uncorrected proofs

REVIEW Sustainable Green Solutions

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Munazzam JawadQ1 Shahid, Muhammad Arslan, Shafaqat Ali, Muhammad Siddique, and Muhammad Afzal* or degrade toxic compounds. The history of using NFWs to treat contaminated water dates back to the classical era.[6] Nevertheless, first comprehensive description was provided by Russel in 1942 who used the term “flotant” and described the formation of these thick root mats that allows emergent macrophytes to grow on the floating mat.[7] Later, van Duzer compiled the bibliography on NFWs from about 1800 literature sources in 20 languages and described their important features such as development scheme, basis of buoyancy, ecological characteristics, control and management, wildlife habitat, and enhancement of water quality.[8] In recent years, variants of NFWs are exploited to treat anthropogenic discharge such as municipal wastewater, agriculture runoff, industrial effluents, etc.[9–13] This includes constructed wetlands (CWs), floating treatment wetlands (FTWs), hydroponic root mats (HRMs), etc. (Table 1).[14–16] In FTWs, plants are grown hydroponically on floating structures such as tethered buoyant mats/rafts while their roots are extended down to the pelagic zones touching the system’s bottom (Figure 1).[17] The treatment system shares features of both pond and wetland where hydraulic gradient is established between the root network and the pond bottom, and pollutants are trapped, filtered, and/or degraded by the combined action of roots and their associated microbial communities.[18] The commercial application of FTWs was first seen for the remediation of eutrophic lakes in Germany and for rivers in Japan.[19,20] Currently, they are being used to treat hypertrophication,[21–26] sewage and domestic wastewater,[27–34] combined sewer overflow,[35] contaminated groundwater,[36,37] stormwater,[38–47] acid mine drainage,[48–50] poultry effluent,[51] piggery effluent,[52] boron enriched water,[53] polluted river water,[54] and nutrients enriched water.[55,56] Recently, the plant–bacteria partnerships have been exploited in FTWs for the maximum remediation of contaminated water.[14,34,57,58] This review presents insights into the structure and design of FTWs, the role of plants and bacteria and their synergism in the removal of pollutants from wastewater, and the factors affecting treatment performance. Moreover, benefits and limitation of using FTWs in multiple contexts are also discussed.

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1. Introduction

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Natural floating wetlands (NFWs), also known as floating marshes, are present in many areas of the world. They comprise emergent vascular plants that grow over the surface of free water by a mat of alive and dead roots, peat, and/or detritus.[1–3] In nature, they perform a variety of ecosystem services related to hydrological, biological, and biogeochemical processes.[4,5] An increasingly important service is treatment of wastewater due to the ability of plants to absorb nutrients and/

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M. J. Shahid, S. Ali, M. Siddique Department Q2of Environmental Sciences and Engineering Government College University Faisalabad, Pakistan M. Arslan Department of Environmental Biotechnology Helmholtz Centre for Environmental Research Leipzig, Germany Dr. M. Afzal Soil and Environmental Biotechnology Division National Institute of Biotechnology and Genetic Engineering (NIBGE) P. O. Box 577, Jhang Road, Faisalabad, Pakistan E-mail: [email protected]

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Floating treatment wetland (FTW) is an effective and sustainable technology for wastewater treatment. It has been widely adopted for treating various kinds of polluted water including agricultural runoff, stormwater, industrial effluents, etc. In FTWs, plants are vegetated on a floating mat while their roots are extended down to the contaminated water acting as biological filters. Nutrients and potentially toxic metal(s)/element(s) are taken up from the wastewater by plants through their roots whereas organic matter is degraded by the microorganisms forming biofilms on the roots and mat surface. Additionally, organic contaminants which are already taken up by the plants are degraded by endophytic bacteria in planta. This article provides an overview of FTWs and its application for wastewater treatment. The key factors which have an impact on the performance of FTWs are also described. Lastly, potential role of combined use of plants and bacteria in FTWs for the maximum remediation of polluted water is emphasized.

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Floating Wetlands: An Innovative Tool for Wastewater Treatment

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www.clean-journal.com

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/clen.201800120.

DOI: 10.1002/clen.201800120 Clean – Soil, Air, Water 2018, 1800120

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© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Table 1. Diverse terminology for floating treatment wetlands.

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3. Role of Plants

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In FTWs, plants perform several functions necessary for the wastewater treatment.[62] This includes stabilization of the pond bed, reducing water flow and turbulence, increasing sedimentation, trapping/filtering suspended particles, and providing habitat for the development of microbial communities.[62,63] The biological processes are more effective in FTWs as compared to the constructed wetlands due to the free suspension of roots in the water column, which establishes direct contact between contaminants and the root-associated microbial community.[46] Microorganisms decompose organic matter into simple nutrients which are removed by plants through direct uptake.[64,65] Moreover, the roots grew horizontally and vertically developing a large surface area for the nutrients uptake.[66] Plants also release various kinds of organic compounds (root exudates) that regulate the biological processes such as denitrification.[67] This is particularly effective for nutrient-rich surface water in which nitrate reducers convert nitrates into N2 gas and release it into the atmosphere. Furthermore, plants take up nutrients for their growth and development and hence lower the eutrophication level. It has been reported that emergent plant species in floating wetlands are able to take up nitrogen from 200 to 2500 kg ha1 per year.[65,67] Accordingly, plants boost the removal of fine suspended particles by adhering them to the root system and cause a prominent reduction in turbidity of water. Plants also produce bioactive compounds that undergo physicochemical changes in water body leading to enhanced sorption and sedimentation processes.[17,63,68] A few plant

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Artificial floating beds

[48]

Floating island system

[19]

Floating beds

[43]

Floating bioplato

[61]

Hydroponic root mats

[44]

2. Structure of FTWs

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The structure of FTWs is strikingly similar to the other traditional wetlands except that, in FTWs, plants are supported by artificial buoyant mats. These floating mats keep plants crown elevated above the water level thus letting them establish their roots in the deeper zones. Buoyancy in FTWs is maintained either by using low density plant holding material or by using plants with aerenchymatous abilities.[59–61] For instance, variety of construction materials, such as polyester sheets, PVC pipes, and bamboo containing meshes, have been used to construct the floating structures (Figure 2). On the other hand, helophytic plants are preferred in designing FTWs as they possess natural ability of entrapping gases within the rhizomes, making them to float on the water surface. Nevertheless, durability, functionality, weight, environmental sensitivity, anchoring, flexibility, and cost are the principal factors in designing buoyant mats.[39] Comprehensively speaking, an artificially designed floating

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[11,25,28,38–41,49,50,58–60]

Floating treatment wetlands

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[23,47]

Floating island

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[27]

Constructed floating wetlands

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Specific term

mat should complement the natural floating mat that is made up of intertwined roots, rhizome, organic matter, and plant litter.[16]

Figure 1. Schematic representation of floating treatment wetlands elucidating pollutants removal by different mechanisms.

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4. Plant–Bacterial Synergism in FTWs

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Interactions between plants, water, and microorganisms are not fully explored in FTWs, yet bacteria play a vital role in such systems.[75] Bacteria degrade organic pollutants, produce phytohormones, increase disease resistance in the host plant, and remove stress due to the presence of contaminants.[81–83] In return, they get nutrients and residency in different parts of the plant. Floating treatment wetlands technology, an in situ solar powered remediation in synergism with bacteria, has great potential for ecosystem restoration with minimum site disturbance, maintenance, and financial input.[34,76] Although plants can partially degrade the organic compounds, nevertheless complete degradation is indispensable without effective partnership with microorganisms.[76,77] The partnership can be neutral, commensal, and dormant.[75,78–80] Plant roots are the primary site for bacteria to colonize plant tissue. However, they do not remain limited to roots and enter the plant to dwell inside and to get nutrients.[81–84] Bacteria that colonize on the root surface are known as rhizobacteria whereas those who colonize in the plant interior are known as endophytic bacteria.[81,84] These bacteria render degradation of toxic organic compounds into CO2 and H2O which are subsequently used by the plants or released from the system with no toxic effects (Figure 3). Biodegradation of hexachlorobenzene (HCB) in water has been effectively accomplished by rhizosphere effect of Typha angustifolia and specific bacterial community found in the rhizosphere.[96] In an evaluation of using vetiver plantlets (Vetiveria zizanioides) grown on a floating platform for degradation of phenol, it was found that rhizomicrobial growth was 100 times more on roots of vetiver

5.1. Water Depth

37

Water depth is an important factor to be considered while designing the FTWs. This is because of the reason that the FTW system is devoid of soil/substrate and plants solely depending on their rooting structure in the free water column for nutrients acquisition. For root development, at least 0.8–1 m of water depth is required for horizontal and vertical growth.[16] However, increased water depth enhances the treatment performance of FTWs due to increased contact time of pollutants with roots and the microbial biofilm.[17] Additional factors such as type of wastewater, treatment purpose, and inflow variations also determine the choice of the water depth. For instance, low water depth is suitable to remove fine particles and suspended solids due to increased contact between roots, wastewater, and microorganisms.[36] On the other hand, high water depth is more suitable to remediate coarse suspended solids by sedimentation due to the formation of a free water zone below the mat down to the bottom of the water column.[90]

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5. Factors Affecting FTWs Performance

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species such as helophytes release atmospheric oxygen into the rhizosphere.[69,70] This release of oxygen takes place mostly in daylight due to photosynthesis.[71,72] The oxygen released by roots regulates redox potential,[73] which affect the nitrogen fate, oxidation of specific phytotoxins, and aerobic degradation of organic matter by microorganisms.[74]

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Figure 2. Schematic representation of a floating mat with vegetation.

plantlets which increased the degradation rate of phenol by more than threefold.[97] Similar to the rhizospheric degradation, inoculation of endophytic bacteria, Microbacterium arborescens (TYSI04) and Bacillus pumilus (PIRI30), helped Typha domingensis to degrade organic pollutants present in the textile effluent, along with reduced toxic effects and improved plant health.[85] Accordingly, endophytes inoculation to Pisum sativum enhanced removal of 2,4-dichlorophenoxyacetic acid without its accumulation in plant’s tissues.[83] The pollutant degradation by endophytic bacteria is not limited to organic contaminants only but can be equally exploited in remediation of metal(s)/element(s). For example, application of endophytic bacteria, previously isolated from Prosopis juliflora grown in soil contaminated with tannery effluent with a high concentration of potentially toxic metals (Cr, Cd, Pb, Cu, and Zn), expressively enhanced the uptake of Cr in ryegrass.[86,87] Similarly, uptake of Zn was increased considerably by inoculation of two grass species, Festuca rundinacea and Lolium perenne, by the endophytic bacterial strain Neotyphodium.[88] Besides degradation, plant–bacteria synergism can also improve the nutrients removal from contaminated water. Li et al.[89] investigated the potential of plants–microorganism synergism by using two species of perennial grasses (L. perenne) and a microbial consortium. The results showed that plant–microbial synergism enhanced the removal of total nitrogen (TN), ammonium (NHþ 4 –N), and total phosphorus (TP) from wastewater.[89] In another study, tall fescue plants (F. arundinacea) in combination with denitrifying polyphosphate accumulating microorganisms exhibited significantly more TN, TP, orthophosphate, NHþ 4 -N, and nitrate (NO3 ) removal from wastewater as compared to fescue plants without bacterial inoculation.[99] More examples of plant–bacteria partnership for enhanced removal of pollutants are described in Table 2.

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Various plant types have been used to establish FTWs, for example, Agrostis alba, Canna flaccid, Carex lasiocarpa, Chamaedaphen calyculata, Glyceria maxima, Menyanthes trifoliate, Myrica gale, Phragmites australis, Panicum hemitomon, Pontederia cordata, Torilis japonica, Typha latifolia, T. angustifolia, etc.[18,91,92] Their choice largely depends on type of contamination, ability of plants to make dense root structures, adoptability to water composition and climatic condition, and local availability and influence. Nevertheless, plants having a fibrous root system, high transpiration rate, release of oxygen through roots in the water, and high total nitrogen removal rate are extensively employed in FTWs.[49,89] This is particularly true when the wastewater contains high organic matter and poor dissolved oxygen. Helophytes such as Juncus effuses and P. australis are the most suitable choices because of their ability to transport atmospheric oxygen into the plant rhizosphere. Similarly, A. alba, C. flaccid, P. cordata, and T. japonica were found to be effective for nutrient removal.[93,94] In another study, T. japonica has shown better nutrient removal rates via biomassbased method compared to area-based method.[93] The plant growth and gain in biomass depend on plant species, age, type of wastewater, nutrients concentration, redox condition of water, trophic level of water, and supporting mat.[95] High nutrient load and toxic compounds may affect development of plants especially at the young stage.[96] In an earlier study, root development in six species namely Oenanthe javanica, Gypsophila sp., Salix babylonica, Gardenia jasminoides, Rohdea japonica, and Dracaena sanderiana were observed to be 10.9, 10.8, 9.0, 6.1, 4.5, and 4.1 cm, respectively.[54] O. javanica exhibited more roots than the other plants and hence was considered as a suitable plant type for FTWs. Likewise, another study reported Juncus edgariae to develop intensive root network (root length of

87 cm) during the treatment of storm wastewater.[43] Some wetland plants are found to be sensitive to the variation of water level such as T. domingensis showing about 52% decrease in biomass due to water level fluctuation.[97] Whereas some species, such as Juncus spp., Phalaris arundinacea, and P. australias, have great morphological adaption to the fluctuation of water levels.[95,98] These plants can be utilized in stormwater ponds which show great fluctuations of the water level.

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5.3. Season and Temperature

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Solar radiations and air temperature have a prominent effect on wetland performance.[145] This is because of the reason that pollutants are degraded from the wastewater via a combination of biogeochemical processes governed by meteorological parameters. It is not easy to measure their effect individually[72,99]; nevertheless, seasonality has been considered a prominent factor in the pollutant removal. During spring, microbial proliferation and plant growth are enhanced which promote pollutant degradation whereas during fall and winter, reduced plant growth and bacterial metabolism result in less removal of contaminants. Picard et al. conducted a study to analyze this effect and reported that nutrient removal was maximum during plants growing season (Jun–Oct) and minimum in winter (Nov–Mar).[100] This positive relationship between temperature and nutrients removal was also reported in few other studies.[85,101] It has been further argued that the treatment performance for all pollutants does not solely depend on temperature.[99] For instance, nitrogen removal is directly related to temperature and season because bacteria involved in biological nitrification– denitrification processes are highly sensitive to temperature changes. The highest removal of TN has been noted for a

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Figure 3. Degradation of organic compounds by plant-bacteria partnership.


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Table 2. Plant bacteria partnership for pollutants remediation. Plant

Bacterium

Ricinus communis

Pseudomonas sp. M6; Pseudomonas jessenii M15

Panicum virgatum

Burkholderia xenovorans LB400

Festuca pratensis, Festuca arundinacea

Neotyphodium coenophialum

Leptochloa fusca

Pantoea stewartii ASI11, Microbacterium arborescens HU33, and Enterobacter sp. HU38

Populus trichocarpa

Pseudomonas putida W619-TCE M. arborescens HU33, P. stewartii ASI11, and Enterobacter sp. HU38

Populus trichocarpa

Methylobacterium populi

Lolium multiforum

Enterobacter ludwigii strains

Solanum nigrum

Serratia sp. LRE07

Lolium multiflorum

Pseudomonas sp. strain ITRI53, Pseudomonas sp. strain MixRI75

Brachiaria mutica

M. arborescens HU33, P. stewartii ASI11, and Enterobacter sp. HU38

Sorghum bicolor

Bacillus sp. SLS18

Populus trichocarpa

Enterobacter sp. Strain PDN3

Leptochloa fusca

P. stewartii ASI11, Enterobacter sp. HU38, and M. arborescens HU33

Sedum alfredii

Sphingomonas SaMR12

Salix alba

Pseudomonas putida PD1

Amaranthus hypochondriacus, Amaranthus mangostanus

Rahnella sp. JN27

Noccaea caerulescens, Thlaspi perfoliatum

Microbacterium sp. NCr-8, Arthrobacter sp. NCr-1, Bacillus sp. NCr-5, Bacillus sp. NCr-9, Kocuria sp. NCr-3

Leptochloa fusca, Brachiaria mutica Axonopus affinis

Reference

Ni, Cu, and Zn

[100]

Organic pesticides; PCBs

[101,102]

Cd

[103]

Cr, Mn, Ni, Pb, and TOC

[104]

TCE

[88]

COD, BOD and Cr

[105]

TNT, RDX, and HMX

[106]

Diesel

[107]

Cd, Cr, Pb, and Cu

[108]

Diesel

[109]

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Typha domingensis

Pollutant

[110]

Cd and Mn

[111]

TCE

[112]

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[113]

Zn

[114]

Cd

[115]

Cd

[116]

Ni

[117]

Acinetobacter sp. strain BRSI56, Pseudomonas aeruginosa strain BRRI54, and Klebsiella sp. strain LCRI87

Crude oil

[118]

Pseudomonas sp. ITRH25, Pantoea sp. BTRH79, and Burkholderia sp. PsJN

Diesel

[119]

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Cr, Mn, Ni, Pb, COD, BOD, and TOC

HMX, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine; PCB, polychlorinated biphenyl; RDX, 1,3,5-trinitro-1,3,5-triazacyclohexane; TCE, 1,1,1-trichloroethane; TNT, 2,4,6trinitrotoluene.

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temperature range from 5 to 15 C.[33] By contrast, temperature change had a low impact on the process of phosphorus removal, that is, even in temperature range of 5 to 15 C during summer and autumn, phosphorus removal appeared to be unaffected.[100–103] Seasonal differences in combination with plant species also strongly affect total organic carbon (TOC) removal and oxidation of the root zone. Increased temperature of >35 C showed a decline in TOC content of the wastewater which was the result of increased microbial activity.[104–106] In contrast, efficiency of plants to remove organic matter was reduced significantly with change in temperature from 24 to 4 C.[150] Lastly, chemical oxygen demand (COD) can also be influenced by the combined effect of season, temperature, and vegetation.[104]

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5.4. Aeration

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The nutrients removal in ecological approaches for wastewater treatment such as wetlands can be maximized by improving the aeration of wastewater.[107,108] The aeration develops the aerobic micro-zones which stimulate the biofilm production in the substrate. It promotes individual as well as simultaneous removal of ammonium and nitrate by the aerobic processes. In a


study, nitrate/nitrite removal was improved from 1.7% to 33.8% with an increase in gas–water ration from 0 to 5 units in FTWs.[26] The aerated wetlands have been found to have lower CH4 fluxes than non-aerated units as aeration helps efficient release of oxidized nitrogen over ammonium in the effluent. The oxidized nitrogen is better than ammonium due to its low toxicity and easy removal by denitrification in the natural system.[109] Likewise, COD reduction in winter as compared to summer can be improved by aeration system.[109,110]

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5.5. Plants Harvesting

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Active plant harvesting can be utilized to expedite additional nutrients removal and to limit internal nutrient cycle in FTWs.[44] Previous study reported that about half of the nutrients are accumulated in the shoots of wetland plants.[17] The aerial biomass has been positively linked to the removal of nitrogen and phosphorus. As plant shoots can accumulate more nutrients during vegetative stage,[111] it is important to observe the temporal accumulation in order to optimize the harvesting strategy. A study on picker weed and soft-stem bulrush recommended harvesting in July/August and in October,

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In FTW systems, microorganisms attached to the roots, rhizomes, and substrate, contribute vitally to the removal of organic matter from wastewater.[33,119] Nevertheless, other processes such as filtration, nutrient assimilation, and oxygenation also contribute to the removal of organic matter from the water column.[118,120,121] The biochemical oxygen demand (BOD) and COD ratio is a key driver to investigate the presence of organic matter and its degradability. It has been reported that wastewater with a BOD/COD ratio >0.5 contains higher load of organic matter and hence are easily biodegradable.[122,123] Since most of the wastewater has BOD/COD ratio ranging between 0.6 and 0.8[124]; application of FTWs can reduce high proportion of COD and BOD without any additional procedure.[29,34,125] Nevertheless, based on the literature, Chen et al.[14] reported that efficiency of FTWs for COD and BOD removal varies from 17% to 84% and 36% to 90%, respectively.

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6.3. Potentially Toxic Metals

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In FTWs, processes like adsorption, metal sulfides formation, direct uptake by plants, bacteria, and algae, and entrapment into the roots biofilm play key role in potentially toxic metals removal (Figure 4).[18] Several potentially toxic metals can bind to tiny clay particles which are precipitated to the system’s bottom.[117,144–147] This precipitation is stimulated by roots exudates by forming metals sulfides and hydroxides.[148–150] The roots in the water column increase the humic content of suspended particles by releasing of rhizodeposits such as dead tissues, exudates, excretion, and lysates, and enhance complexation and flocculation of dissolved metals.[151,152] The dead parts of plants and release of organic compounds in the water increase the sorption of dissolved potentially toxic metals.[117] The precipitated potentially toxic metals are often taken up by the plant tissues, particularly the roots.

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The application of FTWs can remove both organic and inorganic (nutrients, potentially toxic metals, and suspended solids) pollutants from wastewater by different mechanisms as explained below.

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Nutrients removal in FTWs takes place through a number of biogeochemical processes.[25] First of all, plants develop strong root systems which can dynamically uptake nutrients from the water column.[126–129] This is mainly true for nitrogen as ammonia and NO 2 are adsorbed quickly by higher plants and microalgae which are commonly found in water ponds.[130,131] Nevertheless, some plants also demand NO 3 at their specific stages of life cycle for its reproduction process.[132] Nitrogen removal efficiency of FTWs depends upon growth characteristics of plant species, plant age, climate, wastewater, and other environmental circumstances.[179] The average nitrogen absorption rate by plants is highly variable, reported in the range of between 4% and 90%. Since direct uptake cannot exceed beyond a certain limit, further nitrogen removal is aided by microbial biofilm developed on the root surface through biological nitrification–denitrification processes.[134,135] Ammonia is oxi dized to NO 3 through NO2 by nitrifying bacteria in aerobic conditions, and NO3 is restored to molecular nitrogen by denitrifying bacteria. This nitrogen is released into the atmosphere in the form of N2 gas. Denitrification in eutrophic water takes place in the availability of sufficient amount of NO 3 and easily available organic compounds (Figure 1).[136,137] The removal mechanisms for phosphorus in FTWs are sorption, complexation, precipitation, and assimilation into microbial and plant biomass.[89,138,139] The soil and litter particles perform a key role in long-term storage of phosphorus in wetlands. The rates of phosphorus uptake by vascular plants have been studied by many researchers and found that biomass removal with its accumulated phosphorus is an effective way to control eutrophication in freshwater.[140,141] The phosphorus removal potential of FTWs depends upon the plant growth rate and phosphorus uptake by tissues.[142] Wen and Recknagel[129] reported a phosphorus removal rate of 0.086 g2 per day under controlled conditions in an FTW study. In another study, FTW planted with J. ffuses removed 48% phosphorous within 7-day retention time from nutrients enriched water.[143] In a recent study, Keizer-Vlek et al. reported a 60% removal of phosphorous by FTW system applied for the remediation of eutrophic water.[55]

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respectively, to obtain maximum nutrients removal. Another study also suggested harvesting in July and October for maximum removal of phosphorus and nitrogen.[44] Interestingly, external changes in the environment such as temperature may cause the transfer of nutrients from above to below ground tissues.[112] Therefore, harvesting should be performed when high accumulation of nutrients is observed in harvestable parts of plants in FTWs.[38] In a study conducted in Singapore, above ground tissue of Chrysopogon zizanioides and T. angustifolia were repeatedly harvested within a year and analyzed. Phosphorus and nitrogen contents in the shoots tissues were decreased when shoots acquired maturity.[113] Therefore, it was recommended that shoots should be harvested before the culmination of growing season to achieve the maximum removal of nutrients. On the contrary, it is important to consider that harvesting increases organic carbon removal[114,115] and hence may result in a potential decrease of carbon available to nitrogen processing bacteria.[84] The early shoot harvesting, on the other hand, may decrease the nutrients removal efficiency of the plants thus reducing their ability to remobilize nutrients from the stem to the storage organs.[116] Moreover, it is noteworthy that harvesting should not affect the plant health. Harvesting of the whole plant, as mentioned by White and Cousin,[46] is more violent and less sustainable practices as compared to harvesting of only above mat tissues for nutrients removal. Harvesting of entire plants may lead to the complete removal of plants from floating structure and growth media, hence, increases operational and maintenance costs making this approach un-economical.[46]

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Figure 4. Remediation of heavy metals by bacterial interaction (re-drawn from ref. [180]).

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6.4. Suspended Solids

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metals in an FTW pond may have a little adverse effect on the biota, however, the toxicity of metals in an FTW pond depends upon metals bioavailability. About 80% of Cu and Zn bound to organic matter and sulfide was found stable under reduced conditions in an FTW pond.[58] Although sediment re-suspension may affect the bioavailability of metals, very limited resuspension was noticed in FTWs.[169] A summary of previous research work about the removal of organic matter, nutrients and potentially toxic metals by FTWs is described in Table 3.

Total suspended solids (TSS) are mainly removed by physical settling and filtration process in FTWs. Minimized turbulence and free water layer between root mats and system’s bottom create ideal conditions for the particles sedimentation.[95] Moreover, roots of plants allow entrapping/filtering of the suspended particles. In an earlier study, FTWs showed a successful potential to remove TSS and sediment-bound pollutants from stormwater.[47] Another study reported the success of FTWs applied to an acid mine drainage pond, which was able to capture 2.2 kg of solids/m2 of floating vegetation after two growth seasons.[48] The trapped solids sporadically sloughed from roots and settled to the pond’s bottom which opened up more root surfaces for further entrapment. Tanner and Headley reported 34–44% reduction in fine suspended solids while using FTWs for stormwater treatment.[17] Likewise, a pond with floating plant mats showed 41% more reduction in TSS than a pond without any FTW system.[117] The average TSS removal rates in many earlier studies were reported between 2.7 and 7.1 g m2 per day with a highest removal rate of 45 g m2 per day.[19,24,41,164] Besides entrapment, role of microbial community is evident only when a major portion of suspended solid is organic.[118]

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7. Benefits of FTWs

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The FTWs can be installed in any pond or existing water body without any digging/earth moving,[47] and can remove pollutants from the water without additional land acquisition.[47,90] The FTWs do not decrease the storage volume of the pond due to their floating nature.[47] While utilizing the power of nature and sunlight, polluted water can be cleaned by FTWs in a sustainable

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Ladislas et al.[41] reported successful uptake of Ni and Zn in Juncus and Carex species grown in an FTW experiment. Additionally, rhizo- and endophytic bacteria are found to have a key role in the potentially toxic metals remediation. Inoculation of these bacteria enhanced potentially toxic metals removal rate due to their capacity of the sorbing metallic ions on the cell walls.[153] Moreover, they increased the bioavailability of potentially toxic metals and hence uptake by plants.[87,154] A number of attempts have been made to establish successful partnerships between plant and metal-resistance bacteria for effective treatment of potentially toxic metals contaminated water.[84,155–159] In a recent study, FTWs vegetated with Brachia mutica removed potentially toxic metals (Cd, Fe, Cu, Cr, Mn, Co, Pb) from the sewage effluent and the iron removal was 79–85%.[34] The dissolved oxygen in FTW also plays important role in the removal of metals by creating suitable conditions for the formation of particulate metal sulfides.[18,160] Depletion of oxygen by microbes on the rhizoplane also prompts the reduction of potentially toxic metals such as Cr, Mn, and Fe.[95] Moreover, the combined action of microorganisms and oxygen prompt the formation of iron and manganese plagues on the roots of the plants.[161,162] These plagues are responsible for binding metals such as Zn or Cu.[163] The removal of Zn and Cu by plague was 2% and 10%, respectively. This removal is strongly associated with the amount of dissolved oxygen, pH, and specific conductivity of the water column. In an earlier study, roots accumulated 40% of Cu and 80% of Zn of the total amount of metals accumulated in the plants.[58] In another earlier study, roots were found to accumulate almost twofold higher amount of metals as compared to shoots in wetland studies receiving agricultural and highway runoff.[117] Although plants remove metals through uptake, it is not a prime process for removal of potentially toxic metals from the FTW system.[17,18,164,165] The decay of the dead plant parts and reduced conditions are supposed to be liable for Zn removal by enhancing sorption, flocculation, and stable storage of Zn by binding to organic matter and sulfides and subsequent settlement on the bottom of the pond.[166] In the sediment of a wetland, the binding of metals with organic carbon and sulfide affects the metal sequestration as well as control their bioavailability. In the sediment of a retention pond, Cu is more efficiently sorbed by organic matter. This binding of Cu with humic acid is more stable than binding of Zn with luvic acids.[167,168] The sediment with accumulated

U

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Table 3. Removal of organic matter, nutrients, and heavy metals from wastewater by floating treatment wetland. Study level

Floating mat

Plant

Pollution reduction

Domestic

Pilot scale

Composed of plastic pipes with foam to create buoyancy covered with rough messed wire netting and on the top of netting coconut coir was put to hold vegetation.

Care spp., Lythrum salicaria, Phragmites australis, and Juncus effusus

TN: 42%, NH4-N: 35%, P: 22%, COD: 53%

[27]

Domestic

Laboratory scale

Composed of porous commercial mats made from 100% recycled plastic/ shredded carpet fibers contained a porous mesh.

Cyperus ustulatus

TN: 50%, COD: 90%

[23]

Storm water

Pilot scale

Floating mats made of inter-wined Carex virgate, Cyperus ustulatus, Juncus polyester fiber with approximately 95% edgariae, and Schoenoplectus porosity, and patches of polystyrene foam tabernaemontani for buoyancy, supported with fiberglass rods arranged criss-crossed from corner to corner of tanks.

Cu: 5.6–7.7 mg m2 per day, Zn: 25– 104 mg m2 per day P: 3–27 mg m2 per day, FSS: 34–44%

[11]

Domestic

Field scale

Pipe-rope bed supported with PVC pipes, ropes fixed around PVC pipes, each rope having knots to grow and fix plants.

O. javanica Oj), Gypsophila sp. (Gy), Rohdea japonica (Rj), Dracaena sanderiana (Ds), Gardenia jasminoides Var. grandiflora (Gg), Gardenia jasminoides Var. prostrata (Gp), and Salix babylonica (Sb)

TN and TP mass uptake (g m2): Oj: 2.17 and 0.167, Gy: 3.92 and 0.247, Rj: 0.51 and 0.062, Ds: 1.72 and 0.078, Gg: 0.61 and 0.075, Gp: 0.64 and 0.071, Sb:4.48 and 0.331

[48]

Polluted river water

Field scale

Made form PVC pipes, two adsorptive biofilm materials composed of multifaced hollow ball made of bamboo slice hollow ball and elasticity packing were set under the plastic nets. Plants transplanted into nylon rhizo-bags filled with soil and charcoal. At upper side plastic net was set to prevent plant lodging.

Eichhirnia crasslpes, Pisitia stratiotes, Jussiaea reppens, Hydrocotyle verticillata, Hydrocharis dubia, Myriophyllum aquaticum, Pontederia cordata, Canna indica, and Calla palustris

NHþ 4 -N: 36.9%, NO3 : 44.8%, NO2 -N: 25.6%, TP: 53.2%, Cr: 87%, Cu, AS, Cd, Hg, Pb: 43–55%, and E. coli: 61–71%

[19]

Refinery

Pilot scale

Floating bed made form polyethylene foam.

Geophila herbacea, Caddieshack, and Lolium perenne

TN: 60%, TP: 56%, COD: 52–67%, and TPH: 40–55%

[43]

Storm water

Field scale

Floating mat was made of polyethylene.

Carex virgata

TSS: 41%, Zn: 40%, Cu: 39%, and Cu: 16%

[58]

Storm water

Field scale

Mats constructed from extruded plastic woven together with injected closed cell foam and internally supported with PVC pipes.

Juncus effuses, Carex stricta, Andropogon gerardii, and Hibiscus moscheutos

TN: 48%, TP: 39%, and TSS: 78%

[41]

Storm water runoff

Pilot scale

Floating mats were Beemats: Buoyant interlocking mat square joined using nylon connectors and secured with locking waster to maintain integrity.

Juncus effuses and Canna flaccida

N: 28.5 g m2, and P: 1.69 g m2

[40]

Subtropical storm water

Field scale

Floating mats (BioHaven Floating Island) Juncus effuses and Pontederia cordata were used. Floating mats were composed of fine inter-wined polymer strands of 5% fiber and 95% pore space bonded to provide a 3-dimentional non-woven matrix with an area of 7.4 m2.

TN: 15.7%, nitrate and nitrate-nitrogen: 20.6%, ammonia-nitrogen: 51.1%, TP: 47.7%, and OP: 79%

[50]

Urban wastewater

Field scale

Tech-IA floating element each having rectangular lattice shape, made from ethylene-vinyl acetate

Iris pseudacorus

NO3-N: 82%, NH4-N: 20%, COD: 13%, and BOD5: 5%

[25]

Storm water

Field scale

Floating mat was made of ethylene-vinyl acetate

Carex virgata

Cu: 17–83 mg kg1 and Zn: 37–785 mg kg1

[58]

Domestic

Field scale

Floating mat made of polyethylene

Pontederia cordata

TN: 68.6% and TP: 70.3%

Storm water

Field scale

Floating mat made of polyethylene

Carex virgata

TP: 27% and Fe: 13.48%

O

O

PR

D TE

EC

R

R

U

N

C

O

Reference

FS

Wastewater

[207]Q3 [32]

(Continued)


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Table 3. (Continued) Study level

Floating mat

Plant

Urban runoff

Pilot scale

Floating mat was constructed from PVC pipes, plastic mesh and pot holders. Plants planted in hydroponic pots filled with coir fiber

Pontederia cordata, and Schoenoplectus tabernaemontani

N: 21.4 mg/plant, and P: 3.8 mg/plant

[38]

Lake water

Pilot scale

Floating mats prepared from styrofoam

Iris pseudacorus and Typha angustifolia

N: 277 mg m2 per day and P: 9.32 mg m2 per day

[49]

Domestic

Pilot scale

Beemat and Bioheaven

Juncus effusus

TN: 25%, TP: 4%, TN: 0.025 g m2 per day, TP: 0.0076 g m2 per day

[59]

Sewage effluent

Pilot scale

Floating mats were made from Jumbolon Board, five holes of diameter 5 cm in each were made in each mat for plant holding

Brachiaria mutica with three endophytic bacterial strain

TN: 33%, PO4: 18%, COD: 80%, BOD5: 90%

[28]

Boron enriched water

Pilot scale

Floating island manufactured by Floating Island International, Shepherd, MT, USA.

Typha latifolia

Boron removal by first cycle: 12.5–21.4% and by second cycle: 11.0–14.2%

[47]

Waste quarry waters

Field scale

Frame made from plastic pipes with foam plastic floaters. Carpet like grass mats were grown floating mats

Ereophorum polystachion, Ereophorum scheuchzeri, and Equisetum palustre

Total nitrogen: 53–90%

[61]

Acid mine drainage

Lab scale

Floating mat was prepared with polyethylene

Juncus effusus

SO2 4 : 87%, Fe: 100%, Al: 99.8%, Mn: 97.4%, Zn: 99.6%

[44]

Domestic

Lab scale

Plastic foam with five holes on each sheet

Phragmites australis, Acorus calamus

Nitrogen removal by P. australis: 91.5% and 84.2% and by A. calamus: 84.2% and 82.2%

[60]

O

O

PR

Reference

can be increased further by using flowering plants.[55,56] Floating treatment wetlands have potential to balance water temperature of the aquatic pond due to coverage of large surface area and long retention time.[130] In a comparison study between FTWs and natural wetland, a slight decrease in temperature of FTW outflow was noted due to shading of water column.[174,175]

1 2 3 4 5 6

8. Limitations of FTWs

7

Despite the fact that FTW has various advantages and capability to treat various kinds of wastewater, it also has some limitations which might hinder its application and performance in the field and industry. For example, emergent macrophytes grown on a floating mat require a support to fix the floating raft with plants on it at the water surface.[176] The anchoring to the floating rafts is a difficult task. Moreover, for the proper functioning of FTWs, a minimum water depth should be maintained to avoid plant roots to grow deeper into the sediments at the basin of FTWs. If the water rises like during storm events, the plant rooted to basin will cause the floating rafts to sink in water. It can lead to the death of macrophytes as well as severe damage to the floating structure.[16] The huge size of floating rafts may reduce the area of the pond for recreation activities like boating and fishing. Besides, plants should be harvested as a whole or above ground tissues periodically.[44] Proper harvesting and management of macrophytes may need some labor work. The invasive and undesirable plants may harm the macrophytes on the FTWs and minimize its efficiency. Although FTWs have been proved efficient to treat

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

U

N

C

O

R

R

EC

TE

way, with minimum maintenance and operational cost. Floating treatment wetland does not require any complex technological tool for its installation. Moreover, its operation and maintenance do not need any synthetic chemical input. Therefore, low capital and minimum/no operational costs make this technology an affordable and applicable approach, especially in developing countries, for the remediation of sewage and industrial wastewater. Overall, FTW is a low-cost technology compared with constructed wetlands and conventional wastewater treatment technologies. The hanging roots of plants below mat provide a large area for biofilms development and wastewater treatment.[17] The FTWs even have been proved effective for treating of polluted surface water, sewage, industrial, and agricultural wastewater.[10,14,34,65,95,170] Retention ponds are efficient to remove coarse and particulate forms of contaminants but not capable of removing dissolved contaminants effectively.[171] Although constructed wetlands can efficiently remove soluble pollutants and associated fine particles, these wetlands need large area and are unable to tolerate extended duration of high water levels. Floating treatment wetlands can efficiently address all these issues.[172] These can be used in flooding areas as the fluctuation or inundation does not have any detrimental effect on FTWs due to their floating nature.[16] Additionally, FTWs can provide a sustainable wildlife habitat for fish, birds, and invertebrates.[55] Moreover, FTWs can effectively be used to solve problems associated with eutrophication which are common in other wetland systems due to free-floating aquatic plants such as water hyacinth, water lettuce, and duckweed.[55,173] The FTWs emerged as promising eco-technology due to great aesthetic value which

D

FSS, fixed suspended solids; TPH, total petroleum hydrocarbon.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Pollution reduction

FS

Wastewater


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51

Abbreviations

The authors would like to thank Higher Education Commission, Pakistan (grant number 1-52/ILS-UITSP/HEC/2014 and 20-3854/R&D/HEC/14), and International Foundation of Science, Sweden, and the Organization for the Prohibition of Chemical Weapons (grant number W/5104-2).

5 6 7 8

Conflict of Interest

9

The authors declare no conflict of interest.

10

FS

Floating treatment wetlands system is an innovate tool for the treatment of water containing various kinds of pollutants. Although its pollutants removal mechanisms are similar to surface flow wetlands, its floating nature made it applicable for ponds, lake, and rivers with fluctuating water level. Most of the plant species used in FTWs are perennial species, which possess different properties such as root biomass, root length, height, and nutrient uptake. Plant roots system plays a crucial role in the removal of pollutants from wastewater by their direct uptake, provide residency and nutrients for the development of microbial biofilm, and filtration of the polluted water. The below-floating mat portion of macrophytes is reported to accumulate more nutrients as compared to the upper portion of plants. Although FTWs are efficient in both tropical and temperate climates, low temperature in temperate climate can reduce the performance of FTWs. Floating treatment wetlands are designed depending upon knowledge of researchers, availability of local material of floating mat, and indigenous plants. There is a need to develop some standards about the type of macrophytes, their nutrients uptake potential, structure and material of floating mat, plants density, plants harvesting and disposal procedure, hydraulic loading rate, retention time, and suitability of FTWs for their application at pilot scale for different types of wastewater. The microorganisms play an important role in degradation of organic pollutants. The investigation about the type of microorganisms specific for various kinds of pollutants, their organic pollutants degradation capacity, plant growth-promoting activities, performance, and synergetic relation with plants can help to improve the efficiency of FTWs. Harvesting of the plants in FTWs plays a vital role to remove nutrients from wastewater permanently. Further research is needed to investigate the removal of the whole plant or harvesting of specific plant portions for the maximum removal of pollutants from wastewater. Factors such as season for harvesting, multiple harvests in one season, and possible damage due to harvesting need further exploration. The plants used in FTWs mostly belong to grass family and can serve as a potential feed for livestock. Moreover, there is need of further research about nutritional values of these harvested plants and the ultimate effect on livestock health and products such as milk and meat.

4

Keywords

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8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

Acknowledgments

bacteria, floating treatment wetlands, plants, plant-bacteria synergism, pollutants, wastewater

O

9. Conclusions and Future Prospects

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7

NFW, natural floating wetland; NHþ 4 –N, ammonium nitrogen; TN, total nitrogen; TOC, total organic carbon; TP, total phosphorus; TSS, total suspended solids.

Received: March 5, 2018 Revised: June 22, 2018 Published online:

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D

various kinds of wastewater even containing different potentially toxic metals,[18,160,177,178] urban/agricultural runoff containing a high concentration of oil and herbicides may harm the aquatic plants and microorganisms.[179] Any damage to macrophytes and biofilm will ultimately decrease the pollutant removal efficiency of the FTWs.

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52 BOD, biochemical oxygen demand; COD, chemical oxygen 53 demand; CW, constructed wetland; FTW, floating treatment 54 wetland; HCB, hexachlorobenzene; HRM, hydroponic root mat;


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REVIEW Floating treatment wetlands can be exploited to treat various kinds of polluted water. In this system, plants are vegetated on a floating mat and their roots are extended down to the contaminated water acting as biological filters. The plants take up nutrients and trace metals whereas microorganisms degrade organic matter.

Sustainable Green Solutions M. J. Shahid, M. Arslan, S. Ali, M. Siddique, M. Afzal*................ 1800120

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Floating Wetlands: An Innovative Tool for Wastewater Treatment