## 4 Conclusions  
The performance of the FDA-based sensor faults identification approach was proposed and assessed for the key DO sensor belonging to the automatically controlled A\({}^{2}\)O WWTP with the typical dissolved oxygen and the nitrates and nitrites concentration control loops. The DO feedback control system performance is responsible for the WWTP nitrification process, directly determining the spent energy, effluent quality and GHG emissions of the entire plant. The FDA fault identification was investigated for six different types of faults. They were: bias, drift, wrong gain, loss of accuracy, fixed value and complete failure minimum and complete failure maximum faults. The data sets of variables for both the normal \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Emissions Type**} & \multirow{2}{*}{**Source/Technology**} & \multirow{2}{*}{**Emitted Gas**} & \multicolumn{6}{c}{**Daily GHG Emissions**} \\ \cline{3-10}  & & & & **Normal** & **Bias** & **Drift** & **Wrong** & **Loss of Gain** & **Fixed Value** & **Complete Failure** & **Complete Failure** \\ \hline \multirow{4}{*}{\begin{tabular}{c} Off-site \\ emissions \\ \end{tabular} } & \multirow{4}{*}{\begin{tabular}{c} Power generation \\ \end{tabular} } & CO\({}_{2}\), & \multirow{4}{*}{3481} & \multirow{4}{*}{3158} & \multirow{4}{*}{3290} & \multirow{4}{*}{3317} & \multirow{4}{*}{2195} & \multirow{4}{*}{833} & \multirow{4}{*}{4669} & \multirow{4}{*}{833} \\  & & CO\({}_{2}\)/day & & & & & & & & \\ \cline{1-1} \cline{6-10}  & Biological & N\({}_{2}\)O, & \multirow{4}{*}{3.61} & \multirow{4}{*}{2.47} & \multirow{4}{*}{2.89} & \multirow{4}{*}{2.97} & \multirow{4}{*}{14.39} & \multirow{4}{*}{21.49} & \multirow{4}{*}{6.50} & \multirow{4}{*}{21.49} \\  & degradation in the & \(P_{\text{N}_{2}\text{O}/day}\) & & & & & & & & \\ \cline{1-1}  & WWT effluent & & N\({}_{2}\)O/day & & & & & & & \\ \hline \multirow{4}{*}{\begin{tabular}{c} On-site \\ emissions \\ \end{tabular} } & \multirow{4}{*}{\
\begin{tabular}{c} Water-line aerobic \\ biological \\ processes \\ \end{tabular} } & CO\({}_{2}\), & \multirow{4}{*}{13,689} & \multirow{4}{*}{30,459} & \multirow{4}{*}{17,851} & \multirow{4}{*}{16,178} & \multirow{4}{*}{461,439} & \multirow{4}{*}{921,028} & \multirow{4}{*}{10,604} & \multirow{4}{*}{921,168} \\  & & & & & & & & & \\ \cline{1-1}  & & N\({}_{2}\)O, & \multirow{4}{*}{10.35} & \multirow{4}{*}{10.81} & \multirow{4}{*}{10.64} & \multirow{4}{*}{10.05} & \multirow{4}{*}{6.07} & \multirow{4}{*}{3.27} & \multirow{4}{*}{9.20} & \multirow{4}{*}{3.27} \\  & & N\({}_{2}\)O/day & & & & & & & \\ \hline \multirow{4}{*}{
\begin{tabular}{c} Total emissions \\ \end{tabular} } & CO\({}_{2}\), & \multirow{4}{*}{17,170} & \multirow{4}{*}{33,617} & \multirow{4}{*}{21,141} & \multirow{4}{*}{19,495} & \multirow{4}{*}{463,634} & \multirow{4}{*}{921,861} & \multirow{4}{*}{15,274} & \multirow{4}{*}{922,001} \\  & & CO\({}_{2}\)/day & & & & & & & \\ \cline{1-1}  & & N\({}_{2}\)O, & \multirow{4}{*}{13.96} & \multirow{4}{*}{13.28} & \multirow{4}{*}{13.53} & \multirow{4}{*}{13.02} & \multirow{4}{*}{20.46} & \multirow{4}{*}{24.76} & \multirow{4}{*}{15.70} & \multirow{4}{*}{24.76} \\ \cline{1-1}  & & N\({}_{2}\)O/day & & & & & & & \\ \cline{1-1}  & & CO\({}_{2}\), & \multirow{4}{*}{21,330} & \multirow{4}{*}{37,574} & \multirow{4}{*}{25,173} & \multirow{4}{*}{23,375} & \multirow{4}{*}{469,731} & \multirow{4}{*}{929,239} & \multirow{4}{*}{19,953} & \multirow{4}{*}{929,379} \\ \cline{1-1}  & & & & & & & & & \\ \cline{1-1}  & & & & & & & & & \\ \cline{1-1}  & & & & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: GHG emissions due to DO sensor defect. and fault-affected operation of the DO sensor originated from simulations of the calibrated plant model in which the scenario of WWTP influent variations emerged from plant measurements. The prompt and reliable FDA fault identification methodology was successful.