## 3 Anaerobic Digestion Plant Technologies
### Monitoring and Control of Plant Efficiency
\begin{table}
\begin{tabular}{l l l l l} \hline \hline  & & \multicolumn{2}{c}{**(c) membrane reactor [22]**} \\ \hline
**Description** & **Advantages** & \multicolumn{2}{c}{**Drawbacks**} & \multicolumn{2}{c}{**Description**} \\ \hline
**Anaerobic filter** & Biofilm to separate biomass from effluent, & \(\bullet\) & Excellent adaptability for biomass to a new carbon source and to organic load fluctuation & \(\bullet\) & Clogging of filter media \\
**reactor—AF** & \(\bullet\) & \(\bullet\) & Simpler solution for industrial application & \(\bullet\) & Higher investment costs \\  & \(\bullet\) & \(\bullet\) & Higher solution for industrial application & \(\bullet\) & Not suitable for wastewater \\ \hline
**Anaerobic** & Small-inert particles & \(\bullet\) & Higher OLR & \(\bullet\) & Membrane fouling, especially due to proteins at low temperature \\
**fluidized bed** & used as the medium for & \(\bullet\) & Greater resistance to inhibitors & \(\bullet\) & Fluidization is still an empirical science \\
**reactor—AFBR** & bacterial attachment & \(\bullet\) & Good mass transfer efficiency & \(\bullet\) & Process improvement at large scale is needed \\  & & \(\bullet\) & More effective than AF & \(\bullet\) & scale is needed \\  & & \(\bullet\) & Better hydraulic circulation & \(\bullet\) & \\  & & \(\bullet\) & Greater surface area & \(\bullet\) & \\ \hline
**Expanded** & \(\bullet\) & \(\bullet\) & Suitable for small-medium size industry & \\  & & \(\bullet\) & Smaller footprint than UASB & \\  & Modification of UASB reactor, used when & \(\bullet\) & Higher mixing due to higher & \\  & volumetric gas & \(\bullet\) & \(\bullet\) & \(\bullet\) \\  & production rate is low & \(\bullet\) & \(\bullet\) & \\  & and mixing is & \(\bullet\) & Higher organic and & \\
**blanket—EGSB** & insufficient & \(\bullet\) & hydraulic loadings & \\  & It can separate gas, & \(\bullet\) & Suitable for wastewater & \\  & liquid and solid & \(\bullet\) & containing lipids and & \\  & biomass & & toxic/inhibitory compounds & \\  & simultaneously & \(\bullet\) & Deviores for precipitation & \\  & & & separation, extra degassing and & \\  & & & reflux not required & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._measurement, as several variables are involved in the process. Generally, a very small number of parameters are controlled, due to the complexity and cost of advanced monitoring. [PERSON] and [PERSON] [26] visited approximately 400 full-scale AD plants, mostly used for wastewater treatment. In 95% of them, in-situ and on-line monitoring was limited to pH, temperature, water flow rate and biogas flow rate, level and pressure. Only 10% had continuous monitoring of COD, TOC, VFAs, alkalinity and biogas composition. [PERSON] et al. [27] verified that many plants operate on the basis of ex-situ analysis and only sensors for pH, redox potential and percentage gas production were used either in situ or on-line. The main issue is the identification of the most critical parameters for early detection of instability. For example, in [PERSON] et al. [28], a combination of volatile fatty acids (VFAs), VFAs/total alkalinity (TA) and alkalinity per bicarbonate ion (BA)/TA was proposed as a parameter setting for early warning. [PERSON] et al. [29] showed how the formation of hydrogen H\({}_{2}\) and hydrogen sulphide H\({}_{2}\)S were also very sensitive to changes in organic load, and useful for evaluating overload problems that would cause a pH decrease. Recurring problems are also related to the type of sensors chosen and the best positioning to ensure correct monitoring and facilitate maintenance. Several electrochemical, chromatographic and spectroscopic devices (UV, VIS, IR/NIRS) can be used for on-line monitoring in order to generate early warning signals and permit specific actions of correction. Among them, the most promising method is NIRS spectroscopy because a large number of parameters can be measured at the same time [30]. However, in the case of small plants (i.e., below 100 kW) this method is still too expensive.