## 3 Results and Discussion
### Environmental Assessment of CO\({}_{2}\) and N\({}_{2}\)O Emissions  
The on-site and off-site emissions of CO\({}_{2}\) and N\({}_{2}\)O, as main contributors to the Green House Gases released by the water line of the WWTP, were estimated for each type of faulty operation and also for the normal one. Their daily mean values are presented in Table 5.  
The data in Table 5 revealed that on-site emissions are the most significant, accounting in the case of normal operation for about 75% of the total emissions, both for \(P_{CO_{2}\_total}\) and \(P_{N_{2}O\_total}\). Computed total CO\({}_{2}\) emissions, i.e., the sum of the on-site and off-site values, showed increased values for all cases of the DO sensor faults, with the exception of the complete failure minimum fault type. Very large total CO\({}_{2}\) emissions (more than one order of magnitude higher than those of the normal operation) were produced in the cases of the following faults: fixed value, complete failure maximum and loss of accuracy fault. Complete failure minimum fault type showed the least emission values, while drift and wrong gain had emissions that were slightly higher than those of the normal operation case.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & & & & \multicolumn{3}{c}{**Daily Operation Costs (EUR)**} & & \\ \cline{2-9}
**Source/Technology** & \begin{tabular}{c} **Normal** \\ **Operation** \\ \end{tabular} & **Bias** & **Drift** & **Wrong Gain** & \begin{tabular}{c} **Loss of** \\ **Accuracy** \\ \end{tabular} & **Fixed Value** & \begin{tabular}{c} **Complete** \\ **Failure** \\ **Minimum** \\ \end{tabular} & \begin{tabular}{c} **Complete** \\ **Failure** \\ **Maximum** \\ \end{tabular} \\ \hline \
\begin{tabular}{c} Lignite \\ Coal \\ CCGT \\ Nuclear \\ Onshore wind \\ Offshore wind \\ Offshore wind \\ Offshore wind \\ Solar PV \\ commercial \\ Solar PV \\ residential \\ Solar thermal \\ Solar thermal \\ (CSP) \\ Hydro reservoir \\ Hydro run of river \\ Geothermal \\ Biomass \\ \\ \end{tabular} &
\begin{tabular}{c} 1671 \\ 1612 \\  
Assessment of the total N\({}_{2}\)O emissions for the fixed value, complete failure maximum and loss of accuracy faults also reveal increased values for the N\({}_{2}\)O emissions. They are characterized by a factor ranging from 1.4 to 1.7 when compared to the normal operation case. It may be noticed that, contrary to the total CO\({}_{2}\) emissions observed trend, complete failure minimum fault type led to increased values of the N\({}_{2}\)O emissions. N\({}_{2}\)O emissions produced due to bias, drift and wrong gain faults were marginally smaller when compared to the normal reference case, especially due to the decreased values of the off-site N\({}_{2}\)O emissions.  
The overall CO\({}_{2}\) emission values were computed by cumulating the total CO\({}_{2}\) and total N\({}_{2}\)O (as CO\({}_{2}\) equivalent) emissions. According to the results of the overall CO\({}_{2}\) emission, all faults show higher CO\({}_{2}\) emission levels than the normal operation case. The only exception is the complete failure minimum type of fault, but in this particular case, the effluent quality deteriorates by more than 15%, and the sum of the aeration and pumping energy has the highest values of all investigated cases. Assessment of the CO\({}_{2}\) and N\({}_{2}\)O emissions, issued due to different DO sensor faults, provides valuable quantitative information on the extent and ranking of the most unfavorable sensor fault types that may affect the sustainable operation of the WWTP.  
The results validate the logical assumption that the presence of all faults is detrimental to the WWTP operation, and the severity assessment of their consequences has to be considered in an integrated approach of energy, costs, water effluent quality and GHG issues. These evaluations constitute the foundation for the control and safety systems design aimed at achieving the plant sustainability objective.