# Thermal Hydrolysis of Sewage Sludge: A Case Study of a WWTP in Burgos, Spain
## 4 Conclusions  
The analysis of four different thermal hydrolysis processes that are assumed to operate in the WWTP of the city of Burgos (Spain) indicates that Solidstream(r) technology reports better results regarding economic and energy balance. Factors such as biogas production, net energy generation, polyelectrolyte consumption, dewatered sludge production and operating costs were evaluated. The main advantage of thermal hydrolysis is the significant  
Figure 4: Economic balance and feasibility analysis for Cases I and II. (**a**) Initial investment. (**b**) Net annual cash flow. (**c**) Payback. (**d**) Net present value (NPV) at 25 years. The payback period for Scenarios 2 and 3 is not represented because the net annual cash flow was negative for these scenarios.  
decrease in the total amount of sludge that needs final disposal. The operating conditions of the different technologies reported great variations regarding NPV. When a newly constructed WWTP was considered, the main advantage obtained was the significant decrease in capital investment due to the construction of smaller digestion units. However, in the two cases evaluated, Solidstream(r) reported the best economic parameters and energy performance among the different technical options studied.  
The exclusive treatment of secondary sludge instead of the whole sludge stream significantly improves the energy balance because of the lower energy demand of this configuration. This leads to reasonably good results for plants of new construction. Intermediate thermal hydrolysis has reported results that make this technology unfeasible for direct application to a currently operating WWTP. However, performance parameters for plants of new constructions are better.  
The increments in energy demand associated with the additional operating equipment of the thermal hydrolysis process offset the benefits obtained from better reactor performance. Therefore, the main advantage of any of these hydrolysis processes is more closely related to the decrease in the volume of sludge needing final disposal rather than an increase in energy production.  
The following are available online at [[https://www.mdpi.com/2076-341](https://www.mdpi.com/2076-341)]([https://www.mdpi.com/2076-341](https://www.mdpi.com/2076-341)) 7/11/3/964/s1. Table S1: Parameters obtained from the sludge line of the economic balance (Case II); scenarios are abbreviated as S1 to S5 to stand for Scenario 1 to Scenario 5. Table S2: Parameters obtained from the biogas line of the economic balance (Case II). Table S3: Breakdown of total self-consumption of the economic balance (Case I). Table S4: Breakdown of total self-consumption of the economic balance (Case II). Table S5: Total self-consumption, electric power balance and specific energy parameters used in the economic balance (Case II). Table S6: Main results from the economic balance under Case II assumptions; second row for each category contains the difference in the corresponding values with regard to Scenario 1. Table S7: Breakdown of initial investment of the economic balance under Case I. Table S8: Breakdown of the capital investment associated with the economic balance under Case II assumptions.  
Conceptualization, X.G.B. and E.J.M.; methodology, J.G.-C.; software, J.G.-C.; validation, X.G.B.; formal analysis, X.G.B.; investigation and resources, J.G.-C.; data curation, X.G.B. and E.J.M.; writing--original draft preparation, J.G.-C.; writing--review and editing, X.G.B. and E.J.M.; visualization, X.G.B.; supervision, X.G.B. and E.J.M; project administration, X.G.B.; funding acquisition, X.G.B. All authors have read and agreed to the published version of the manuscript.  
This research was possible thanks to financial support from the Ministry of Economy and Competitiveness and European Regional Development Funds (ERDF) through project UNLE15-EE-3070.  
Not applicable.  
The authors declare no conflict of interest.