## 3 Results
### Online COD Sensor  
The temporal series of influent and effluent COD concentrations are reported in Figure 4, together with the COD effluent concentration registered by the online sensor every 30 min for almost all the whole monitored period. As visible from Figure 4, the online sensor gave excellent results in following the increasing and decreasing patterns of effluent COD concentration. For instance, the online sensor properly measured the sharp increase (but still far below the effluent COD water quality target of 160 mg L\({}^{-1}\)) of effluent COD concentration observed from November 2017 to December 2017. Similarly, \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Pollutant**} & \multirow{2}{*}{**IN—OUT**} & **OUT HF** & **OUT HF** & **OUT HF** & **OUT** & **OUT** \\  & & **FRB** & **1—OUT** & **FRB—OUT** & **1—OUT** & **FWS—OUT** & **IN—OUT** \\ \hline pH & \(t\) value & \(<\)0.01\% & _39.68\%_ & _5.02\%_ & _0.05\%_ & _9.10\%_ & **\textless{0.01\%**} \\ \hline COD & \(t\) value & \(<\)0.01\% & _27.99\%_ & \(<\)0.01\% & _4.01\%_ & _0.17\%_ & **\textless{0.01\%**} \\  & Relative & 70.23\% & & & 72.46\% & 50.09\% & 49.27\% \\  & Absolute & 70.23\% & & & 21.57\% & 3.05\% & 2.43\% & **97.50\%** \\  & rem. eff. & & & & & & \\ \hline TP & \(t\) value & 42.18\% & _66.14\%_ & _10.76\%_ & _12.80\%_ & _0.28\%_ & **0.25\%** \\  & Absolute & & & & & & & **45.55\%** \\  & rem. eff. & & & & & & & **84.65\%** \\ \hline N-NO3- & \(t\) value & _0.04\%_ & 50.30\% & _0.01\%_ & _18.82\%_ & _0.32\%_ & **0.32\%** \\  & Relative & 40.45\% & & & 43.71\% & & & \textless{100.19\%} & **39.94\%** \\  & Absolute & & & & & & & \\  & rem. eff. & 40.45\% & & & 26.03\% & & \textless{30.06\%} & \\ \hline MBAS & \(t\) value & \(<\)0.01\% & 65.44\% & _0.68\%_ & _1.14\%_ & _0.73\%_ & **\textless{0.01\%**} \\  & Relative & 73.45\% & & & 36.15\% & 11.77\% & 42.09\% & \\  & Archolute & 73.45\% & & & 9.60\% & 1.73\% & 5.01\% & **93.11\%** \\  & rem. eff. & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Results of _t_-test (expressed in percentage and italics) and removal efficiencies (absolute in comparison to influent wastewater and relative among the stages) of the CW of Cecchi winery: influent (IN); effluent from the 1 st stage FRB beds (OUT FRB); effluent from the 2 nd stage refurbished HF bed (out HF 1); effluent from the 2 nd stage HF beds built during the 2009 upgrade (OUT HF 2); effluent from the 3 rd stage FWS bed (OUT FWS); effluent from the official effluent sampling point, after the optional sand filter (OUT fin). Data from 19 January 2017 to 14 June 2019. Statistically significant results (\(<\)5%) are evidenced by a gray background cell. Results of the overall CW WWTP are reported in bold. the subsequent slight decreasing pattern until June 2018 was properly caught. The maintenance procedure also guaranteed good performance in time as visible, for instance, by the proper measurement of a slight increase of COD concentration from June 2019 to July 2019. The qualitative goodness of the online sensor measurement is also confirmed by the analysis of errors between the sampled COD effluent concentration and the daily mean COD effluent concentration monitored by the online sensor. On the 26 samples available for direct comparison, the error range was \(-\)160%-+60%, with a mean error of \(-\)17% \(\pm\) +62%. More relevant, 73% of the samples fallen within the error range \(-\)50%-+50% (Figure 5). It must be noted that the monitored effluent concentration was very low, with a mean value of \(29\pm 19\) mg L\({}^{-1}\), i.e., often near the bottom measurable threshold for COD measurement (10 mg L\({}^{-1}\)). On the other hand, the frequency density function of the errors (Figure 5) evidences a more frequent underestimation of effluent concentrations. Figure 4: Samples and online sensors results for the COD monitoring of the CW of Cecchi winery.