## 3 Results and Discussion
### Energy Costs Assessment for Operation Affected by Faults  
Operating a sewage treatment plant is highly energy intensive, with implicitly high economic costs. The price of energy is determined by a variety of supply and demand factors, such as the geopolitical situation, the national particular energy mix, distribution network expenses, environmental protection taxes, severe weather conditions or excise and taxation levels. The cost of energy also depends on the energy source. Energy prices vary from 4.8 to 12.10 eurocents depending on the energy source, as can be seen in Table 3. These values were extracted from an International Energy Agency (IEA) report on the mean costs of generating energy. This report provided data for a total of 243 plants in 24 countries from Europe, Africa and Asia [75].  
The daily costs of normal operation and each type of DO sensor malfunction were determined for the sewage treatment plant considered in this study. The daily costs were individually evaluated for different potential energy sources or energy production technologies. The data in Table 4 were obtained by multiplying the costs of different energy sources [75] with the total energy demand values computed for the normal and faulty operation cases. Table 4 summarizes the findings.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Source/Technology** & **Eurocents/kWh** \\ \hline Lignite & 9.12 \\ Coal & 8.80 \\ Gas (CCGT 1) & 6.81 \\ Nuclear & 6.62 \\ Wind onshore & 4.80 \\ Wind offshore & 8.45 \\ Solar PV 2 commercial & 5.38 \\ Solar PV residential & 12.10 \\ Solar thermal (CSP 3) & 11.62 \\ Hydro reservoir & 6.91 \\ Hydro run of river & 6.53 \\ Geothermal & 9.50 \\ Biomass & 11.33 \\ \hline \hline \end{tabular} \({}^{1}\)CCGT—combined cycle gas turbines, 2 PV—photovoltaic, 3 CSP—concentrating solar power.  
\end{table}
Table 3: Costs of different energy sources.  
By analyzing the costs of electrical energy spent for the different cases of the faults, it may be observed that complete failure minimum fault of the DO sensor implies the largest energy costs, as the reduced faulty value of the DO sensor determines the control system to considerably increase the airflow rate. On the contrary, the fixed value and complete failure maximum faults imply the smallest and comparable energy costs due to the large values of the DO sensor signal that make the DO controller reduce the airflow rate. However, for these latter cases, the quality of the effluent dramatically deteriorates by a factor higher than 20 and ranks these faults as having the worst overall effect.  
Values of the daily energy costs presented in Table 4 also reveal that onshore wind, solar PV commercial and hydro run of rivers are the most favorable sources of energy to be used from the economic costs point of view, while the solar PV residential, solar thermal (CSP) and biomass are the most expensive ones. The lignite (CSS) and coal (CSS) energy sources may be as well considered as having high costs.  
Depending on the technical solution availability, switching to the alternative cheaper energy sources and implementing a customized program for the use of a mixed energy source in case of faulty sensor operation might reduce the implied energy costs before the replacement of the defective sensor.