## 5 Improving Reactor Performance
\begin{table}
\begin{tabular}{c p{284.5 pt}} \hline \hline
**Process Parameter** & **Effect** \\ \hline Temperature & The increase in temperature accelerates degradation rates, fluid dynamics and settling characteristics of particles [181]. The improvement in microbial activity increases the reactor treatment capacity of organics reducing the digester volume needed [182]. \\ \hline pH and alkalinity & pH values should be close to neutral conditions. The stability of the digestion is closely related to the capacity of buffering acid intermediaries, the release of CO2 and the presence of ammonia. The interaction between the ionic species and free forms attenuates pH deviations making the process more robust to organic loading fluctuations [27]. \\ \hline Organic loading rate (OLR) & Represents the amount of organic material entering into the digester with the influent. Increasing the volumetric flow or increasing the solid content of the feeding material leads to an increase in organic loading. Biogas production is directly associated with the amount of organics fed into the reactor, and any increment in OLR is usually associated with an improvement in the biogas production rate. The increase in solid content attained by adding a co-substrate in anaerobic digestion is one of the main reasons for obtaining a better volumetric efficiency of the reactor. However, an excess in OLR may also cause process imbalances due to the accumulation of acid intermediaries associated with disturbances in the acidogenic and methanogenic phases. \\ \hline Hydraulic retention time (HRT) & Refers to the time the fluid spends in the reactor. This time is calculated as the ratio between the volume of the reactor and the volumetric flow applied. HRT and OLR are linked by the volumetric flow, thus increasing the incoming flow also leads to an increase in OLR and a decrease in HRT. The time needed for the substrate to be fully degraded depends on the characteristics of the material, complexity in the structure of organic compounds and the activity of the microflora. Co-substrates characterized by a limited hydrolysis phase will need a higher retention time in the anaerobic reactor. Inhibitory conditions lead to poor performance of the microbial activity, with the digestion system not being able to degrade organics in the time given by the HRT. \\ \hline Volatile fatty acids (VFAs) & Short-chain fatty acids are produced as intermediary compounds during the anaerobic conversion of organics. Process imbalances lead to the accumulation of these acids, inhibition of methanogens, and therefore a decline in biogas evolution along with pH variations when the buffer capacity of the system is surpassed [183]. \\ \hline Volatile fatty acids (VFAs) & Process inhibition has been reported to occur at VFA concentrations in the range of 2000–4000 mg/L [184] depending on the type of substrate evaluated. However, co-digestion with high N-containing organics allows the maintenance of process stability even though high levels of VFA may be present. Stable performance was reported by [PERSON] et al. [185] when studying co-digestion of pig manure, reporting as inhibitory the VFA range of 16.5–18.0 g/L \\ \hline \hline \end{tabular} This compound is derived from the conversion of protein-rich material. The toxicity of ammonia in the digester is linked to the level of free ammonia, which is dependent on the system pH. Nitrogen is an essential nutrient for the process, but excessive levels lead to methanogenic inhibitory conditions. The ammonium concentration found in the reactor liquor depends on substrate C/N ratio, HRT and OLR applied to the reactor, and the degradability of the substrates (hydrolysis performance). \\ \hline Ammonium & Amb ratio plays a relevant role in the buffer capacity of the system by attenuating pH drops through the equilibrium ammonia–ammonium reaction. However, a high concentration of ammonium ions may be detrimental to the anaerobic microorganisms. [PERSON] et al. [186] reported that a threshold for stability is found at 1 g NH3-N/L (free ammonia), irrespective of the OLR studied. \\ \hline \hline \end{tabular}  
* Acclimation of the microflora to high ammonia levels may attain stable performance when treating high-nitrogen-containing wastes. [PERSON] et al. [187] reported stable performance of a full-scale chicken manure digestion plant under ammonium-N levels of 6.2 g/L and [PERSON] et al. [188] indicated that 8.5 g NH4+N/L was the threshold for experiencing inhibitory conditions, with this value being associated with free ammonia nitrogen (FAN) values greater than 800 mg NH3-N/L.  
\end{table}