## 3 Anaerobic Digestion Plant Technologies
Advantages and drawbacks of each type of reactor are reported in Table 2a. The most common AD plants are composed of one or more vertical heated reactors with an internal mixing system based on a wet process operated in mesophilic conditions. This type of digester is referred to a complete stirred tank reactor (CSTR) and represents the first generation of high-rate anaerobic digesters, which is widely used in the case of co-digestion of wastewater, high-dense liquid animal manure and organic industrial waste. The main limitation, if no recirculation is performed, is the failure to retain high microorganism concentrations inside the reactor. In fact, the key factor for a good anaerobic high-rate treatment is the retention of the slow-growing microorganisms. A new generation of reactor, such as the up flow anaerobic sludge blanket (UASB) reactor contains a well-settleable methanogenic sludge. The formation of a dense sludge bed permits the microorganism's retention inside the system. The advantages of UASB reactors over CSTR are compactness, higher loading rates, lower retention times, lower operational costs, lower sludge production and higher methane production rates (Table 2b). Since the anaerobic microorganisms can form granules through self-immobilization of the cells, the performance of the system is highly dependent upon the granulation process, together with the characteristics of the wastewater. However, substrates with a high fraction of particulate organic material cannot be treated using this technology [23]. Membrane bioreactors (MBR) technology is also able to promote the retention of the active biomass within the system (Table 2c). The membrane forms a selective barrier, thus allowing certain components to pass through while retaining the others. In particular, cells are retained, while inhibitory compounds, such as furan aldehydes and carboxylic acids, negatively affecting the biological process, are separated. However, when the particulate and cell concentrations are high, fouling can occur. The combination of anaerobic membrane technology with specific systems (coagulant addition, adsorbent addition, use of granular materials improving aeration) lead the membrane to work with high flow rates and may represent a further improvement of the technology. These integrated systems show several advantages, such as improved methane production and less frequent fouling problems. They can be used to treat \begin{table}
\begin{tabular}{l l l l l} \hline \hline  & & **Advantages** & & **Drawbacks** \\ \hline \multirow{6}{*}{**Mesophilic process**} & \multirow{6}{*}{*} & \multirow{6}{*}{Free ammonia decreasing} & \multirow{6}{*}{*} & Higher retention time (30–50 days) \\  & & & Free ammonia decreasing & * & Bigger digesters size \\  & & & Less thermal energy required by & * & Less degradation efficiency \\  & & & digester, even if larger digester & * & Lower pathogen removing from \\  & & & size should be considered & & substrate, post-treatment is needed \\  & & & Less nitrate concentration in & * & Higher viscosity of influent, \\  & & the sludge & & pre-treatment is needed \\  & & & Better process stability & * & More clogging risk \\  & & & & Low methane yield \\ \hline \multirow{6}{*}{**Thermophilic process**} & \multirow{6}{*}{*} & \multirow{6}{*}{Lower retention time (15–16 days)} & \multirow{6}{*}{*} & Higher free ammonia concentration \\  & & & Small size of digesters & * & Higher thermal energy required, but \\  & & & Higher organic & & higher quantity of energy is produced \\  & & & load-bearing capacity & & from biogas \\  & & & Higher efficiency of degradation & * & Higher nitrate concentration in \\  & & & Higher biogas production rate & & the sludge \\  & & & Higher pathogen removing & * & More sensitive to \\  & & & from substrate & & environmental changes \\  & & & Lower viscosity & * & Decreased stability process \\  & & & Less clogging risk & * & Larger investment \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mesophilic versus thermophilic process *.