Treatment of Water Using Water Hyacinth, Water Lettuce and Vetiver Grass-A Review

Phytoremediat ion techniques for the treatment of different types of wastewater have been used by several researchers. These techniques are reported to be cost effective compared to other methods. Various contaminants like total suspended solids, dissolved solids, electrical conductivity, hardness, biochemical oxygen demand, chemical oxygen demand, dissolved oxygen, nitrogen, phosphorous, heavy metals, and other contaminants have been minimized using water hyacinth, water lettuce and vetiver grass. In this paper, role of these plant species, origin and their occurrence, ecological factors and their efficiency in reduction of different water contaminants have been presented.


Introduction
Phytoremediat ion is one of the b iological wastewater treatment methods [1], and is the concept of using plants-based systems and microbiological processes to eliminate contaminants in nature. The remediation techniques utilize specific planting arrangements, constructed wetlands (CW), floating-plant systems and numerous other configurations [2]. The removal of wastewater constituents are achieved by different mechanis ms like sedimentation, filtrat ion, chemical precipitation, adsorption, microbial interactions, and uptake of vegetation [3], among wh ich, the most effective technology is phytoremediation strategy using CW technology. Besides water quality imp rovement and energy savings, CWs have other environmental protection features such as promoting biodiversity, providing habitat for wetland organis ms and wild life (e.g. birds and reptiles in large systems) [4], serving climatic (e.g. less CO 2 production [4]; hydrological functions and biomethylation [5]). These systems are generally cost effective, simp le, environ mentally non-disruptive [1,6] ecologically sound [7] with low maintenance cost [8] and low land requirements [9].
The principles of phytoremediat ion system are to clean up contamin ated wat er, wh ich include iden t ificat ion and imp le men t at ion o f efficien t aqu at ic p lant ; u pt ake o f dissolved nutrients and metals by the growing plants; and harvest and beneficial use of the plant biomass produced fro m the remediation system [9]. The most important factor in imp le menting phytoremed iation is the selection of an appropriate plant [1,10], which should have high uptake of both organic and inorganic pollutants, grow well in polluted water and easily controlled in quantitatively propagated dispersion [1]. The uptake and accumu lation of pollutants vary from p lant to plant and also from specie to specie within a genus [11]. The economic success of phytoremediation largely depends on photosynthetic activity and gro wth rate of plants [7], and with lo w to moderate amount of pollution [12].

Role of Macrophytes in Water Contamination Removal
Macrophytes play important roles in balancing lake ecosystem. For the first time , they were recognised during 1960s and 1970s in water quality improvement [32]. Aquatic macrophytes treatment systems for waste-water are the need of developing countries, because they are cheaper to construct and a little skill is required to operate [20]. They improve the water quality by absorbing nutrients with their effective root system [15]. Macrophytes not only retain nutrients by biomass uptake, but also increases sedimentation [33]. These are utilized for nutrient and metal removal fro m water in the forms of CW or retention ponds because of their fast growth rates, simp le requirements, and ability to accumulate biogenic elements and toxic substances [9].
Aquatic plants are grouped into submerged, emergent, and floating-leaved based on their leaf's relation with water. During selection, bio mass production, growth rate, and easiness of management and harvest should be taken into account [9]. Wetlands are main ly dominated by the floating aquatic macrophytes [34][35][36]. Floating aquatic p lants can grow in vertical as well as horizontal direction, thereby increasing the photosynthetic surface area. These factors altogether ma kes floating aquatic plants, one of the earth's most productive communities [9]. The most common aquatic macrophytes among the floating-leaved, being employed in wastewater treatment are water hyacinth and water lettuce [37][38][39]. Imp ressive removal rates of inorganic nit rogen [nitrate (NO 3 -N), ammoniu m (NH 4 -N), and total N)] and phosphorus (PO 4 -P and total P) have been reported using aquatic plants especially when water hyacinth were utilized in nutrient or meta l-rich wastewaters [9]. A wuah et al. [25] found 70% of TDS reductions by water lettuce. Vetiver can also be used as hydroponics. Hydroponics is cultivating the crops in nutrient solution or media without soil. Th is species is known for its wide capacity to adapt various water regime conditions, but probably its temperature requirements were not fully satisfied [10]. Ebrah im et al. [40] found that vetiver root can remove TDS in water about 55.93% by the adsorption method. In India, as there is plenty of abundant and agriculture based lands, therefore, these techniques for wastewater treatment can be used for safe disposal of contaminated water.

Plant Origin and Geographical Distribution
Water hyacinth is fast growing perennial aquatic macrophyte (requiring a wet habitat) [41] and prolific free floating aquatic weed [42]. It is a me mber of p ickerelweed fa mily -Pontederiaceae and Genus -Eichhornia. Its name Eichhornia was derived fro m well known 19th century Prussian politician J.A.F. Eichhorn [43]. This tropical plant spread throughout the world in late 19th and early 20th century [44]. According to M itchell [45], the water hyacinth is indigenous to South America, particu larly to the Amazonian basin. It reached Australia in 1895, India in 1902, Malaysia in 1910, Zimbabwe in 1937 and the Republic of the Congo in 1952 [46]. It is abundantly found in India, Bangladesh and South East Asia [47]. This plant is rounded, upright with shiny green leaves, lavender flowers [48] with dark blue root system [49,50].
It has the great reproduction potential as it gro ws double in 5 to 15 days [14,16,49,51]. On ly ten plants in just eight months can produce population of 655,330 indiv iduals [14]. It co mmonly forms dense, interlocking mats due to its rapid reproductive rate and complex root structure [52]. It reproduces both sexually and asexually [53]. It is availab le naturally in wide region over 33 0 N to 33 0 S of the equator and grow rapidly fro m 220 kg/ha-day to 600 kg/ha-day seasonally in pond with density from 224 to 412 tons/ha [54]. The inflorescence bears 6-10 lily-like flowers, each 4-7 cm in diameter [46] and the flo wering period lasts for about fifteen days. When flowering cycle ends, flo wer stalk bends due to that spike reaches under the water surface and seeds are released directly into the water [14,55]. The height fro m flower top to root top of water hyacinth usually reach upto 1.5 m and more [56]. Arceivala [57] stated that individual plants ranges from 500 to 1175 mm fro m the top of the lavender flo wer to root tips.

Eco logical Factors
For a phytoremediation system to work efficiently, optimal plant growth is the key parameter. Many environmental factors can influence plant growth and its performance, such as temperature, pH, solar radiat ion, and salinity of the water. The weight and size of aquatic plants are a function of these factors. Its growth can be described by two ways: first by reporting the percentage of water surface covered for a period of time and second but more useful is by reporting the plant density in units of wet plant mass per unit of surface area [14,48]. Nutrient availability also affects the growth and performance of aquatic p lants. As per Makhanu [58] it co mprises of 95% water and 5% dry matter, out of which silica, potassium, nitrogen and protein is 50%, 30%, 15% and 5%, respectively. It also provides breeding space and sanctuary for birds, fish, snails, insects and other wildlife [59]. Zhao et al. [60] found that nutrient concentration, mean nu mber of ra mets, mean height and total biomass of water hyacinth significantly increased with increasing nutrient level. Shorter time is required to reach ma ximu m bio mass yield in summe r with high growth rate [61] whereas optimal water temperature for its growth is 28-30 ℃. Temperature above 33 ℃ inhib its further growth [55]. If temperature of -3 ℃ lasts for 12 hours, it will destroy all leaves and temperature of -5 ℃ for the period of 48 hours will destroy whole plant [48]. Other researchers also found similar results about water hyacinth sensibility to low temperatures. According to Stephenson et al. [62], it can survive 24 hours at temperatures between 0.5 and -5 ℃ but it will die at -6 to -7 ℃ and cannot be grown in open where average winter temperature drops under 1 ℃. Therefo re, it is not suitable for temperate or frig id areas due to their sensitivity to cool temperature [63]. Overall nutrient uptake is greater in summe r when temperatures are h igher and mo re favorable fo r p lant growth [64]. Low air humidity fro m 15% to 40% can also be limiting factor for undisturbed growth of water hyacinth [65]. It tolerates drought as well because it can survive in mo ist sediments up to several months [55]. Generally, plant grows best in the pH range of 5.5-7.0 [9]. Optimal water pH fo r g rowth of this aquatic p lant is neutral but it can tolerate pH values fro m 4 to 10 [66].
The growth rate of water hyacinth is strongly dependent upon the concentration of dissolved nitrogen (N) and phosphorus (P) in the water [67][68][69]. Sato and Kondo [70] reported that its ma ximu m growth rate can be achieved at 28 mg/ L of total N and 7.7 mg/ L of total P. The levels of available n itrogen and phosphorus have often been cited as the most important factors in limiting water hyacinth growth [69,[71][72][73][74]. According to Reddy et al. [69,71,72], 5.5 mg of N/ L and 1.06 mg of P/ L is required for survival of water hyacinth growth whereas to achieve ma ximu m growth N, P and K (potassium) are added at the rate of 20 mg N/L, 3 mg P/L and 52 mg K/ L, respectively. Therfore, to get the ma ximu m gro wth usually N is added as ammoniu m nit rate, sodium phosphate as P, and potassium chloride for K. Reddy and Tucker [75] suggested not only its nutrient concentration but also ratios between nutrients play an important role in plant growth. The h ighest production occurs when the N:P ratio in the water was close to 3.6. Plants need nitrogen for their metabolism to grow and to reproduce. According to Delgado et al. [76], water hyacinth prefers ammoniu m ions rather than nitrate ions. Water hyacinth absorbs ammonia by their roots to incorporate it in their biomass [31]. However, in the absence of ammoniu m N, a high growth can also be achieved with nitrate as the only source of N [77]. Reddy et al. [71] suggests that the overall P requirement of p lant is very lo w in comparision of N. Th inner and longer root enhance geometry for uptake of nutrients from the environment, hence P uptake depends on root length, diameter and surface area in contact with the environment [78]. Morphological plasticity in root system is, thus favorable for water hyacinth to adapt to lo w-P environment [79]. Reddy et al. [72] stated that nitrogen content of hyacinth tissue is inversely related to the K supply rates. It has one of the highest K tissue concentrations when compared with other aquatic plants [80][81] which ranges fro m 10 to 83 mg o f K/g. Such a wide range of tissue K content suggests that it has high K requirement and a high K uptake capability [71]. Reddy et al. [72] found that ma ximu m water hyacinth biomass 3.1 Kg (dry weight)/m 2 at a concentration of 52 mg of K/ L. Kn ipling et al. [80] found ma ximu m N, P, K concentration in the leaf, stem and root tissue of water hyacinth, respectively.
The plant biomass (the sum of leaf, stem and root volumes) relates closely to evapotranspiration potential [82]. Over the years, some researchers have found evaporation fro m open water to exceed evapotranspiration of vegetated surfaces [83][84][85]. Water hyacinth evapotranspiration loss is in between 3 to 10 times [54,86,87] in co mparison of open water.

Efficiency of Water Hyacinth In Reduction of Water Contaminants
Information in the literature about plant yields and growth rates are varying. The productivity of water hyacinth cultured in nutrient enriched waters and wastewaters has been found to be in the range of 40-88 mt (dry wt) /ha/yr [88,89]. Jo et al. [90] evaluated the growth of water hyacinth after 30 days and reported yield of 6402.5 g/ m 2 whereas Sooknah and Wilkie [91] used hyacinth in dairy manure for 31 days and found its yield was 1608 g/ m 2 . DeBusk et al. [92] evaluated hyacinth in secondarily treated municipal wastewater and reported plant productivity of 16 g/m 2 /day. Similarly, Ayyasamy et al. [93] observed biomass which increased fro m 75 to 101-106 mg/ L, with 37% increase in 10 days. Snow and Ghaly [94] found water hyacinth yields were 83 and 49 g/ m 2 with hydraulic retention times (HRT) of 6 and 12 days, respectively. It has been estimated by Reed et al. [95] that 10 individual plants can spread and cover one acre pond within 8 months.
Richards [96] reported poor growth in d istilled water because it produced small leaves with inflated petioles. According to Valipour et al. [18] water hyacinth is unable to survive in salinities above 2 ppt. Haller et al. [97] reported that it can withstand upto 2500 mg/kg (equivalent to 4040 μS/c m) and selenium concentration of more than 10 mg/L has phytotoxic effects on the water hyacinth [98].
The use of water hyacinth as the functional unit in wastewater treatment systems has been increasingly demonstrated and treatment regimens developed as a result of successful pilot projects [35,99].
The water hyacinth has successfully resisted of its eradication by chemical, bio logical, mechanical, or hybrid means [100]. Adeniran [101] observed that the water hyacinth of CW based requires only 13% of the energy as co mpared to conventional sewage treatment plant for the same quantity of sewage and concluded that is a viable and cost effective option for the treat ment of do mestic sewage in a developing economy. It has a huge potential for removal of the vast range of pollutants from wastewater [42,[102][103][104] and has the ability to grow in severe polluted waters [105]. It is also used to improve the quality of water by reducing the levels of organic, inorganic nutrients [106] and heavy metals [19,[107][108][109][110]. Presence of its fibrous root system and broad leaves help them to absorb higher concentrations of heavy meta ls [111]. It read ily reduces the level of heavy meta ls in acid mine drainage water [112] and silver fro m industrial wastewater in short time [113]. Th is capability ma kes them a potential bio logical alternative to secondary and tertiary treatment for wastewater [35,[114][115][116].
Water hyacinth has been found to stabilize temperature in experimental lagoons, thereby preventing stratification and increasing mixing within the water colu mn [117]. Water hyacinth can convert alkaline pH into neutral [20,23]. The reduction in p H is due to absorption of nutrients or by simu ltaneous release of H+ ions with the uptake of metal ions [20]. Borges et al. [118] obtained EC reduction by 18.1% in 5 days and TDS removal by 39.1% in 20 days. Lissy and Madhu [14] observed an increase in TDS when plant placed in the tank. Th is increase was due to the presence of clay or other fine particles present in the plant roots and or the presence of high Cr concentrations. On subsequent days, it showed that the TDS value considerably decreased by the accumulat ion process.
The reduction in pH favors microbial action to degrade biochemical o xygen demand (BOD) and chemical o xygen demand (COD) in the wastewater. According to Reddy [119], the presence of plants in wastewater depletes dissolved CO 2 during the period of photosynthetic activity and an increase in DO of water, thus creates aerobic conditions in wastewater, which favors the aerobic bacterial activ ity to reduce the BOD and COD [20]. Dar et al. [16] and Shah et al. [120] observed increase in DO level after using water hyacinth in wastewater whereas Mangas-Ramirez and Elias-Gutierrez [121] and Perna and Burrows [122] found lowered DO concentrations beneath the hyacinth mats. Trivedy and Pattanshetty [17] found that systems with shallow depth were more efficient in removing dissolved solids, suspended solids, BOD, COD, n itrogen and phosphorus. According to Valipour et al. [13] and Sooknah [123], higher pond depth can raise the anaerobic zones resulting slow organic degradation rate and foul odour emission. Many researchers [93,124,125] have found that removal of nutrients is more efficient in young plants as compare to old; hence, regular harvesting of old plants is essential. If not harvested at an appropriate time, nutrients from the plants are leached back into the water and old plants after death cause anaerobic conditions in water [93]. Ga mage and Yapa [126] used water hyacinth in text ile mill and monitored for a period of one year. They observed BOD and COD removal was 75% and 81.4% respectively whereas Kulatillake and Yapa [127] reported 99% BOD and 80% COD removal for rubber factory effluents. Snow and Ghaly [94] found that the COD reductions decreased as HRT was increased and ammoniu m (NH 4 ) reductions were significantly affected by plant type, but were not significantly influenced by HRT. They also concluded that plant type and HRT both have significant effects on nitrate reductions. Koottatep and Polprasert [128] obtained 71.0% (1 day) and 83.0% (5 days) efficiency in COD reduction whereas Jing et al. [129] found 13.0 to 51.0% COD removal in river water. Mohamad [130] observed a rapid heavy metal uptake during first four days of contact time, and such uptake being decreased with time until it reached saturation.
Elias et al. [131] observed 87.0% efficiency in the reduction of ammon ia, while as per Jing et al. [129], the efficiency in treat ing river water is 78.0-100.0%. Koottatep and Polprasert [128] obtained 84.0-86.0% removal efficiency of total nitrogen in 8 weeks of treat ment; Schulz et al. [132] reported 19.0% efficiency in 14 days and 30.0% in 70 days whereas Cornwell et al. [133] reported only 8.4% removal of nitrate-N in 10 months. Ingersoll and Baker [134] reported a removal efficiency of over 90% with an init ial n itrate concentration of 30 mg/ L. For inorganic N, Reddy et al. [135] reported a reduction of about 80%, wh ile Sheffield [136] observed 94% inorganic N and 40-55% ortho-P reduction. For total P, Reddy et al. [135] measured about 32% reduction, while Ornes and Sutton [137] achieved a higher removal rate of 80%. Bramwell and Devi Prasad [138] observed during a pilot scale study an average decrease in total N and total P by 27.6% and 4.48%, respectively. Sheffield [136] reported that pond with an air stripping unit, a flocculation and settling unit, removes >99% ortho-P, 99% n itrate, and >99% ammon ia. According to Knipling et al. [80], harvesting of one acre of hyacinth would remove 170 kg of N and 60 kg of P and in ma ximu m gro wth, one hectare of hyacinths could remove about 2500 kg of N/yr [139] and as high as 7629 kg of N/ha/yr [75]. According to Ayyasamy et al. [93], n itrate removal efficiency of water hyacinth was increased to 64, 80 and 83% with in itial nitrate concentrations of 100, 200 and 300 mg/ L, respectively, but it is decreased with 400 and 500 mg/ L. This was due to osmotic pressure at higher concentrations not supporting the uptake of nitrate [140]. In the ground water samples, the nitrate removal was greatly dependent upon the presence of other nutrients, such as sulphate and phosphate, which caused lower n itrate uptake by water hyacinth [93].
Mane et al. [98] ind icated that at lower concentrations (5 mg/ L) of heavy metals, the p lant growth was normal and removal efficiency was greater. Concentrations greater than 10 mg/ L, the plant started wilting and removal efficiency was reduced due to toxicity at higher metal concentrations. O'Keefe et al. [142] found similar nature of metal uptake for cadmiu m. Water hyacinth without reduction in growth have high removal rates for iron (Fe), zinc (Zn), copper (Cu), chromiu m (Cr), cad miu m (Cd), manganese (Mn), nickel (Ni), mercury (Hg) and arsenic (As) [143][144][145][146]] fro m aqueous solutions besides absorbing organic substances such as phenol, formaldehyde, formic , acetic and o xa lic acids [144][145][146]. Liao and Chang [146] found that the absorption capacity for water hyacinth, as 0.24 kg/ha for Cd, 5.42 kg/ha for lead (Pb), 21.62 kg/ha for Cu, 26.17 kg/ha for Zn, and 13.46 kg/ha for Ni. Valipour et al. [18] stated that if heavy meta ls exceed the saturation limit of 268 and 2152 mg/kg for Cd, 381 and 3372 mg/ kg for Cu, 229 and 1850 mg/kg for Ni, 462 and 2764 mg/ kg for Zn in shoots and roots, respectively, it can lead to mo rphological deformity. It is the best species as Cd accu mulators [110,147,148]. In California, water hyacinth leaf tissue was found to have the same mercury concentration as the sediment beneath, suggesting that plant harvesting could help mediate mercury contamination if disposed of properly [149]. M ishra et al. [111] used hyacinth for coal min ing effluent for the removal of heavy metals and observed 70.5 ± 4.4, 69.1 ± 3.9, 76.9 ± 1.4, 66.4 ± 3.45, 65.3 ± 2.4 and 55.4 ± 2.9 percent Fe, Cr, Cu, Cd, Zn and Ni, respectively was removed. The study revealed that plant roots accumulates heavy metals appro ximately 10 times of its init ial concentration whereas Chandra and Kulshreshtha [150] reported 18.92 (g dry tissue wt) Cr accu mulat ion in roots of water hyacinth.
According to Lindsey and Hirt [151], water hyacinth can be used like food for people or fodder because its leaves are rich in proteins and vitamin A. But it is not reco mmended to consume if used for removal of heavy metals and to xic substances as it can cause problems when enter in food chain [102]. Its biomass is rich in nitrogen and other essential nutrients. Apart fro m biogas [152], its sludge contains almost all nutrients and can be used as a good fertilizer with no detrimental effects on the environ ment [51]. After harvesting, it can be used for co mposting, anaerobic d igestion for production of methane, fermentation of sugars into alcohol [48], green fertilizer, co mpost and ash in regenerating degraded soils. These operations can help in recovering expenses of wastewater treatment.

Plant Origin and Geographical Distribution
Pistia stratiotes (L.) is a floating perennial co mmonly called water lettuce belonging to the family Araceae. It floats on the surface of the water, and its roots hanging submerged beneath floating leaves [23]. While it may have orig inated in South America but the origin of water lettuce is uncertain. It has been used in Africa as a medicine and fodder for cattle for centuries being recorded in Egypt in 77 A.D. [153]. It has spread over the rest of Africa and parts of Asia and in the 1970s also found its way to Australia [154]. The leaves can be up to 14 cm long and have no stem. They are light green, with parallel veins, wavy margins and are covered in short hairs wh ich form basket-like structures and help in trapping air bubbles, increasing the plant's buoyancy. The flowers are dioecious and are hidden in the middle o f the plant among the leaves. The plant can be reproduced by both vegetatively and sexually [23,155].
Water Lettuce is non winter-hard plant, having a minimu m growth at temperature 15 ℃ [156]. In general, the specific growth rate of water lettuce is slightly higher as compared to the water hyacinth in dry season. However, the rainy spell reduces the growth of the water lettuce because of the lower solar rad iation, which is needed for its growth [157]. Fonkou et al. [21] stated that lettuce doubles its biomass in just over 5 days; triples it in 10 days, quadruples in 20 days and has its original bio mass multip lied by a factor of 9 in less than one month. Th is evolution indicates that 25 days is the ma ximu m period to allow the plant in the system. Because this plant reproduces rapidly and decays, the efficacy of the system is intimately lin ked to its careful management trough periodic harvesting of part of the bio mass produced.
Especially in tropical or subtropical areas, water lettuce (large-leaved floating plant) is used in phytoremediation water systems [158][159]. Th is is because, compared to native plants, this invasive plant show a much higher nutrient removal efficiency with their high nutrient uptake capacity, fast growth rate, and big bio mass production [41].

Eco logical Factors
Water lettuce is superior in productivity as compared to other small aquatic weeds such as Le mna spp [61]. Knowledge on salinity tolerance of p lant can help better utilize the p lant(s) without bringing disaster because it has significant effects on growth and performance. According to Haller et al. [97], floating plants such as water lettuce have higher survival rate, at higher levels of EC having a killing strength (>4000 μS/cm). Th is indicates that water lettuce withstand higher salinity conditions but does not grow at higher COD levels [91].
Although water lettuce can produce high biomass and remove large amounts of nutrients and metals, they may not be suitable for temperate or frig id areas due to their sensitivity to cool temperature wh ich significantly affects their performance [63]. Lu [9] suggested that Fe, Cu, and Ni are essential for p lant growth, but when present at high concentrations, they are toxic to plant. Lu et al.[22] also reported that low concentration of nutrients may reduce the performance of p lant in remov ing nutrients.

Efficiency of Water Lettuce in Reduction of Water Contaminants
Plants are known to accumulate large quantities of nutrients during period of rapid growth [160]. Fonkou et al.
[21] observed in 25 days, the b io mass increased in 7 ponds fro m the original 518 g/ m 2 (average) to 2488, 2578, 2925, 5379, 6176, 6793 g/ m 2 respectively whereas Ayyasamy et al. [93] observed during the 10 day experiments, the biomass of water lettuce increased fro m 75 to 92-94 mg/ L with 24% increases. Water lettuce of 1.25 mg N/ L treat ment doubled its init ial b io mass. Plant showed healthy bright green without yellowing of the old leaves. Lowe r N and P requirements ma ke water lettuce desirable for a polyculture system. The N, P and ash contents of biomass were about 1.5 t imes lesser in water lettuce than in water hyacinths [161]. A 200-fo ld difference in dry weight of water lettuce was reported by Aoi and Hayashi [161] between cu ltivated in rain water and treated sewage water. Fonkou et al. [21] indicated that the number of leaves per plant decreases, as a result of the decay of the basal leaves that fall back into water, then releasing the substances that were absorbed after 15 days in all the treatment ponds. It was found that EC, DO and ammonia are poorly removed. Unpublished works reported total bacteria, faecal Streptococci and Salmonella sp. to be fairly eliminated in the system by 52.7-64.3 %, 45.6-79.5 and 35.5-66.4 % respectively. DO was increased fro m 0.75 to 6.02 with imp rovement of 87.5%.
Dipu et al. [23] found that alkaline pH was changed into neutral using lettuce. Similar results were also reported by Mahmood et al. [20]. The reduction in pH is due to absorption of nutrients and other salts by plants or by simu ltaneous release of H+ ions with the uptake of meta l ions [20]. Awuah et al. [25] used lettuce in their study of bench-scale continuous-flow wastewater treatment system with feed of sewage. They observed that lettuce removed TDS by 70%, fecal coliform by 99%, BOD by 93%, COD by 59%, n itrate by 70%, total phosphorus by 33% and a mmon ia by 95%. Water lettuce is reported to reduce the ammoniu m ions from the water as it utilizes ammoniu m (NH 4 -N) prior to n itrate (NO 3 -N) as nit rogen source and does not switch on the utilizat ion of NO 3 -N until NH 4 -N gets consumed entirely [161]. Ingersoll and Baker [134] reported nitrate removal efficiency of water lettuce ranged from 31 to 51%. However, according to Aoi and Hayashi [161], at an initial nitrate concentration of 5.5 mg/ L, water lettuce had a similar nitrate removal capacity to water hyacinth in batch culture experiments [93]. It has been extensively used to remove meta ls like Zn, Ni, and Cd fro m the water co lu mn [162]. However, at 20 mg/ L Cr, plants of lettuce showed 100% death after three days [163].
Preliminary study by Lu [9] revealed that water lettuce growth decreased the EC in the treatment plot due to salt removal fro m the waters by plant uptake or root adsorption and it was concluded that water quality in ponds was improved by phytoremediation with water lettuce, as evidenced by decreased turbidity, total solids, NH 4 -N, NO 3 -N and total Kjeldahl N, and nutrient concentrations. Reductions in ortho P, total d issolved P, and total P concentrations was found by 18-58% co mpared to the control plots. Metals were substantially accu mu lated in the roots of water lettuce. A larger proportion of Ca, Cd, Co, Fe, K, Mg, Mn, and Zn were attached to external root surfaces by adsorption or surface deposition while more Al, Cr, Cu, Ni, and Pb were absorbed and accumulated into the roots.
A study conducted by Lu et al. [22] indicated that total suspended solids in the water column were decreased by approximately 10% in treat ment p lots compared to control plots. Water lettuce growth decreased water pH, which was not expected for it is well known fact that water pH rises with plant photosynthesis. Besides plant uptake, denitrification may also contributed to the decreased NO 3 -N concentration in the treatment plots as a more anaerobic condition (dissolved oxygen <1.5) which was created by the growing plants at the water's surface and other anaerobic micro -sites [164,165]. Alu min iun (Al), calciu m (Ca), Fe, K, and Mn concentrations in the remediat ion plots were significantly (P<0.01) reduced by the growth of water lettuce. Water lettuce can be considered a hyperaccumu lator for trace meta ls such as Cr, Cu, Fe, Mn, Ni, Pb, and Zn [166]. Periodic harvesting of water lettuce is necessary not only for maintaining an optimal g rowth density, but also for effective removal of nutrients (N and P) and meta ls fro m the waters, otherwise the nutrients and metals would be released back into the water system after the p lant died and decomposed [21,166].
Mukhopadhyay et al. [167] found that the removal is dependent both on the contact time and the init ial arsenic concentration. He observed a rapid in itial uptake upto 48 hours and gradual attainment of equilibriu m after 120 hours. Such concentration and duration dependent removal were also obtained for cadmiu m using water hyacinth [142] and water lettuce [168] and for Hg (II) using lettuce [169]. According to Mukhopadhyay et al. [167] and O'Keefe et al. [142], metal uptake was higher fo r lo w metal concentration and decreased thereafter with increase in metal concentration. So me researchers found similar nature of meta l uptake in water lettuce for cad miu m and for arsenic. Mishra et al. [170] found water lettuce removed 80% of mercury (i.e. fro m 10 µg/ L to 2µg/ L) fro m the coal mining effluent in 21 days. Mercury accumulat ion in the roots of lettuce was about four t imes higher than the shoots at lower concentrations [111,143,171,172]. Maine et al. [42] found water lettuce efficiently removed Cr fro m water at the concentrations of 1, 2, 4, and 6 mg Cr/ L.

Plant Origin and Geographical Distribution
Vetiver grass (Chrysopogon zizanioides) belongs to the Gramineae family. The vetiver is a unique tropical plant that has been proven and used in some 100 countries for soil and water conservation, land rehabilitation, pollution control, water quality improvement and many other environ mental applications, particularly the loo ming food crisis in many parts of the developing world. The vetiver System is easy to use and low cost [173,174]. It is tall erect and native to India, South and South-East Asia [175]. It is found throughout the plains, lower h ills of India part icularly on the riverbanks, in ma rshy soils and it is widely used in Karnataka, India [27]. It is an herbaceous perennial plant, the leaves are erected and rather stiff with height ranges from 0.5 to 1.5 m. It has a deep and resistant root system with fast growth [10]. According to Dulton et al. [176], it is characterized by its large bio mass and having a dense root system extending up to 3 m in depth. It has fine purple flowers and an architectural aesthetic that can be well incorporated in landscape designs [173].
Vetiver system is based on the use of vetiver grass, which was first recognized early in the 1990s for having "super absorbent" characteristics suitable for the treatment of wastewater and leachate generated from landfill [177].

Eco logical Factors
Vetiver grass is an "ecological-climax" species [10] with a deep dense spongy root system that binds soils together. Vetiver can withstand drought [28,178] and is not affected by flood [178]. A lthough vetiver is a tropical grass [179] can also tolerate extreme temperatures, fro m -15 ℃ to 60 ℃ [28]. According to Zhang [180], it g rows rapid ly above 25 ℃. Many researchers have used vetiver for extreme cold conditions like in Australia, vetiver gro wth was not affected by severe frost at -11 ℃ and it survived fo r a short period at -22 ℃ in northern China. In Georgia (US), vetiver survived in soil temperature of -10 ℃ but not at -15 ℃ [181] whereas Maffei [178] records vetiver having an absolute minimu m temperature of -15 ℃ belo w which death occurred. Vierit z et al. [179] reported that although very little shoot growth occurred at the soil temperature range of 15 ℃ (day) and 13 ℃ (night), root growth continued at the rate of 126 mm/day, indicating that vetiver grass was not dormant at this temperature. It was concluded that under frosty weather, its top growth is killed but its underground growing points survived, plants grow more slowly under colder conditions and the growth stages are better defined on the basis of therma l t ime rather than chronological time. Maffei [178] describes vetiver as growing lu xu riantly in areas with temperatures ranging fro m 21-45 ℃. According to Maffei [178] and Zhang [180] root length, root and shoot dry weight increased with increasing temperature fro m 15/ 13 to 35/ 30 ℃ (day/night) and min imu m daily air temperature for growth should be less than 12 ℃.
Even though it is not an aquatic plant, vetiver can be established and survive under hydroponic conditions. However, vetiver cannot be established directly in leachate ponds, as it does not float as alligator weed (Alternanthera philoxeroides); it needs a floating platform to grow on. Its high affin ity for both organic and inorganic chemicals shows that the grass could be used to develop a cost effective and environment friendly remediation for waste water [28]. Xia et al. [182] suggested that for sustainable removal of pollutants fro m leachates, vetiver shoots should be trimmed 2-3 times per year.

Efficiency of Vetiver Grass in Reduction of Water Contaminants
Vetiver grows rapid ly and has a huge biomass [182]. It can purify eutrophic water [183], garbage leachates [182] and wastewater fro m pig farms [184]. It is excellent for the removal of heavy metals fro m contaminated soil [185,186] and rehabilitating landfills [187]. It has proven to be exceptionally successful in urban environ ments by demonstrating its ability to absorb pollutants into its foliage [173]. According to Xia et al. [182], vetiver has high level of tolerance for polluted water and very effective in removing pollutant fro m landfill leachates, particularly N and P. Nitrogen and Phosphorus absorption is also expedited because roots have direct exposure to effluents. It tolerates wide range of pH, salinity, sodicity [175,188], acidity and heavy metals such as As, Cd, Cu, Pb and Zn [188] It could also absorb higher N, P and K [196]. Jayashree et al. [189] used this system upto 60 days for the treatment of text ile water and found that pH reduced fro m 8.6 to 7.8, EC fro m 1.34 to 0.22 dS/ m, total kjeldahl nitrogen fro m 8.85% to 0.53%, P fro m 5.9% to 0.81%. Researchers [178,188] found that it has high level of tolerance to salin ity.
Vetiver can be used in phytoremediation o f contaminated water system and has been reported to adsorb many heavy meta ls [187]. Ho wever, the concentration of heavy metals in wastewater played an important role in vetiver growth. The vetiver ecotypes absorbed Fe>Mn>Zn>Cu>Pb, and they concentrated these meta ls more in roots than in shoots [1]. It has been used successfully for contaminants removal in many countries such as Australia, China, Thailand, Vietnam and Senegal [181]. Truong et al. [175], soils that can even be loaded with very high levels of alu miniu m (>68 Al/cation exchange capacity %), iron, manganese (>578 ppm) and other heavy metals often associated with acidic soils such as As, Cd, Cu, Cr and Ni. It can also withstand high levels of pesticides and herbicides and also to a wide range of toxics [10].
Girija et al. [28] stated that the higher temperature favors their growth and mu ltiplication. Lo w values of pH beco me almost neutral after one month of its planting. EC of polluted water is directly proportional to its dissolved mineral matter content and after planting vetiver, the EC decreased to a very low value. Lakshmana et al. [27] also found the same result. Hardness was found to be ranging from 106-206 mg/ L but after planting vetiver, a 60% removal was observed in 2 months, which is in agreement with Truong and Hart [190]. DO was increased fro m 0 to 4.5 mg/L after 1 month which is in agreement with Stefanie et al. [191]. W ith an increase in EC, coliforms too increased in nu mber. DO have an inverse relationship with the coliform and is directly proportional to COD and BOD. As the organic matter is the food of coliform bacteria, Boonsong and Chansiri [192] observed higher BOD and COD removal efficiency.
Mane et al. [98] found that shoot length of vetiver grass was increased by 18.6% at 200 mM NaCl concentration whereas; increase in root length about 24.8% was observed at 50 mM NaCl. The average leaf area also increased under saline conditions. Dry weight and fresh weight bio mass was less effective under salinity stress. They also observed increased levels of polyphenols at elevated salin ity due to the accumulat ion of secondary metabolites. Linear increase in the EC and TDS of the soil was found at increasing salinity and the vetiver is tolerant upto 100 mM o f salinity because of increase in growth and photosynthetic parameter. Ebrah im et al. [40] indicated decrease of TDS by 55.93% in hard water with the help of vetiver root by using adsorption method.
Depart ment of Natural Resources and Mines, Queensland research showed that vetiver grass has a fast and very high capacity for absorption of nutrients, particularly nitrogen and phosphorus in wastewater [26,193]. Wagner et al. [194] found that both N and P supplies increased vetiver growth significantly (<1% level). Gro wth increased mainly with the level of N supplied. However, very little growth response occurred at rates higher than 6000 kg/ha/year although rates up to 10,000 kg/ha of N did not adversely affect vetiver growth. Vet iver requirement for P was not as high as for N, and no growth response occurred at rates higher than 250 kg/ha/year. However, its growth was not adversely affected at P up to 1000 kg/ha/year. Anon [195] and Zheng et al. [196] found 98% removal for total P in 4 weeks and 74% for total N after 5 weeks in polluted river water. Wagner et al. [194] used vetiver in hydroponic system using sewage effluent and observed that both N and P removal over 90% fro m the effluent; it also reduced algae growth and faecal colifo rms. Truong and Hart [190] used vetiver for do mestic effluent treatment for 4 days and the removal in total nitrogen was 94%, total P was 90%, EC by 50%, change in pH was (fro m 7.26 to 5.98), faecal colifo rm changes were 44% and E. co li changes were 91%. Therefore, vetiver has high potential to be used for industrial wastewater treat ment.

Concluding Remarks
It has been observed that phytoremediat ion of wastewater using the floating plant system is a predo minant method which is economic to construct, requires little maintenance and increase the biodiversity. Many researchers have used water hyacinth, water lettuce and vetiver grass for the removal of water contaminants but their treat ment capabilit ies depend on different factors like climate, contaminants of different concentrations, temperature, etc. Vetiver grass can be grown as floating in water without the soil med ia (hydroponic way). The removal efficiency of contaminants like TSS, TDS, BOD, COD, EC, hardness, heavy metals, etc varies fro m plant to p lant. Plant growth rate and hydraulic retention time can influence the reduction of contaminants. Therefore, an available knowledge and techniques for removal of water contaminants and advances in waste water treat ment can be integrated to assess and control water pollution.