar effects to that reported for activated carbon and adsorbents [217, 218, 24]. [PERSON] et al. [219] studied the behavior of digestion systems under high-solid conditions and biochar addition. The improvement in digestion obtained was attributable to the presence of biochar causing an enhancement in food waste hydrolysis thanks to the promotion of butyric acid degradation pathways. These authors demonstrated the involvement of the DIET mechanism by proving a relationship between the _Syntrophomonas_ and _Methanosarcina_ species. The introduction of bio-electrodes also provides a similar effect in digestion systems. [PERSON] et al. [220] described the improvements in VFA degradation thanks to the role played by soft-carbon-felt electrodes in overloaded batch reactors. However, adding just a conductive material to create a biofilm attached to its surface is an even more efficient way of increasing digestion performance as [PERSON] et al. [83] were able to prove. These authors evaluated anaerobic digestion under fed-batch mode operation, introducing a large carbon brush device, electrodes and a combination of brushes and electrodes. As a result, it was observed that a completely different microbial community structure was formed in the large-size brushes, with _Methanothrix_ being predominant in the biofilm. The reactor containing these brushes was highly effective in improving digestion performance, demonstrating a superior efficiency compared with the system using microbial electrodes with an applied voltage. Nanoparticles have also been studied as a way for improving biogas production. [PERSON] et al. [221] studied the addition of three different types of iron oxide nanoparticles reporting increments in biogas production, doubling the yields obtained from the conventional system.