Seasonal variations of COD, TN, and NH
4+-N removal efficiency and distributions of NOx (NO
3−-N and NO
2−-N) at different treatment stages of the studied hybrid system during summer and winter seasons is presented in
Figure 3. The average concentration of inflow COD was almost similar during both seasons but total removal efficiency was significantly higher (
p < 0.05) during summer (88% ± 6%) that in winter (77% ± 11%). The present COD removal efficiency during both seasons was much higher than the full-scale AA hybrid system reported by Pan et al. [
7]. Compared with the other treatment stages, the first treatment stage (ST unit) contributed to the highest COD reduction during both seasons. The same phenomenon was also reported by the other researchers studied on hybrid systems that consisted of multiple treatment stages [
30]. However, due to the high COD removal in ST unit during summer (43% ± 19%) in contrast with winter (27% ± 17%), total COD removal efficiency was significantly better during summer. Otherwise, COD removal rates in all treatment units showed similar performance during both seasons. In fact, in wetland beds (VBFW and HSFW), COD removal efficiencies were slightly higher during winter because of the low removal rate in pre-treatment stages.
AA influenced the OT unit to perform better COD reduction after the ST stage. Moreover, the MA-VBFW contributed more to the COD reduction than the HSFW because of AA [
7]. Previous studies also reported that various aeration strategies significantly increased COD removal in various types of full-scale and lab-scale CWs [
2,
7,
10,
27]. Organic removal in CWs generally occurs through the sedimentation, filtration, interception, and microbial degradation in both aerobic and anaerobic conditions [
5], but aerobic degradation is predominant for dissolved COD removal [
6]. AA assists in improving mixing and increases aerobic condition in CWs, thus enhancing COD removal especially in winter [
10,
13,
27].
Nitrogen was found mainly in the form of NH
4+-N. Inflow NH
4+-N on average accounted for 80% ± 4% and 78% ± 6% of TN in warm and cold periods, respectively. On the contrary, outflow NH
4+-N concentrations were found to be 39% ± 19% and 63% ± 14% of TN during warm and cold seasons, respectively. The inflow TN and NH
4+-N were almost similar during both seasons but the outflow concentrations were significantly higher during winter (
p < 0.05), suggesting that the low temperatures had a negative effect on both TN and NH
4+-N removals.
Figure 3b,c represented stepwise mean removal efficiencies of TN and NH
4+-N in the investigated hybrid system during high and low temperatures. NH
4+-N removal efficiency was found to be higher than TN removal in both seasons. TN and NH
4+-N removal efficiencies during the warm period were 73% ± 6% and 86% ± 8%, respectively, whereas during cold period 56% ± 12% and 63% ± 18%, respectively. Compared with the cold period, the average removal efficiencies of TN and NH
4+-N were significantly increased (
p < 0.05) during summer of approximately 17% and 23%, respectively. Maximum TN and NH
4+-N removals were observed in the OT and MA-VBFW units during both seasons where AA was applied, which suggests that the aeration had a positive influence on nitrogen removal [
2,
5,
9,
10]. In addition, the HSFW performed significantly in removing TN during both seasons (17% ± 9% summer and 13% ± 5% winter). However, NH
4+-N removal efficiency at the HSFW stage was higher in winter (10% ± 5%) than that in summer (4% ± 9%). This was due to the comparatively low NH
4+-N inflow concentrations (9 mg/L) in the HSFW stage and high NH
4+-N oxidations in previous OT and VBFW stages during summer. However, the HSFW significantly decreased NO
3−-N (summer from 10 ± 1 to 5 ± 1 mg/L and winter 8 ± 3 to 5 ± 2 mg/L) concentrations during both seasons (
Figure 3d). Since then, the inflow NOx concentrations were less than 1 mg/L throughout the experimental period, but the OT and MA-VBFW units significantly increased NO
3−-N concentrations and limited increase of NO
2–-N. These phenomena indicated that the proper nitrification occurred in these steps due to AA [
6,
9,
10]. In contrast, the HSFW subsequently decreased NO
x (mostly NO
3−-N) concentrations, which suggested that denitrification occurred in the HSFW stage [
10]. However, the outflow NO
3−-N concentrations of the wetland during both seasons indicated an incomplete denitrification in the HSFW stage [
10] which can be attributed to the lack of available organic carbon sources [
9,
31].
In addition to the microbial nitrification and denitrification, ammonia volatilization especially in the OT unit, plant nitrogen uptake and ammonia adsorption in the VBFW and HSFW units also contributed to the TN removal in the studied system [
2,
6,
8,
9]. The average pH measured at all treatment stages of the wetland was 7.2 ± 0.1 during both seasons, which should therefore limit the contribution from ammonia volatilization [
8,
29]. Plants uptake can significantly remove nitrogen in CWs (around 20% of total nitrogen removal) [
6,
9]. Vymazal and Kröpfelová [
30] reported that plants uptake in experimental multistage hybrid CW treated municipal sewage (saturated vertical flow, free-drain vertical, and horizontal flow units in series) can remove up to 26% of the nitrogen inflow load. The current study showed that AA improved nitrification in the OT and MA-VBFW stages and the subsequent denitrification in the HSFW stage during both summer and winter seasons. Generally, TN removal efficiency in CWs varied between 40% and 70% [
8,
27]; considering this range, the nitrogen removal in the studied system during both seasons was good.