## 3 Results
### PF200 Operation and Comparative Tests  
The total duration of the pilot's operation was equal to 5 months. Turbidity was measured at three points--at the inlet (raw water taken from the adjacent river), after the clarifier, and after the filter. The extensive duration of the tests (1100 h total operation time) combined with a high sampling frequency of once per second resulted in a significant amount of data. A convenient way to represent this information is as a frequency distribution on a percentile plot. This removes the influence of the periodic variations of the analyzed process parameters, allowing for a comparison with the data collected by the SCADA system at the CWFP. Data downloaded from SCADA for the same period are also converted to the percentile plot and directly compared with the results for PF200.  
Over the period of the tests, the average dosing of PACI coagulant was equal to 18.15 ppm, equivalent to 1.02 mg\({}_{\rm{AJ}}\)/L, with the minimum and maximum dose over that period ranging from 13 to 50 ppm. Raw water turbidity in that period averaged 12.2 NTU, ranging from 0.24 to 500 NTU. Sludge blanket clarification resulted in up to a 10-fold decrease in turbidity (1-log removal efficiency according to the definition established by the EPA). During the experiments, on average, it resulted in a turbidity of 0.2 NTU, and it ranged from the detection limit of the system at 0.02 NTU to 2.1 NTU. Stacked rapid sand filters provide up to another 1-log turbidity removal. Consequently, the overall turbidity decrease over the entire system approaches 99%. However, these values could only be  
Figure 4: Clean bed head loss (**top**) and clean bed log turbidity removal (**bottom**) are used as potential indicators for the filter bed cleanliness; no correlation between the backwash time and the head loss was observed, and optimal backwash time was set as the backwash time needed for clean bed particle concentration to be equivalent to influent turbidity (here called the clean bed turbidity removal).  
achieved in conditions of high raw water turbidity. Over the 5 months of operation, the filter produced water meeting a requirement of less than 0.3 NTU only 75% of the time, while the US EPA legislation requires turbidity below 0.3 NTU for 95% of the time and below 1 NTU for the remaining part of the operation.  
For the evaluation against CWFP, the turbidity removal efficiency of both systems is used to assess their performance. In comparison, the performance of the full-scale CWFP system is superior to the tested pilot at each stage of the process. Over the period of experiments, the clarifier reliably allowed for more than a 1-log decrease of turbidity, while the filter served as a polishing step, bringing the final treated water turbidity to the range from the detection limit of 0.02 NTU to 0.12 NTU, with an average value of 0.05 NTU. This performance corresponded to CWFP meeting the EPA turbidity standards 100% of the time. Figure 5 shows the performance comparison between the two systems, revealing that the water quality at each stage of the pilot process is inferior to the water quality of the benchmark conventional system. It is important to keep in mind that CWFP operates below its design capacity and has a longer flocculator residence time, a long track record of good performance, and well-established good operation practice compared to the PF200.