Lytle, C.M., Lytle, F.W., Yang, N., Qian, J.H., Hansen, D., Zayed, A., Terry, N., 1998: Reduction of. Cr(VI) to Cr(III) by wetland plants: potential for in situ heavy ...
Potential Use of Water Hyacinth (E. CRASSIPENS) for Wastewater Treatment in Serbia 1
Nevena Nesic1, Ljubinko Jovanovic2 Institute of Forestry, Center for multidisciplinary studies, University of Belgrade Belgrade, Serbia and Montenegro 2
Abstract The aim of this paper is to determine the feasibility of water hyacinth in treating wastewater. This floating perennial has been used in aquatic systems for wastewater purification for many years worldwide. A lot of interests have been shown for this plant in last few years in Serbia. Water hyacinth is very efficient in removing vast range of pollutants, from suspended materials, BOD, nutrients, organic matter to heavy metals and pathogens. At the same time E. crassipens is one of the most notorious weeds worldwide. When introduced to aquatic ecosystem it spreads very quickly thanks to its high reproduction potential. Therefore water hyacinth tends to eliminate all other living organisms in surrounding. An opinion will be given about potential usage of E. crassipes for wastewater treatment in Serbia through comparison of results obtained from various studies and scientific papers which represent experience of other countries in using water hyacinth and aquatic systems. These studies and scientific papers also describe this plant as notorious weed and propose different control measures. Key words: Eichhornia crassipes, water hyacinth, aquatic systems, wetlands, wastewater, aquatic weed
Introduction Water pollution is one of the most serious problems of today’s civilization. The consumption of water has been doubling on every twenty years but the reduction of this period is expected if today’s trends in water use continue (Velasevic and Djorovic, 1998). These two statements justify people’s fear that whole areas of the world will remain without biochemical safe water suitable for drinking and other needs. One can say situation is already alarming if it is known that because of fresh water disposition on Earth only one third of its territory is well provide with water, and if drastic efforts in water protection are not made by year 2025 2.3 billion people will live in areas with chronic water shortage (WHO, 2005). There are many technologies for wastewater treatment that can help in re-establishing and preserving physical, chemical and biological integrity of water. All of these technologies can be classified in two basic groups: 1. Conventional methods for purification of wastewater (wastewater treatment is carried out by physical, chemical and biological processes) and 2. Alternative methods for purification of wastewater (wastewater treatment is carried out by imitating self-purification process present in natural wetlands). Today these conventional wastewater treatment facilities fail in satisfying all demands of ecologically aware societies. This is because they: do not harmonize with basic principals of water conservation, do not enable reclamation and reuse of water and nutrients, generate toxic sludge as by product and use chemicals, harmful to environment and people, in the treatment process (Davis for EPA, 2004). So scientist sought for other solutions that will go beyond all problems mentioned above. All of the answers were found in natural wetlands which then served as model for construction of systems for wastewater purification by aquatic plants. There are many different types of these alternative systems (constructed wetlands, aquatic systems …) but all of them have the same major characteristic - thanks to symbiotic relationships between their basic components, aquatic plants (Peterson and Teal, 1996), microorganisms (Perkins and Hunter, 2000), algae, substrates and water they have the ability to remove organic and inorganic matter, nutrients, pathogens, heavy metals and other pollutants from wastewater (Naranjo, 1993; Peterson and Teal, 1996; Redding et al., 1997; Knight et al., 1993; Hammer, 1989) in a completely natural way (House et al., 1999; Verhoeven and Meuleman, 1999). In last few years a great interest has been shown for research of aquatic macrophytes as good candidates for pollutant removal or even as bioindicators for heavy metals in aquatic ecosystems (Aoi and Hayashi, 1996; Maine et al., 1999). Water hyacinth (Eichhornia crassipes (Mart.) Solms.), just one of the great
number of aquatic plant species successfully used for wastewater treatment in decades, was of particular importance. It is important to emphasize that E. crassipes has a huge potential for removal of the vast range of pollutants from wastewater (Chua, 1998; de Casabianca and Laugier, 1995; Maine et al., 2001; Mangabeira et al., 2004; Sim, 2003) and that a great number of aquatic systems with water hyacinth as basic component were construct (U.S. EPA, 1988; Aoi and Hayashi, 1996), but this macrophyte is also one of the most dangerous and the most invasive aquatic weed in the world (Wilson et al., 2005; U.S. EPA, 1988; Maine et al., 1999). Serbia is facing the same problems in planning, management and conservation of waters as most countries of the world. Facts like Serbia will have enough reserves of its own water only until year 2000 and after that year underground or transit waters will be used for satisfying people requirements (Velašević and Đorović, 1998) point that for relatively small country like Serbia dispose of water resources have a special significance. So in last year a great deal of interest was shown in Serbia for introduction of water hyacinth and construction of aquatic systems for wastewater treatment. And because of that the aim of this paper is to offer answers on some of the key questions like: Can this tropical plant grow in Serbia?; Will it impose a threat to existing wetlands?; Is it efficient enough in removal of different pollutants? and many other. Given answers could help interested in making final decision.
Water hyacinth (Eichhornia crassipens (Mart.) Solms.) Water hyacinth (E. crassipens) is fast growing perennial aquatic macrophyte (Reddy and Sutton, 1984). It is a member of pickerelweed family (Pontederiaceae) and its name Eichhornia was derived from well known 19th century Prussian politician J.A.F. Eichhorn (Aquatics, 2005). This tropical plant spread throughout the world in late 19th and early 20th century (Wilson et al., 2005). Today it is well known for its reproduction potential (de Casabianca and Laugier, 1995) and as a plant that can double its population in only twelve days (APIRIS, 2005). Water hyacinth is also known for its ability to grow in severe polluted waters (So et al., 2003). E. crassipens is well studied as an aquatic plant that can improve effluent quality from oxidation ponds and as a main component of one integrated advanced system for treatment of municipal, agricultural and industrial wastewaters (U.S. EPA, 1988; Sim, 2003; Wilson et al., 2005; Chua, 1998; Mangabeira et al., 2004; de Casabianca ang Laugier, 1995; Maine et al., 2001). To regret water hyacinth is often described in literature as serious invasive weed (Wilson et al., 2005; U.S. EPA, 1988; Maine et al., 1999; So et al., 2003; Singhal and Rai, 2003) and it is ranked on eight place in the list of world’s ten most serious weeds (Reddy and Sutton, 1984). Plant origin and geographical distribution Water hyacinth is South American plant, growing worldwide in regions between 40o north and sought latitude (Center et al., 2002). Its first appearance in North America was at the end of 19th century (1884). About the same time it was spotted in Egypt (Ghabbour et al., 2004; El Zawahry and Kamel, 2004) where it invaded many of African rivers and lakes such as Lake Victoria (Ghabbour et al., 2004). Till then water hyacinth spread to countless tropical and subtropical countries (Julien, 2001) usually with catastrophic socioeconomic and ecological consequences (Center et al., 2005; Ghabbour et al., 2004). E. crassipens is wide distributed in south of USA in states like Alabama, California, Florida, Louisiana, Texas and others (Reddy and Sutton, 1984). Water hyacinth has not yet been noticed on territory of Serbia. Taxonomy Cronquist (1988), Thorne (1992) and Takhtajan (1997) suggest following water hyacinth taxonomic placement (Center et al., 2002): Division: Magnoliophyta Class: Liliopsida Subclass: Commeinidae Superorder:Commelinanae Order: Pontederiales Family: Pontederiaceae Genus: Eichhornia Specific epithet: crassipes (Martius) Solms-Laubach. Morphology Water hyacinth is aquatic vascular plant with rounded, upright and shiny green leaves and lavender flowers similar to orchids (U.S. EPA, 1988).
Individual rosette is erect and free floating with numerous stolons (Center et al., 2005). Each one carries six to eight spirally arranged succulent leaves that are produced sequentially on a short vertical stem. Petioles are bulbous and spongy with many air spaces (U.S. EPA, 1988) which allow plants to float on a water surface. But floating leaves can vary in size and morphology primary according to growth conditions and the stage of colony development. Leaves with bulbous petioles are dominant in open water whereas elongated petioles (to 1.5 m in height) predominate in dense colonies (Center et al., 2005). The inflorescence consists of ten to thirty flowers with six violet blue or violet pink petals (Center et al., 2002). Top petal has gold yellow spot bordered with blue line which resembles the pattern of peacock eye (Aquatics, 2005; APIRIS, 2005). Root system of water hyacinth is dark blue in colour (Aquatics, 2005; APIRIS, 2005) with numerous stolons. New plants are formed at the end of these stolons. Measured from flower top to root top E. crassipens usually reach height of 1.5 m and more (Center et al., 2005). When grown in wastewaters water hyacinth is smaller and it often reaches heights no more then 0.5 to 1.2 m (Reddy and Sutton, 1984). Growth, development and reproduction As mentioned above E. crassipens is fast growing perennial with great reproduction potential. Growth of water hyacinth is primary dependant on: ability of plant to use solar energy, nutrient composition of water, cultural methods and environmental factors (U.S. EPA, 1988). Plant growth is described in two ways: first is by reporting the percentage of water surface covered of a period of time; second and more useful method is by reporting the plant density in units of wet plant mass per unit of surface area (U.S. EPA, 1988). So under normal conditions loosely packed water hyacinth can cover the water surface at relatively low plant density (10 kg/m2 wet weight) and it can reach maximum density of 50 kg/m2 wet weight before growth ceases (Reddy and Sutton, 1984). According to other authors this aquatic macrophyte can totally clog aquatic ecosystems by reach density of 60 kg/m2 wet weight outside of its native range (Julien and Orapa, 1999). Just like all other biological processes growth of E. crassipens depends on various ecological factors. For the purpose of this paper water and air temperatures are considered as main limiting factors for regular plant development and growth. Water hyacinth is growing fastest at temperatures from 20 to 300C, but growth fully stops at temperatures from 8 to 150C (Stephenson et al., 1980). This aquatic plant can reproduce in both generative and vegetative ways. That means new plants can be produced from seeds or they represent clones derived from stolon elongation due to division of auxiliary meristems of mother plant (Center et al., 2005). At first these new rosettes are attached to mother plant but stolons are very fragile so they easily break enabling young individuals to float away and colonise new areas (Wilson et al., 2005; Center et al., 2005). How fast in this colonisation? Only ten plants in just eight months can produce population of 655,330 individuals (Babu et al., 2003). So theoretically, by vegetative reproduction one plant can colonise water surface during one month by creating 8,191 new individuals. Water hyacinth is mainly reproduced by generative means in its natural habitat and it produces large number of seeds (Wilson et al., 2005; Center et al., 2005). Numerous authors (Penfound and Earle, 1948; Gopal, 1987; Bock, 1966; Ueki and Oki, 1979; cit. Wilson et al., 2005) who have been studying water hyacinth in its flowering period had obtained very similar results about its fenology and generative reproduction. The flowering period lasts for about fifteen days. When flowering cycle ends flower stalk bends and the spike is now under the water surface and seeds are released directly into the water (Center et al., 2002). Each inflorescence contains normally 1 to 20 seed capsules and capsule caries 3 to 250 seeds. The authors mentioned above also agree that in spite of the production of this large number of seeds there are only 3 to 3.4 seeds per plant each year that are eventually able to germinate (Wilson et al., 2005). Seeds usually germinate within 6 months but in wet sediments at the bottom they can contain germination for 15 to 20 years (Center et al., 2002). Seeds germinate in moist environment in sediments or in warm shallow water (Center et al., 2002) and after 30 to 40 days seedlings have 4 to 8 leaves (Wilson et al., 2005). Metthews (1967, cit. Wilson et al., 2005) points even assuming a relatively low germination rate with only 3 seeds per plant per year density of new seedlings can be higher then density of mature plants reaching levels of 11,000 plants/m2. Assuming every seedling with 4 to 8 leaves weights 10g and that seedling density is 100 seedlings/m2 then in two months, when conditions for germination and growth are suitable, the fresh weight of biomass can easily be over 1 kg/m2 (Wilson et al., 2005). So when maintaining and monitoring aquatic ecosystems or aquatic systems one must have in mind that where ever water hyacinth can produce new plants from seeds generative reproduction must not be
underestimate as E. crassipens can colonise areas again very fast after applied control measures, dramatic changes in water level or after winter. Ecological factors Water hyacinth is heliophyte plant growing best in warm waters rich in macronutrients (Center et al., 2002). Optimal water pH for growth of this aquatic plant is neutral but it can tolerate pH values from 4 to 10 (Haller and Sutton, 1973; cit. Center et al., 2002). This is very important fact because it points that E. crassipens can be used for treatment of different types of wastewater. Optimal water temperature for growth is 28-30oC (Center et al., 2002). Temperatures above 33oC inhibit further growth (Knipling et al., 1993; cit. Center et al., 2002). Optimal air temperature is 21-30oC (U.S. EPA, 1988). If lasting for 12 hours temperature of -3oC will destroy all leaves and temperature of -5oC during the period of 48 hours will destroy whole plant (U.S. EPA, 1988). Work of other authors also presents similar data about water hyacinth sensibility to low temperatures. E. crassipens can survive 24 hours at temperatures between 0.5 and -5oC, but it will die at 6 to -7oC and can not be grown in open where average winter temperature drops under 1oC (Stephenson et al., 1980). So if aquatic systems with water hyacinth are constructed in colder climates it would be necessary to build greenhouses for maintaining optimal temperature for plant growth and development (Reed and Bastian, 1980). Low air humidity from 15% to 40% can also be limiting factor for undisturbed growth of water hyacinth (Allen, 1997). E. crassipens tolerates drought well because it can survive in moist sediments up to several months (Paraja, 1934; cit. Center et al., 2002). Salinity is the main obstacle for growth of water hyacinth in costal areas (Olivares and Colonnello, 2000). De Casabianca and Laugier (1995) have studied the production of this aquatic macrophyte in relation to different effluent salinity value. They were also tracking plant reactions to high levels of salinity by observing symptoms of a different intensity (de Casabianca and Laugier, 1995). They have concluded that production of water hyacinth was reducing and necroses on leaves and bulbous petioles were occurring earlier with increase of salinity. In their experiments at maximum salinity of 6 g NaCl/l effluent there were no production and necroses on leaves and bulbous petioles were occurring just after couple of hours. After 3 days all seedling decayed (de Casabianca and Laugier, 1995). Results of other authors (Penfound and Earle, 1948; Zhenbin et al., 1990; cit. de Casabianca and Laugier, 1995) differ from results mentioned above and they define lethal toxicity at salinities of 2 and 3g NaCl/l effluent. It was assumed that these differences are due to water hyacinth adaptation (de Casabianca and Laugier, 1995).
Aquatic systems for wastewater treatment Aquatic systems are special type of constructed wetlands that treat industrial, municipal and agricultural wastewaters by floating and submerged aquatic plants (U.S. EPA, 1988). They can remove suspended materials, nutrients and heavy metals from wastewater with great efficiency (de Casabianca and Laugier; 1995; Grodowitz et al., 1997; Center et al., 2005; Ghabbour et al., 2004; El Zawahry and Kamel, 2004; So et al., 2003). Water hyacinth systems were used mostly in regions with warm climate because of plant sensitivity to low temperatures and frost. Sought parts of the USA have the largest number of these alternative facilities (U.S. EPA, 1988). First experimental aquatic system in open spaces in USA appeared in early 1970s in Texas (U.S. EPA, 1988, Grodowitz et al., 1997; Center et al., 2005). Today they can be seen in south of France (de Casabianca and Laugier, 1995), in Brazil (Mangabeira et al., 2004), Argentina (Maine et al., 2001), India (Babu et al., 2003; Singhal and Rai, 2003), Egypt (Ghabbour et al., 2004; El Zawahry and Kamel, 2004), China (So et al., 2003) and other countries where winter temperatures are not too low. The aquatic systems consist of one or more shallow rectangular basins in which one or more aquatic vascular plan species are grown (Tchobanoglous, 1987). Wastewater purification is principally carried out by bacterial metabolism and physical sedimentation (U.S. EPA, 1988). Aquatic macrophytes themselves, do not contribute much in pollutant removal (Tchobanoglous, 1987). Their role is in providing other components of aquatic system that improve wastewater treatment capability (Reed and Bastian, 1980). The root of water hyacinth acts like living substrate for attached microorganisms which then provide a significant degree of treatment thanks to their metabolism (U.S. EPA, 1988). Besides enabling growth of microbial colonies root system is also good medium for filtration and adsorption of suspended materials, nutrients and heavy metals (Center et al., 2002).
As mentioned above water hyacinth systems are very efficient in pollutant removal from wastewaters. Efficiency of nitrogen removal ranges from 10 to 90 % (U.S. EPA, 1988). Results from Iron Bridge water hyacinth system in Florida, USA demonstrated that phosphorus removal was from 35 to 80 % (U.S. EPA, 1988). The same facility successfully removed about 60% of BOD5 and 43% of suspended materials from wastewater. These systems can also remove heavy metals like chrome, cadmium, copper and other effectively. In their experiments Maine et al. (1999) have shown that 72% of cadmium was removed from wastewater by water hyacinth. Accumulated nutrients and heavy metals are removed from aquatic systems by plant harvesting and sediment dredging (U.S. EPA, 1988; Reddy and Sutton, 1984). There are many speculations on the use of water hyacinth upon harvesting. According to some authors (Lindsey and Hirt, 1999) it can be use like food for people or fodder. But it is not recommended to consume water hyacinth if it was used for removal of heavy metals, rare earth elements or other toxic substances that can cause problems if they enter food chain (Chua, 1998). Upon harvesting water hyacinth can be used for composting, anaerobic digestion for production of methane, and fermentation of sugars into alcohol (U.S. EPA, 1988).These operations can help in recovering expenses of wastewater treatment. But this kind of compensation is often feasible only in large scale operations since it is unlikely that small facilities will approach the economic break even point form methane gas production (U.S. EPA, 1988). Construction, operation, maintenance and monitoring costs of aquatic systems are quite high (Sim, 2003) but in comparison to same costs of conventional treatment facilities they are still acceptable (U.S. EPA, 1988). According to data of U.S. Environmental Protection Agency (1988) total construction costs of water hyacinth systems range from 2,000,000 to 2,800,000 $, and maintenance costs are about 550,000 $ per year.
Control of water hyacinth Excessive growth of water hyacinth populations can be inhibited by biological, mechanical and chemical control measures. Biological control is the most environmentally friendly measure but there are some very important things to think about before using it. First, when choosing species that will perform biological control it is not enough for them to be just natural enemies of water hyacinth in South America. There are many cases where introduced species did not perform well in new habitat because they have found new hosts and instead of destroying E. crassipens they started to reduce other aquatic macrophytes populations. Second problem can be their acclimatisation especially if they were introduced to colder climates. In early 1970s, the USDA and CIBS (now CABI-Bioscience) have released three natural enemies for the purpose of biological control of this aquatic weed in the USA. These biological agents were two weevils Neochetina eichhorniae Warner and N. bruchi Hustache, and later the pyralid moth Niphograpata albiguttalis (Warren) (Center et al., 2002; Grodowitz et al., 1997; Wilson et al., 2005). Beside these three insects of Argentinean origin now widely used is also the mite (Orthogalumna terebrantis Wallwork) (Center et al., 2002). The two weevil species are used at the most in many countries worldwide. There are numerous data about their excellent efficiency. After introducing biological control agents water hyacinth now occupies only one third of its former area in the Gulf Coast states and in Uganda, Tanzania and Kenya after only four years upon introduction of Neochetina sp. to Lake Victoria about 75% of dense water hyacinth mats have sunk (Center et al., 2002). Similar results were attained in lagoons of Sepic River in Papua New Guinea and Mexico where after three years upon introduction Neochetina sp. reduced 3,041 ha of water hyacinth by 62% (Julien and Orapa, 1999; Center et al., 2002). Today there is also a great deal of interest for the use of plant pathogens, phytotoxins or their derivates as agents for biological control of this notorious aquatic weed. The most studied and used are fungus Alternaria eichhorniae Nag Raj and Ponnappa Sp. Nov., A. alternata (Fr.) and Cercospora piaropi Tharp. (Center et al., 2002; Babu, 2003). So many insects, mites and fungus have been released in many locations in tropics and they have drastically reduced further spread of water hyacinth (Wilson et al., 2005; Julien and Orapa, 1999). But there are still many questions about their efficiency worldwide and water authorities are sometimes pointing to fact that biological control is too slow (Wilson et al., 2005; Center et al., 2002). Because of that there is still tendency for applying mechanical and chemical control measures which are not always acceptable.
Mechanical control is often reduced to cutting and sometimes burning of water hyacinth (APIRIS, 2005; Babu, 2003). When using cutting as control measure the major problem is great plant biomass. Cutting machines are quickly filled so the whole operation is very slow and expensive. This problem was solved to a certain degree by different construction of cutting machines (APIRIS, 2005) but there is still affinity for use of quicker and cheaper methods. And that is usually the use of chemical control measures. Of course the instantaneous remove of aquatic weed will be attained quickest by applying herbicides but it can lead to additional load and damage of sensitive aquatic ecosystems and pollution of whole environment. Long term application of same herbicides can also lead to appearance of plant resistance (Babu, 2003). All control measures mentioned above have many advantages and constrains so in the future it would be wise to use integrated control measures against water hyacinth. It should be pointed that fast growth and rapid spread of water hyacinth are often consequences of large quantities of nutrients and other pollutants presented in water contaminated with effluents from agriculture, industry, municipalities and other sources. So the presence of E. crassipens is not always a problem itself but clear sing that something else is not right or that ecosystem is out of balance. In this case spending the whole wealth on different control measures would be just avoiding the real problem.
Potentials and constrains in using water hyacinth Water hyacinth is plant with many advantages firstly because it can be used for many purposes, but it has one major fault. E. crassipes is one of the most invasive weeds that can destroy precious aquatic ecosystems in a short time which can lead to series of other problems. Because of that it is very difficult to answer the question – Is E. crassipes the golden plant or the world’s worst aquatic weed? One can often read that people have a moral imperative to think about potential utilization of abundantly available biomass of water hyacinth in tropical countries for the benefit of people for whom E. crassipes has created many problems or even has destroyed their lives (Lindsey and Hirt, 1999). There are many examples around the world of how communities or individuals have used water hyacinth to great advantage. In regions where it can be found in abundance water hyacinth can be used like food for people because its leaves are rich in proteins and vitamin A (Lindsey and Hirt, 1999). It can be also utilized as green fertilizer or as mulch, compost and ash in regenerating degraded soils. In regard to its ability to remove different pollutants from water it can positively influence on fish populations in natural waters or on fish growing in artificial accumulations. It can easily replace straw as substrate for mushroom growing or it can be use as fodder (Lindsey and Hirt, 1999). It is interested that it can be used in energy production thereby combating deforestation. As briquettes or biogas it can be used for lighting a furnace and cooking in schools, restaurants or government institutions (Lindsey and Hirt, 1999). In settlements where water hyacinth has destroyed economy (fishing, river transport …) population can reorient to manufacturing of art paper, crafts, ropes, furniture and other things from water hyacinth which will positively influence to reduction of unemployment and increase of income (Lindsey and Hirt, 1999). The major characteristic that limits wider use of water hyacinth is its sensitivity to low temperatures (U.S. EPA, 1988). If aquatic systems with water hyacinth are built in colder climates it would be necessary to place them in the greenhouses. Construction of greenhouses can have significant impact on the extent of construction costs. Because of that it is presumed that it would be economically unjustified to build these alternative systems in regions where they can not operate in open the whole year (Reed and Bastian, 1980). On the basis of data obtained from Republic Hydrometeorological Service of Serbia (2005) it can be concluded that in concern to water hyacinth Serbia falls into the category of regions with colder climate. The coldest month is January with average month temperature in intervals between -6oC and 0oC (RHMZ, 2005). If temperature limit for growing E. crasssipes in open is average winter temperature of 1oC then it can be said that it would be very difficult for this aquatic macrophyte to acclimatize in Serbia that is plant will probably freeze every winter and die. Eichhornia crassipes is very productive photosynthetic plant (Reddy and Sutton, 1984; Julien and Orapa, 1999) so its growth is causing many problems in rivers and lakes. Water hyacinth can impose exceptionally serious ecological problems in this way. It creates dense mats of biomass which are reducing light to submerged vegetation (Ultsch, 1973; cit. Center et al., 2002; Babu, 2003; Wilson et al., 2005) which is then pushed out of aquatic community. In addition E. crassipes
influence the depletion of oxygen in aquatic ecosystems (McVea and Boyd, 1975; cit. Center et al., 2002). As a result the number of phytoplankton is decreasing and that will lead to change in structure of invertebrate community (O’Hara, 1967, Hensen et al., 1971; cit. Center et al., 2002) and eventually cause negative impact on fish fund. Almost all of autochthonous vegetation can be expelled from aquatic ecosystem (U.S. EPA, 1988). Water hyacinth can endanger survival of all living organisms in aquatic environment. The reduction of biodiversity is clearly evident wherever E. crassipes appears. Hydrophilic and hydrophilic vegetation that is ecosystems of wetlands, swamps and marshes with classes Phragmitetea, Lemnetea and Potametea fall in category of extremely endangered vegetation in Serbia (Jovanovic, 2001). Because of that introduction of water hyacinth could be considered as negative action. The reduction of biodiversity is one of the major problems especially if it is known that changes in biodiversity at the end always have a global negative impact. In African countries like Uganda water hyacinth has also influenced on much frequent occurrence of diseases (dysentery, malaria, schistosomiasis) related to content of different pathogens in water (National Geographic, 2004). It is strongly considered that this is happening because the dense mats of E. crassipes prevent normal water circulation, thus creating places with stagnant water suitable for development of various pathogens (National Geographic, 2004). Beside ecological damages E. crassipes is also causing considerable economic damages in regions where it spreads uncontrolled. Dense mats of water hyacinth create impenetrable barriers and prevent navigation (Center et al., 2002). They also block drainage systems causing flooding or preventing withdrawal of floodwaters. This aquatic macrophyte obstructs irrigation by preventing predicted water flow in channels, clogging irrigation pumps and destroying dams (Penfound and Earle, 1948, cit. Center et al., 2002). It is blocking access to recreational areas and reduces values of waterfront properties. Beside that water hyacinth often creates suitable environment for mosquitoes because it disrupts application of insecticides, drainage and water circulation. All of this has negative impact on economics of communities whose main earnings come from fishing and tourism (Center et al., 2002). Following examples point economical losses due to water hyacinth infestation in USA. For growth control and prevention of further spread of this aquatic weed authorities in Florida spent over $43 million during 1980 to 1991 (Schmitz et al., 1993, cit. Center et al., 2002). Currently annual costs for water hyacinth management range from $ 500,000 in California to $3 million in Florida (Mullin et al., 2000, cit. Center et al., 2002).
Conclusion After all facts presented in this paper it is very hard to give the final conclusion or irrefutable stand about potential use of water hyacinth (Eichhornia crassipes (Mart.) Solms.) for wastewater treatment is Serbia. Water hyacinth is undoubtedly aquatic macrophyte with great potential but the first constrain to be noticed is it’s incapability of adjusting to climatic conditions in Serbia. Thus it is presumed that plantation growing of E. crassipes in Serbia would not be very appealing and finally not economically justified. This fact raises another question – Are described advantages of water hyacinth in tropics also its advantages in regions where it could not be found in abundance which will certainly be the case in Serbia? If it is then presumed that E. crassipes will be mostly use for wastewater purification in Serbia major concerns would be low winter temperatures. If this is a plant that can not be grown in open where average winter temperature is below 1oC and if it would be necessary to build expensive greenhouses the profitability of aquatic systems in Serbia is questionable. If thinking about recovering some expenses of construction, operation, monitoring and maintenance of aquatic systems it can be said that this would be feasible only in large scale operations. Apparently aquatic systems in Serbia would be small because of high construction costs thus it would not be enough biomass for methane and alcohol production or for composting. The same case would also be for other accompanying purposes of water hyacinth. If it is presumed that major source of water hyacinth in Serbia would be aquatic systems for removal of different pollutants from wastewater the question would be if it is then suitable for animal and human diet? Removed plants will for sure contain larger or smaller concentrations of heavy metals and other toxic substances so using water hyacinth like food is not advisable. The same would be for using it like fertilizer, mulch or compost because some pollutants can leak to soil or underground water. It would be possible to use it like ash for soil improvement but only if heavy metals are firstly extracted. The question is again - Is this operation economically justified?
Perhaps growing of E. crassipes in fish ponds where it would be use for water purification should also be avoided. Due to fast spreading it can cause anaerobic conditions in pools which are not suitable for fish breeding. Aeration pumps should then be installed so the question is – Would it be economically justified to grow water hyacinth and fish in same ponds? And the last advantage described in this paper is using water hyacinth instead of straw like substrate for mushroom growing. There is a plenty of straw in Serbia. Basically water hyacinth could be used but would it be worth a risk of carrying seeds to new areas while transporting bales? In essence the biggest NO for introduction of Eichhornia crassipes (Mart.) Solms. in Serbia is the fact that this is one of the most invasive weeds in the world which have cause a lots of damages in areas where it was introduced. One can say that water hyacinth will be isolated if greenhouses and that it would not impose any threats to sensitive aquatic ecosystems in Serbia. To regret there are many examples of how some species were unintentional or unconscious brought to new areas thus it is very unlikely that E. crassipes will not be able to “escape” from isolation. Of course there are always different control measures for reduction of vast populations that “fugitive” plants would create. But is it necessary to burden ecosystems with additional toxic substances or introducing new living organisms which could impose new problems in environment? Even if genetically modified plants with dead gene are introduced there is always a question about legality of import of such plants in Serbia and of course what will happen if they enter a food chain. At the end it can be concluded that it is necessary to be very careful with introduction of Eichhornia crassipes (Mart.) Solms. and construction of aquatic systems with water hyacinth in Serbia. It would be best to carry out detailed studies about plant behaviour in our climate and real dangers that it can impose. It is very important to point that there are over 45 aquatic plant species that can be successfully used for wastewater purification in Serbia. These plants are autochthonous, fully acclimatised and which is much important they are not aquatic weeds.
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