## 3 Results and Discussion
### Cooling Circuit Energy Balance  
Figure 7 shows the diagram of the main, emergency and auxiliary cooling circuits of the engines. Each circuit consists of a primary and a secondary section as previously described, both separated by a plate heat exchanger. When the digestion section worked as a conventional mesophilic system, the energy from this circuit was used to increase the sludge temperature and keep the digester at 37 \({}^{\circ}\)C. However, after installing the thermal pre-treatment unit, this heat recovery was no longer needed. Part of the energy available in the main circuit--secondary section--is now used to preheat the feeding water of the recovery mixed boiler. The exceeding heat is dissipated in cooling towers. Due to the physical proximity between the exchangers of the main cooling circuit and the boiler feed water preheating circuit, the minimum approach point considered for calculations was 5 \({}^{\circ}\)C. A significant amount of low-grade energy does not find any application in the WWTP when the thermal hydrolysis unit is operating. This assessment highlights the relevance when evaluating biogas production enhancement. All sets of additional equipment must be considered, along with changes in heat recovery and the quality of the energy needed after implemented modifications. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline \(MP_{\text{eng}}\), mechanical power (kW) & 1072 \\ \(DL\), degree of loading (\%) (2 engines) & 86.9 \\ Effective average pressure (kPa) & 1193 \\ _Specific TP_biogas-eng, engine specific biogas consumption (g/kWh) & 468 \\ \(\eta_{\text{mec-eng}}\), engine mechanical efficiency (\%) & 39.3 \\ \(\eta_{\text{el-eng}}\), engine electrical efficiency (\%) & 38.2 \\ \(\eta_{\text{thermal-eng}}\), engine thermal efficiency (\%) & 49.8 \\ \(\eta_{\text{energy-eng}}\), engine energy efficiency (\%) & 88 \\ _UTEF_, usable thermal energy factor of the engines (\%) & 84.4 \\ (\(W\)/Q)\({}_{\text{eng}}\), work/heat ratio & 0.77 \\ Thermal losses & \\ Exhaust gases thermal power at 25 \({}^{\circ}\)C (kW) & 858 \\ Exhaust gases thermal power at 120 \({}^{\circ}\)C (kW) & 679 \\ Thermal power losses in the auxiliary water circuit (kW) & 211 \\ Oil exchanger (kW) & 143 \\ _TP_IC, thermal power of the intercooler (kW) & 68 \\ Combustion and emissions & \\ Exhaust gases dry oxygen (\%) & 8.9 \\ Stoichiometric air–biogas ratio (\%) & 17.1 \\ \(A\), air–biogas ratio (\%) & 10.7 \\ \(E_{\text{air}}\), excess air (\%) & 59.4 \\ \(T_{4}\), exhaust gas temperature (\({}^{\circ}\)C) & 483 \\ \(M_{\text{air}}\), inlet air mass flow (kg/h) & 5371 \\ \(M_{\text{eg}}\), exhaust gas mass flow (kg/h) & 5878 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Parameters of biogas engines at the operating point. The emergency exchanger only comes into service in the event of insufficient power, breakdown, or failure of the primary exchanger. In contrast, its secondary circuit is cooled by water coming out of the secondary exchanger from the auxiliary circuit, both being positioned in series and fully cooled in cooling towers. The secondary section of the auxiliary circuit also dissipates its energy in cooling towers. The three-way thermostatic valves in both the main and auxiliary circuits allow the water temperature to be kept at 90 \({}^{\circ}\)C at the outlet and 55 \({}^{\circ}\)C at the inlet of the engine, by recirculating water through the existing bypasses without going through any of the heat plate exchangers located in the two cooling circuits. The heat exchangers of the block, crankcase and cylinder jackets are shown in Figure 7 as external independent units for the sake of simplicity to facilitate process description. This representation eases heat transfer calculation and aids in understanding the different circuits considered. The cooling water is fed at the bottom part of the engine, where the crankcase is located, and it circulates up through the cylinder jackets, exiting through the cylinder heads. All these components are located inside the engine.