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Industrial wastewater generated from various production processes is often associated with elevated pollutant concentrations and environmental hazards, necessitating efficient treatment. Floating wetlands (FWs) have emerged as a promising and ecofriendly solution for industrial wastewater treatment, with numerous successful field applications. This article comprehensively reviews the removal mechanisms and treatment performance in the use of FWs for the treatment of diverse industrial wastewaters. Our findings highlight that the performance of FWs relies on proper plant selection, design, aeration, season and temperature, plants harvesting and disposal, and maintenance. Welldesigned FWs demonstrate remarkable effectiveness in removing organic matter (COD and BOD), suspended solids, nutrients, and heavy metals from industrial wastewater. This effectiveness is attributed to the intricate physical and metabolic interactions between plants and microbial communities within FWs. A significant portion of the reported applications of FWs revolve around the treatment of textile and oily wastewater. In particular, the application reports of FWs are mainly concentrated in temperate developing countries, where FWs can serve as a feasible and costeffective industrial wastewater treatment technology, replacing highcost traditional technologies. Furthermore, our analysis reveals that the treatment efficiency of FWs can be significantly enhanced through strategies like bacterial inoculation, aeration, and coplantation of specific plant species. These techniques offer promising directions for further research. To advance the field, we recommend future research efforts focus on developing novel floating materials, optimizing the selection and combination of plants and microorganisms, exploring flexible disposal methods for harvested biomass, and designing multifunctional FW systems
Environmental Science and Pollution Research https://doi.org/10.1007/s11356-023-31507-3 REVIEW ARTICLE Industrial wastewater treatment using floating wetlands: a review Jianliang Mao1 · Guangji Hu2 · Wei Deng1 · Min Zhao3,4 · Jianbing Li1,4 Received: 13 August 2023 / Accepted: December 2023 © The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2023 Abstract Industrial wastewater generated from various production processes is often associated with elevated pollutant concentrations and environmental hazards, necessitating efficient treatment Floating wetlands (FWs) have emerged as a promising and ecofriendly solution for industrial wastewater treatment, with numerous successful field applications This article comprehensively reviews the removal mechanisms and treatment performance in the use of FWs for the treatment of diverse industrial wastewaters Our findings highlight that the performance of FWs relies on proper plant selection, design, aeration, season and temperature, plants harvesting and disposal, and maintenance Well-designed FWs demonstrate remarkable effectiveness in removing organic matter (COD and BOD), suspended solids, nutrients, and heavy metals from industrial wastewater This effectiveness is attributed to the intricate physical and metabolic interactions between plants and microbial communities within FWs A significant portion of the reported applications of FWs revolve around the treatment of textile and oily wastewater In particular, the application reports of FWs are mainly concentrated in temperate developing countries, where FWs can serve as a feasible and cost-effective industrial wastewater treatment technology, replacing high-cost traditional technologies Furthermore, our analysis reveals that the treatment efficiency of FWs can be significantly enhanced through strategies like bacterial inoculation, aeration, and co-plantation of specific plant species These techniques offer promising directions for further research To advance the field, we recommend future research efforts focus on developing novel floating materials, optimizing the selection and combination of plants and microorganisms, exploring flexible disposal methods for harvested biomass, and designing multi-functional FW systems Keywords Floating wetlands · Pollutant removal · Industrial wastewater · Wastewater treatment Abbreviations ADMI American Dye Manufacture Institute AMD Acid mine drainage BOD Biochemical oxygen demand Responsible Editor: Alexandros Stefanakis * Jianbing Li Jianbing.Li@unbc.ca School of Engineering, Environmental Engineering Program, University of Northern British Columbia (UNBC), 3333 University Way, Prince George, British Columbia V2N 4Z9, Canada School of Environmental Science and Engineering, Qingdao University, Qingdao 266071, Shandong Province, China School of Life and Environmental Sciences, Wenzhou University (WZU), Wenzhou 325035, Zhejiang Province, China WZU‑UNBC Joint Research Institute of Ecology and Environment, Wenzhou University (WZU), Wenzhou 325035, Zhejiang Province, China COD Chemical oxygen demand CW Constructed wetland DO Dissolved oxygen EC Electrical conductivity FW Floating wetland PVC Polyvinyl chloride TAN Total ammonia nitrogen TDS Total dissolved solids TKN Total Kjeldahl nitrogen TN Total nitrogen TP Total phosphorus TPH Total Petroleum Hydrocarbons TS Total solids TSS Total suspended solid 13 Vol.:(0123456789) Introduction A variety of wastewater is generated in large volumes from different industrial production processes, such as chemical, petroleum, textile, and mining industries (Zhang et al 2015) Industrial wastewater contains a diverse array of organic, inorganic, and biological pollutants, which will deteriorate the receiving waterbodies if not treated properly (Bi et al 2019) Conventional technologies for industrial wastewater treatment include gravity separation, screening, gas flotation, flocculation, activated sludge, sequential bioreactor, membrane bioreactor, anaerobic baffled reactor, membrane filtration, and advanced oxidation (Shrestha et al 2021; Toczyłowska-Mamińska 2017; Yu et al 2017) However, these technologies are associated with several shortcomings such as high cost, sludge production, low operating pH, high maintenance requirement, and secondary pollution (Ijaz et al 2016; Jain et al 2020) Consequently, there is an urgent need for developing effective and eco-friendly methods for industrial wastewater reclamation In response to this need, natural wetlands have emerged as promising agents for the treatment of polluted water, and in light of this, various artificial wetlands have been designed and used to treat anthropogenic Fig. 1 Classification and typical structure of FW 13 Environmental Science and Pollution Research discharges such as municipal wastewater, agricultural runoff, and industrial effluents (Ijaz et al 2016; Li et al 2021a, b; Naeem et al 2020; Rahi et al 2020) Artificial wetlands mainly include subsurface flow wetlands, free water surface flow wetlands, and floating wetlands (FWs) (Stefanakis 2018) Among them, FWs are a prominent type that can be established easily using soilless planting technology in a flexible and cost-effective manner (Shahid et al 2018) Various terminologies have been used for FWs (Fig. 1), including floating treatment wetland, constructed floating wetland, artificial floating wetland, artificial floating island, ecological floating bed, floating hydroponic system (Davamani et al 2021; Karstens et al 2021; Oliveira et al 2021) As illustrated in Fig. 1, the structure of FWs mainly consists of plants and a floating mat Notably, the key distinction between natural and artificial FWs is that plants grow on the artificial buoyancy materials in the latter The roots attached with biofilm can thus grow under the water surface and the crown of plants is supported above the water surface Rhizosphere is the environment that interacts closely with the plant root system Instead of soil, the water is the main environment to the root system in FWs, and bacteria are also observed in rhizosphere (Saleem et al 2018) A variety of buoyancy materials have been employed in the Environmental Science and Pollution Research construction of floating mats, such as bamboo-based meshes, wire meshes, fibrous material, polystyrene foam, polyvinyl chloride (PVC) pipes, and polyester sheets (Shahid et al 2018) PVC-based floating materials are regarded as the new-generation materials with better stability and buoyancy, and it is also assessed to be environmentally safe because of the insignificant release of microplastic particles (Ziajahromi et al 2020) Additionally, mat materials are also influential to other critical factors affecting the mat design, including anchoring, cost, durability, flexibility, functionality, and local availability (Samal et al 2019) Ensuring the stability of floating mats to withstand sunlight radiation, water wave action, and wind over extended periods is essential Additionally, the parenchymatous ability of plants could also increase the buoyancy of FWs, which can entrap gases in tissues and therefore make plants float on the water surface (Rehman et al 2019a) Typically, FWs rely on interactions among plants, water, atmosphere, and microorganisms to treat pollutants (Shahid et al 2020a, b) The main mechanisms of FW for pollutants removal include entrapment of solids, uptake of nutrients and metals, development of biofilms, degradation of organic pollutants, and flocculation of suspended matter (Samal et al 2019) Different types of FWs have been installed in reservoirs, ponds, rivers, and lakes for natural water quality improvement (Hu et al 2010; Saeed et al 2016) With the development of this technology, FWs have been employed to treat industrial wastewater, such as acid mine drainage (Gupta et al 2020), effluent from the pulp and paper industry (Ayres et al 2019), and oily wastewater from the petrochemical industry (Darajeh et al 2016) Previous literature reviews have extensively explored the utilization of constructed wetlands (CWs) for the treatment of industrial wastewater (Stefanakis 2018; Vymazal 2013) These reviews have demonstrated the effectiveness of CWs in mitigating industrial pollutants However, it is important to note that traditional CWs often necessitate significant land resources for their implementation In contrast, floating wetlands (FWs) present an alternative approach that relies on the robust plant system and associated biofilm to directly remediate pollutants, eliminating the need for additional land allocation (Colares et al 2020) Recent publications have begun to unveil the considerable potential of FWs in achieving high treatment efficiency, even for complex industrial pollutants Nonetheless, there exists a noticeable gap in the literature—a comprehensive discussion of the application of FWs for industrial wastewater treatment and a thorough exploration of their advantages and limitations It is within this context that this review aims to make a valuable contribution The primary objective of this review is to provide an encompassing summary of the development and practical application of FWs in the treatment of industrial wastewater This review will summarize the structural components of FWs, the critical factors influencing the performance of FWs, the removal mechanism of various industrial pollutants by FWs, and the applications of the FWs of treating different industrial wastewater Moreover, the limitations of FWs in their current state and avenues for their future development will be discussed in detail, the review scope and framework are shown in Fig. 2 Factors affecting the performance of FWs Various factors can impact the FWs treatment performance (Fig. 3), and they are discussed below Fig. 2 Review scope and framework 13 Environmental Science and Pollution Research 60% to 80%) (Rehman et al 2018) Aquatic plants are more commonly used in FWs because the plants are well-adapted to the complex water environment characterized by different nutrient concentrations, redox conditions, and trophic status of water (Vymazal 2013) Plants can also be used in combination because different species have different treatment capacities for different pollutants and concentrations The combination of plants can achieve higher efficiencies in the treatment of wastewater with complex pollutants (Dzakpasu et al 2014) However, it is difficult to assess the function of individual species and the synergistic effect of plants Design Design of FWs is critical for effective application Some important considerations in designing an FW system are as follows: Fig. 3 Influential factors on the performance of FWs Plant selection The choice of plants is essential to the effective removal of pollutants from industrial wastewater There are some key principles of plant selection in FWs application: the plants should be native perennial species that have a good growth rate in hydroponic environments, an extensive root system, high tolerance to pollutants, and a large capacity to uptake pollutants (Calheiros et al 2020; Shahid et al 2020b) Various species of plants have been used in FWs, such as vetiver (Chua et al 2012), Typha domingensis (Di et al 2019), Brachiaria mutica (Ijaz et al 2015), Najas minor (Zhou et al 2016), Salvinia natans and Phragmites australis (Huang et al 2017), Oenanthe javanica (Wang et al 2018), Iris pseudacorus (Zhang et al 2019), Pistia stratiotes (Samal et al 2021), and Typha orientalis (Ansari et al 2017) For industrial wastewater treatment, plant selection depends on the type of pollutants, water quality, and climatic conditions (Shahid et al 2018) Although different plant species have different phytoremediation potentials, the main pollutant removal mechanisms include accumulation, exclusion, translocation, osmoregulation, distribution, and concentration (Rezania et al 2016) It is important to choose proper plants for target pollutants under given conditions For example, Pistia stratiotes and water hyacinth can remove Pb, Cu, and Cd from industrial effluent (Aurangzeb et al 2014; Volf et al 2015) Vetiver can remove organic matters from wastewater (Darajeh et al 2014) Under the same conditions, Phragmites australis removed 20% more chemical oxygen demand (COD) from oily wastewater than Brachiara mutica (around 13 i) Baseline monitoring of wastewater: Baseline water quality and flow direction assessment is necessary for the design of FWs because the types and concentrations of pollutants are important to the selection of plants and application sites (Winston et al 2013) For example, a baseline analysis can help researchers select study sites with a high pollutant load in the inflow (Borne et al 2013) ii) Retention period: FWs are often designed or assessed under varying retention periods depending on the treatment scale In laboratory-scale settings, retention periods typically range from several hours to weeks, while large-scale investigations may extend over several months to years iii) Self-buoyancy: Floating plants can achieve self-buoyancy by entrapping gases in their rhizomes and tissues, enabling them to grow on the water surface (Rehman et al 2019b) Floating rafts and mats can also serve as stable platforms to support non-floating plants growing on the water surface (Chow et al 2019; Shin et al 2015) iv) Depth of water: Compared with the other types of constructed wetlands, FWs are lack of soil and thus the plants heavily rely on their roots for nutrients uptake from the water body (Shahid et al 2018) Therefore, the water depth is crucial for root structure development For most of the plants in FWs, a water depth of at least 0.8–1.0 m is required for vertical growth (Headley and Tanner 2008) Increased water depth can promote the treatment performance of FWs due to increased contact time of roots and microbial biofilm with pollutants (Tanner and Headley 2011) Water depth is flexible for different pollutants as a low water depth is preferred for removing small particles and suspended solids due to more frequent contact between wastewater and biofilm While deep water is suitable to treat coarse suspended Environmental Science and Pollution Research solids by sedimentation because there is sufficient free water zone for solids precipitation (Chen et al 2012) Moreover, deep water creates layers with different dissolved oxygen concentrations in the water column, and thus nitrification and denitrification can be achieved under aerobic and anaerobic conditions, the co-existence of both process can efficiently remove N from wastewater (Colares et al 2020) Other important design considerations include the size of wetland and plant coverage (the area ratio of plants to water) (Chen et al 2016) Surface area has a significant impact on the efficiency of FWs; a greater surface area would harbor a larger bacterial population and bring a higher nutrient removal efficiency (Stewart et al 2008) An increase in plant coverage can also increase the treatment efficiency (Winston et al 2013) However, increasing plant coverage sometimes may limit the pollutant removal because this can reduce the dissolved oxygen level in wastewater, resulting in decreased aerobic bacteria bioactivity and plant growth rates (Wei et al 2020) FWs with 100% plant coverage were seen with a low dissolved oxygen level compared with 50% planting coverage (Chang et al 2012) Dense plant coverage also limits the gaseous exchange process, signifying that gas exchange most likely occurs in the uncovered portions of the water The shortage of oxygen and light caused by dense plant coverage could affect the biofilm attached to plant roots (Chang et al 2012; Headley and Tanner 2008) The plant coverage ranges from 5% to > 50% in the reported experimental designs and field applications (Chang et al 2012), but it is suggested to be 700℃) condition (Kathi 2016) Pyrolysis decomposes biomass at a relatively lower temperature (400–700℃) in an oxygen-free environment into useful products including pyrolysis oil, gases, and biochar (Muradov et al 2010) After proper modification, biochar can be used for wastewater treatment In a study on municipal wastewater treatment, harvested reed straw was pyrolyzed into biochar, which was used as reed biochar substrate and a carbon source for microorganisms (Huang et al 2020) After adding the biochar to FWs, the average removal efficiencies of TN and TP increased by 57.6% and 46.7%, respectively, and the microbial species for nitrogen removal were also enriched with the additional carbon source (Huang et al 2020) 13 Environmental Science and Pollution Research Installation and maintenance of FWs In the field application of FWs, fertilizers can be used at the early stage of plants growth to help plants grow better until the development of floating plants is complete (Arslan et al 2017) Plants are usually cultivated in less-polluted or tap water for adaption to the new environment, and this can increase the survival rate of the selected plants in wastewater treatment later (Hefni et al 2017) Floating mats can be anchored to the edge of the aquatic environment by ropes to reduce mats drifting caused by strong winds and waves (Borne et al 2015) However, appropriate flexibility is also needed to adjust the position of floating mats with the change of water level (Wei et al 2020) To keep the stability of FWs in windy seasons or areas, the height of plants should be limited because tall plants may cause the turnover and drifting of floating mats (Chen et al 2016) Installing small floating islands with low-height plants is more suitable for windy environments After installation, the main maintenance includes weeding, clearing the blockage of floating matters on water surface, and repair of broken parts (Pavlineri et al 2017) Trimming and removing shoots can effectively prevent plants from withering due to the shortage of oxygen (Hawes et al 2016) Harvesting, mentioned above for improvement of pollutants removal, is also important for FWs maintenance; periodic harvesting of plants can provide enough space for plants to regrow (Hussain et al 2018; Wei et al 2020) Additionally, regular monitoring of FWs is essential to prevent the planting hole or water area from being blocked caused by the accumulation of plant branches, plastic species, and other materials and maintain the longterm operation (Borne et al 2015) Summary of impact factors Plant selection is a crucial factor in the effective removal of pollutants from industrial wastewater using Floating Wetlands (FWs) Key principles for selecting suitable plants include choosing native perennial species with strong adaptability to hydroponic environments, extensive root systems, high pollution tolerance, and efficient pollutant uptake capabilities The choice of plant species should be based on the specific pollutants, water quality, and climatic conditions of the treatment site Factors like water depth, plant coverage, aeration, seasonal variations, and temperature also significantly impact the pollutant removal efficiency of FWs Additionally, proper plant harvesting, and disposal strategies are essential to maintain the effectiveness of FWs, preventing the release of captured pollutants The installation and maintenance of FWs require careful consideration, including initial fertilization, plant adaptation, and routine maintenance tasks like weeding and clearing Overall, effective pollutant Environmental Science and Pollution Research removal in FWs relies on a combination of plant selection, thoughtful design, aeration, seasonal factors, and proper maintenance practices Industrial wastewater pollutant removal using FWs Industrial wastewater is a collective term, and the species and loads of pollutants vary drastically by the sources The main pollutants in industrial wastewater include organic matter, nutrients, heavy metals, and total solids These pollutants are commonly generated from textile, oil, food processing, mining, chemical, and pulp and paper industries In general, mining wastewater contains high concentrations of heavy metals Wastewater from textile and food processing industries contain high levels of organics and solids, and food processing effluents contain high concentrations of nutrients (Fig. 4) Industrial wastewater usually contains high concentrations of organics, TS, and heavy metals, while the concentrations of nutrients are relatively lower (Bi et al 2019) Compared with municipal wastewater, industrial wastewater is toxic, colored, smelly, and foamy FWs have different removal mechanisms for different pollutants by employing plants, bacteria, and plant-bacteria synergism Organic matter Organic pollutants in industrial wastewater are mainly from tannery, palm oil, dairy, beverage, pharmaceutical, textile, and food processing industries (Mutamim et al 2012) Organic pollutants can be used by microorganisms and plants as a carbon and energy source to increase biomass; however, some organic pollutants such as phenolic compounds, benzene, toluene, polycyclic aromatic hydrocarbons, and other types of hydrocarbon are toxic and carcinogenic (Jain et al 2020) In FWs, biodegradation of organic pollutants by microorganisms attached to roots is active near the root area Biofilm and plant roots can assimilate dissolved organic matter directly, while large organic compounds can be transformed into smaller compounds by microorganisms so that the smaller compounds can be taken up by plants (Barbara 2009) CODis used to measure the oxygen demand of organic and inorganic pollutants in water, and BOD is employed to gauge the ability of microorganisms in water to degrade organic substances In the application of FWs, they are the key parameters to assess the degradability of organic matter A BOD/COD ratio > 0.5 indicates that the wastewater is suitable for biodegradation treatment (Kumar et al 2010) Nevertheless, effective removal of organic pollutants has been achieved for different types of wastewater with BOD/COD ratios ranging from 0.6 to 0.8 (Benvenuti et al 2018; Li and Guo 2017; Prajapati et al 2017) For industrial wastewater Fig. 4 Characteristics of industrial wastewater treated using FWs: a) main types and pollutants and concentration ranges of b) solids and organics and c) heavy metals in industrial wastewater (using Data from Table 1) 13 Environmental Science and Pollution Research Table 1 Pollutants initial concentration on studies of FWs treating industrial wastewater Wastewater Textile wastewater Wastewater characteristics ADMI = 1285 ± 1.55, COD = 1438 ± 12.7 mg/L, BOD = 1230 ± 10.2 mg/L, TDS = 8230 ± 8.8 mg/L, TSS = 5175 ± 0.7 mg/L Textile wastewater ADMI = 1142 ± 21, COD = 1495 ± 20 mg/L, BOD = 1135 ± 20 mg/L, TDS = 4706 ± 28 mg/L, TSS = 734 ± 8 mg/L Textile wastewater ADMI = 1308 ± 11, COD = 1794 ± 7 mg/L, BOD = 1350 ± 13 mg/L, TDS = 5143 ± 21 mg/L, TSS = 1900 ± 15 mg/L Textile wastewater ADMI = 1638 ± 1.6, COD = 1734 ± 1.8 mg/L, BOD = 1478 ± 1.5 mg/L, TDS = 9060 ± 4.6 mg/L, TSS = 6438 ± 4.1 mg/L Textile wastewater TS = 55.67 ± 5.72 mg/L, COD = 150.13 ± 39.06 mg/L, BOD = 317 ± 16.73 mg/L Textile wastewater COD = 177.76 mg/L, BOD = 87.178 mg/L Textile wastewater TDS = 2560.83 mg/L, TSS = 131.61 mg/L, BOD = 107.46 mg/L, COD = 187.1 mg/L Textile wastewater TDS = 400 mg/L, TSS = 92 mg/L, BOD = 121 mg/L, COD = 310 mg/L, Color = 40 m−1 Textile wastewater TDS = 4961 mg/L, TSS = 4569 mg/L, BOD = 249 mg/L, COD = 471 mg/L, Color = 35.5 m−1 Textile wastewater TDS = 5251 ± 404 mg/L, TSS = 324 ± 29.7 mg/L, BOD = 283 ± 17.9 mg/L, COD = 513 ± 37.6 mg/L, Color = 66 ± 4.4 m−1 Oil and grease COD = 538 ± 83 mg/L, BOD = 228 ± 65 mg/L, Oil and grease = 17.4 ± 2.7 mg/L Palm oil mill effluent COD = 210 mg/L, BOD = 42.91 mg/L Palm oil mill effluent COD = 790–810 mg/L, BOD = 350–400 mg/L Diesel Oil = 10,000 mg/L, COD = 10000 mg/L, BOD = 3500 mg/L Refinery wastewater TPH = 1720 mg/L, COD = 142.8 mg/L Oil field-produced wastewater COD = 1336 ± 58.74 mg/L, BOD = 405 ± 19.43 mg/L, hydrocarbons = 316 ± 7.84 mg/L Crude oil contaminated water COD = 1316 ± 73.5 mg/L, BOD = 365 ± 15.4 mg/L, hydrocarbons = 319 ± 9.7 mg/L Oil field wastewater COD = 1324 ± 66.5 mg/L, BOD = 475 ± 15.5 mg/L, oil = 325 ± 10.1 mg/L Crude oil spilled water COD = 150–160 mg/L, BOD = 20–25 mg/L, Oil and grease = 0.15 mg/L Palm oil mill effluent COD = 750 mg/L, BOD = 350 mg/L, Oil and grease = 15 mg/L Acid mine drainage Cd = 0.02 mg/L, Cu = 4.78 mg/L Acid mine drainage Fe = 81.4 ± 1.16 mg/L, Al = 70.3 ± 1.11 mg/L, Mn = 21.9 ± 0.27 mg/L, Zn = 1.372 ± 0.0436 mg/L, Ni = 0.697 ± 0.0413 mg/L, Cu = 0.184 ± 0.00435 mg/L, Pb = 0.08 ± 0.0341 mg/L, Cr = 0.01 mg/L Acid mine drainage Fe = 12 mg/L, Al = 11.3 mg/L, Zn = 0.385 mg/L, Ni = 0.388 mg/L, Cu = 0.0218 mg/L, Pb = 0.0105 mg/L Pulp and paper mill effluent TDS = 1840 mg/L, EC = 2.64 dS/m, BOD = 475.1 mg/L, COD = 880.5 mg/L, TKN = 192.65 mg/L, PO43− = 145.6 mg/L, Cd = 2.45 mg/L, Cr = 1.38 mg/L, Cu = 5.64 mg/L, Fe = 8.95 mg/L, Mn = 3.66 mg/L, Pb = 1.74 mg/L, Ni = 1.02 mg/L, Zn = 6.9 mg/L Chemical contaminated water Benzene = 13 ± 3 mg/L, MTBE = 2.2 ± 0.5 mg/L Dairy wastewater TS = 1671.58 ± 177 mg/L, COD = 5866.67 ± 924 mg/L, BOD = 2282.75 mg/L Sugar mill effluent EC = 5.44 ± 0.05 dS/m, TDS = 1932.2 ± 11.22 mg/L, BOD = 947.88 ± 6.44 mg/L, COD = 1620.3 ± 10.23 mg/L, TKN = 126.43 ± 3.12 mg/L, TP = 124.32 ± 2.14 mg/L Paperboard mill wastewater EC = 1.98 ± 0.06 dS/m, TDS = 1000 ± 16.3 mg/L, TSS = 200 ± 3.26 mg/L, BOD = 44.0 ± 1.43 mg/L, COD = 256 ± 4.17 mg/L, TN = 25 ± 0.81 mg/L, TP = 8.50 ± 0.28 mg/L, Pb = 0.96 ± 0.02 mg/L, Cd = 0.42 ± 0.01 mg/L Paper mill wastewater TP = 0.02–0.88 mg/L, TN = 1.8–7.4 mg/L Saline industrial wastewater TDS = 5000 mg/L, COD = 350 mg/L, TN = 13.2 mg/L, TP = 4 mg/L, Batik Wastewater TSS = 1424.04 ± 166.62 mg/L, COD = 273.88 ± 24.93 mg/L, BOD = 37.95 ± 8.14 mg/L, Cr = 0.81 ± 0.06 mg/L Batik Wastewater TSS = 1183 mg/L, TAN = 2.38 mg/L, NH4+ = 0.96 mg/L, NH3 = 1.42 mg/L 13 Reference Kadam et al (2018a) Chandanshive et al (2020) Chandanshive et al (2020) Kadam et al (2018b) Charoenlarp et al (2016) Tusief et al (2020) Qamar et al (2019) Nawaz et al (2020) Tara et al (2019b) Tara et al (2019a) Ijaz et al (2016) Tan (2019) Darajeh et al (2014) Fahid et al (2020) Li et al (2012) Rehman et al (2019b) Afzal et al (2019b) Rehman et al (2018) Effendi et al (2017) Darajeh et al (2016) Palihakkara et al (2018) Kiiskila et al (2019) Kiiskila et al (2017) Kumar et al (2016) Chen et al (2012) Queiroz et al (2020) Kumar et al (2019) Davamani et al (2021) Ayres et al (2019) Gao et al (2020) Tambunan et al (2018) Effendi et al (2018) Environmental Science and Pollution Research treatment, a floating treatment wetland was installed to treat palm oil mill effluents, and the reduction of BOD and COD was reported to be 96% and 94%, respectively, with aeration under a BOD/COD ratio of 0.46 (Darajeh et al 2016) In another study, FWs were used to remediate water contaminated by spilled crude oil, and significant reductions in BOD (81%), COD (81%), and oil content (91%) were observed after 4 weeks treatment (Effendi et al 2017) In a recent study, an FW augmented with bacteria was used to treat dye-enriched synthetic wastewater, and the results showed that the FW significantly reduced COD from 471 to 30 mg/L and BOD from 249 to 31 mg/L; also, the treatment efficiency of the augmented FW was much higher than non-vegetated and only vegetated (i.e., nonaugmented) FWs (Tara et al 2019a) Total solids Total solids (TS) include total suspended solids (TSS) and total dissolved solids (TDS) TSS indicates the total quantity of suspended particulate matter in water and usually consists of phosphates, nitrates, carbonates, and a series of bicarbonates, which can increase the turbidity of wastewater and reduce light availability for submerged macrophytes and microorganisms (Bi et al 2019; Wei et al 2020) In addition, many harmful substances, such as heavy metals, polycyclic aromatic hydrocarbons, and organic matter, can be adsorbed on TSS, and thus the increase of TSS in water may have ecotoxic effects on aquatic organisms (Rossi et al 2006) TDS represents the total amount of dissolved inorganic and organic solids in water and is related to the high conductivity and salinity of wastewater Textile effluents usually contain higher concentrations (1,000–10,000 mg/L) of TDS than other industrial wastewater (Wei et al 2020) TSS is mainly removed via physical sedimentation and filtration processes (Borne et al 2013), while TDS removal is also associated with the uptake by plants Vegetation and associated roots can speed up the removal of TSS and TDS (Shahid et al 2019) Plant roots play an important role in the removal of suspended solids and particles in wastewater (Benvenuti et al 2018) The network of plant roots can entrap suspended solids (Hawes et al 2016) It also forms an ideal area for the development of biofilm (Cao et al 2012), which is active to entrap and filter fine particles TSS can also be removed by sedimentation and precipitated to the bottom (Chen et al 2016) It is necessary to clean the sediment layer to avoid pollutants resuspending (Headley and Tanner 2012) For textile wastewater, results of many studies indicated that FWs can successfully achieve 53–80% and 53–85% removal of TSS and TDS, respectively (Charoenlarp et al 2016; Kadam et al 2018a; Nawaz et al 2020; Zhang et al 2015) Nutrients Nitrogen and phosphorus in wastewater are referred to as nutrients, and both are necessary elements for the growth of plants (Carey and Migliaccio 2009) However, high levels of nutrients in the aquatic ecosystem can lead to eutrophication and ultimately result in the degradation of the ecosystem FW is a low-cost and sustainable approach for nutrient removal from industrial wastewater In FWs, the main mechanisms of phosphorus removal are physical, including complexation, sorption, filtration, precipitation, and fixation (Lynch et al 2015; Schwammberger et al 2019; Stewart et al 2008) Compared with constructed wetlands, phosphorus sorption to FWs is limited because of the absence of soil-based barriers and other filtration materials Adding suspended substrate in FWs can improve the removal of phosphorus (Huang et al 2020) Assimilation by plants and microorganisms can also reduce phosphorus concentrations, especially the organic phosphorus (Spangler et al 2019) Bacteria degrade organic phosphorus into dissolved forms, which can be readily taken up by plants and other microorganisms (Bi et al 2019) The main process for nitrogen removal is biological nitrification–denitrification Nitrifying bacteria on biofilm O2−, which can be transform ammonia into N O3− and N assimilated by plants (Borin and Salvato 2012) During the nitrifying-utilization cycle, nitrate can be restored to N2 and N2O by denitrifying bacteria and released into the atmosphere A constructed FW was used for treating nitrogen-rich mining wastewater in a cold climate, and the results showed that macrophyte root-associated denitrification was the main pathway for nitrogen removal, and the nitrogen removal rates ranged between 32 and 2250 mg N 2O-N/(m2 *day) through the denitrification (Choudhury et al 2019) Moreover, there is an oxic-anoxic gradient in the rhizosphere of FWs, and this gradient helps to form a diversity of microorganism communities and therefore achieve better nitrification–denitrification (Oliveira et al 2021) Other nitrogen removal processes include sedimentation and uptake by plants Aquatic plants can adsorb ammonia and NO2− to support their growth (Li et al 2008), and the average nitrogen adsorption rates were reported in a wide range under different conditions, such as plant growth stage, plant species, types of wastewater, concentrations of dissolved oxygen, season, and other environmental factors (Emparan et al 2019) For instance, Spangler et al (2019) investigated five plant species in FWs for nutrients removal and found that plants selection and timing of harvesting affect nutrients removal Another research reported that the uptake of nitrogen was higher in spring and summer (Dong et al 2011) 13 The N/P ratio is helpful to generate an effective FW design The optimum N/P ratio usually depends on the specific plant species (Li et al 2017) A study found that the optimal N/P ratios for nutrient removal by M elatinoides, E crassipes, and A philoxeroides were 10:1, 20:1, and 20:1, respectively (Li et al 2021b) pH is also influential to the removal of nutrients A recent study reported that different pH conditions impacted nitrogen and phosphorus removal by aquatic macrophytes For example, almost 100% ammonium nitrogen and phosphate phosphorus were removed at pH 6.5 and 7.0, respectively, while the removal efficiency decreased to 60% at pH 5.0 and 6.0 (Qian et al 2020) FWs enhanced by other methods can achieve higher nutrients removal in wastewater treatment A study investigating the combination of FW islands with micro-bubble aeration and filtration media found that the combined system had a high removal rate for nutrients as micro-bubbles can prevent the formation of the anaerobic area in the lower zones of the wastewater (Yoon et al 2016) Another study reported that maximum reduction in TN and phosphate was achieved using plants assisted with endophyte (Ijaz et al 2015) In a study, water hyacinth and water lettuce were used to treat mixed industrial wastewater in an industrial park, and the phytoremediation reduced 71% of NH4+, 74% of TN, 57% of PO43−, and 64% of TP (Victor et al 2016) Another research investigated endophyte-assisted floating treatment wetland for the remediation of mixed wastewater (70% of domestic and 30% industrial effluent from an industrial park), and up to 90% and 39% reduction of nitrogen and phosphorus were obtained, respectively (Ijaz et al 2016) Heavy metals Industrial wastewater from mining, electroplating, steel and non-ferrous metallurgy and some chemical production usually contains high concentrations of heavy metals and some metal species (e.g., Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn) are significantly biologically toxic (Shrestha et al 2021) Heavy metals in industrial wastewater are in two forms: dissolved and particulate Plants can take up the dissolved fraction (Bi et al 2019) The uptake process is impacted by a series of factors, such as plant species, temperature, pH, heavy metal concentration, and the types of heavy metals (Dhir et al 2009) Meanwhile, high salinity was observed to reduce heavy metal removal efficiency because sodium ions can compete with heavy metals during the uptake by plants (Leblebici et al 2011) In FWs, the physicochemical processes affecting the transport and fate of heavy metals include adsorption, complexation, chelation, ion exchange, reduction–oxidation, uptake of plants and bacteria, entrapment into the biofilm, and metal sulfides formation (Ali et al 2020; Rezania et al 2016) 13 Environmental Science and Pollution Research Plant roots play a significant role in heavy metals removal as roots and attached biofilms can trap particulate heavy metals (Borne et al 2014) Moreover, plant roots can release exudates, excretion, lysates, and dead tissues to increase the humic content in water (Canellas et al 2020), which promotes the flocculation and complexation of dissolved metals (e.g., Cd, Cu, Ni, Pb, Zn) (Hankins et al 2006; Kulikowska et al 2015) The exudates of roots can also adsorb heavy metals, facilitating the formation of metal sulfides and hydroxides (Kim et al 2010) The decay of dead plant parts can enhance heavy metals removal by binding to sulfides and organic matter, metals associated with sulfides form insoluble metal sulfides (Borne et al 2015; Hankins et al 2006) It has been reported that Cu can also be combined with organic matter (e.g., humic acid) to form stable compounds (Fuentes et al 2013) Roots can accumulate a higher amount of metals than other tissues For example, roots were found to accumulate 80% of Zn and 40% of Cu in plants (Saleem et al 2019) Biofilm is another important FW component for metal removal It was found that the precipitation of several metals can be enhanced by the function of rhizosphere microorganisms (Ijaz et al 2015; Shahid et al 2020) A variety of plants are capable of removing heavy metals from wastewater A study screened the capacity of 34 plants to remove metals in water (Schück and Greger 2020), and their results showed that the highest removal efficiency can be up to 52–94% after 0.5 h and up to 98–100% after 5 days FWs implanted with different plants have been widely used to remove metals from various types of industrial wastewater and waterbody polluted by industrial effluent (Table 2) In Canada, floating cattail mats were used for acid mine drainage treatment (Chen et al 2016), and after 131 days, the system removed 88% of Fe and 77% of Ni, respectively The system also achieved 98% of Ni and 95% of Fe removal efficiency after operation for 2 years Recent studies showed that the inoculation of bacteria can effectively promote the heavy metal removal efficiency by aquatic plants in wastewater treatment For example, bacteria augmentation was found to significantly enhance the heavy metals removal from (46.4–70.0%) to (65.5–89.7%) of a FW in textile wastewater treatment after 20 days (Nawaz et al 2020) Summary of pollutant removal using FWs Floating Wetlands (FWs) hold great potential for a wide range of applications in treating pollutants in industrial wastewater, including organic compounds, total solids, nutrients, and heavy metals The treatment mechanisms involve the activities of root-associated microorganisms and plants, the formation of biofilms, physical sedimentation, adsorption, ion exchange, redox reactions, and various other processes These processes work synergistically to provide pathways for the efficient removal of pollutants 13 Floating material Wastewater characteristics (concentration in mg/L) COD = 1438 ± 12.7, PVC pipe, aluminum metal A baccifera BOD = 1230 ± 10.2, wire gauze, and PVC plastic F dichotoma TDS = 8230 ± 8.8, sheet (floating in tap water for two TSS = 5175 ± 0.7 months) PVC pipes, elbows, thermacol COD = 1495 ± 20, Vetiveria zizanioides BOD = 1135 ± 20, sheet and aluminum metal (floating in tap water for three TDS = 4706 ± 28, wire gauze months) TSS = 734 ± 8 PVC pipes, elbows, thermacol COD = 1794 ± 7, Vetiveria zizanioides, BOD = 1350 ± 13, sheet and aluminum metal Ipomoea aquatica TDS = 5143 ± 21, wire gauze (floating in tap water for three TSS = 1900 ± 15 months) PVC pipe and elbows COD = 1734 ± 1.8, Chrysopogon zizanioides, BOD = 1478 ± 1.5, Typha angustifolia TDS = 9060 ± 4.6, (planted over floating bed for TSS = 6438 ± 4.1 a month) PVC pipe rafts TS = 55.67 ± 5.72, Vetiveria zizanioides (NA) COD = 150.13 ± 39.06, BOD = 317 ± 16.73 Free floating COD = 177.76, BOD = 87.178 Eichhornia crassipes, Pistia stratiotes (NA) Free floating TDS = 2560.83, Eichhornia crassipes, TSS = 131.61, Pistia stratiotes BOD = 107.46, (stored in water tub) COD = 187.1 Polystyrene-based sheets TDS = 400, TSS = 92, Phragmites australis BOD = 121, COD = 310 (seeding on floating sheets supported by coconut shavings and soil) Diamond Jumbolon Roll, TDS = 4961, TSS = 4569, Phragmites australis, aluminum foil BOD = 249, COD = 471 Typha Domingensis (seeding on floating sheets supported by coconut shavings, gravel, sand and soil) Polystyrene sheets TDS = 5251 ± 404, Phragmites australis TSS = 324 ± 29.7, (seeding on floating sheets BOD = 283 ± 17.9, supported by coconut shavCOD = 513 ± 37.6 ings, and soil) Plant Species (Arrangement) Table 3 Treatment of textile wastewater using FWs 77 72 74 79 74 15.87 21.29 41.07 45.39 72 h - 91 73 54 1 year 8 days Pakistan Tara, et al (2019a) 92 - 86 - Pakistan Tara, et al (2019b) 92 87 97 Pakistan Qamar et al (2019) Pakistan Tusief et al (2020) Pakistan Nawaz et al (2020) 72 h 5 weeks 83–85 89–90 84–85 84–85 74–79 20 days - 88 Kadam, et al (2018b) Chandanshive et al (2020) Chandanshive et al (2020) Kadam, et al (2018a) Thailand Charoenlarp et al (2016) 31.69 57.78 - - 4 days 57 77 70 36 67 43 75 India India India 75 72 h 9 days 82 47 56 TSS India 66 66 TDS Retention Period Location Reference 61–76 74–79 71–84 75–83 31–51 5 days 81 BOD Color COD Removal efficiency (%) Environmental Science and Pollution Research Crude oil contaminated water Oil field-produced wastewater Refinery wastewater Diesel Palm oil mill effluent 52–67% - 93 98.9 52.18 90 97.4 Jumbolon sheets Polyethylene foam, covered with peat Diamond Jumbolon Role Jumbolon role (cells of polyethylene resins), aluminum foil, and polypropylene random copolymer pipes 90 days 2 weeks 14 days 3 days 99.1 95 18 months 3 months Pakistan Pakistan China Pakistan Malaysia Malaysia Pakistan Retention Period Location 40–55% 35 days 72.28 73.48 90 94 Polystyrene sheets 45 92.78 - 87.5 BOD TPH 25.24 87 COD Removal efficiency (%) NA Diamond Jumbolon Roll Typha domingensis COD = 538 ± 83, BOD = 228 ± 65, Oil and (seeding on floating sheets supported by tap grease = 17.4 ± 2.7 water for 30 days) COD = 210, BOD = 42.91 Eichhornia crassipes (NA) COD = 790–810, Vetiveria zizanioides BOD = 350–400 (grown in a hydroponic solution for 5 weeks) Cyperus laevigatus L Oil = 10,000, (seeding on floating COD = 10,000, sheets supported by tap BOD = 3500 water for 30 days) TPH = 1720, COD = 142.8 Geophila herbacea O Kumtze (GHK), Lolium perenn CV Caddieshack (LPT), Lolium perenne Topone (LPT), Lolium perenne L (LPL) (seedling establishment for 10 days) Typha domingensis, LepCOD = 1336 ± 58.74, tochloa fusca BOD = 405 ± 19.43, (seeding on floating sheets TPH = 316 ± 7.84 supported by coconut shavings, gravel, sand and soil, grown for 1 month) Phragmites australis, COD = 1316 ± 73.5, Typha domingensis, BOD = 365 ± 15.4, Leptochloa fusca, BraTPH = 319 ± 9.7 chiaria mutica (grown using plastic pots in a garden) Oil and grease Palm oil mill effluent Floating materials Wastewater characteristics Plant Species (mg/L) (Arrangement) Wastewater Table 4 Treatment of industrial oily wastewater using FWs Afzal et al (2019b) Rehman et al (2019b) Li et al., (2012) Fahid et al (2020) Darajeh et al (2014) Tan (2019) Ijaz et al (2016) Reference Environmental Science and Pollution Research 13 Darajeh et al (2016) Malaysia 4 weeks 96 94 Polystyrene platform 90.28 81.1 Palm oil mill effluent COD = 150–160, BOD = 20–25, Oil and grease = 0.15 COD = 750, BOD = 350, Oil and grease = 15 Crude oil spilled water Chrysopogon zizanioides (grown in a temporary hydroponic nursery for 5 weeks) Pakistan 42 days 97 97 Brachiara mutica, Phrag- Diamond Jumbolon Role, 93 and aluminum foil mites australis (seeding on floating sheets supported by coconut shavings, gravel, sand and soil, grown for 1 month) Vetiveria zizanioides Plastic pots with rockwool 81.69 (NA) inside COD = 1324 ± 66.5, BOD = 475 ± 15.5, oil = 325 ± 10.1 Oil field wastewater BOD TPH COD 4 weeks Reference Retention Period Location Removal efficiency (%) Floating materials Wastewater characteristics Plant Species (mg/L) (Arrangement) Wastewater Table 4 (continued) 13 Indonesia Effendi et al (2017) Environmental Science and Pollution Research Rehman et al (2018) effluent by FWs planted with chrysopogon zizanioides in different conditions (Darajeh et al 2016) The study validated the model established using response surface methodology under three factors (i.e., oil concentration, plant density, and time), and showed the optimum decreases in BOD and COD were 96% and 94%, respectively, after 4 weeks using 30 plants The model showed that plant density has the most significant effect on the BOD and COD removal and had good prediction performances in terms of COD (adjusted R2 = 0.974) and BOD (adjusted R 2 = 0.957) removal (Darajeh et al 2016) Bacteria-plant synergism was reported to be able to enhance the treatment efficiency of FWs A study quantitatively assessed the performance of endophyte-assisted FWs in the treatment of oil and grease-contaminated wastewater (Ijaz et al 2016), and it found that the plants inoculated with a consortium of pollutant-degrading and plant growthpromoting endophytic bacteria reduced COD and BOD by 87.0% and 87.5%, respectively Another study investigated the plant synergism in FWs for remediation of oil field wastewater (Rehman et al 2018), while FWs were used in combination with hydrocarbon-degrading bacteria Their results showed that both experimental plants can remove organic and inorganic pollutants from oily wastewater, and bioaugmentation significantly increased the pollutants removal efficiency from 76 to 97% for oil, 80% to 93% for COD, and 85% to 97% for BOD in the wastewater Moreover, the analysis of alkane-degrading gene abundance and its expression profile further validated a higher microbial growth and degradation activity in the water around plant roots and shoots (Rehman et al 2018) Another study used similar bioaugmented FWs to remediate oil field wastewater, and the highest reduction efficiencies were observed as 95% for TPH, 90% for COD, and 93% for BOD (Rehman et al 2019b) Their study also found that the average fresh biomass, dry biomass, and length of the plants increased by 31%, 52%, and 25%, respectively, after the treatment FWs were installed at a large scale for remediation of oil-contaminated wastewater, and the plants in the FWs were inoculated with a consortium of 10 different species of hydrocarbondegrading bacteria (Afzal et al 2019b), with the treatment reducing 97.4% of COD, 98.9% of BOD, 82.4% of TDS, 99.1% of hydrocarbon content, and 80% of heavy metals within 18 months Wastewater from other industries FW has been applied for treating other types of industrial wastewater (Table 5) FWs have been effectively employed for the treatment of acid mine drainage (AMD), paper industry effluents, batik industry effluents, and food industry wastewater, demonstrating their potential in reducing pollutants and improving water quality In a recent study, Cd = 0.02, Cu = 4.78 Fe = 81.4 ± 1.16, Al = 70.3 ± 1.11, Mn = 21.9 ± 0.27, Zn = 1.372 ± 0.0436, Ni = 0.697 ± 0.0413, Cu = 0.184 ± 0.00435, Pb = 0.08 ± 0.0341, Cr = 0.01 Fe = 12, Al = 11.3, Zn = 0.385, Ni = 0.388, Cu = 0.0218, Pb = 0.0105 Acid mine drainage Acid mine drainage TDS = 1840, EC = 2.64 dS/m, BOD = 475.1, COD = 880.5, TKN = 192.65, PO43− = 145.6, Cd = 2.45, Cr = 1.38, Cu = 5.64, Fe = 8.95, Mn = 3.66, Pb = 1.74, Ni = 1.02, Zn = 6.9 Benzene = 13 ± 3, MTBE = 2.2 ± 0.5 TS = 1671.58 ± 177, COD = 5866.67 ± 924, BOD = 2282.75 Pulp and paper mill effluent Chemical contaminated water Dairy wastewater Acid mine drainage Wastewater characteristics (mg/L) Wastewater Floating materials Phragmites australis (NA) Eichhornia crassipes Eichhornia paniculata Polygonum ferrugineum Borreria scabiosoides (conditioned in containers with fluvial) Water caltrop Water hyacinth (NA) Polyethylene terephthalate NA NA Chrysopogon zizanioides NA (grown in potting soil) Water hyacinth Free floating (NA) Chrysopogon zizanioides Plywood and PVC pipes (maintained in hydroponic media for up to 4 months) Plant species (Arrangement) Table 5 Treatment of different types of industrial wastewater using FWs High plant uptake of Fe (74.6%), Zn (30.1%), and Cu (140%), with relatively lower plant uptake of Pb, Al, and Ni (