## 3 Findings and Discussion  
### Effects of the Initial Organic Concentration on the Treatment Efficiency of CW  
The findings showed that in all the first three runs of the experiment, most of the selected water quality parameters, namely UV-254, DOC, sCOD, NH\({}_{3}\)-N experienced a significant decrease  
Figure 4: The layout of the prototype of a combination system (CW+PCR). (**1**) Influent tank; (**2**) Peristaltic pump; (**3**) Influent pipe of CW; (**4**) Constructed Weltand; (**5**) Mixture Sand and Soil; (**6**) Gravel; (**7**) Baffle; (**8**) Plants; (**9**) Influent pipe of PCR; (**10**) Beaker 1 L; (**11**) TiO\({}_{2}\)/\(\alpha\)-Al\({}_{2}\)O\({}_{3}\) balls; (**12**) Magnetic stir bar; (**13**) UVA lamp; (**14**) Magnetic stirrer; (**15**) Box covered with aluminium foil; (**16**) Effluent pipe; (**17**) Effluent tank. in concentration after treatment in the CW, with removal efficiencies in the ranges of 55.2-80.8%, 28.1-71.9%, 72.1-79.7% and 60.1-61.7%, respectively (as shown in Figure 5). The removal efficiencies of UV-254 and DOC (as organic matter (OM) indicators) rose with an increase of the initial DOC concentration. At the lowest initial concentration of DOC (3-4.5 mg/L - Run 1), the efficiency in removal of OMs was relatively low (ranging 55.2 \(\pm\) 2.4% and 28.1 \(\pm\) 2.4% for UV-254 and DOC respectively) because most of the removed OMs were compensated by soluble microbial products released by microorganisms [25; 26]. On the contrary, in Run 2 and Run 3, the CW showed better performance in the removal of OMs, with 68.2 \(\pm\) 4.0% and 41.8 \(\pm\) 2.1% in Run 2, followed by 80.8 \(\pm\) 0.6% and 71.9 \(\pm\) 1.5% in Run 3, for UV-254 and DOC, respectively. Also seen from Figure 5, sCOD removal efficiencies had a slightly decreasing tendency from 79.7% to 79.3% and to 72.1% for Run 1, Run 2, and Run 3, respectively. An explantion for this may be that through the last time of operation, plants roots may be decomposed and dissolved and therefore sCOD of effluent may increase to a higher value at the end of this experiment [27]. From the literature review, it is clear that the treatment process of CW is based on a complex number of biological and physical processes such as adsorption, precipitation, filtration, nitrification, decomposition, plant uptake, etc. Therefore, it can be explained that the reduction of OM in this study was achieved by these above mechanisms. Microorganisms attached in the substrates (soil, sand, and gravel) and in the plant root had responsibility on the degradation of labile OM through aerobic, anoxic, and anaerobic processes [28], while stable OM fractions may be accumulated on the surface of substrates and/or biofilm through an adsorption process. In terms of the adsorption process, the reduction of non-biodegradable organic contaminants in the influent could be contributed by the adsorption of biofilm and/or substrates in CW. In fact, as observation, the color of the substrate medium (sand) changed from white to light brown, indicating that there was some adsorption of HA onto the surface of substrates and/or biofilm, and clearly, adsorption was one of the main OM removal mechanisms in the CW, which was in accordance with the findings of [PERSON] et al. [25], who pointed out that the adsorption onto the biofilm was one of the NOM removal mechanisms in their experiments. Furthermore, as reported by [PERSON] [29], over 90% of the OM accumulated in a gravel-bed constructed wetland over a period of 5 years consisted of recalcitrant OM fractions, indicating that CW operation in treating stable OM was probably due to the accumulation of refractory OM in the gravel bed. Along with the substrates in the CW, plant roots might contribute to the adsorption of OM in the influent. According to the review of [PERSON] et al. [30], there are different hypotheses which favor the adsorption of humic substances by plant roots. The available evidence proved that HA was absorbed in the root surface or accumulated passively in the \"apparent free space\" [31].