Urban and industrial wastewater discharges remain a major source of pollution worldwide. Urban runoff, stormwater overflows, and untreated sewage discharges are increasingly important sources of pollution. Population growth and the change in annual rainfall patterns associated with climate change make it increasingly difficult to meet the growing demand for recreational, industrial, agricultural, and domestic purposes. Thus, the regeneration of used water with the aim of giving it a second use is increasingly imperative ^[1][2][1,2].

Solutions based on nature, such as constructed wetlands (CWs), offer high possibilities for the sustainable use of water, facilitating its treatment and reuse in situ, as well as contributions to adaptation to climate change through the use and promotion of vegetation, both in urban and rural areas ^[3][4][3,4]. The circular economy criteria and objectives require opting for technologies and configurations that allow the recovery of nutrients and other resources contained in wastewater while allowing the reuse or recycling of the water itself for different uses. CWs offer very interesting benefits regarding both sustainability and circularity [5].

The reuse options are conditioned by the destination that will be given to the treated or pre-treated wastewater. For a few reuse destinations, a pretreatment may be sufficient, while CW effluents can be reused in a greater number of applications by achieving efficiencies similar to or higher than those of conventional treatments in the removal of organic pollutants, while enabling a partial or advanced removal of nutrients, pathogens and persistent pollutants ^[5][6][5,6]. The latter depends on the configuration, type, and operating conditions of the CWs. Pathogenic pollution is one of the factors that limits many of the potential uses of reclaimed water, so it must also be a factor to be considered.

2. Treatment of Gray Water and Runoff Using CWs for Water Reuse

Gray water is produced from sinks, laundry, and showers and might represent up to 75% of the total domestic wastewater, with approximately 250–300 L generated per person per day in developed countries and 100–120 L in developing countries [2]. Gray water presents a lower concentration of organic matter, nutrients, and pathogens than black water and, consequently, is easier to be treated ^[7][8][15,16]. Considering the diversity of effluents generated and the possibility of proposing decentralized treatment systems, CWs stand out as a suitable technology for the treatment of gray water and runoff aiming at their reuse. Regarding urban runoff, CWs also contribute to the removal of highly toxic contaminants such as heavy metals that might present risk in the reuse of the treated effluent. Due to their landscaping characteristics, for using ornamental plants, experiments are carried out seeking to use CWs in urban spaces, thus enabling the effluent reuse in situ, which is allied to energy production ^[1][9][1,18].

2.1. Treatment of Gray Water in CWs

Lakho et al. ^[7][15] investigated a system that combined VFCW, filtration, and disinfection for the treatment of gray water from a restaurant in Belgium. The VFCW was built and operated for six months. After going through an activated charcoal system, ultrafiltration, reverse osmosis, ionic exchange membrane, remineralization, and ultraviolet disinfection, the effluent reached a level of potability in accordance with the Belgian potable water regulation and was reused in the restaurant ^[7][15]. Another innovation proposed for the use of CW in the treatment of gray water from a restaurant was the operation of a pilot system combining a constructed wetland microbial fuel cell (CWMFC) and a biological filter (FB) for the continuous treatment and recycling of hand washing water ^[9][18]. The CWMFC system reached a full bacterial removal for an E. coli influent load of 4 log and 432 mg L⁻¹ chemical oxygen demand (COD). The final effluent quality met the South-African standards for noble reuse and was also able to generate 4.33 mW m⁻³-treated effluent. However, those authors ^[9][18] reported the need for further studies related to pathogen removal. Kotsia et al. ^[10][19] investigated a VFCW pilot system conceived as a treatment garden, in which they used ornamental plants to treat synthetic gray water aiming to improve the aesthetics and acceptability of the system. A high organic matter removal efficiency was observed; on the other hand, the total phosphorus (TP) removal reduced gradually from 100% during the first year of operation to 15% throughout the second year. Their results showed that Pittosporum tobira and Hedera helix can grow in VFCW treating gray water without any changes in their physical aspects. The biochemical oxygen demand (BOD) and total suspended solids’ (TSSs) final concentration in the effluent was below 10 mg L⁻¹ and met the Australian criterion for reuse in toilet flush, except for pathogen quantity. Thus, the use of a simple disinfection system such as the visible ultraviolet (UV/Vis) or chlorination is recommended for the elimination of pathogens ^[10][19]. Due to the difficulty of finding large areas needed for the construction of CWs for gray water treatment in urban environments, a solution would be the use of containers for VFCWs. An installation like that might provide the treatment of gray water and their reuse as irrigation water for urban façades, provided that it is followed by a disinfection treatment to comply with irrigation standards set by the WHO ^[11][20]. Another great challenge in the use of CWs to treat gray water is the presence of excess personal care products (PCPs) found in water from toilets and showers. Ren et al. ^[12][21] developed studies on a pilot scale associating a membrane bioreactor (MBR) and VFCW for gray water PCPs. The removal of PCPs in the MBR was mainly through the adsorption and biodegradation of activated sludge. PCPs were removed >80% in the CW system, through plant uptake and biodegradation, among other mechanisms. The VFCW presented lower removal efficiency in winter since several plants died, which led to a continuous decrease in the number of microorganisms adhered to the roots of the plants and reduction in the oxygen transport capability. Despite that, the final effluent met Chinese reuse requirements regarding organic matter and solids. In addition to analyzing the efficiency of the removal of the main contaminants, studies have given more and more importance to the phytotoxic effects that gray water effluents reused for the irrigation of different crops might cause. An analysis carried out on gray water effluents from washing machines and kitchen sinks treated using biological minireactors and HFCW showed that tomatoes irrigated with that water did not suffer any negative effects in relation to their growth, photosynthetic activity, hydric state, osmotic potential, or productivity. Moreover, their results showed that treated gray water did not affect soil salinity and even improved plant height. Despite the positive results, those authors recommended further studies to monitor the long-term effect on the soil and health of those consuming the tomatoes ^[13][22]. Among the contaminants recently studied, research involving CWs in the treatment of gray water have shown certain concerns with the removal of antibiotic resistant bacteria. Results from the treatment of laundry water using a VFCW showed that an increase ranging between 36.34% and 40.79% in the bacteria resistant to ciproflaxin and ceftriaxone might occur. Another important observation was the strong association of a lack of the degradation of ciproflaxin and ceftriaxone with a lack of the removal of surfactants, which required the use of a disinfection method before the use of the treated water. To guarantee an efficient removal of these surfactants in CWs, an efficient COD removal is also important. In addition, the highest correlation between COD and LAS removal was obtained regarding an organic surface loading rate, suggesting that the area plays a more important role than the volume of the system. Pseudomonas spp. predominated in the degradation of such substances ^[14][23]. Other studies reported that to meet higher quality requirements for the noble reuse of gray water effluents, the implementation of an pilot advanced system of disinfection is always required ^[15][16][24,25]. With this purpose, an HFCW and ultraviolet-visible (UV/Vis) disinfection were used to treat gray water from the washbasins of a primary school in Morocco. After the treatment, the effluent produced was used to irrigate lawn areas favoring plant growth. With the UV/Vis disinfection in 50 mWs/cm², the effluent met the unrestricted irrigation requirements set by the WHO, Morocco, and California (USA), which is one of the strictest guidelines for fecal coliforms (23 CFU/100 mL) ^[16][25]. It can be observed in recently published works on the use of CW in gray water treatment that there is a predominance of the use of unsaturated VFCW. This preference may be due to the smaller area required to install these units due to more efficient oxidative processes that also allow for the efficient removal of ammonia.

2.2. Treatment of Runoff Water in CWs

Studies related to the treatment of runoff water have shown that those effluents showed a concentration of heavy metals and that the CWs might provide a good efficiency in the removal of such substances. An HFCW-SCW hybrid system built on a pilot scale was used to treat runoff water from the parking lot of a retail shop in Eastern Sicily, Italy ^[17][26]. The hybrid system showed good efficiency in the removal of heavy metals, mainly lead (Pb), zinc (Zn), and copper (Cu). The removal efficiency for Clostridium perfringens was observed in the HFCW unit. Algi growth occurred in the SCW unit, which reduced the efficiency of the TSS, BOD, and COD removal, compromising the water quality. Preliminary results suggested the reliability of the technology in the treatment of runoff water for urban reuse according to Italian parameters ^[17][26]. In another study, Tuttolomondo et al. ^[18][27] investigated an VFCW on a pilot scale for the treatment of the first discharge runoff water and verified the effect of such water on the reuse to irrigate pepper and rosemary. Their results showed a good removal of organic matter, nutrients, and heavy metals, such as nickel and chrome. The Escherichia coli (E. coli) concentrations were low in the effluent and levels were always below 100 CFU/100 mL, reaching the values required for agricultural use. Regarding the plants irrigated, both showed positive results, mainly in relation to metal removal [2].

2.3. Main Characteristics of CW Systems Used for the Treatment of Gray Water and Runoff Aiming for Water Reuse

Table 12 summarizes the main characteristics of the studies involving the treatment of gray water and runoff using CWs. Many of these studies were carried out on a pilot or field scale, but some small-scale studies developed in a laboratory were also included. Out of the 10 systems included in Table 12, 3 investigated runoff water and 7 investigated gray water. As for the type of technology, most were simple VFCW (seven cases), while there was one simple HFCW and two hybrid CWs. The vegetable species used were quite diversified, both defined as single-crop or polycrop farming, as well as parallel comparative studies, so that the 10 systems included in Table 12 used 17 different plant species. Out of those, only Phragmites australis appeared in three reports and Typha latifolia in two reports; the remaining 14 species were found only once in the reports. Gravel was the most frequently used substrate to prepare the CW beds. It could be used alone or in a combination with sand or other materials such as zeolite. There were also reports of the use of volcanic origin substrates, clay, or bioceramics. The HLR of the VFCW systems ranged between 10 and 130 mm/d, with a 63 mm/d (data number, n = 6) average. When comparing VFCW with other types of CW, they present a higher capacity; for this reason, they can operate greater hydraulic and organic loads. However, the reduced appearance of the other technologies in the reports in Table 12 do not allow for a deeper comparison. The VFCW COD removal resulted in 82 ± 18% on average (n = 7). The general BOD₅ and TSS removals were similar or slightly higher than that of COD (85% and 84% on average, with n = 6 and n = 3, for BOD₅ and TSS, respectively). Regarding the removal of nutrients and pathogens, only 4 out of the 10 studies surveyed presented information about these contaminants. Thus, the small number of systems and the variability of factors did not allow us to obtain indicative removal values or the appropriate operating conditions for efficient treatment, highlighting that more research is still needed. Finally, irrigation was the major water destination, reported in 6 out of the 10 studies, followed by use for toilet flushing in 3 reports. Other reported destinations included washing machines and potable water uses.

3. Treatment of Domestic Wastewater and Industrial Effluents by CWs for Water Reuse

Despite having been initially designed to treat domestic wastewater on a small scale, recently, CWs have been used to treat effluents from agriculture and dairy industries, winery, tannery, paper, and pulp, among others. Regarding industrial effluents and their reuse, CWs contribute to a system that can treat water containing high organic loads and different contaminants, while producing a small amount of sludge ^[20][21][28,29]. The efficiency of the removal of the main contaminants of domestic and municipal wastewater in CWs is comparable to that of modern technologies of wastewater treatment, such as the activated sludge process (ASP), sequence batch reactor (SBR), mobile bed biofilm reactor (MBBR), upflow anaerobic sludge blanket (UASB), etc. CWs can remove BOD (85–90%), COD (65–80%), TSS (90–95%), TN (65–80%), and TP (35–50%) ^[22][30]. Furthermore, if CWs are used in their hybrid configuration and are associated with disinfection systems, they can be highly efficient in removing pathogenic microorganisms. Thus, CWs can contribute the purpose of domestic and industrial wastewater recovery aiming at reusing it, and present the benefits of low maintenance and operational costs, robustness, and efficiency in the removal of several emergent substances that are introduced in the environment by anthropic action every year ^[23][24][31,32]. The use of CWs results in a more acceptable landscape for the population that live around the sewage treatment stations, turning the spaces into gardens, which transform a gray landscape into a green one with gains for the local flora and fauna ^[25][33]. In addition, there is evidence that CWs can be used on a real scale by aiming to recover water in places of water shortage such as the Gaza Strip ^[26][34]. Therefore, in the last few years, many studies have focused on the use of CWs for the treatment of domestic wastewater aiming at its reuse, mainly as a post-treatment to conventional technologies, to provide greater efficiency and ecological benefits ^[27][35].

3.1. Efficiency of CWs in the Reuse of Domestic Effluents

Landscape Integration of CWs and Aesthetic Gains from the Use of Ornamental Plants

Studies have been developed showing that CWs operated in large-scale facilities present efficiency in the treatment of effluents and result in ecological and aesthetic gains due to the use of ornamental plants. De Anda et al. ^[28][36] studied a unit designed to treat domestic sewage in a large research center in Spain for 2 years. The unit included a septic tank, anaerobic filter, HFCW, and chlorine disinfection. The system also allowed the production of Agapanthus africanus as an ornamental plant. Chlorination was required to meet the Spanish guidelines regarding fecal coliform removal for the irrigation of green areas. The water was reused to irrigate grass close to the research center, and the HFCW provided an environment where several species of birds, lizards, butterflies, and bees could live. A study carried out by Kaushal et al. ^[29][37] evaluated three large-scale SCWs used for the treatment of effluents from urban, rural, and industrial facilities for 1.5 years. They investigated the removal of E. coli, Enterococci, and total coliforms for agricultural reuse in India. Their results showed that although the microbiological removal was over 70%, it was not possible to meet the requirements for agricultural reuse in India regarding those parameters. Thus, it was necessary to promote a disinfection process. On the other hand, they contributed to the wildlife habitat and improved the aesthetics of previously degraded territories. Sandoval–Herazo et al. ^[30][38] evaluated the process of removing wastewater pollutants on an HFCW microcosm scale using different ornamental plants and substrates obtained from recyclable material. In that experiment, the ornamental plant Lavandula sp. was not able to adapt and died 45 days after sowing without producing flowers; the Spathiphyllum wallisii produced 12 flowers, while the Zantedeschia aethiopica produced 10 flowers. Their results revealed that the use of substrates originated from PET bottles is a viable alternative to be implemented in CWs. The plants Spathiphyllum wallisii and Zantedeschia aethiopica contributed noticeably to the removal of wastewater pollutants, resulting in good quality for type C agricultural reuse, pursuant to the EU Commission Norms. HFCWs were used to treat effluents in three resorts located in Thailand. Those HFCWs were designed to receive effluent from a septic tank and decanters. The project included different types of plants aiming to promote a decorative aspect, since it was built in a high circulation area. Despite the effluent having reached Thai standards for reuse in buildings, a low removal of fecal coliforms was observed due to the temperature reaching (30 ± 5 °C), and a disinfection process had to be added ^[31][39]. Dell’osbel et al. ^[32][40] evaluated the performance of a pilot hybrid system combining VFCW and HFCW in the treatment of urban effluents. A primary screening treatment and secondary biodigester treatment were employed. The use of five ornamental plants was investigated seeking to aggregate the landscaping potential and better acceptance of the treatment station. The system presented a good capability of removing nutrients from the hybrid system. Pursuant to the Brazilian regulations, the results obtained showed that the final effluent could be used to wash cars and applied to other uses in which the users had direct contact with the water and in which the operator might be exposed to spray aspiration. Achieving the established effluent quality for these uses required recirculation as well as post-treatment disinfection ^[32][40].

The Challenge of Eliminating Pathogenic Microorganisms

As verified in previous reports, one of the greatest challenges in the use of CWs was the removal of pathogenic microorganisms, which usually required the use of disinfection to meet the quality requirements for reuse. Thus, a great part of the studies on the domestic effluent treatment using CWs in the last few years has been directed towards this purpose. Quartaroli et al. ^[33][41] compared the use of calcium hypochlorite and sodium hypochlorite as disinfectant agents in a laboratory HFCW system. They carried out batch disinfection tests, using three hypochlorite dosages (5, 10, and 15 mg L⁻¹) and the contact times ranged between 5 and 50 min. All disinfection dosages used showed microorganism final results below 103 CFU/100 mL, meeting the requirements set by FAO for water reuse in agriculture. Trihalomethane was found after disinfection tests; there was also a possibility of the formation of other potentially harmful chlorination byproducts such as haloacetic acids, haloacetonitrile, haloketone, and trichloronitromethane. Therefore, a higher-quality effluent regarding COD removal can be obtained by associating CWs with other treatments to reduce the risk of effluent contamination by those byproducts. Ali et al. ^[34][42] compared the quality of effluents obtained from full-scale HFCW and VFCW as a post-treatment to anaerobic reactors aiming at pathogen removal. Both systems resulted in over 90% removal of the bacterial population. With a tertiary system involving a SCW, a 50% increase was observed in the removal of pathogenic microorganisms, thus enabling the use of such a treated effluent in irrigation. With a similar objective, Russo et al. ^[35][43] used a HFCW as the tertiary treatment of a large-scale sewage treatment station in Italy. Their objective was to obtain a suitable effluent to be used in agriculture, considering the regulations in force in Italy and the European Union. Although CWs have shown a high efficiency in E. coli reduction, the results are not enough to meet the limits for the reuse of wastewater in agriculture set by the Italian regulations (10 CFU/100 mL with a maximum value accepted of 100 CFU/100 mL) and were higher than the EU water quality value as required for classes A, B, C, or D. The combination of CW treatments with disinfection by UV treatment was investigated by the same authors ^[35][43], who obtained highly effective results with a complete removal of E. coli, somatic coliphages, and C. perfringens spores. Conversely, low efficiency was observed for enterococci. Thus, although E. coli removal from the effluent after the UV treatment met the Italian requirement for reuse, the total removal of enterococci was not possible. Even if neither the Italian regulations nor the EU have set a limit for enterococci, the risk associated with their environmental dispersion is hard to estimate; thus, their removal is needed to obtain better water quality with the least harmful impact to human’s health and the environment ^[35][43]. Stefanakis et al. ^[36][44] verified that one possibility of removing coliforms and enterococci in CW systems involving domestic effluent treatment would be the system aeration. This procedure was implemented in a real-scale VFCW. The effluent was generated from sedimentation tanks and slow biological filters. The number of fecal coliforms, E. coli, and enterococci after the aerated VFCW treatment phase met the WHO guidelines for the reuse of wastewater in agriculture and eliminated the need for a final disinfection. A higher efficiency of aerated VFCW in the removal of microbiological contamination when compared to passive CW systems was reported for the first time, which might have implications in the selection of processes and CW technology for reuse. This also shows that aeration might be a new and efficient treatment scheme to be employed in new treatment stations or to update existing ones, seeking to improve pathogen removal. It has been suggested that high concentrations of dissolved oxygen likely alter the characteristics of the microbial consortium, including the development of groups that feed on pathogens ^[36][44]. Gonzales-Gustavson et al. ^[37][45] investigated the removal of different types of highly pathogenic viruses from an effluent originated in a large-scale domestic sewage treatment station, which included the coagulation, flocculation, and low-pressure UV disinfection phases or SCW as a tertiary treatment. The system served 112,000 inhabitants in Northeastern Spain and the objective of that study was to reuse that water to irrigate vegetables. The SCW was more efficient in reducing virus concentrations compared to the conventional post-treatment, although the effluent showed more variable virus concentrations, probably due to the variability of conditions in the CW. Their results showed that the viral load found in the final effluent did not allow its use in lettuce irrigation according to WHO recommendations. The authors also indicate that the CW land surfaces that would be required to achieve the effluent quality proposed by the WHO would make this option unfeasible in practice for large flows. Gonzalez-Flo et al. ^[38][46] evaluated the performance and quality of water obtained using a combined full-scale system involving an HFCW and chlorine disinfection in Granollers, Barcelona, Spain. The whole system produced over 100 m³·d⁻¹ reuse water for the irrigation of gardens and streets, and sewage network cleaning. The effluent met the water reuse requirements set by Catalonia for pH, electrical conductivity, TSS, E. coli, Legionella eggs, and nematoids in 90% of the analyses. Studies developed in India by Thalla et al. ^[39][47] compared the efficiency of real HFCW and VFCW systems allied to disinfection in the treatment of a domestic wastewater with a concentration of 500 mg COD L⁻¹ and 300 mg BOD L⁻¹. The VFCW showed better efficiency in the general removal than the HFCW. After chlorination disinfection, the water could be reused according to the American Environmental Protection Agency (EPA) in landscaping, impoundments, building, and industrial reuse such as tower cooling and recirculation, as well as environmental reuse in groundwater recharge ^[39][47]. Otter et al. ^[40][48] reported a study combining a VFCW with a chlorine generation pilot system in loco. The VFCW received domestic effluent originated from the treatment with activated sludge and from ponds in a Spanish city. The VFCW reduced the chlorine demand in 85%. During the effluent passage through the VFCW, increased conductivity and chlorine concentration were observed due to the planted vegetations’ high evapotranspiration rates. The system was considered an alternative of efficient disinfection in decentralized applications of effluent reuse in remote locations with limited access to the electricity network and with restricted requirements for pathogen indicators ^[40][48]. Therefore, the disinfection stage associated with CWs is extremely important with a view to using the recovered water for some applications. To this end, chlorination is proposed (the intervention most commonly used) for reuse in urban environments ^[28][39][40][36,47,48]. For agricultural reuse, care must be taken with residual concentrations, as the presence of chlorine in irrigation water is responsible for contamination of the soil, causing toxicity mainly in the leaves of irrigated crops ^[41][42][49,50].

Use of Microalgae in SCW to Improve the Effluent Quality

When systems including SCW are used, recent studies indicate perspectives to improve the effluent quality based on the use of microalgae. Yehia et al. ^[43][51] investigated the treatment of a mixture of domestic effluent with agricultural and industrial material using four pilot SCW units in the presence and absence of microalgae in Northern Egypt (Delta). Their results showed that the best BOD and COD removal was achieved using Chlorella reaching 88% and 84% removal efficiency, respectively. Those authors also reported that Azolla was the microalgae providing the best removal of effluents with the highest TN concentration, while Spirulina was considered to be the most efficient microalgae in the removal of metals such as aluminum (Al), iron (Fe), and manganese (Mn). When compared with other plants used in SCW, microalgae present the advantage of fast growth and ability to absorb nutrients, and good tolerance to temperature changes, in addition to the potential economic benefit of the biomass of the harvested algae. The effluent BOD in all units was below 15 mg L⁻¹ for most of the year. This allowed the application of the treated effluent in the irrigation of agricultural products and green landscapes in education and recreation facilities, according to Egyptian regulations. Some reports indicated that the conventional SCW without microalgae did not meet the requirements for unrestricted irrigation ^[43][51]. The combined use of VFCW with microalgae was investigated in the post-treatment of anaerobic reactor effluents ^[44][52]. According to the Brazilian regulation ABNT 13969/97, the treated effluent could be reused to wash floors and pavements and irrigate gardens and landscapes. In systems involving large-scale (35,000 m²) SCW without microalgae, results revealed that there was an increase in the sulfate concentration (SO₄⁻²) ^[45][53]. Such an increase might be due to the denitrifying bacteria activity, since chemolithoautotrophic bacteria reduce nitrate, while the S-oxidant bacteria oxidize sulfide to return SO₄⁻² during denitrification. This bacterial activity might explain the high NO⁻³ removal and the SO₄⁻² release in CW, which requires post-treatment so that the effluent can be reused ^[45][53].

CWs as Post-Treatment of Domestic Effluents

CWs also appear as a good proposal in the post-treatment of domestic effluents in systems involving ponds, UASB reactors, and Imhof tanks, among other technologies. Ergaieg et al. ^[46][54] used two HFCW as a post-treatment for maturation ponds in a real-scale tertiary treatment of domestic effluents in Tunisia. The proposal aimed to improve the quality of the water used by farmers in that region regarding the removal of BOD and pathogens. One of the problems found by those authors was the risk of clogging the system due to higher hydraulic loads observed in some moments, which despite meeting the requirements of the Tunisian regulation for reuse as irrigation water, was not in compliance with the WHO regulations regarding coliform removal rates. Omidinia-Anarkoli et al. ^[47][55] studied a pilot-scale hybrid CW (HFCW + VFCW) in the post-treatment of domestic effluents from stabilization ponds. The performance of two substrates and the effect of the presence and absence of plants were compared. When gravel was used as substrate, the VFCW showed maximum BOD removal efficiency. Phosphate removal (PO₄⁻³) showed seasonal dependency, and the highest values were observed in hot seasons. Their study also showed that VFCW as a post-treatment for ponds is a solution to obtain effluent for several reuse applications in developing countries that face hydric crisis, such as Iran. The fecal coliform concentrations in the effluent in cold periods tended to be over the maximum standard, while in hot periods they met the reuse requirements ^[47][55]. A study comparing the VFCW use and its absence as a post-treatment to UASB reactors verified that when the technologies were used separately, they did not achieve water quality to meet the Indian requirements for reuse. However, the UASB reactor removal capacity allied to the VFCW post-treatment reached up to 98% removal regarding organic matter parameters and could be reused ^[48][59]. In terms of combining CWs with anaerobic reactors, several applications have gained significant interest ^[34][44][42,52]. Biogas produced from anaerobic reactors could be converted into electricity and used to operate an aerated CW, favoring better performance in the removal of conventional and emerging contaminants. It should be noted that some risks also existed in some integrated processes and deserve attention. For example, although the formation of free chlorine is a fundamental step in the disinfection process adopted, the simultaneous existence of ammonia and free chlorine could produce chloramines which are considered very toxic substances in the environment and for aquatic life ^[49][50][60,61]. Furthermore, some other possible by-products, such as chlorate and perchlorate, are also toxic to humans or deteriorate in the post-treatment process of CW effluents. Therefore, a considerable risk assessment must be carried out in order to select the optimal integrated system in terms of the type of wastewater and the intended reuse purpose.

Hybrid CW Configurations and Combined Technologies Facing Emerging Pollutants Removal

When seeking to use CWs to obtain reuse water without employing other technologies, the best alternative has been found in hybrid configurations. Hybrid CWs have the advantage of combining aerobic, anoxic, and anaerobic environments and related treatment processes. The results show the potential for the long-term operation of hybrid CWs for the treatment of domestic–industrial-mixed wastewater. Studies have shown that when VFCW + HFCW is used to treat domestic wastewater, effluents reached reuse conditions in gardening, agriculture, and cleaning with sanitary purposes ^[51][62]. However, even when employing hybrid systems including VFCW + VFCW + HFCW, the salt removal difficulty still remains with increased electrical conductivity in the effluent when the hybrid system is used in a hot climate due to the excess evapotranspiration. The advantage of use in such climate conditions is that the VFCW does not require a resting time, and the effluent can meet quality requirements for use, being classified as Classes C and D according to the EU (irrigation of indirect consumption crops and in drip irrigation), which also results in a reduction in construction and operation costs ^[52][63]. Another possibility of optimizing processes involving CWs, reducing the time needed to remove contaminants in VFCW, is the use of biochar from the plant used in the treatment unit. When using biochar obtained from Phragmits australis, a significant reduction was observed in fecal coliforms, COD and BOD, and the metals Cu, Mn, Cd, and Al in only 72 h ^[53][64]. Those results indicated that the wastewater quality was highly improved by the biochar treatment. However, it was still not enough to reach reuse levels, with stricter requirements regarding the removal of the main physical, chemical, and biological parameters. Moreover, contaminant desorption mechanisms for this type of biochar must be considered in further investigations ^[53][64]. The application of hybrid CWs to various wastewaters has demonstrated that the combination could improve pollutant removal efficiency. Hybrid CWs could cover the limitation of each CW. For example, by combining HFCW for VFCW, the desirable condition for the nitrification–denitrification process would be created due to the aerobic, anoxic, and anaerobic environment. Some operational and design parameters such as HLR, bed material, system configuration (number of beds; system layout), influent pollutant concentrations and effluent recirculation can affect the performance of hybrid CWs. It is interesting to note that hybrid CWs are effective in removing organic matter (BOD; COD) and suspended solids, while in terms of pathogens and nutrient removal like N and P components, removal efficiencies depend on system properties and operating conditions.

3.2. Efficiency of CWs in the Reuse of Industrial Effluents

The studies published reported that CWs are employed in the treatment of effluents of industries such as car manufacturing, oil, tanning and batteries, winery, cooling, brewery, and petrochemical, aiming at the reuse of wastewater. CWs were observed to be quite efficient in the removal of contaminants present in industrial effluents such as heavy metals and toxic organic compounds. A treatment proposal for the reuse of effluents with heavy metals in a large automobile manufacturer in Italy was developed on a pilot scale using a hybrid CW VF + HFCW ^[54][68]. Wastewater collected from industrial and civil buildings was subjected to physical–chemical and biological pre-treatments prior to the hybrid CW system. The authors concluded that the CW systems implemented were highly efficient in the removal of TSS and heavy metals such as Fe (97.9% removal) and Zn (92.9% removal). However, electrical conductivity, alkalinity, and calcium did not meet the requirements for reuse according to the EPA regulations. There are studies on the use of CW in the treatment of industrial effluents with high organic load such as those from oil, wine, and beer production. Due to the fact that effluents originated from the olive oil production present up to 4000 times more COD and high phenol concentrations, pre-treatment was applied in open tanks for sedimentation or even pH correction. The removal rates of COD, TSS, TKN, and phenol in the pilot VF + SCW hybrid system reached 54.1%, 52.0%, 44.4%, and 60.1%, respectively. On the other hand, the pilot SCW system (with enhanced pre-sedimentation and pH correction) achieved COD, TSS, TKN, and phenol removals of 49.4%, 72.0%, 26.9%, and 51.1%, respectively. Even if a hybrid CW with VFCW + HFCW was used, it was only possible to produce effluent for agricultural reuse in crops that did not require contact, that is, further treatment or lower surface loading rates would be required for more restrict uses ^[55][71]. In studies on wine cellars, the use of hybrid CW was investigated regarding agricultural use standards ^[56][72]. The treatment system was subjected to a pre-treatment phase (coarse sieving), followed by a Imhoff tank and an equalization tank. The hybrid CW presented a total area of around 230 m² and consisted of a VF + HF + SCW bed, producing water for reuse in suitable conditions for food crops to be consumed raw with edible parts produced above the ground, as described by the EU regulation ^[57][73]. When treating effluents from a brewery, a system made of an HFCW in two stages was used to produce tomato irrigation water ^[58][69]. The HFCW system was used in the post-treatment of an UASB reactor effluent. Those authors observed that the effluent going through the UASB and HFCW treatments reached the irrigation water standard as required by the Ethiopian environment protection agency, showing a high tomato yield capability ^[59][74]. Recent studies have also demonstrated the use of CWs in the treatment of effluents to help the production of water to be reused in cooling systems and petrochemical industries. Wagner et al. ^[60][70] investigated the use of a pilot hybrid CW (HF + VF + SCW) in the pre-treatment of a membrane system for the reuse of industrial synthetic effluents for tower cooling. The study also verified the removal of phenolic contaminants. Their results showed that the HFCW removed PO₄⁻³, TSS, and TOC as a result of adsorption and filtration. Regarding VFCWs, they outstood in the removal of phenolic compounds resulting from biodegradation. The SCW did not contribute to the removal of substances and even increased the effluent salinity. However, they provided some options of water storage and habitat for the aesthetic improvement of the unit benefitting the local flora and fauna. Another study developed by the same research group ^[61][75] sought the reuse of water from a cooling tower of a system made of green and gray technologies. The treatment plant showed a configuration that included a SCW as a pre-treatment, followed by a system composed of VF + HFCW. After leaving the hybrid CW, the effluent was subjected to nanofiltration, electrochemical oxidation, and reverse osmosis. In addition, they studied the removal of corrosion inhibitors. The pre-treatment using hybrid CWs before the nanofiltration resulted in the removal of phosphate, nitrate, and benzotriazole, but increased TOC and electrical conductivity was also observed. Thus, for the water reuse in cooling towers, the use of reverse osmosis was required ^[61][75]. CWs have been increasingly applied in treating various industrial wastewaters with specific characteristics. Considering the specific characteristics of various industrial effluents with low or high pH, soil materials with a neutralizing ability but low cost for potential use in full projects should be selected. For the use of CW in the treatment of huge industrial effluent flows, the main drawback is the large area that would be required. However, this problem could be minimized by associating it with the economic production of plant biomass and its energy recovery.