# Constructed Wetlands for the Wastewater Treatment: A Review of Italian Case Studies
## 3 Discussion  
The application of CWs in wastewater treatment becomes more common in different parts of the world. Similarly, this review found that the number of published results in the first 11 years (2001-2011) was small in number than the following 12 years (2012-2023), showing how familiar the CWs technique becomes in Italy. Of the 76 articles reviewed, 2 were about laboratory experiments, 30 were large-scale experiments, and the remaining 44 referred to pilot projects. It has been reported that the pollutant removal efficiencies of all the CWs reviewed in this article showed remarkable potential, even though there are biotic/abiotic (physical, chemical, and biological processes) factor-driven performance variations among them. For instance, photolytic degradation, sorption, plant uptake, microbial degradation, type of CWs, plant type, operational mode, soil matrix, hydraulic retention time, hydraulic loading rate, research location, climate, etc. might have caused the observed removal efficiency differences among the CWs. Table 1 summarizes the types of CWs, locations, plants used, wastewater treated, pollutants, and their removal efficiency (RE%), which are the common operational parameters reported in all the 76 reviewed articles. Biotic and abiotic factors can have a significant effect on the pollutant removal efficiency of CWs. For instance, according to [57], nutrient removal is controlled by the pH of the treatment system in CWs. A reduction in the NH\({}_{4}\)\({}^{+}\)-N removal rate from \(7.8\pm 1.2\) g/m\({}^{3}\)/d to 6.4 \(\pm\) 1.3 g/m\({}^{3}\)/d corresponding to a reduction in pH from 8.1 to 7.6 was reported [58]. Similarly, research results indicated significant differences in wastewater treatment capacities among plants [59; 60; 61]. Some researchers [62] reported removals of ammonium and phosphate ions from a pig industry effluent that ranged between 59-84% and 32-92%, respectively, in CWs with _Phragmites australis_, and 62-75% and 7-68% in CWs with _Typha latifolia_. In the same manner, differences in the removal of pollutants among plants were also observed in this review. For example, the benzene removal potential (%) of a horizontal subsurface CW in South Italy showed different values using _Phragmites australis_ (39.78%) and _Typha latifolia_ (35.14%) [63]. The types of microorganisms present in a CW also affect the removal efficiency. Organic matter biodegradation is correlated to autotrophic and heterotrophic bacteria, fungi, yeast, and protozoa [64; 65]. The presence or absence ofthese microorganisms in the CWs of the reviewed Italian works possibly contributed to the observed performance variations. Temperature determines the rate of metabolic activities and impacts microbial populations [66]. Ref. [67] stated a significant (\(p<0.05\)) positive impact of temperature on the rate of organic matter degradation, nitrification, and denitrification processes in less time. The success of treatment in a CW often declines at cold temperatures, mostly due to decreased biotic activity [15]. For instance, the rate of ammonium oxidation is greatly reduced when the temperature drops below 10 \({}^{\circ}\)C [68]. Moreover, ref. [69] reported 7% and 9% improved removal efficiencies of ammonia nitrogen and nitrate nitrogen, respectively, in constructed wetlands in summer than in winter. The reviewed Italian case studies were conducted under different temperatures and seasons, which are expected to cause performance variations. Hydraulic retention time or the amount of time contaminants are in contact with the substrate and the rhizosphere of plants is widely known to be a significant controlling element in determining the effectiveness of pollutant removal [33; 70; 71]. Ref. [67] confirmed a decrease in organic matter (as BOD and total suspended solid) and nitrogen removal efficiency with a reduced hydraulic retention time because of less contact time of pollutants in the system. Ref.