**4. Discussions**

The evaluation of the main chemical properties of the SW treated in the combined system using lagoons and constructed wetlands has produced important indications in terms of depuration efficiency and, therefore, of its sustainability.

Concerning the depuration performance of the lagoon system, the decrease in the TSS concentration due to the lagoon process is well known [51]. The presence of a nonaerated tank promoted the activity of anaerobic bacteria, which degraded the organic matter concentration, and this also reduced the amount of the TSS [49,52,53]. Furthermore, the effect of aeration in the upstream tank promoted the flocculation process, due to the accelerated bacterial activity [52].

Although the reductions in the amounts of the OM and TKN, as a result of the lagoon system, were noticeable, the concentrations in the effluent noticeably exceeded the accepted amount for the discharge limits permitted by the main national rules. For example, according to the Italian environmental regulation (Legislative Decree 152 of 2006), the concentrations of nutrients and OM were about two orders of magnitude for the limits, equal to 20 mg/L for N and 160 mg/L for COD.

Concerning the depuration performance of the constructed wetland, the reduction in the pH of SW, which was close to neutrality, was in accordance with Boas et al. (2018) [54], who worked on CWs combining H-SSF and V-SSF systems, which favor the microbial activity of OM degradation and nutrient conversion.

The noticeable decline detected in the TSS concentration for both RRs is expected, since TSS is the water parameter that is strongly modified by CW treatments, as a consequence of the sedimentation, filtration, and adsorption processes that occur in CWs [33].

The COD and TKN removal in the CW were high, presumably due to the synergistic effects of both physical, chemical, and microbial processes, also under heavy loads of OM and nutrients, as is presented in this study. Organic and N compounds are removed in CWs of the SSF type by a combination of adsorption, nitrification/denitrification, volatilization, and ionic exchange [3]. Nitrification and denitrification are considered essential mechanisms for N removal [1,37], having an efficiency of more than 60% [55]. According to Vidal et al. (2018) [3], denitrification is the most effective process to remove nitrogen in CWs, and the aerated and non-aerated treatments prior to CW in the experimental plant have been beneficial for these processes. The aerated treatment has oxidized part of the TKN in SW, which was converted to nitrate. The aerated treatment should have denitrified part of this nitrate, but the remaining part was made available for denitrification in CW, which is nitrate-limited [3,27]. The nitrification of SW before the CW enhances N removal, and increases the nitrate available for denitrification [45]. Moreover, the absence of aeration should have provided anaerobic bacteria that were already adapted to the denitrification process. Denitrification is more desirable than ammonia volatilization in CWs treating the wastewater of animal origin, since ammonia is a pollutant for atmospheric, aquatic, and terrestrial environments through dry and wet deposition [45]. In our study, although not being directly measured, ammonia volatilization may have been limited, due to the pH level that was lower than eight [1,56,57], while the nitrification process should have been presumably low, due to the limited oxygen supply from the plants. Therefore, denitrification may have been the dominant process in TKN removal, in close accordance with Hunt et al. (2002) [27]. An important role in nutrient and OM removal is played by bacteria, which create a biofilm around the soil particles, allowing the catalysis of chemical reactions [33]. Effluent recirculation enabled the wastewater to flow repeatedly over this biofilm, enhancing the contact between the pollutants and microorganisms [33,34].

Plant uptake helps nitrogen removal, but its influence is lower compared to the other processes, and depends on the specific species. Plants remove ammonia nitrogen due to the stimulation of nitrifying bacteria and the uptake of nitrogen compounds [8], but these mechanisms seem to be marginal in many examples. *Typha latifolia* L. prefers slightly acidic environments, but ammonium uptake is conditioned by its toxicity (>0.2 g/L) [23] and COD concentrations of 0.6–0.8 g/L (that inhibit photo-synthesis and, consequently, nutrient incorporation) [23,58], as was evident in many stages of our study. Gonzales et al. (2009) [36] stated that the macrophyte species did not significantly contribute to the overall efficiency of V-SSF CWs in N removal, especially in the dry season. These authors attributed this minor contribution of plants to the high concentrations of contaminants. In contrast, planted CWs clearly show higher efficiencies for organic compounds, with removal efficiencies of up to 70% in wetlands planted with *Typha latifolia* L. compared to 60% of unplanted beds [36].

In relation to the variations in SW parameters and the removal of pollutants, the present study has demonstrated that the CW was more efficient in removing TSS, COD, and TKN compared to the lagoon. The lower efficiency for TSS removal in the lagoon system compared to the CW can be attributed to the grea<sup>t</sup> solid content of the raw SW. This low efficiency is close to the value of 25% experienced by Stone et al. (2004) [59] for SW lagoon treatment in North Carolina. To increase the system ability to remove TSS, a pre-treatment to remove further amounts of TSS in the raw SW is still necessary because it can prevent the soils of CW from being rapidly clogged. The very high efficiencies of the CW system in removing TSS are in close accordance with the values (97–99%) reported by [14,33]. In the experiences of other authors, TSS removal efficiencies between 40–50% [3] and 70–80% [35,36] were detected. Literature reports COD removal efficiencies in the range

of 50–80% [33,35,36,60] with an extreme value of 99.5% detected by Masi et al. (2017) [14] working in a CW combined system (as in our study). N removal varies between 60% and 80% according to many authors [3,8,11,27,36,54], but extreme values are also reported (10–40% [33,35] to 99% [14]). In our study, the TKN removal efficiency for the CW system with RR = 4:1 is in the range reported by the majority of studies, and it is not far from the optimal value of [14] in the CW with RR = 10:1.

The analysis of the depuration efficiency of the combined system (lagoon + CW under the two RRs) suggests the adoption of an RR equal to 10:1, in order to increase the TKN removal, while the efficiency of reducing the pH and EC, and removing the TSS and COD, is comparable. Similar to the observations of Lee et al. (2006) and He et al. (2004) [33,34], the effluent recirculation in the system supplies a considerable amount of oxygen in the SW, promoting the reductions in COD and TKN. Concerning the experiences using V-SSF-CWs systems with recirculation, He et al. (2006) [34] showed that this operation strategy increased the average removal efficiencies of NH4-N, COD, and TSS to 62%, 81%, and 77%, respectively, compared to the values of 36%, 50% and 49%, without effluent recirculation. With an RR of 100%, the average removal efficiencies were 91% for COD and 96% for TSS [61].

Regarding the effects of COD and TKN on *Typha latifolia* L., the irrigation of the CW with SW effluents from the lagoon treatment did not affect plant survival in the dry season, especially at the higher RR. In contrast, the higher plant mortality detected in the CW with the lower RR can be attributed to a peak in the nitrogen load, which exceeded the tolerance limits of *Typha latifolia* L. These limits were quantified by De los Reyes et al. (2014) [7] between 0.2 and 0.4 g/L of NH4 +-N, which correspond to 60–80% of TKN, and therefore the expected phyto-toxic effects may have been realistic.
