# Thermal Hydrolysis of Sewage Sludge: A Case Study of a WWTP in Burgos, Spain
## 3 Results and Discussion
### Mass and Energy Balances for the Sludge and Biogas Line
The resulting cycle time of reactors for Scenarios 3-5 is relatively high, negatively affecting the injection coefficient and, therefore, the thermal power recoverable from \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Parameter** & **Scenario 1** & **Scenario 2** & **Scenario 3** & **Scenario 4** & **Scenario 5** \\ \hline
**Digesters** & & & & & \\ \hline Production of biogas (kg/h) & 516 & 677 & 632 & 457/266 & 806 \\ V\({}_{\text{biogas}}\), production of biogas (m\({}^{3}\)/h) & 465 & 609 & 569 & 411/240 & 726 \\ TP\({}_{\text{biogas}}\), thermal power of biogas (kW) & 2741 & 3590 & 3355 & 3837 & 4279 \\ Thermal power of SRE (kW) & 1037 & 0 & 528 & 966 & 970 \\ Thermal power of biogas flow (kW) & 9 & 12 & 11 & 7/5 & 13 \\ Transmission losses (kW) & 190 & 73 & 146 & 127/73 & 219 \\ Pipes and exchangers losses (kW) & 151 & 82 & 141 & 144/59 & 186 \\ \hline \multicolumn{6}{c}{**Engines**} \\ \hline TP\({}_{\text{biogas-eng}}\), biogas consumption (kW) & 2741 & 2720 & 2977 & 3266 & 3907 \\ TP\({}_{\text{eg-useful}}\), useful thermal power from & 526 & 523 & 603 & 653 & 758 \\ N\({}_{\text{eng}}\), number of engines operating & & & & & \\ (units) & 2 & 2 & 3 & 3 & 3 \\ DL, degree of loading (\%) & 87.7 & 86.9 & 58.5 & 65.9 & 82.4 \\ TP\({}_{\text{main-circuit-fw}}\), thermal power of the & 531 & 528 & 577 & 640 & 759 \\ main circuit (kW) & & & & & \\ \(<\)TP\({}_{\text{main-circuit-fw}}\), average thermal & & 105 & 47 & 73 & 50 \\ power of the boiler’s main circuit (kW) & & & & & \\ \hline \multicolumn{6}{c}{**Recovery mixed boiler**} \\ \hline \(<\)TP\(>_{\text{biogas-rb}}\), biogas consumption (kW) & & 870 & 378 & 571 & 372 \\ Useful average thermal power of exhaust & & 173 & 89 & 150 & 119 \\ gases (kW) & & 33 & 14.8 & 23 & 15.7 \\ k\({}_{\text{i}}\), injection coefficient of live steam (\%) & & & & & \\ \hline
**Gasometers** & & & & & \\ \hline TP\({}_{\text{biogas-sq}}\), thermal power supplied (kW) & & 1767 & 2170 & 1919 & 2000 \\ TP\({}_{\text{biogas-st}}\), thermal power stored (kW) & & 870 & 378 & 571 & 372 \\ \hline \multicolumn{6}{c}{**Total process**} \\ \hline TP\({}_{\text{digester}}\), thermal power required for & & & & & \\ digesters (kW) & 1037 & 576 & 999 & 392 & 999 \\ TP\({}_{\text{total-losses}}\), total losses of thermal power & & 656 & 1912 & 1242 & 2159 & 1700 \\ \({}_{\text{thermal-process}}\), thermal efficiency (\%) & 37.8 & 16 & 29.8 & 10.2 & 23.3 \\ \({}_{\text{electric-process}}\), electric efficiency (\%) & 38.2 & 27.1 & 29.4 & 27.6 & 31.9 \\ \({}_{\text{Energy-process}}\), energy efficiency (\%) & 76 & 43.1 & 59.2 & 37.8 & 55.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Parameters obtained for the biogas line under assumptions of the economic balance case (Case I). exhaust gases. This fact results in greater consumption of biogas by the boiler. It was also found that the thermal power available from the engine's main cooling circuits is insufficient to satisfy the heat demand of digesters in Scenarios 4 and 5; thus, additional thermal power must be provided from different elements (i.e., heat from the use of auxiliary biogas by the boiler, increasing the concentration of thickened sludge in the flotation thickeners). Scenario 4 needs an additional amount of 326 kW (thermal power), whereas Scenario 5 requires 211 kW. Under the conventional configuration of the WWTP, the mesophilic digestion process obtains additional thermal power whenever needed from the heat recovery systems of the engines' exhaust gases and the main cooling circuit of the engines. The decrease in the mass flow of dewatered sludge under all scenarios is noteworthy when compared with the values obtained from Scenario 1. However, the consumption of polyelectrolytes is significantly increased in Scenarios 2-4. Scenario 5 requires null polyelectrolyte addition because dewatering in this scenario is performed at high temperatures, eliminating the need for a coagulating agent.