## 4 Discussion and Conclusions
Indeed, four of the 28 samples collected during the monitored period evidenced the need for the FWS 3 rd stage to respect the limit for discharge in terms of COD. Only one time the system needed also the sand filter to make the effluent in agreement with the required water quality. The multistage CW WWTP of Cecchi winery performed with an overall mean COD removal efficiency of 97.5%, confirming the importance to use multistage CWs for the treatment of high strength winery wastewater in case of stringent effluent targets, in agreement with [PERSON] et al. [5]. The monitored data evidenced the suitability of the online sensor in catching the effluent COD patterns. This is giving practical help to the CW WWTP owner, which has an instrument to continuously monitor the performance and to manage the optional recirculation of FWS 3 rd effluent toward the sand filter. In terms of full precision, the results cannot lead to general and definitive conclusions. The fullprecision was definitely good, with low mean error (\(-17\%\)) and with the majority of the error being less than \(\pm 50\%\). To maintain good performance, the CW WWTP owner follows a strict maintenance procedure. This confirms the need for strict protocol for the use in online sensors and to maintain the reliability of the sensor measurement, in agreement to what was evidenced by [PERSON] et al. [17] for using online sensors to monitor nitrogen pollutants. On the other hand, the range of errors and the greater number of underestimations in comparison to overestimation seem to suggest to avoid a full substitution of manual sampling with an online sensor for the monitoring of the CW WWTP performance. Therefore, the possibility to reduce the operational and maintenance costs of CW WWTPs by substituting water quality samples expenditure with online monitor, as suggested by [PERSON] et al. [16], seems to be viable in case of CWs treating industrial wastewater, even though in several countries the related, more stringent water quality standards of the effluents (as in this case) will anyway require a minimum level of analysis with legal validity. The main advantages offered by the online and continuous data acquisition are linked to several opportunities, as for instance the design of smart CW WWTPs where sensors are couples to information technologies (ITs) that can be programmed for starting active aeration of the reactor, recirculation, prolonged or shortened HRTs, different HLRs, and so on. In terms of operation, the obtained results at Cecchi WWTP are very interesting because they demonstrate, by comparison with simultaneous samples analyzed in the laboratory, that sensors are able to properly identify the highest peaks and fluctuations of the substance concentrations. This kind of continuously updated information is extremely relevant for performance optimization, for understanding the behavior of the WWTP in different operative scenarios, and finally for driving the powering on or off of the eventual process enhancement tools. These conclusions need to be confirmed by a greater number of full-scale CW WWTPs equipped with online sensors and monitored, especially for the optimization of the maintenance protocol and the estimation of the confidence level of the instrument. The use of online sensors are hoped also to monitor interesting application of CWs other than wineries, as well as conventional municipal wastewater from medium-big scale CW WWTPs, but also to monitor the performance under other complex conditions, such as high hydraulic load variation (e.g., combined sewer overflow, or stormwater) or different industrial wastewater. [PERSON] elaborated the data, made the analyses, and wrote the manuscript; [PERSON] and [PERSON] were the designers of the WWTP, contacted the owner of the winery, and collected the data; [PERSON] was the Technical Director of the WWTP, and revised the manuscript from the point of view of both scientific validity (robustness of references and discussion) and English readability.