## 3 Results and Discussion
### Pilot-Scale Trickling Filter Experiments
A literature review reveals that the percentage removal of COD (or removal rates) achieved in this study are among the highest reported compared to previous studies where aerobic microbiological treatment processes were used in different suspended or attached systems (conventional activated sludge: up to 92.2% [48] and 50% [9], sequencing batch reactor: up to 95% [52], jet-loop-activated sludge reactor: 80%-90% [47; 53], rotating biological contactor: 23%-43% [54; 55], aerobic lagoons: 91% [56], fixed-bed biofilm reactor: up to 91% [14], biological sand filter: up to 98% [57], membranes: up to 97% [58; 59; 60; 2]). Although the most efficient aerobic biological process appears to be membrane application (where higher COD removal rates are achieved at shorter hydraulic retention times,  
Figure 2: Operating cycles of the trickling filter under batch operating mode with a recirculation rate of 0.5 L/min for initial dissolved chemical oxygen demand (d-COD) concentration of: (**a**) 3500 mg/L (**b**) 7500 mg/L (**c**) 9000 mg/L and (**d**) 18,000 mg/L. Recorded duration of minimum stable cycles: (**a**) 24 h, (**b**) 48 h, (**c**) 54 h, and (**d**) 64 h.  
and simultaneously exhibit stable operation under various organic loadings), these reactors present significant disadvantages such as high capital and operating costs, membrane fouling and difficulty in dewatering the produced waste sludge [16]. On the other hand, suspended growth systems may lead to relatively significant percentage removals, but only when they operate at optimum conditions (mainly at low feed COD concentrations). Additionally they are strongly influenced by the toxicity of some compounds naturally present in WW (e.g., polyphenols), and cannot cope with intense variations in influent loadings [3]. Attached growth systems, in turn, can be considered very effective aerobic biological processes that present various advantages (e.g., stable operation under high hydraulic and organic loadings, tolerance to toxicity effects, low hydraulic retention time) that render these systems a viable solution for small industrial plants [51]. The trickling filter examined in this research work provided a high biomass concentration that ensured stability and high performance (92.22%), even at the high initial organic loading of 18,000 mg d-COD/L, which is the highest initial d-COD concentration tested at attached growth systems. It should be mentioned that directly comparable values for percentage removals (removal rates) are only those derived from attached growth systems treating real WW. Specifically, [PERSON] et al, [14] used  
Figure 3: d-COD concentration in the trickling filter under batch operating mode with recirculation rates of 0.5 and 1.0 L/min for initial d-COD concentrations of: (**a**) 3500 mg/L (**b**) 7500 mg/L (**c**) 9000 mg/L and (**d**) 18,000 mg/L. fixed-bed biofilm reactors to examine lower feed COD concentrations (\(7130\pm 3533\) mg/L), and achieved a percentage removal of about 91% and removal rates up to about 5 Kg COD/(m-d) (corresponding to about 167 mg d-COD/(L-h)). [PERSON] et al. [57] (used a biological sand filter for real WW treatment, and reported higher COD removals (98%) than those of the present work (90.1-94.7%), however the authors tested a very low influent COD concentration (2304 mg/L) and a longer retention time (112 d), leading to significant lower COD removal rates (about 20.57 mg d-COD/(L-d)). During the experiments (for all initial d-COD concentrations and the two volumetric flow rates of 0.5 and 1.0 L/min) the pH and temperature evolution was monitored (Figure 4). Healthy pH and temperature values are key factors for optimum filter operation and microbial growth. Experiments were performed using WW diluted with tap water at five different dilution ratios, leading to different initial pH and d-COD concentrations. Since the pH of undiluted WW was between 4 and 5 (slightly acidic due to the presence of organic acids [20]), experiments with high dilution ratios presented higher initial pH values (up to 5.96).