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Keywords:

  • wastewater;
  • bioaugmentation;
  • designer rhizospheres;
  • biostimulation;
  • constructed wetlands

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. Constructed Wetlands
  5. Future Prospects And New Trends
  6. Conclusion
  7. Acknowledgment
  8. LITERATURE CITED

Treatment of different wastewater using macrophytes-vegetated constructed wetland reveals its potential in terms of significant reduction in BOD, COD, suspended solids, total solids, total nitrogen, heavy metals along with remediation of xenobiotics, pesticides and polyaromatic hydrocarbons. The rhizosphere of macrophytes such as Phragmites, Typha, Juncus, Spartina and Scirpus serves as an active and dynamic zone for the microbial degradation of organic and sequestration of inorganic pollutant resulting in successful treatment of domestic, textile and other effluents. Up to 2049–6648 µg metal per gram dry weight of plant biomass are found to accumulate in plant parts i.e. shoots and roots. Major metal removal mechanisms are bioaccumulation in plant parts, phytoextraction and phytostabilization. Different wastewaters treated through this technology are industrial, domestic, dairy, pesticides, PAHs, and xenobiotics containing effluents. Loading limits of the wetland, removal efficiency, biomass disposal and variation in seasonal growth are some of the limiting factors which can be overcome by stimulating the plant microbe interaction through designer rhizospheres involving pigmentation, biostimulation and genetic alterations of plant and associated microbial community. © 2013 American Institute of Chemical Engineers Environ Prog, 33: 9–27, 2014


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. Constructed Wetlands
  5. Future Prospects And New Trends
  6. Conclusion
  7. Acknowledgment
  8. LITERATURE CITED

Comprising over 70% of the Earth's surface, water is the most precious natural resource that exists on our planet [1]. Most water pollutants are eventually carried by rivers into the large water bodies no longer leaving them clean or pure; posing human health risks. Water is referred to as polluted when it is impaired by anthropogenic contaminants and either does not support a human use (like serving as drinking water) or undergoes a marked shift in its ability to support its constituent biotic communities [2]. For water pollution two general categories exist: direct and indirect. The former include effluent outfalls from factories, refineries, and waste treatment plants etc., that emits fluid of varying quality directly into urban water supplies. The latter includes contaminants that enter the water supply from soil/ groundwater systems and from the atmosphere via rain water. Soils and ground waters contain the residue of human agricultural practices (fertilizers, pesticides, etc.) and improperly disposed of industrial wastes [3]. Some major pollutants found in contaminated waters are heavy metals, xenobiotics, nutrients, organic matter and acidifying gases such as sulfur dioxide. The discharge of effluent from domestic and industrial sources has detrimental effects on the aquatic ecosystem [4] as this outfall can deposit large amount of organic matter, nutrients and pollutants leading to eutrophication (fertilization of surface water by nutrients that were previously scarce), temporary oxygen deficits and accumulation of pollutants into receiving waterways.

In the last few decades, researchers have tried to adopt an eco-technological approach to clean up or remediate wastewater using plants. This use of plants termed phytoremediation (phyto meaning plant and remedium meaning to clean or restore) actually refers to diverse collection of natural or genetically engineered plants for cleaning contaminated environments [5]. Eventually combining the existing biological and engineering strategies to improve the applicability of phytoremediation has come up in the form of constructed wetlands (CW) using plants termed macrophytes which according to USEPA are aquatic plants, growing in or near water that are emergent, submergent, or floating. Present review aims to sum up different aspects of constructed wetlands, design, construction and its applications for treating various effluents. Some attempts to improve the model system using novel techniques are also discussed.

Constructed Wetlands

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. Constructed Wetlands
  5. Future Prospects And New Trends
  6. Conclusion
  7. Acknowledgment
  8. LITERATURE CITED

The Ramsar convention brought wetlands to the attention of the world and proposed the following definition: Wetlands are areas of marsh, fern, peat land or water whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed 6 m [6]. Constructed wetlands are complex biological system that mimics natural self-cleansing processes [7] by reducing pollutant level to a dischargeable limit. In fact these can be treated as nature's kidneys. Root morphology and depth are important plant characteristics for phytoremediation. A fibrous root system (found in grasses e.g., Fescue), has numerous fine roots spread throughout the soil and provides maximum contact with the soil due to the high surface area of the roots. A tap root system (such as in alfalfa) is dominated by one larger central root. Root depth directly impacts the depth of soil that can be remediated or depth of ground water that can be influenced, as close contact is needed between the root and the contaminant or water [8]. Some common plants used in wetlands are listed below in Table 1.

Table 1. List showing common macrophytes used in constructed wetlands
FamilyPlantCommon nameReference
PoaceaePoa pratensisKentucky Bluegrass[9]
 Phragmites spp.Reed[10-12]
 Oryza sativaCommon rice[13]
 Paspalum distichumKnotgrass[11]
 Phalaris arundinaceaReed canary grass[14]
 Glyceria fluitansFloating Sweet-grass[14]
 Spartina spp.Coralgrass[15]
CyperaceaeSchoenoplectus spp.Club-rush[16]
 Cyperus sppPapyrus sedges[16]
 Carex spp.Sedges[17]
 Scirpus sppBullrush[18, 19]
SalicaceaePopulus sppPoplar Trees[20]
 Salix sppWillow Trees[20]
TyphaceaeSparganium erectumBur-reed[14]
 Typha latifoliaCattail[21]
AlismataceaeSagittaria spp.Common Arrowhead[22]
AmaranthaceaeKochia sppForage Kochia[23]
CeratophyllaceaeCeratophyllum spp.Coontail[24]
FabaceaeMedicago sativaAlfalfa[9]
JunaceaeJuncus spp.Rush[25]
LemnaceaeLemna minorDuckweed[26, 27]
PontederiaceaeEichhornia crassipesWater Hyacinths[27, 28]
PotamogetonaceaePotamogeton spp.American pondweed[29]

A universally used plant species is Phragmites [30, 31] commonly called reeds, which contribute to wastewater cleaning processes in many different ways: increasing the permeability and porosity of substrate [32], creating micro sites with reducing conditions by releasing oxygen from the roots [33, 34] termed as ROL (Radial oxygen loss). Through these oxygenated and oxygen poor micro sites even resistant chemicals get affected [32]. The withered parts insulate the root zone during the cold period. So in the temperate climates the pollutant removal capacity is affected only slightly the seasons [35]. Actually, the plant forms a thick root-mass, and by virtue of aerial stems transports oxygen to the root zone, thus aiding microbial activity and microbial digestion.

A typical system can be divided into three sections: inlet, vegetation and outlet. Prior to inlet section is a presettlement tank with a continuous inflow. Vegetation section, the principal component of constructed wetlands is composed of plants having the ability to accumulate some compounds in large concentration as compared to environment and also removes nutrients in wetlands [36-39]. It is composed of gravel bed on sides and inside planted with macrophytes which lead to outlet section connected to the collection tank. HSSF (Horizontal subsurface flow) and VSSF (Vertical subsurface flow) and hybrid system (using is the combination of a vertical and horizontal flow subsurface) constructed wetland or a acombination of any two concepts provides a means for more effective treatment efficiency [39].

Remediation Process

Often termed green remediation, botano-remediation, agro-remediation, vegetative remediation, it involves a continuum of processes each occurring to differing degrees for different conditions, media, contaminants, and plants [8]. Five main processes have been identified in remediation process: Phytoextraction [40], Phytodegradation [41], Rhizofiltration [42], Phytostabilization [43], Phytovolatilization [44, 45]. These processes tend to overlap to some degree and occur in varying proportions during phytoremediation [8].

Bioremediation using plants has been naturally coupled with microbial remediation in the form of rhizospheric bacteria termed as “rhizoremediation”, where organics and nutrient removal is mostly performed by attached micro biota [34]. Rhizoremediation involves interactions of plant roots and associated microbes to remediate elevated concentrations of some compounds; present as solid, liquid or gaseous substrates [46]. Such interactions offer very useful means for treating water contaminated with recalcitrant organic compounds [47]. The success of a plant species as the spot of rhizoremediation depends on (1) highly branched root system to harbor large number of bacteria, (2) primary and secondary metabolism, and (3) establishment, survival, and ecological interactions with other organisms [48]. Some co-metabolized (Cometabolism is defined as the oxidation of non growth substrates during the growth of an organism on another carbon or energy source) recalcitrant pollutants such as pesticides are only transformed and not effectively mineralized by microorganisms [49]. Microbes living in the rhizosphere termed rhizomicrobia also promote plant health by stimulating root growth (regulators), enhancing water and mineral uptake and inhibiting growth of pathogenic and non-pathogenic soil microbes [46, 48]. Rhizomicrobia may also accelerate remediation processes by volatilizing organics such as PAHs or by increasing the humification of organic pollutants [50].

The rhizosphere of plants acts as a microcosm where microbial activity is enhanced leading to active degradation of recalcitrant compounds and reduction in parameters like BOD (Biological Oxygen Demand), COD (Chemical Oxygen Demand), TS (Total Solids), and salt level from various effluents like acid mine drainage, agricultural landfill and urban storm-water runoff. Some contaminants are also released into the environment as a result of spills of fuel and solvents, military explosives, chemical weapons, agricultural uses (pesticides, herbicides), industrial (chemical, petrochemical), and wood treatment activities and get degraded in the root zone of plants or taken up, followed by sequestration or volatilization. Organic pollutants that have been successfully phytoremediated include organic solvents, for example, trichloroethylene [51], herbicides like atrazine [52], explosives such as trinitrotoluene, petroleum hydrocarbons, oil, gasoline, BTEX [53], monoaromatic hydrocarbons, and PAHs, polyaromatic hydrocarbons, MTBE, PCBs [54]. Inorganic pollutants mostly occur as natural elements and human activities such as mining and traffics promote their release into the environment, leading to toxicity. Inorganics like plant trace elements (Cr and Zn), non-essential elements (Cd and Hg) and radioactive isotopes cannot be degraded but are transformed via stabilization or sequestration in harvestable plant tissues [46] as shown in Table 2.

Table 2. Remediation potential of different metals using wetland plants
MetalVegetated plantsRemoval mechanismRemoval statisticsCase studyReferences
Pb, CdTypha domingensis, Lemma minorBioaccumulation by plantsPotential metal removal rates are 3–8 mg Pb/m2 day and 2–4 mg Cd/m2 dayWetland microcosms[26]
Zn, Pb, CdPhragmitesMetal tolerance, uptake and accumulationZn in shoots: 47 2049 µg g−1 d. wtZn in roots: 100–6684 µg g−1 d. wtPb in shoots: 2.5–80 µg g−1 d. wtPb in roots: 8.4–830 µg g−1 d. wtCd in shoots: 0.3–7.4 µg g−1 d. wtCd in roots: 2.5–49 µg g−1 d. wtMetal accumulation in seedlings from two different sources under glass-house conditions[10]
Zn, Pb, CdTypha latifoliaHeavy metal uptakeLeaves: Zn: 22–122, Pb: 4–7–40 and Cd:0–2–0–8 //g g˜ d. wtSoil-sediments: Zn: 86–3009, Pb: 26–18894 and Cd: l–4–26//gg-i d. wtRoots: Zn: 46–946, Pb: 25–3628 and Cd: 10–17//g g"^ d. wtSeedlings were grown in the metal treatment solutions or in the metal-contaminated media under laboratory conditions[10]
CuPhragmites australisPhytoextractionBioconcentration factor (BCF) increased from 349 to 1931 on increasing Copper concentration from 7.85 to 78.5 µmHydroponic experiment at different Cu concentrations[55]
Zn, PbTyphaAccumulation in tissues, precipitation as iron-hydroxides in root zones0–99% and 0–64% reduction for Zn and Pb, respectively, in pond 1 and 94–99% for Zn and 25–60% for Pb in pond 2, 69% removal rates of sulfate in eachSeries of subsurface flow ponds filled with spent mushroom substrate constructed at Navan, Ireland[56]
CuScirpus californicusCopper immobilization in wetlandShoots and roots of S. californicus sorbed 0.6% and 1.9%, respectively, of copper entering the systemEight-acre constructed wetland treatment system receiving copper-contaminated water[19]
N, PPhragmitesN- reduction, P- immobilization, physical settlement of solids51% reduction for total N, 13% total P, 84–90% for suspended solids and 49% for BODConstructed surface flow wetlands at Ireland[57]
CuPhragmites australisMetal tolerance, Uptake, AccumulationCu concentrations in the PM shoots were higher than in the FS, WB and PB shoots, but lower than in the HK shootsTwo mine sites (Parys UK and Belgium)contaminated with Cu and three ‘clean’ sites (UK, Hong Kong)were studied under field and glasshouse conditions.[58]
Cd, Cu, ZnJuncusPhytoextraction, PhytostabilizationPlant bioaccumulation was only observed for Cd, Cu, and Zn, being similar for Cd at the two sites and significantly higher for Cu and Zn, nine and four times higher, respectivelyEstuarine environment[25]
Cr, Pb, Zn, CuSpartina alterniflora, Phragmites australisMetal uptakeCu: 300 microg/g in leaves, <200 microg/g in stems Zn: 500 microg/g in sediments, Cu: 200 microg/g Metal concentrations lower in stems than in leaves, and Cr, Pb, and Zn were lower in P. australis than in S. alternifloraMetal-contaminated salt marshes study[59]
Pb, ZnPhragmites,Typha latifolia, Paspalum distichumReduction in TSS, Pb, Zn, Cd, Cu %99, 98, 75, 83, and 68% reduction in Pb, Zn, Cd, Cu, and TSS, respectivelyReduction rates of contaminants in a treatment wetland, South China[18]
CrTypha latifolia, Phragmites australisAbiotic reduction, precipitation and accumulation of Cr (III) in the sedimentsCr (VI) removal rates were 0.005 to 0.017 mg L−1 d−1, 0.0003–0.08 mg L−1 d−1, and 0.004–0.13 mg L−1 d−1 for the control, T. latifolia, and P. australis microcosms, respectively.Greenhouse and bench-scale microcosm experiment[21]
SePhragmites, TyphaPhytoextraction, Phytostabilization25–74 µg L−1 reduction in treated waterEstablished outdoor SSF wetland[60]
Cu, Cd, Ni, Pb, ZnPhragmites australisAdsorptionAfter three cycles of adsorption-elution, the adsorption capacity regained completely and desorption efficiency of metal around 90%Reed biomass used as biosorbent from aqueous solution[61]
HgTransgenic Spartina alternifloraConverting ionic Hg into elementary Hg and volatilization from the plant[15]
Mn, Ni, Cu, Zn, PbScirpus littoralisMetal uptake by plantAccumulation of Mn, Ni, Cu, Zn, Pb upto 494.92, 56.37, 144.98, 207.95, and 93.08 ppm dry wt in 90 days timeMetal accumulation studied under water-logged and field conditions for 90 days[18]
Pb, Mn, CrScirpus americanus, Typha latifoliaAccumulation in plant partsNearly 100 % Removal of Pb, Cr and 71-100 % for Mn during 6-8 days of experimentationAccumulation of metals by in vitro raised plants in supplemented MS media[62]
Cu, Zn, CdTyphaActivity of the indigenous soil microflora and plant enzymesConstructed wetland[63]
Cd, Cu, Zn, Pb, Cr, Ni, Al, Fe, MnPhragmitesAccumulation in plant parts12–62 mg m-2 y-1 accumulation of metals in VSSF in leaves and 38–88 mg m-2 y-1 in stems23–56 mg m-2 y-1 accumulation of metals in HSSF in leaves and 38–88 mg(m−2 yr−1) in stemsCombined CW in Belgium treating domestic wastewater[12]
ZnSpartina densifloraHeavy metal uptake100–4800 ppm ZnGlasshouse experiment[64]
As, Cu, Fe, Mn, Pb, ZnSpartina maritima, Spartina densifloraPhytostabilization, BioaccumulationZn in tissues: 27 to 1249 ppm and from 42 to 2326 ppm for S. densiflora and S. maritimaCu in tissues: 22 to 2546 ppm and from 27 to 4933 ppm for S. densiflora and S. maritimaPb: 0.1 to 217 ppm and from 0.1 to 292 ppm for S. densiflora and S. maritimeTo study metal accumulation by spartina species in two marshes with different levels of pollution[65]
Cd, Zn, Cr, Cu, Ni, PbPhragmitesMetal mobility and uptakePlumes: 19–117 μg kg−1 DM Cd, 98–408 μg kg−1 DM for Cr, 3.1–7.0 mg kg−1 DM for Cu, 0.5–2.3 mg kg−1 DM for Ni, 0.4–4.5 mg kg−1 DM for Pb, 36–132 mg kg−1 DM for ZnIntertidal marshes in the Scheldt estuary[66]
Al, Fe, MnPhragmites australisMetal accumulationRoot: Al(OH)3 > Al2O3 > Fe3O4 > MnO2 > FeOOHHeavy metals in the sediment of constructed wetlands[67]
Cd, Zn, Cr, Cu, Ni, PbPhragmites australisAccumulation in plant partsRhizomes > Stems > LeavesExperimental constructed wetland (CW) sited in Castelnovo Bariano[68]
Cu, ZnTypha latifolia, Phragmites australisAccumulation in plant partsCu, Zn: 80 and 91% for unplanted control, 83 and 92% for cattail, and 83 and 92% for reed wetlandRiver water contaminated by swine confined-housing operations[69]
Al, As, B, Ba, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Se, Sn, V, U, ZnPhragmitesAccumulation in plant partsRoots > Rhizomes > Leaves > StemsConstructed wetlands with horizontal sub-surface flow (HF CWs) designed for treatment of municipal sewage in the Czech Republic[70]
Cd, Cr, Cu, Hg, Mn, Ni, Pb, ZnPhragmitesAccumulation in plant partsRoot Cd:1.13 ± 0.08, Cr: 6.97 ± 0.19, Cu:14.98 ± 0.93, Hg: 5.22 ± 0.38, Mn:475.80 ± 11.91, Ni:9.12 ± 0.20, Pb:16.54 ± 0.97, Zn :104.10 ± 9.28 Rhizome Cd:1.00 ± 0.08, Cr:1.52 ± 0.06, Cu:4.33 ± 0.32, Hg:3.19 ± 0.26, Mn:37.51 ± 2.82, Ni:1.67 ± 0.14, Pb:15.30 ± 0.93, Zn:32.67 ± 2.36Stem Cd:0.68 ± 0.06, Cr:0.40 ± 0.04, Cu:2.31 ± 0.28, Hg:1.05 ± 0.12, Mn:27.92 ± 2.34, Ni:0.48 ± 0.08, Pb: 9.87 ± 0.80, Zn:10.04 ± 0.87 Leaf Cd:1.05 ± 0.10, Cr:0.69 ± 0.04, Cu:4.13 ± 0.19, Hg:1.73 ± 0.23, Mn:308.30 ± 11.47, Ni:1.69 ± 0.15, Pb:13.20 ± 0.74, Zn:28.40 ± 1.72Phragmites australis and the corresponding water, sediment samples from the mouth area of the Imera Meridionale River (Sicily, Italy)[71]
Pb, Cu, ZnPhragmites[72]
ZnPhragmites australis, Acorns calamus, Scirpus tabernaemontaniAccumulation in plant partsScirpus tabernaemontani: Removal effects were 31,050.84 mg/kg (10,206.67 mg/kg in above-ground parts and 20,844.17 mg/kg in under-ground parts) Acorus calamus and Phragmites australis, the highest accumulation concentrations of zinc ion were 54,130.67 mg/kg (16,774.00 mg/kg in above-ground parts and 37,356.67 mg/kg in under-ground parts) and 25,423.34 mg/kg (4506.67 mg/kg in above-ground parts and 20,916.67 mg/kg in under-ground parts)Hydroponically cultured[73]
Cu, Zn, Cd, PbPhragmites australisPhytoextractionCu, Zn, Cd: soil > Phragmites australis of aerial part>phragmites australis of underground partPb: Soil > Phragmites australis of aerial part≈phragmites australis of underground partRiver wetland system[74]
Cd, Cr, Cu, Fe, Ni, PbPhragmites cummunis, Typha angustifolia, Cyperus esculentusAccumulation in plant partsP. cummunis was in the order of Fe (2813) > Mn (814.40) > Zn (265.80) > Pb (92.80) > Cr (75.75) > Cu (61.77) > Ni (45.69) > Cd (4.69) T. angustifolia:Fe > Mn > Zn > Cr > Pb > Cu > Ni > Cd C. esculentus:Fe > Mn > Zn > Pb > Ni > Cu > Cr > CdPlants grown in aqueous solution[75]
Cr, Cu, Pb, Fe, ZnSpartina alternifloraPhytoextractionCr: 3.0 ppm, Cu: 7.0 ppm, Fe: 410 ppm, Pb: 0.5 ppm, Zn:28 ppm in dry soilSediments in various locations in Bayou d'Inde in Southwest Louisiana a waterway (industrial and municipal waste streams)[76]
CdTypha angustifoliaHyperaccumulation in plantsCd conc in Root- 1962.31 ± 32.70 mg L−1Cd conc in Leaf −39.66 ± 1.76 mg L−1Lab scale green-house study[77]
Al, Fe, Zn, PbTypha domingensisRhizofiltrationPb2+ > Fe3+ > Al 3+ > Zn2+Raising plants hydroponically and transplanting them into metal-polluted waters[78]
As, Cd, PbTypha orientalisToleranceAccumulationGreenhouse study[79]
Cr, Cd, PbTypha angustifoliaHeavy metal uptakePot experiment[80]
Co, Cr, NiSpartinaPhytostabilization BioaccumulationCo in tissues: B0.1–35.8 and from B0.1–43.4 lg g−1 for S. densiflora and S. maritima Cr in tissues: 2–18.8 and from B0.1–25.2 lg g-1 for S. densiflora and S. maritima Tissue Ni: B0.5–11.1 and from B0.5–15.6 lg g-1 for S. densiflora and S. maritimeMetal contaminated site study[81]
Cr, Ni, ZnTypha domingensisHeavy metal uptakeBFs: Cr 0.181, Ni 0.247, Zn 0.857 TFs: Cr 0.152, Ni 0.030, Zn 0.197Primary treatment wetland (wastewater from industrial processes and sewage from the factory)[82]
HgJuncus maritimus, Scirpus maritimusPhytostabilization PhytoaccumulationHg-contaminated salt marsh sediment chemical environment[83]
CrSpartina argentinensisHyperaccumulationGlasshouse experiment[84]
Zn, Cd, CuPhragmites australisMetal accumulationAccumulation in reeds (Phragmites australis) in urban sediments from two stormwater infiltration basins[85]
Cu, Zn, Cd, PbPhragmites karkaMetal tolerance, Uptake,AccumulationCu> Zn> Cd> Pb[86]
Al, Pb, Cd, Co, Ni, Cr, Fe, Mn, Zn, CuPhragmites australisMetal accumulationAl > Pb > Cd > Co > Ni > Cr micronutrients: Fe > Mn > Zn > CuPhragmites australis growing at 4 selected sites along the bank of the lower River[87]
CdJuncus subsecundusMetal accumulationCadmium accumulation and removal (except for Cd removal at 20 mg Cd kg−1) by plants was significantly higher in Cd treatments with than without PAHGlasshouse experiment was conducted to investigate effects of Cd) without orwith PAHs on growth of Juncus subsecundus[88]
HgPhragmites australisAccumulation in plant partsRoot (0.321 ± 0.05 BCD) exhibited the highest Hg accumulation followed by rhizome (0.245 ± 0.04 BCD) and leavesHg-contaminated coastal lagoon[89]

Combinations of the various phytoremediation processes may occur simultaneously or in sequence for a particular contaminant, or different processes may act on different. The identifying characteristics associated with wastewater are high BOD and COD value, suspended and dissolved solids, heavy metals and xenobiotics. Biological characteristics include coliforms, and other types of bacteria. When such water was subjected to a standardized retention time in a constructed wetland, the pollutant/contaminant load comes down to allowable limits to be discharged into environment. For fecal coliforms one log to two log reductions have been achieved. Removal of nitrogen in the form of ammonia and organic nitrogen requires a supply of oxygen for nitrification, which comes from plant roots that don't penetrate completely. The reduction obtained in BOD is 85.71%, COD 86.14%, TSS 87.58%, TS 87.64%, total N 81.55% along growing in constructed wetland for distillery effluent [90]. Removal of heavy metals occurs mainly by binding to soils, sediments and particulate matter or precipitation as insoluble salts and uptake by bacteria, algae and plants. The major proportion of heavy metal removal is accounted for by binding processes within wetlands [91]. Because of their positive charge, the heavy metals are readily adsorbed, complexed and bound with suspended particles, which subsequently settle on the substrate. The precipitation of heavy metals as insoluble salts such as carbonates, bicarbonates, sulfides and hydroxides is another process that leads to their long term removal. These salts are formed as a result of reaction between heavy metals with other chemicals and lead to precipitation and settling of metal salts [92]. Some wetland plant species have been found to have a property of heavy metal tolerance, for example, Typha latifolia, Glyceria fluitans and Phragmites australis. Tables 2-4 depicts the capacity of remediation potential of different macrophytes to various contaminants.

Table 3. Remediation potential of different wastewater using wetland plants
Type of wastewaterVegetated plantsRemoval MechanismRemoval StatisticsCase studyReferences
Industrial wastewaterPhragmites, Schoenoplectus, Cyperus, TyphaUptake by plants50% of influent metal loadConstructed wetlands[16]
Mineralization and pathogen containing wastewaterPhragmitesTreatment in wetland by adsorption to biofilmsTwo to three log cycle reduction in counts of indicator bacteriaField scale gravel bed hydroponics constructed wetland[93]
Mine effluentTyphaAccumulation in tissues, precipitation as iron-hydroxides in root zones0–99% and 0–64% reduction for Zn and Pb in pond 1 and 94–99% for Zn and 25–60% for Pb in pond 2, 69% removal rates of sulphate in each pondSeries of subsurface flow ponds filled with spent mushroom substrate constructed at Navan, Ireland[56]
Dairy wastewaterPhragmites australis, Scirpus validusTreatment by wetland plants and residing bacteriaRemoval rate: TKN 25%, ammonium level 16%, BOD 73%, SS 91%, COD 38%, Faecal coliforms 99%CW for dairy wastewater[94]
Nitrogen and Bacterial contaminated waterPhragmites, TyphaPlant ammonia assimilation, nitrification, restitution of stored nitrogen in the vegetal tissuesRemoval rate: 27% in Kjeldahl Nitrogen, 19% ammonia nitrogen, 4% nitrate-nitrite, 90% for bacteriaTwo wetland combined system- one vertical and other horizontal[95]
Textile effluentPhragmitesMineralization and degradation70% removal efficacyDegradation in VFCW[96]
Dilute farm effluent, dirty waterPhragmitesBiological treatment of wastewater in activated sludge and wetland conditionsReduction in pH value from 10.9 to 7.6, BOD 821 to 65 mg/L, COD 2005 to 210 mg/L, and ammonium 0.3 to <0.1 for each wetland bedAerated sequencing batch reactor containing activated sludge followed by a series of constructed wetlands[97]
Removal of bacteria in sand columnsJuncus effuses, Phragmites australisPredation and lysisEfficiency of removal upto four orders of magnitude of cfu obtainedRemoval of bacteria in planted and unplanted sand columns[98]
Domestic wastewaterPhragmitesSediment accumulationConcentrations of Cd, Cu, Pb, and Zn in the sediment generally decreased along the treatment path of the CWCombined CW: two VSSF reed beds followed by two HSSF reed beds[12]
Urban runoffPhragmitesTreatment in wetlandRemoval performance of planted filters was more efficient and stable after the filters matured compared to that of unplanted filtersExperimental temporarily flooded vertical-flow wetland filters treating urban runoff[99]
Nitrate-dominant wastewaterTypha, ScirpusEnhanced biological denitrification by fueling heterotrophic microbial activityNitrate removal were around 500 mgN/(m2 d). Areal removal rate 25% higher in cattail versus bulrush mesocosms. DO in bulrush between 0.5 and 2 mg L−1, while DO in cattail mesocosms below 0.3 mg L−1Batch wetland mesocosms[100]
Industrial wastewaterTypha latifolia, Phragmites australisUptake by plants and reedbed aerationHigh removal of organics from tannery wastewater, up to 88% of BOD5 (from an inlet of 420–1000 mg L−1) and 92% of COD (from an inlet of 808–2449 mg L−1)Two-stage constructed wetlands planted with Typha latifolia and Phragmites australis[101]
High-strength wastewaterTypha angustifolia, Cyperus involucratusAeration by plant rootsAverage mass removal rates of COD, TKN and total-P at a HLR of 80 mm d−1 were 17.8, 15.4, and 0.69 gm−2 d−1Vertical flow (VF) constructed wetland systems to treat high-strength wastewater under tropical climatic conditions[102]
Sludge stabilizationPhragmitesReedbeds aerationReed bed pilot plant for sludge stabilization[103]
Domestic wastewaterPhragmites australis, Phalaris arundinaceaAeration by plant rootsNH4-N concentration of 29.9 mg/L was reduced to 6.5 mg/L average removal efficiency of 78.3%. Removal of BOD5 and COD amounted to 94.5 and 84.4%Phosphorus removal amounted to 65.4%Three-stage experimental constructed wetland[104]
Domestic wastewaterPhragmites australisDecontamination effect of Phragmites australisDecontamination rate: 64.5% for BOD, 68% for COD, 79.7% for SS, 21.0% for Total Phosphorus, 20.7% for total nitrogenPilot subsurface horizontal flow constructed wetland[105]
Nutrient removalPhragmites mauritianusRemediation by plantsConductivity values decreased by 24 and 28% in wetland 1 and wetland 2, TDS decreased by 32% and 28%, Ammonium nitrogen increased by 5% in wetland 1 and 12% in wetland 2, nitrate nitrogen decreased by 62 and 56%, Reactive phosphorus concentrations were reduced by 4 and 3%, in wetland 1 and wetland 2Horizontal Subsurface Flow Constructed Wetlands[106]
Municipal wastewaterCanna, Phragmites, CyprusRemediation by plantsRemoval efficiency: COD 88%, BOD 90%, TSS 92%Vertical flow constructed wetlands[107]
Table 4. Remediation potential of Xenobiotics in various wetland plants
XenobioticsVegetated plantsRemoval mechanismRemoval StatisticsCase studyReferences
Glycol-based deicing agentScirpus spp., Medicago sativa, Poa pratensisDegradation60.4, 49.6, and 24.4% of applied [14C]EG degraded to 14CO2 in the Medicago sativa, Poa pratensis, and nonvegetated soils, Scirpus spp. enhanced the mineralization of [14C]PG by 11–19% and [14C]EG by 6–20%.Vegetated water incubation systems[9]
AtrazinePhragmitesRetention in wetlandBetween 17–42% of measured atrazine mass was obtained within 30–36 m of wetlandAmendment of atrazine in a constructed wetland[52]
Metalaxyl and Simazine (hydrocarbons)TyphaAccumulation in plant partsMetalaxyl and simazine activity in solution was reduced 34 and 65%[108]
PAH-degradationPhragmitesDegradation[54]
Crude oilTypha latifolia, Typha angustifolia, Phragmites communis, Scirpus lacustris, Juncus spp.Degradation, microbial dissimilatory sulphate reduction and biosorptionOil content of the water after treatment was decreased to less than 0.2 mg/L from 2–10 mg/L, and the concentrations of heavy metals decreased below the relevant permissible levelsPassive system of the type of the constructed wetlands[109]
2,6-dimethylphenol, 4-chlorophenol, NaphthaleneCarex gracilis, Juncus effususDegradationConcentrations of 20 mg/L organic pollutant in the case of 4-chlorophenol, about 30 mg/L naphthalene and 50 mg/L 2,6-dimethylphenol were efficiently eliminatedHydroponic cultures using sand-bed reactors planted under batch and flow-through conditions[41]
Oil SpillSpartinaDegradation and accumulationConstructed wetlands[110]
Benzene, Toluene, Ethylbenzene, Xylenes (BTEX)Scripus cyperinus, Juncus effuses, Carex lurida, Typha latifoliaPhytodegradation, Phytovolatilization90 % of the BTEX removedConstructed wetlands[53]
Fenpropimorph, Linuron, Metalaxyl, Metamitron, Metribuzin, Propachlor, PropiconazoleSparganium erectum, Phragmites australis, Phalaris arundinacea, Glyceria fluitans, Typha latifoliaRetention of pesticides in arable soils3–67% retention of pesticides (Fenpropimorph, Linuron, Metalaxyl, Metamitron, Metribuzin, Propachlor, Propiconazole)Constructed wetlands[71]
Azo dyesPhragmitesMineralization and degradationNearly 70% removal efficacyDegradation in VFCW[96]
DDT, PCBsPhragmites australis, Oryza sativaAccumulation in plant parts and transformation by reductive halogenation92.0–95.0 ng DDT in roots and 70.5–78.0 ng in stem of reedsGlasshouse experiments under hydroponic conditions[71]
Organic pollutantPhragmitesBiodegradation and plant uptake>90% removal of Lindane, Pentachlorophenol, Endosulphan and Pentachlorobenzene; 80–90% for Alachlor and Chlorpyriphos; 20% for Mecoprop and SimazineHSSF pilot plant in Spain[111]
Textile azo dye acid orange 7Phragmites australisDegradation by plantVFCW treatment[112]
Bisphenol A, Bisphenol FPhragmites australisBiodegradation by rhizosphere bacteriaBPA and BPF degradation in the sediment[113]
Ibuprofen, Carbamazepine, Clofibric acidTyphaAdsorption on LECA and phytodegradationRemoval efficiencies of 96, 97, and 75% for ibuprofen, carbamazepine and clofibric acid in summer, 26% in removal efficiency was observed for clofibric acid in winterMicrocosm constructed wetlands systems established with a matrix of light expanded clay aggregates (LECA) and planted with Typha[114]
CarbamazepineTyphaUptake and metabolizationCB removal: 56% for the CB initial concentration of 2.0 mg L−1 1 to 82%Hydroponic conditions[114]
Eenrofloxacin (ENR), Ceftiofur (CEF), Tetracycline (TET)Phragmites australisAccumulation in plant partsLevels of 6 ± 2 lg L−1 for SWP and 43 ± 5 lg L−1 for control samples after 7 days,resulting in 94 and 57% of drug removalWastewater treatment plants (WWTPs)[115]
PAHJuncus subsecundusPAH degradationCadmium accumulation and removal (except for Cd removal at 20 mg Cd kg−1) by plants was significantly higher in Cd treatments with than without PAHs,whereas accumulation of PAHs by plants (except for pyrene in roots at 0 added Cd)Glasshouse experiment[88]
Terbuthylazine (TER)Typha latifoliaDegradation by plantDegradation pathways of terbuthylazine (TER) by Typha latifolia in constructed wetlands[116]
4C-labeled 1,4-dichlorobenzene (DCB), 1,2,4-trichlorobenzene (TCB), γ-hexachlorocyclohexane (γHCH)PhragmitesPlant uptakePlant uptake of DCB, TCB, γHCH was significant with bioconcentration factors reaching 14, 19, and 15Hydroponic conditions[117]
Lindane (HCH), Monochlorobenzene (MCB), 1,4-dichlorobenzene (DCB), 1,2,4-trichlorobenzene (TCB)PhragmitesPlant uptake[118]

From the review of literature as compiled in above tables, an inference can be easily made about the trend of different metals being remediated by wetland plants. Major metals to be remediated in constructed wetlands are Pb [7, 26, 70], Zn [7, 71], Cd [7, 26, 70]), Cu [71], Cr [71].These metals can be termed as most common heavy metal pollutant present in wastewater/soil followed by Al > As > B > Ba > Co > Fe > Hg > Mn > Ni > P, Mo > N > As > Se > Sn > U > V.

The presence of these metals can be attributed to discharge of industrial, gases, effluents and solid waste into the environment. The above case studies have been done in lab scale, pilot scale, glass house or hydroponically grown cultures later treated with metal spiked water. Most of the literature cited over time explains the data on metal accumulations in various plant parts (root, stem, leaves) and in soil/sediments. So far there has been no focus on technology mechanisms/ microbe's role/enzymatic processes involved in the remediation processes. The comparative analysis of remediation of different types of wastewater (as compiled in Tables 1-4) reveals that Phragmites are most commonly used plant species. Around 41% of of CWs are vegetated with Phragmites solely or in combination with other plant, for example, Acorns calamus, Scirpus tabernaemontani [73], Typha latifolia [69], Cyperus esculentus [25], Spartina alterniflora [59]. Typha (23%), Spartina (15%), Scirpus (8%) and Juncus (5%) are other important plants after Phragmites. Plants less used include Cyperus, Acorns, Lemna, Paspalam. For the treatment of xenobiotics, Phragmites are majorly used plants (32%), followed by Typha (22%), Juncus (11%). Other species less used are Medicago, Glyceria, Phalaris, Oryza, Carex, Sparganium, Poa, Scirpus, and Spartina. Major remediation processes are accumulation, degradation, mineralization and metabolism due to microbial activities. Xenobiotics largely include pesticides Fenpropimorph, Linuron, Metalaxyl, Metamitron, Metribuzin, Propachlor, Propiconazole), PAHs [Benzene, Toluene, Ethylbenzene, Xylenes (BTEX) and dyes (Textile azo dye, acid orange 7)]. Along with degradation, some of these are also retained within soil/sediments along the length of wetland [71, 114].

Being eco-friendly, Phragmites (Reeds) constructed wetland is often termed as “Ecoflo reed bed system.” It has many advantages over the other water treatment technologies for primary, secondary or tertiary wastewater treatment in private housing/communal/commercial developments because of easy integration in new and existing housing schemes. Being a living system, reed beds work in harmony with the environment blending visually with the natural landscape which is particularly important in scenic areas [32, 119]. These are extremely durable and provide a reliable, long-term solution to wastewater/sewage treatment as per their effectiveness in preventing faecal contamination from reaching wells, reservoirs and surface waters is concerned. A well-constructed wetland can effectively remove all suspended solids from wastewater. Moreover reed beds require low-maintenance as its maturation with time essentially looks after itself. These systems are very competitively valued when compared to other treatment systems because unlike conventional systems, these improve with age as the roots mature and expand with time becoming more efficient for biologically filtering wastewater. After the plants have been allowed to grow for some time, they are harvested from the wetlands and incinerated [27]. This procedure is practiced till the contaminant level in water comes down to allowable limit. Constructed either with horizontal or vertical flow these wetlands act as a mechanism to treat non-point source pollution before it reaches lakes, rivers and oceans. Experiments carried out with planted and unplanted reedbeds on same substrate have shown a significant influence on nutrient removal [120, 34]. Moreover, the aquatic vegetation in wetlands plays an important role in removing nutrients [121, 36-38, 122, 123]. Plants take up nutrients primarily through their root systems, only some uptake occurs through immersed stems and leaves from the surrounding water. Aerial stem by virtue of large internal air spaces transport oxygen to the root area of the soil to enable aerobic microbes to decompose the pollutant [124] and aid in the settling of suspended material by reducing the rate of sewage flow [125].

Future Prospects And New Trends

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. Constructed Wetlands
  5. Future Prospects And New Trends
  6. Conclusion
  7. Acknowledgment
  8. LITERATURE CITED

Increasing the efficiency of system and bringing the experimental results at par with the field activities has always been a big challenge to all environment engineers and scientists. To increase the efficiency of constructed wetlands, there is a need for better knowledge of the biological processes involved in plant-microbe-contaminant interactions, novel genes for bioremediation in plant and bacteria, molecular and biochemical approaches in the degradation pathways and whole mobile genetic pool (consisting of plant and rhizospheric bacteria) in a constructed wetland. Introducing certain novel amendments can meet the high standard of reclamation in a given environmental matrix. Apart from the biological processes, an understanding of physical and chemical processes occurring in the system also offers a great help in understanding the system and increasing its efficiency manifolds. Amendments can be in terms of planting two or more species, for example, planting Sparganium erectum, Phragmites australis, Phalaris arundinacea, Glyceria fluitans, Typha latifolia together in a wetland. Such a combination has been used in retention of pesticides like Fenpropimorph, Linuron, Metalaxyl, Metamitron, Metribuzin, Propachlor, Propiconazole in soils [14]. Using two or more plant species together in a wetland adds to scenic beauty and also increases efficiency of wetland as there may be more than one organic pollutant specific to different plants for accumulation and remediation purpose.

The primary filtering and aeration of wastewater can be done so that there is a pre settlement of suspended particulate matter prior to releasing it into Phragmites root zone. The aeration pumps can be applied at site of collection tanks and aeration can be provided at different levels in the tank with the help of spurges or nozzles. The aeration can be done for 20–30 h to make the environment partially aerobic and to increase the level of dissolved oxygen in the wastewater. Pre-settlement tank and aeration chambers can be provided in vicinity of constructed wetlands which can support for primary treatment of water. Use of soil amendments such as synthetics (ammonium thiocyanate) and natural zeolites have yielded promising results [126-130]. EDTA, NTA, citrate, oxalate, malate, succinate, tartrate, phthalate, salicylate and acetate etc. have been used as chelators for rapid mobility and uptake of metals from contaminated soils by plants. Use of synthetic chelators significantly increased Pb and Cd uptake and translocation from roots to shoots facilitating phytoextraction of the metals from low grade ores [27].

Adding certain microbial components along with these plants which degrades various pollutants and contaminants can be a rationalized approach for treating wastewater with a variety of pollutants. The rhizosphere of wetland plant can be considered as elevated zone in terms of microbial presence and activity. Adding certain inoculants, not native and having higher remediation potential generates a biased rhizosphere termed as Designer Rhizospheres [131]. Bioaugmentation, the addition of microbes to enhance a specific biological activity, has been practiced intentionally for years in wastewater treatment [132]. In the constructed wetlands, the role of rhizospheric microbial population is quite active relative to passive role of vegetation. Certain bacteria having the ability to degrade a particular pollutant/contaminant based on their natural, non-engineered metabolic processes can be employed for the remediation purpose [133]. This use of rhizomicrobial populations present in the rhizosphere of plant for bioremediation is termed as rhizoremediation [48] and when microbial populations are added from outside source, then it is known as bioaugmented rhizoremediation [48, 134-136]. In most of cases, bioaugmentation impact on indigenous microbes is often overlooked keeping remediation as primary goal. Addition of microbes to soil can potentially result in establishment of new microbial population, shifts in microbial population or transfer of genetic material (like plasmids harboring metal/antibiotic resistance genes) to indigenous population which is not its primary goal [131].

The intentional stimulation of resident xenobiotic degrading bacteria by addition of electron acceptors, water, nutrients or electron donor termed biostimulation can also be employed to speed up remediation processes [137]. However in many cases, the fertilization practice of contaminated site using compost, nitrogen, phosphorus and carbon has been unpredictable because it has been reported to either enhance and not the degradation of pollutants [48, 138]. In case, the degradative bacteria is absent in indigenous microbial population, bioaugmentation can be employed by introducing either wild type or genetically modified microbes into soil [48]. The laboratory scale results of seeding microbes for degradation of soil pollutants have been ambiguous [48].

The bioavailability of organic compounds is the most important factor that determines the overall success of a bioremediation process [47, 50]. The availability of pollutant to the organisms or bio-availability depends upon the chemical nature of the pollutant (hydrophobicity, volatility, binding capacity, reactivity) and soil properties (particle size, water and organic content, cation exchange capacity, pH). Many chemicals, plant/microbe exudates and secondary plant products are potential enhancers of pollutant bio-availability. Artificially this can be improved by adding soil amendments in the form of surfactants like Triton-X 100, SDS [46, 48, 50]. In soil polluted with organic chemicals, a combined stress might enhance the degradation [47]. In the field of root technology, certain strains of naturally occurring soil bacteria Agrobacterium tumefaciens has been used to induce root proliferation to increase the length and mass of plant roots and thus the degradation [47].

Genetic alterations of plants and transgenic plants for improved phytoremediation have already been developed and are spreading for field studies [46, 47]. It can be done by two methods (A) gene introduction (b) gene alteration. The most straightforward way is to add a broad host range plasmid having desired gene and a molecular marker which can be screened later on by a differentiable phenotypic trait.

Co-inoculation of a consortium of bacteria/or with algae each with different parts of catabolic degradation route, involved in the degradation of certain pollutant is often found to be more efficient than the inoculation of single strains with the complete pathway [48]. Usually several bacterial populations degrade pollutant more efficiently than a single species or strain due to presence of partners which use the various intermediates of the degradative pathway more efficiently (joint metabolism) [48, 139]. The close proximity of different strains and the formation of mixed micro colonies were observed only in the presence of pollutant naphthalene, illustrating the formation of communities where various activities fulfill each other [48, 134]. However, a few reports have been collected where the direct introduction of microbial strain or consortium for xenobiotic degradation activities is (bioaugmented rhizoremediation) able to efficiently colonize the root [48, 134-136].

Along with these, sample pre-treatment before entering a constructed wetland can be done, for example, aerobic or anerobic digestion, filteration to remove suspended solids, pre-settlement tanks.

Conclusion

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. Constructed Wetlands
  5. Future Prospects And New Trends
  6. Conclusion
  7. Acknowledgment
  8. LITERATURE CITED

The application of constructed wetland harboring macrophytes such as Phragmites, Typha, Juncus, Spartina, and Scirpus is a promising method for cleaning up varied types of effluents starting from domestic, agricultural and industrial sources revealing its potential in terms of significant reduction in BOD, COD, suspended solids, total solids, total nitrogen, heavy metals along with remediation of xenobiotics, pesticides and polyaromatic hydrocarbons as the root zone of these plants serves as an active and dynamic zone for the microbial degradation of organic and sequestration of inorganic pollutant resulting in successful treatment of domestic, textile, and other effluents. A significant amount of metals and other organic pollutants are found to accumulate in plant parts, that is, stem leaves and roots. A progressive and novel approach can be applied to such systems to overcome loading limits and improving removal efficiency due to seasonal variations. Some techniques like bioaugmentation, biostimulation and genetically engineered plant/microbe can be employed in this regard but on the ground of genetic manipulations many ethical issues get raised as the biggest challenge in their use will be the horizontal transfer of plasmids or genes in the environment. The large scale adoption of this technology still requires much fundamental and applied research needed to underpin CW technology but when this would be taken in real conjunction with actual remediation schemes; it will achieve the multipurpose of wastewater treatment, eco-friendly approach and biomass reuse.

Acknowledgment

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. Constructed Wetlands
  5. Future Prospects And New Trends
  6. Conclusion
  7. Acknowledgment
  8. LITERATURE CITED

The authors thank the Director, Thapar University, Patiala, India, for providing the infrastructure and facilities.

LITERATURE CITED

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. Constructed Wetlands
  5. Future Prospects And New Trends
  6. Conclusion
  7. Acknowledgment
  8. LITERATURE CITED
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