# Thermal Hydrolysis of Sewage Sludge: A Case Study of a WWTP in Burgos, Spain
## 3 Results and Discussion
### Economic Balance and Feasibility Analysis  
Table 9 shows the results for a yearly analysis of cash flow components when calculating the economic balance under the assumptions of Case I and the increase observed, using Scenario 1 as a reference. Table 10 shows the results from the economic balance and feasibility analysis, considering Cases I and II in the different scenarios. The estimation of initial investment for economic balance for Cases I and II is detailed in Table S6-S8. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Parameter** & **Scenario 1** & **Scenario 2** & **Scenario 3** & **Scenario 4** & **Scenario 5** \\ \hline Biogas production & 24,020 & 31,458 & 29,443 & 35,022 & 37,192 \\ (MWh/year) & \(\cdots\) & 7438 & 5423 & 11,002 & 13,172 \\ Net electric power & 9180 & 8510 & 8635 & 9268 & 11,974 \\ (MWh/year) & \(\cdots\) & \(-\)670 & \(-\)545 & 88 & 2794 \\ Dewatered sludge & 40,445 & 23,617 & 26,026 & 16,250 & 14,550 \\ (t/year) & \(\cdots\) & \(-\)16,828 & \(-\)14,419 & \(-\)24,195 & \(-\)25,895 \\ Polyelectrolyte & 64.8 & 113 & 90.2 & 100.7 & 24.5 \\ (t/year) & \(\cdots\) & 48.2 & 25.4 & 35.9 & \(-\)40.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Results obtained annually and used for the calculation of cash-flow parameters under Case I. The second row corresponds to the increment obtained when compared with Scenario 1. A negative sign represents a decrease. Figure 4a shows the values of the capital investment for the two economic cases (Cases I and II) considered. Evaluating a time horizon of 25 years, Figure 4b shows the net annual cash flow in euros, Figure 4c represents the payback time expressed in years, and Figure 4d shows the net present value in euros. When obtaining payback values higher than 100 years, it is represented as 100 years in the graph. If the net annual cash flow is negative, then the payback is not represented in the figure. Figure 4b shows that the net annual cash flow from Scenario 2 is negative. This scenario considers the installation of a batch THP. The two cases give negative values for net annual cash flow, resulting in an NPV of \(-\)6,310,937 EUR for Case I at a time horizon of 25 years and a value of \(-\)3,405,279 EUR for Case II. Treating only WAS by thermal hydrolysis (Scenario 3) improves the economic balance, but the NPV is still negative when evaluated under the assumptions of Case I, whereas benefits are obtained in Case II. However, even in this latter case, the benefits are not high enough to consider this technical option as economically feasible. The payback period is 106 years. Scenario 4 considers the performance of an ITHP system. This scenario, along with Scenario 5, has a higher initial investment; thus, it is penalised by this factor when evaluating the payback period. Therefore, it is only under the assumptions of Case II that this option gives a payback period of 31 years, which may be improved if fiscal incentives are available; otherwise, this option, just as in Scenario 2, cannot be considered economically feasible. Conversely, Scenario 5, although having a similar initial investment to that of an ITHP system, presents much higher value for the parameter net annual cash flow and, thus, results in a better economic balance, with payback periods of 12 and 7 years for Cases I and II in terms of economic balance. In the present case study, costs regarding operation and sludge disposal are fixed. These factors have a great impact on the economic balance. Future work will be performed to evaluate how sludge disposal costs, fiscal incentives (regarding electricity production) and the addition of cosubstrates will impact the economic balance. Final sludge disposal costs are highly varied among different countries, and the outcome of economic balance will be greatly affected by the selection of different disposal routes (e.g., landfill, land application, composting, incineration).