# Constructed Wetlands for the Wastewater Treatment: A Review of Italian Case Studies
## 1 Introduction  
Environmental degradation can be caused by both anthropogenic and natural sources of pollution. Anthropogenic pollution associated with the industrial and agricultural sectors, for instance, is contributing immensely to environmental deterioration, especially in the aquatic ecosystem [1, 2]. Domestic and municipal wastewater, sewage from wastewater treatment plants, urban runoff, livestock wastewater, stormwater, and landfill leachate are other major sources of pollution to the aquatic environments. If wastewater coming out from these sources is released into a natural water body without proper treatment, it results in an algae bloom [3, 4] that affects aquatic biodiversity [5]. Moreover, it can contaminate soil and groundwater, endangering human health [6]. Consequently, the remediation of polluted water is vital to both reduce such risk and increase clean water availability. Indeed, as recently highlighted by [7], wastewater treatment could contribute to achieving 11 out of 17 sustainable development goals (SDGs) adopted by the United Nations, considerably reducing the global water crisis. Nowadays, the use of green technologies for such purposes is increasing due to (i) their ability to reduce pollution without compromising environmental sustainability and (ii) low implementation and maintenance costs [8]. Among these emerging green technologies, phytoremediation is being recognized as a promising, low-risk, and environmentally friendly in situ clean-up method, where plants are used to decontaminate the environment by eliminating, holding, or providing nontoxic contaminants in soil or water [9, 10, 11]. Phytoremediation was successfully usedin constructed wetlands (CWs), an artificially built pollutant removal method that utilizes the combined contribution of substrates, macrophytes, and microbial community [9]. CWs are designed and built engineering systems that use the natural processes of emergent/floating/submerged wetland plants, saturated or unsaturated substrates/soils, and associated microbial communities built for water pollution control [12; 13; 14]. They are synthetic systems that have been designed to resemble the biological, chemical, and physical processes that take place in natural wetlands [15].  
With the use of CWs, wastewater remediation can be conducted more affordably, sustainably, and easily, with a high rate of nutrient recovery, and minimal maintenance/operation costs [16; 17; 18; 19] in an eco-friendly way [20; 21]. CWs are capable of treating wastewater from different sources such as municipal, livestock, industrial, agricultural, domestic, acid-mine waste, storm run-off, and landfill leachate [22; 23; 24; 25; 26; 27; 28; 29]. Numerous harmful chemicals, including antibiotics, heavy metals, landfill leachate, textile dyes, pesticides, hormones, petroleum, and explosives are removed or degraded by the phytoremediation technique [30]. With the help of CWs, a variety of pollutants can be eliminated from wastewater, including biochemical oxygen demand (BOD), chemical oxygen demand (COD), suspended solids (SSs), total Kjeldahl nitrogen (TKN), total phosphorus (TP), total coliforms (TCs), and metals by microbial degradation, plant absorption, substrate adsorption, and filtering by the packed media and biological predation [26; 31].  
This review article focuses on the research works conducted with different kinds of CWs, macrophytes, and substrates in Italy from the year 2001 to 2023, in order (i) to assess the current status about the use of CWs for wastewater treatment, and (ii) to provide useful information for future researchers. The bibliographic research was conducted using some of the most important popular and scientific search engines (Scopus, ScienceDirect, Web of Science, SpringerLink, Google Scholar, and ResearchGate) by entering different keywords, i.e., CWs, wastewater, and Italy.