## 3 Results and Discussion  
### Pilot-Scale Trickling Filter Experiments  
Although the biological treatment of WW has been tested mainly in suspension processes [2; 9; 47; 48], promising results have recently been demonstrated when using attached growth systems [2; 13; 14; 16; 49]. The pilot-scale trickling filter was initially inoculated with enriched microorganisms and operated as a batch reactor until their attachment onto the support material. The operating mode with 0.5 L/min recirculation was then examined. The application of this mode provided DO concentrations constantly above 4.0 mg/L without an external oxygen supply, thus reducing drastically the operational cost of the system. The first experiments were performed with an initial WW feed concentration of 3500 mg/L and a recirculation rate of 0.5 L/min. A series of operating cycles was performed until the system reached a constant, efficient performance and recorded a maximum degradation rate of d-COD for at least three cycles. After these operating cycles, a minimum period of 24 h was needed to reach the maximum degradation rate of C\({}_{3500}\) (Figure 2a). The same procedure was followed for the feed concentrations of 7500, 9000 and 18,000 mg d-COD/L (Figure 2b-d) and minimum operating cycle durations of 48, 54 and 64 hours, respectively, were achieved. The effect of volumetric flow rate on the system's performance was also studied, by examining the values of 1 and 2 L/min. It should be noted that with a recirculation rate of 2 L/min the filter was not able to operate sufficiently due to detachment of biofilm from the support material and very low d-COD removal rates (data not shown). Data from the three last cycles of the set of experiments with 0.5 and 1.0 L/min are presented in Figure 3.  
The data presented in Figure 3 clearly show that the trickling filter operated successfully for both recirculation rates (0.5 and 1.0 L/min) as high d-COD removal (above 90%) was achieved for all initial d-COD concentrations tested. Figure 3 also depicts the positive effect that increasing the recirculation rate from 0.5 to 1.0 L/min had on the filter's performance for all feed d-COD concentrations tested. Specifically, for the volumetric flow rate of 0.5 L/min and concentrations of C\({}_{3500}\), C\({}_{7500}\), C\({}_{9000}\) and C\({}_{18,000}\), percentage removals of 90.06%, 90.85%, 90.13% and 90.89% were achieved, respectively, while for the higher volumetric flow rate of 1.0 L/min and the same initial d-COD concentrations, increased percentage removals were recorded: 94.68%, 93.72%, 93.22% and 94.17%, respectively (Figure 3). For the volumetric flow rate of 0.5 L/min the maximum degradation rate of 256 mg d-COD/(L-h) was achieved for C\({}_{18,000}\), while for C\({}_{3500}\), C\({}_{7500}\) and C\({}_{9000}\), the recorded degradation rates were 138.96, 139.45 and 148.77 mg d-COD/(L-h). For the volumetric flow rate of 1.0 L/min, the maximum degradation rate was also reported for the high feed concentration of C\({}_{18,000}\) (284.61 mg d-COD /(L-h)), while for C\({}_{3500}\), C\({}_{7500}\) and C\({}_{9000}\) the degradation rates were 142.41, 152.32 and 155.64 mg d-COD/(L-h), i.e., higher than those observed with the volumetric flow rate of 0.5 L/min. The positive effect of the recirculation rate on d-COD removal in similar packed bed reactors was also reported by [PERSON] et al. [50] and [PERSON] et al. [51]. It is possible that higher recirculation provides a more favorable environment within the trickling filter by ensuring a better distribution of microorganisms, dissolved oxygen and nutrients.