# 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
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Parameter** & **Scenario 1** & **Scenario 2** & **Scenario 3** & **Scenario 4** & **Scenario 5** \\ \hline \multicolumn{6}{c}{* **Thermal hydrolysis**} \\ \hline mTS-reactor, mass of TS outlet reactor & & 768 & 768 & 768 & 768 \\ (kg/batch) & & 15 & 6 & 10 & 7 \\ MPB, flow mass of process gases of & & 56 & 25 & 39 & 27 \\  steam (kg/h) & & 1439 & 647 & 1002 & 685 \\ MTS-reactor, flow mass of reactor sludge & & 11,796 & 5302 & 8203 & 5612 \\ MTS, average consumption of live steam & & 1385 & 623 & 964 & 660 \\ (kg/h) & & 952 & 952 & 952 & 952 \\ MTS-specific, average specific consumption & & 952 & 952 & 952 & 952 \\ of live steam (kg/t TS) & & & & \\ \hline
**Dilution water** & & & & \\ \hline Msludge-dilution water, mass flow of sludge & & 14,813 & 6659 & 10,301 & \\ dilution water (kg/h) & & & & \\ TPdiluted-sludge, thermal power of diluted & & 1279 & 575 & 890 & \\ sludge (kW) & & & & \\ \hline
**HSE** & & & & \\ \hline T\({}_{\rm HSE}\), temperature of HSE sludge (\({}^{\circ}\)C) & & 50 & 51 & 50 & \\ TP\({}_{\rm HSE}\), thermal power of HSE sludge & & 807 & 575 & 559 & \\ (kW) & & -472 & 0 & -331 & \\ Cooling thermal power of HSE (kW) & & & & & \\ \hline
**Mixing ring** & & & & \\ \hline Temperature of the sludge ring (\({}^{\circ}\)C) & 15 & 50 & 32 & 28/50 & 25 \\ Mass flow of sludge ring (kg/h) & 28,138 & 14,813 & 24,493 & 28,138/10,301 & 31,496 \\ Thermal power of the sludge ring (kW) & 474 & 807 & 882 & 474/559 & 889 \\ Solid concentration in sludge ring (\%) & 5.4 & 10.0 & 6.1 & 5.4/10 & 4.9 \\ \hline \multicolumn{6}{c}{**Digesters**} \\ \hline Number of digesters operating (unit) & 3 & 1 & 2 & 2/1 & 3 \\ Volume of digesters operating (m\({}^{3}\)) & 18,000 & 6000 & 12,000 & 12,000/6000 & 18,000 \\ Total VSR (\%) & 45.2 & 61.9 & 56.5 & 68 & 70 \\ HRT, hydraulic retention time (d) & 27.1 & 17.4 & 20.8 & 18/25 & 27.1 \\ OLR, organic loading rate (kg VS/d m\({}^{3}\)) & 1.5 & 4.4 & 2.2 & 2.3/2.6 & 1.5 \\ Concentration of digested sludge (\%) & 3.6 & 5.5 & 3.6 & 3.8/7.5 & 2.3 \\ Thermal power of digested sludge (kW) & 1161 & 651 & 1112 & 1162/456 & 1441 \\ \hline \multicolumn{6}{c}{**Dewatering**} \\ \hline Concentration of dewatered sludge (\%) & 20.3 & 30 & 30 & 41 & 45 \\ Mass flow of dewatered sludge (kg/h) & 4977 & 2696 & 2970 & 1968 & 1694 \\ Reduction rate of dewatered sludge (\%) & & \(-45.8\) & \(-40.3\) & \(-60.5\) & \(-66.0\) \\ Mass flow of polyelectrolyte (kg/h) & 7.4 & 6.9 & 7.6 & 8.1 & 2.9 \\ \hline
**Balance of NH\({}_{4}\)\({}^{*}\)** & & & & & \\ \hline Mass flow of NH\({}_{4}\)\({}^{*}\) (kg/h) & 19.3 & 35.3 & 33.9 & 44.4 & 41.7 \\ Total increment rate of NH\({}_{4}\)\({}^{*}\) (\%) & & 82.9 & 75.6 & 30.1 & 116 \\ \hline
**Total polyelectrolyte** & & & & & \\ \hline Total mass flow of polyelectrolyte (kg/h) & 7.4 & 12.9 & 10.3 & 12.3 & 2.9 \\ Total increment rate of polyelectrolyte & & & & & \\ (\%) & & 74.3 & 39.2 & 66.2 & \(-60.8\) \\ \hline \hline \multicolumn{6}{c}{* Thermal hydrolysis operating with one reactor.} \\ \hline \hline \end{tabular}
\end{table} 
Table 6: _Cont._There are several studies available in the literature that have reported a wide range of values regarding the biogas enhancement of hydrolysed sludge. [PERSON] et al. (2011) reported low biogas increments of 17% to 24% when evaluating sludge under two different temperatures, 150 and 170 \({}^{\circ}\)C. However, other reports have indicated biogas increments between 57-70% ([PERSON], 2013; [PERSON], 2013) for standard thermal hydrolysis. Values that are five times higher were reported by [PERSON] et al. (2015) when compared with the mesophilic digestion of WAS. On the other hand, [PERSON] et al. (2016) evaluated post-thermal hydrolysis with centrate recirculation and reported an enhancement in methane yield of 62%. Based on the great variety of these results, in the present study, the criterion of considering the conversion of destroyed VS into biogas gives a value for biogas enhancement that is in the middle range of the reported values.