# Thermal Hydrolysis of Sewage Sludge: A Case Study of a WWTP in Burgos, Spain
## 3 Results and Discussion
### Economic Balance and Feasibility Analysis
In fact, one of the main benefits of installing a thermal hydrolysis unit in a WWTP is associated with the enhanced degradation of organic matter and improved cake solid content, from 25.2% to 32.7% of TSS (total suspended solids), as reported by [PERSON] et al. (2018), who also indicated that this improvement \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}{**Economic balance and feasibility analysis (Case I)**} \\ Electric power & 844,604 & 782,920 & 794,420 & 852,656 & 1,101,608 \\ (\(\xi\)/year) & \(-\)61,684 & \(-\)50,184 & 8052 & 257,004 \\ Deweated sludge & 465,622 & 271,831 & 299,458 & 198,429 & 170,802 \\ (\(\xi\)/year) & \(-\) & 193,791 & 166,164 & 267,193 & 294,820 \\ Polyelectrolyte & 149,095 & 259,009 & 207,524 & 247,820 & 58,429 \\ (\(\xi\)/year) & \(-\)110,814 & \(-\)58,429 & \(-\)98,725 & 90,666 \\ Operation costs & \(-\)119,584 & 119,584 & 132,267 & 135,321 \\ (\(\xi\)/year) & \(-\)119,584 & \(-\)119,584 & \(-\)132,267 & \(-\)135,321 \\ \hline \multicolumn{6}{c}{**Economic balance and feasibility analysis ( Case II)**} \\ Electric power & 844,604 & 782,920 & 905,280 & 942,172 & 1160,488 \\ (\(\xi\)/year) & \(-\)61,684 & 60,676 & 97,568 & 315,884 \\ Deweated sludge & 465,622 & 271,831 & 299,458 & 198,429 & 170,802 \\ (\(\xi\)/year) & \(-\)193,791 & 166,164 & 267,193 & 294,820 \\ Polyelectrolyte & 149,095 & 259,909 & 207,524 & 247,820 & 116,858 \\ (\(\xi\)/year) & \(-\)110,804 & \(-\)58,429 & \(-\)98,725 & 32,237 \\ Operation costs & \(-\)67,584 & 50,371 & 90,915 & 91,978 \\ (\(\xi\)/year) & \(-\)67,584 & \(-\)50,371 & \(-\)90,915 & \(-\)91,978 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Economic balance and feasibility analysis for Cases I and II. The second row corresponds to the increment obtained when compared with Scenario 1. A negative sign represents a decrease. translates into a 25% reduction in sludge disposal costs. Similarly, [PERSON] and [PERSON] (2020) studied the performance of several industrial hydrolysis plants. They also reported that the increase obtained in biogas yield does not necessarily lead to better overall performance in terms of energy balance. Other parameters are of great relevance. Among these, [PERSON] and [PERSON] (2020) made special reference to the adjustment of the solid content of the feed, digester retention time, and digestion temperature to optimise the overall energy balance of the digestion process. The Solidstream(r) process achieved the best results of the five proposed scenarios after evaluating efficiency performance parameters, also considering the evaluation of both Case I and Case II in terms of the economic balance. Solidstream(r) reported excellent results regarding parameters such as gross and specific consumption of live steam, thermal, electrical and energy performance of the process, average thermal power used from the exhaust gases and percentage of biogas consumption in the mixed recovery boiler. Furthermore, centrates from the dehydration section are sent back to digesters and not directly to the nitrification-denitrification process via nitrite like in the case of the other scenarios. Regarding the four components that make up annual net cash flow, Solidstream(r) obtained the highest production of electricity, the lowest amount of dewatered sludge and the lowest consumption of polyelectrolytes. However, this process presents a higher capital investment.