Xia Hanping, Ao Huixiu, Liu Shizhong He Daoquan (South China Institute of Botany, Academia Sinica, Guangzhou 510650, China)
Abstract. This paper investigated effects of vetiver (Vetiveria zizanioides) in purifying urban garbage leachate, with compared to effects of Alternanthera philoxeroides, Paspalum notatum, and Eichhornia crassipes.. The results showed that the leachate from Likeng Garbage Landfill of Guangzhou exceeded the effluent standard, which could be harmful to plants and environment. E. crassipes died in the two types of leachate, P. notatum could not survive in the high concentrated leachate (HCL) and was severely damaged in the low-concentrated leachate (LCL). Vetiver and A.philoxeroides were also stressed and injured to varying degrees. LCL impacted most on the eutrophication of A. Philoxeroides. On the whole, the purification of LCL by A. philoxeroides was better than that of vetiver and P. notatum, but the growth of vetiver in HCL and its purification of HCL was much better than A. philoxeroides. Of all seven items measured in the study, ammoniac nitrogen was the best cleansed, and its purification rate was between 83%-92%, which suggests that the 3 species were all likely to have strong absorption abilities to ammoniac N dissolved in water. In addition, vetiver showed a quite high purification rate for phosphorus (more than 74%). In sum the purifying effects of the four species of plants on, and their tolerance to leachate were ranked as Vetiveria zizanioides > A.philoxeroides > P. notatum > E. crassipes. Vetiver and A. philoxeroides could be used as plants to assist in purifying garbage liquid.
Key words: Garbage leachate, Vetiver (Vetiveria zizanioides), Alternanthera philoxeroides, Paspalum notatum, Eichhornia crassipes, Purification, Pollution ecology.
1. INTRODUCTION
Many experiments and observations have confirmed that vetiver (Vetiveria. zizanioides), a perennial grass, has an excellent effect in soil and water conservation, extreme soil amelioration, and other environmental mitigating uses[1-3]. Todate it has been widely used for protection of sloping farmlands, orchards, reservoirs, dikes and dams, for improvement of extreme (strongly acid, alkaline or infertile) soils, and reclamation of mine tailings and polluted areas. Vetiver grown in wetlands has good purifying and stabilizing effects on aquaculture sludge[4]. Although vetiver is not a hydrophyte, it prefers wet and water logged habitats; even if its crown and roots, even a large portion of its shoots, are submerged for relatively long periods, the grass can also grow and develop. However, to date there have not been any reports on suspending vetiver directly into water to purge sewage.
The city of Guangzhou is moving rapidly to becoming an international metropolis; however, a series of ecological and environmental problems have emerged with the development of its economy. For example, that life garbage landfill and its leakage pollute the surroundings is becoming increasingly evident. Currently, municipal garbage in Guangzhou is treated mainly through filling and burning, and some measures are taken to prevent garbage and its waste water from polluting nearby areas. However, the leachate from landfills usually has high contents of "pollutants" such as biochemical oxygen demand (BOD), chemical oxygen demand (COD), alkalinity, organic matter and nitrogen compound, in which most plants can not survive. Although this kind of sewage is not discharged until "purified", the proper effluent standards are rarely reached; as a result farmlands and fishponds are polluted on the lower reaches to varying degrees, and the quality of lives, both health and mind, of nearby inhabitants are effected [6]. Sometimes "purified" discharges are far below the effluent standard; as a result, they have to be pumped into the filtration station for a second cycle of cleansing at high cost.
One solution would be to find some plants that could grow well in waste water and have the capability to purify it, thus providing a meaningful way of mitigating the harmful impact of urban garbage. Vetiver grass has a huge biomass and is able to tolerate adverse conditions, and its effects in ameliorating polluted soil, rehabilitating mine tailings, and tolerating poisonous heavy metals are well demonstrated. Since vetiver has so many excellent properties, it might be possible to use it for sewage purification. The question would be as to its effectiveness. This paper records and discusses an investigation into the impact of growth of vetiver by landfill leachates and its effectiveness in their purification, in comparison to Alternanthera philoxeroides, Paspalum notatum, and Eichhornia crassipes, with the objective of identifying one or two species of plants that have the ability to purify waste water.
2 MATERIALS AND METHODS
2.1 Experimental Materials The garbage leachate was taken from the sewage-purifying station of Liken Garbage Landfill of Guangzhou. This landfill, located in the northern suburbs of Guangzhou, is about 15 km from downtown. Established in 1992, it is the largest landfill in Guangzhou, and treats some 2600 t of refuse per day. The sewage-purifying station, lies below the landfill and purifies 300 t of leachate per day. The two kinds of leachate used in the trial were collected from the entrance and exit of the station; they consisted of the highly concentrated leachate (HCL) that flowed out of the landfill prior to purification, and the low concentration leachate (LCL) that had been physically cleansed and ready for discharge into the oxidation pond, respectively. Plant materials used were four herbs: Vetiver (V. zizanioides), A.philoxeroides, P. notatum, and E. crassipes. Vetiver and P. notatum were sampled from the nursery in South China Institute of Botany, and A.philoxeroides and E. crassipes from ditches and ponds.
2.2 Trial Designs and Arrangements The experiment was conducted with a method of water cultivation in buckets, and undertaken in the glasshouse of South China Institute of Botany. The glasshouse was ventilated and pervious to light. The experiment was arranged in three treatments: clean water, LCL, and HCL; and three duplicates for each treatment. The operation was as follows: preparing 36 plastic buckets (4 species x 3 treatments x 3 duplicates) with a volume of about 3 dm3 each, adding 2.50 kg of the 3 liquids above into each bucket, and putting one of the 4 species whose health and weight were basically similar into each bucket. Before the plants were put into the buckets, tops and roots of vetiver and P. notatum were pruned to 20 cm and 10 cm, respectively; A. philoxeroides was also cut into sections of 20 cm long; but only E. crassipes was kept intact; since the whole plant did not exceed 20 cm long. All the plants were weighed and their tiller numbers counted. Water cultivation lasted 66 days (May 12th- July 17, 1997), and observations were made of their growth in the three kinds of water and their efficiencies in purifying waste water. During the period of cultivation, clean water was added to buckets once every 2-4 days to supplement water reduced by transpiration and evaporation, but the amount added each time was no more than that removed. In addition, the two original leachates were also put into four similar buckets, each for 2 buckets and 2.5 kg for each bucket. They were set up at the same time (and were given the same arrangements as the 36 buckets) to investigate the effects of biological and environmental factors, including evaporation, irrigation, vessel absorption, and impacts of sunshine and air, on the water quality of the leachate.
2.3 Observations And Analysis
Items 2.1 Situations of plant growth in water. Including plant height, number of tillers, biomass of shoots, and length of new roots and their net increased weight.
Items 2.3.2 Effects of plants in purifying leachate. The analytical items contained contents of Fe, Pb, Cd, Zn, Ni in the two types of leachate; and pH and contents of COD, total N, ammoniac N, nitrate N, total P, Cl in the 2 liquids, including prior to and after water cultivation.
2.4 Analytical Methods pH was measured with an acidmeter. COD values were acquired through measuring the consumption of dissolved oxygen; the leachates were oxidized with KMnO4; and BOD was also referred to as the consumption of dissolved oxygen after the leachate was incubated for 5 days at 20�C in an incubator. Alkalinity was measured with titration of double indicators, phenothalin and methyl orange. Total N was oxidized with K2S2O8 and analyzed with an ultraviolet spectrophotometer. Ammoniac N and nitrate N were determined with direct distillation and colorimetry of phenoldisulfonic acid (C3(CH) 3 (HSO3) 2OH), respectively. Total P was digestedwith H2SO4HClO4, then measured colorimetrically. Cl3 was determined with titration of AgNO3. Metal elements were all measured with an atomic fluorescence spectroscopy.
3 RESULTS
3.1 Situations of the Water Quality of the Leachate Table 1 shows that the contents of COD, total N, ammoniac N, total P, and Cl3� in the leachate exiting from the purifying station of Likeng Garbage Landfill were quite high, and were many times (dozens) higher than the highest allowable discharge concentrations of industrial sewage. After purifying, the contents of these items significantly decreased; and artificial purifying rates were between 30-80%. However, the concentration of the exit liquid still went beyond the effluent standard, which exceeded 100 mg/L COD, 210-240 mg/L total N, 1.6 mg/L P and 500-600 mg/L Cl3, even compared to the second standard of "the standard for comprehensive discharge of sewage of China (GB 8979-88)"[7]. BOD of the two liquids were relatively low, and were within the effluent standard after purifying. Of all items analyzed, only nitrate N in the exit water was much higher than that in intake leachate, suggesting that the nitrification probably took place in the LCL, for oxygen in the air dissolved in the water when the leachate was mechanically cycled, thus aerobic nitrifying bacteria rapidly and greatly multiplied in LCL. Summerfelt et al. observed the contents of nitrate N in sludge from aquaculture rapidly rose from 0.057 mg/L to 45.41 mg/L[4]. Human derived garbage is different from industrial pollutants with reference to heavy metals. The contents of Zn, Pb, Cd, and Ni in the two leachates were far less than that in industrial effluents, even lower than background values in soil, and therefore they generally did not add to the toxicities of heavy metals to the environment.
Table 1 The water quality of two types of leachate from Likeng Garbage Landfill of Guangzhou and the purifying effect via the purifying station
TABLE NOT INCLUDED
3.2 Impacts of Environmental Factors on the Water Quality of waste water After the two kinds of leachate had remained undisturbed in the greenhouse for 66 days, it was found that the main factors relating to water quality had changed (Table 2). pH in LCL and HCL increased by 0.42 and 0.45 respectively. COD, alkalinity, N, P, and Cl, which are perhaps detrimental to, or over-nutritious to the environment, all went up or down in varying degrees. Four items that fell were, COD, alkalinity, total N and nitrate N -- all were reduced significantly, in the order of 29% - 47% , which indicated that pollutants could be broken down, diluted, oxidized, and evaporated under the impacts of microorganism, rain water, atmosphere, sunshine; and thus the polluted degree of wastewater could be alleviated. The lowering of nitrate N concentration possibly resulted from denitrification.
The amounts of ammoniac N, total P and Cl3 slightly increased (due to environmental factors as well). It is interesting that the contents of total N in the two leachates went up, but ammoniac N went down. This was probably a result of amination and ammonification of organic substances, which was also perhaps the reason why water pH rose. The two chemical reactions broke down complex organic substances into simple organic and inorganic substances, lowering the amount of organic N, and accordingly reducing total N in the leachate. The products of amination, amino acid, amine and amide, increased in ammoniac N; and the product of ammonification, NH3, was susceptible to the combination with H+ in water to form NH4+, which made H+ decrease and comparatively made OH3 increase.
Table 2 The changes of water quality of the 2 liquids 66 days after incubating open in the green house
TABLE NOT INCLUDED
3.3 The growth of the four Species of Plants in Liquids The four species selected all have the characteristics of rapid growth, large biomass, and somewhat or strong tolerance to a poor environment. The water cultivation experiment showed that they all could grow and develop in clean water; vetiver and A. philoxeroides were capable of surviving in the two types of liquor; but P. Notatum died in the HCL and was severely impaired in the LCL; E. crassipes also died in the two leachates.
It is thus clear that: 1) the ability of the four species of plants to resist pollution ranked as: vetiver, A.philoxeroides > P. notatum > E. crassipes, and 2) the sewage from sanitary landfills of Guangzhou indeed polluted the surroundings and poisoned organisms. The growth of vetiver, A. philoxeroides and P. notatum in clean water and the two leachates is tabulated in Table 3, from which it is noted that the plants, no matter what species, showed large differences in growth in the three kinds of water. It is obvious that P. notatum was not suitable to grow in water, and its resistance to pollution was considerably weak. The growth and biomass of A. philoxeroides in the three liquids produced the largest disparities, and were significantly in order of LCL > HCL > clean water. It is suggested that water being no high concentration could make A.philoxeroides eutrophy, which is coincident with the phenomenon that A. philoxeroides widely creeps in filthy sludge or drainage. The growth of vetiver assumed a trend of clean water > LCL > HCL, indicating that it is damaged gradually and becomes more serious with concentration increases.
In addition, the root:shoot ratios of the three species tended to increase with increased liquid concentrations, which was perhaps due to: 1) pollutants inhibiting roots to absorb nutrients and water; and thus their shoots could not obtain sufficient nutritients, and 2) shoots were more sensitive to pollutant toxicity than roots, and therefore the shoot biomass production was diminished more severely than the root. Root:shoot ratio of plants is a useful indicator of environmental stress, e.g. plants, under the condition of water and mineral nutrient deficiency, tend to have increased root:shoot ratios in order to increase the root surface absorption capacity[9].
Table 3 The growth situations (mean SD) of three species in clean water and two leachates
TABLE NOT INCLUDED
The most conspicuous feature of vetiver is its deep and massive root system . In the present study, however, vetiver did not exhibit the characteristics at all, except that it produced a slightly greater root biomass in clean water than the other two species. Vetiver was inferior to P. Notatum in root length, and inferior to A. philoxeroides in relation to net increment of roots in waste water (Table 3). It is most likely that the aquatic environment was unfavorable for the root growth of vetiver. The net increment of biomass of vetiver in clean water was significantly more than that of A. philoxeroides and P. Notatum (Fig. 1),which might be because the former was better than the latter two with special reference to the endurance to infertility. The biomass gained by A. philoxeroides in LCL was far more than in other treatments and by other plants; whereas vetiver had more biomass than A. philoxeroides in HCL. This suggests that: 1)vetiver is an excellent plant in its tolerance of low nutrient status and to pollution; 2) although A. philoxeroides grew better in water than the other two species, it had a poor ability to tolerant nutrient deficiencies, for its growth in clean water was even weaker than that of P. Notatum; 3) A. philoxeroides could take advantage of "pollutants/nutrients" in water, when the concentration was not very high, resulting in a diffusion of growth ; and 4) P. notatum was obviously not appropriate for the aquatic habitat.
Fig. 1 Effects of different water-cultivated treatments on biomass of the three species
FIGURE NOT INCLUDED
3.4 The Purification of Vetiver, A. philoxeroides, and P. notatum for Garbage Sewage After cultivating plants for 66 days, the concentration of almost all "pollutants" measured in the two liquids had decreased substantially, when compared to their original concentration (Table 1), except for Cl3 in the LCL that supported P. notatum, which was the only one to increase (Table 4). The element that declined the most was the content of total N in LCL supporting A.philoxeroides, which decreased from 293.8 mg/L to 23.9 mg/L, a reduction of 92%; the factor that had the highest reduction was alkalinity in HCL supporting vetiver, which dropped from 1882.9 mg/L to 365.5 mg/L, a reduction of 1517.4 mg/L. The results indicated that both vetiver and A.philoxeroides impacted positively in improving the quality of leachates. Most of the seven indicator factors/elements showed decreases in concentration levels due to the purifying effect of plants than did the controls (Table 2). It is also noted in Table 4 that some items varied greatly thus significantly effecting water quality, indicating the different effect of the three species on the leachates were different. For HCL, the water quality of the leachate supporting vetiver was generally better than that cultivating A. philoxeroides, whereas the water quality in LCL assumed a trend of cultivating A. philoxeroides better than cultivating vetiver than cultivating P. Notatum.
Table 4 The water quality (mean SD) of two leachates 66 days after being affected by three species of plants
TABLE NOT INCLUDED
The concentration (Table 2) after open incubating minus the concentration (Table 4) after cultivating plants is equivalent to the purifying efficiency of plants on the leachates. All three species had purifying impacts on the seven "pollutants" in varying degrees, their purifying rates varying from 11% to 91%, but there were no efficiencies in the purifying of P. notatum for Cl3 and nitrate N, and of vetiver on nitrate N. Among the 7 observed items, ammoniac N was removed almost completely, at rates between 83% - 91%, suggesting that ammoniac N was directly taken up by plants as the most available N resource. Next was the purification of vetiver for P, and its purifying rates in the two kinds of leachate all were above 70%, which suggests that vetiver also had a large uptake capacity or a strong purifying impact on P. In addition, the purification by A. philoxeroides for nitrate N was tangibly superior to that of vetiver and of P. notatum. on HCL. The effects of vetiver in purifying the seven "pollutants" were all better than that of A. philoxeroides; particularly in COD and P, the two species produced a significant difference (Table 4,5). In LCL, A. Philoxeroides could make use of "pollutants" as its nutrient resource, as a result its purifying efficiency on sewage was stronger than that of vetiver and P.notatum, especially on nitrate N and total N; but the purifying rates of vetiver on P was still the highest.
Table 5 A comparison of purifying effects of three species on two leachates
TABLE NOT INCLUDED
4 DISCUSSION In the 1950's vetiver was noted to be effective in soil and water conservation. Since the middle 1980's, this plant has been widely used through the subtropics and tropics, following its introduction and recommendation of The World Bank and The Vetiver Network. Up to now there have been at least 140 countries and regions researching, disseminating and applying the vetiver bio-engineering technology. The effects of vetiver in ameliorating soil, rehabilitating mine tailings and industrial polluted areas are also quite good. Presently vetiver has been found to have at least 31 uses (Grimshaw, 1996),and it is extensively called a miracle grass. It is clear from this study that the effect of vetiver in purifying garbage leachates was also pretty ideal, though the plant itself had been stressed and hurt by the leaching liquids. The increased biomass of vetiver gradually reduced in clean water, LCL, and HCL (Fig.1), indicating that the rate of growth of vetiver was gradually reduced with the concentration increases. However, the new increments of biomass of vetiver when grown in clean water and HCL were the highest compared to that of A. philoxeroides and P. notatum, suggesting that : 1) vetiver was the most tolerable to low plant nutrient levels; and 2) vetiver had the strongest resistance to polluted water, as it was damaged least in HCL. In addition, vetiver had a good effect in reducing levels of P (Table 5), probably due to its strong uptake capacity of P.
The previous studies have showed that vetiver planted in infertile soil was effective in improving soil [11,12]; and it could also increase the contents of organic matter, total N, available N and K in soil, but dwindle the content of available P[12]. These results suggest that vetiver is likely to be a "phosphorous-sucking" plant. Although vetiver's ability to cleanse LCL on was not as good as A. philoxeroides, its effect in purifying COD was tangibly superior to A. philoxeroides; furthermore, both had the similar purifying ability to reduce ammoniac N levels by as much as 90%. More importantly, the effects of vetiver in removing the measured seven pollutants in HCL all surpassed that of A. Philoxeroides. Summerfelt etal. even found that vetiver established in wetland could effectively remove extra solids and nutrients in aquaculture sludge, and the removal rates to suspended solids, total COD, dissolved COD, total kjeldahl N, total P, and dissolved P were 96%-98%, 72%-91%, 30%-81%, 86%-89%,82%-90%, and 92%-93%, respectively[4].
A. philoxeroides is a special amphibious plant, having a widespread distribution in such filthy and wet areas as septic tanks and ditches. It too has pretty strong tolerance to adversity. The results obtained from our study showed that the effects of A. philoxeroides in purifying LCL was considered good, particularly its removal rates of nitrate N, total N, and alkalinity were even higher than that of vetiver (Table 5). Moreover, A. philoxeroides produced a far greater biomass than other treatments and other species (Fig. 1), indicating that LCL probably had an action of eutrophication on A. philoxeroides. According to Gao et al. recent report, the removal rates by A. philoxeroides to N and P in run-off were 77% and 64%, respectively [13]. Observations in this paper found that this plant had a higher purifying rate on ammoniac N, up to 83% - 89%, but its removal of P was not so good, and the removal rate was only 39%-48% (Table 5). Many documents have reported that the hydrophyte E. crassipes is an excellent purifying plant, but about its tolerance to adversity has been poorly documented. P. notatum is a xerophyte, and has somewhat strong tolerance to adverse conditions when established in soil. This research work showed that the resistance of E. crassipes to adversity was quite poor as it was in the two leachates; and P. notatum did not grow in water at all, and thus it was impossible to use in polluted water. In addition, both vetiver and A. philoxeroides were severely impaired by HCL, and furthermore the latter was probably "over-nurtured" by LCL. These phenomena all indicate that the leachates, even having been purified by the purifying station, from garbage landfills in Guangzhou were toxic or eutrophic to the environment, and therefore it needs to be further improved.
All in all, the resistance and purifying ability of the four species of plants investigated in the paper were ranked in the sequence of vetiver> A. philoxeroides > P. notatum > E. crassipes, of which vetiver and A. philoxeroides were capable of being used as biological measures to assist purifying waste water. When the concentration of waste water is not high, it had better be purified with A. philoxeroides . However low concentrate waste water perhaps has an action of eutrophication on this plant, so measures should be taken to prevent it from its invasive tendency to block the water course when A. philoxeroides is chosen to purify waste water. As to highly concentrated waste water, vetiver is a better choice. However vetiver is not a hydrophyte, and it can not suspend directly in water to grow as A. philoxeroides, but needs a prop system to fasten it. In addition to this, vetiver shoots should be trimmed for it grows rapidly and has a considerably large biomass, and only by doing so can vetiver sustainably absorb pollutants in water, and make itself become a "super-bioaccumulator".
Acknowledgments
The authors would like to express their sincere gratitude to Mr. Richard G. Grimshaw and The Vetiver Network for their generous financial support to carry out this experiment. Thanks are also due to Likeng Garbage Landfill for providing help.
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