using phytoremediation technologies to upgrade waste water treatment in europe

Phytoremediation Technologies Subject Area 5.3

490© 2007 ecomed publishers (Verlagsgruppe Hüthig Jehle Rehm GmbH), D-86899 Landsberg and Tokyo • Mumbai • Seoul • Melbourne • Paris

Env Sci Pollut Res 1414141414 (7) 490 – 497 (2007)

Subject Area 5.3: Phytoremediation and ecosystem restoration

Discussion Article

Using Phytoremediation Technologies to Upgrade Waste WaterTreatment in EuropePeter Schröder1*, Juan Navarro-Aviñó2, Hassan Azaizeh3, Avi Golan Goldhirsh4, Simona DiGregorio5,Tamas Komives6, Günter Langergraber7, Anton Lenz8, Elena Maestri9, Abdul R. Memon10, Alfonso Ranalli11,Luca Sebastiani12, Stanislav Smrcek13, Tomas Vanek14, Stephane Vuilleumier15, Frieder Wissing16

1 Department of Microbe-Plant Interactions, GSF National Research Center for Environment and Health, Neuherberg, Germany2 Department of Stress Biology, Polytechnical University of Valencia, Spain3 R&D Center the Galilee Society, Shefa-Amr, Israel4 The Jacob Blaustein Institutes for Desert Research, Ben Gurion University of the Negev, Israel5 Faculty of Mathematics, Physics and Natural Sciences, Department of Biology, Pisa, Italy6 Plant Protection Institute, Hungarian Academy of Sciences, Budapest, Hungary7 Institute of Sanitary Engineering and Water Pollution Control, BOKU-University of Natural Resources and Applied Life Sciences,

Vienna, Austria8 Ingenieurbüro Lenz, Ringelai, Germany9 Department of Environmental Sciences, University of Parma, Italy10 TUBITAK Research Institute for Genetic Engineering and Biotechnology, Gebze, Turkey11 Istituto Sperimentale de l`Elaiotechnica, CNR, Pescara, Italy12 Scuola Superiore di Studi Universitari e di Perfezionamento Sant'Anna, Pisa, Italy13 Analytical Chemistry Laboratory, Charles University, Prague, Czech Republic14 Department of Plant Cell Tissue Cultures, Czech Academy of Sciences, Prague, Czech Republic15 Department Microorganisms, Genomes, Environnement, UMR 7156 CNRS, Université Louis Pasteur Strasbourg, France16 ILKON – Engineering Office for Applied Limnology, Bonn, Germany

* Corresponding author: Prof. Dr. Peter Schröder, GSF National Research Center for Environment and Health, Department of Microbe-Plant Interactions, Ingolstädter Landstrasse 1, 85764 Neuherberg, Germany ([email protected])

phytoremediation technologies seem to be most promising to solvethis burning problem.Conclusions. To quantify the occurrence and the distribution ofmicropollutants, to evaluate their effects, and to prevent them frompassing through wastewater collection and treatment systems intorivers, lakes and ground water bodies represents an urgent task forapplied environmental sciences in the coming years.Recommendations. Public acceptance of green technologies is gen-erally higher than that of industrial processes. The EU should stimu-late research to upgrade existing waste water treatment by imple-menting phytoremediation modules and demonstrating theirreliability to the public.

Keywords: Aquatic macrophytes; constructed wetlands; helophytes,personal care products; pharmaceuticals; phytoremediation; recal-citrant organic xenobiotics

DOI: http://dx.doi.org/10.1065/espr2006.12.373

Please cite this paper as: Schröder P, Navarro-Aviñó J, AzaizehH, Goldhirsh AG, DiGregorio S, Komives T, Langergraber G, LenzA, Maestri E, Memon AR, Ranalli A, Sebastiani L, Smrcek S,Vanek T, Vuilleumier S, Wissing F (2007): Using Phytoremedia-tion Technologies to Upgrade Waste Water Treatment in Eu-rope. Env Sci Pollut Res 14 (7) 490–497

Abstract

Goal, Scope and Background. One of the burning problems of ourindustrial society is the high consumption of water and the high de-mand for clean drinking water. Numerous approaches have been takento reduce water consumption, but in the long run it seems only pos-sible to recycle waste water into high quality water. It seems timely todiscuss alternative water remediation technologies that are fit for in-dustrial as well as less developed countries to ensure a high quality ofdrinking water throughout Europe.Main Features. The present paper discusses a range of phytore-mediation technologies to be applied in a modular approach to inte-grate and improve the performance of existing wastewater treatment,especially towards the emerging micro pollutants, i.e. organic chemi-cals and pharmaceuticals. This topic is of global relevance for the EU.Results. Existing technologies for waste water treatment do not suf-ficiently address increasing pollution situation, especially with thegrowing use of organic pollutants in the private household and healthsector. Although some crude chemical approaches exist, such as ad-vanced oxidation steps, most waste water treatment plants will notbe able to adopt them. The same is true for membrane technologies.Discussion. Incredible progress has been made during recent years,thus providing us with membranes of longevity and stability and, atthe same time, high filtration capacity. However, these systems areexpensive and delicate in operation, so that the majority of commu-nities will not be able to afford them. Combinations of different

Introduction

Today, more than 100,000 different chemicals are availableon the European market, and one third of them exceed quan-tities of one tonne per annum. Most of them have been intro-duced for the benefit of daily life, medicine, food productionand industrial purposes, and a good proportion of these com-pounds lack natural counterparts. The majority of these com-pounds have a rather poor biodegradability. Hence, fresh waterresources become more and more contaminated with micro-quantities of these man-made pollutants. Moreover, some ofthese pollutants may possess the undesirable property of ex-erting estrogenic activity on various higher organisms. Europehas to face the issue that many of these foreign compounds or

Subject Area 5.3 Phytoremediation Technologies

Env Sci Pollut Res 1414141414 (7) 2007 491

xenobiotics will increasingly create environmental problemsin all regions of our continent. Due to steadily improving ca-pabilities for environmental analysis, we are nowadays ableto detect compounds in very low concentration ranges (e.g.nanomoles) in water bodies and sediments. Amongst themare well-known pesticides, plasticizers, fuel additives, flameretardants, medicaments and fragrances. Industrial activitiesare a second source of water pollution. Industrial wastewaterdischarged into aquatic ecosystems either directly or becauseof inadequate treatment of process water can lower waterquality of a region by increasing concentrations of pollutantssuch as organic matter, suspended particulates, micropollut-ants, nutrients (phosphorus and nitrogen) or heavy metals,thereby causing adverse effects on human health and unde-sirable changes in the composition of aquatic biota.

It becomes increasingly clear that societal attitude towardswater pollution is associated with rising economic costs be-cause of the ensuing depletion of water resources for spe-cific uses. The reduction of pollution in waste water willdepend on what a given community or an industrial areaallows into the effluent stream, and on the efficiency andeffectiveness with which these effluents are treated. The spe-cial treatment requirements for industrial effluents differ fromthose of municipal wastewater. In case of industrial efflu-ents, where specific pollutants are well defined, targeted treat-ment at source can be applied. Municipal waste water treat-ment may require more diverse technologies.

The ETAP (Environmental Technologies Action Plan) of theEuropean Union claims urgent action for better water qual-ity and protection of our natural resources. High priority isalso given to environmentally sound water treatment tech-nologies that will reduce greenhouse gases, recycle materi-als and provide all partner countries with affordable tech-nologies. The discussion paper on water issues is very specificabout novel green technologies to be adopted in this respect(http://europa.eu.int/comm/environment/etap/pdfs/etapwaterissuefr.pdf). Hence, it would be timely to target on new, en-vironmentally benign, biological technologies for the removalof toxic pollutants from water. This will have the additionalbeneficial effects of a reduced health risk to people and eco-systems, and will exploit available research excellence bycollaboration with industry. Better environmental conditionsas a result of reduced impact of pollution discharges in Eu-ropean countries will also create favorable conditions forsocial development and for overall industrial activity.

The EU Strategy for Sustainable Development (EuropeanCommission 2004) and several EU directives strive to pro-vide Europe with clean water of high purity and stable qual-ity. This addresses all types of water use, but especially con-siders drinking water. Conventional waste water treatmentsystems have not provided adequate solutions for the re-moval of micropollutants, especially pharmaceuticals, per-sonal health products and heavy metals.

Although high-tech solutions are presently available all overEurope, their sustainability is usually not achieved, since theirresilience to numerous parameters is questionable, and clearcut evidence is presented in the ETAP papers that these tech-nical solutions are too expensive for many communities. Itis also evident that, in order to protect resources for future

generations, approaches have to be adopted which will notonly preserve the ecosystem, but also protect biodiversity.

1 Micropollutants in Urban Waste Water and Sludge

Pharmaceuticals have been detected in surface waters of theUS and Europe at concentrations in the ng/L to μg/L range(Kolpin et al. 2002). A German study reported on the occur-rence of 55 active pharmaceuticals and 9 metabolites in thedischarge of 49 sewage treatment plants, as well as in receivingwater bodies, at concentrations of up to several μg l–1 (Ternes1998). Another study reports that 27 out of 32 pharmaceuticalsubstances and 4 of 5 metabolites were detected in the effluentof European wastewater treatment plants, and that surface waterpeak values exceeded 1 μg l–1 (Larsen et al. 2004). Americanstudies came up with similar values, and pointed out that nu-merous compounds found in sewage plants will consequentlyalso be present in potable-water supplies and, hence, representa public health problem of increasing concern (Stackelberg etal. 2004). In other cases, municipal waste water treatment plantsmay receive considerable amounts of pre-treated industrial wastewater, polluted ground water, etc., leading to additional pollu-tion with organic xenobiotics. Reviews on the occurrence, fateand possible effects of pharmaceuticals or their active me-tabolites in the environment from sewage and animal hus-bandry are available and point to the danger of their wide-spread distribution in ecosystems (Daughton and Ternes1999, Sweetman 2002). Few recent papers describe also theoccurrence of perfluorinated surfactants in water (Skutlareket al. 2006). However, the environmental effects of the pres-ence of many other compounds and mixtures thereof in wastewater have not been properly addressed with respect to theirbiological activity (Richardson and Bowron 1985, Halling-Sorensen et al. 1998, Daughton 2001, Ternes 2001).

A second problem arises when surplus wastewater sludge isreused as a fertilizer for agricultural purposes. This sludgemust clearly conform to certain limit values with respect toxenobiotic substances in order to prevent accumulation insoil, plant and drainage to surface and ground water. Thelatest revision of the statutory order in Denmark, for thefirst time, included a list of limit values on organic micro-pollutants in sludge. Four groups of micropollutants wereincluded in this list: the Linear Alkylbenzene Sulfonates(LAS), Polycyclic Aromatic Hydrocarbons (PAH), Nonyl-phenols + Ethoxylates (NPE), and Diethylhexyl-Phthalates(DEHP) (Knudsen et al. 2000). The fate of pharmaceuticalsor personal care products in sludge is rarely addressed. To-day, up to 50% of the sludge used for agricultural purposesdoes not comply with the given standards. Nevertheless, thissludge is used as an amendment to soils and, hence, deliversorganic pollutants to soil, surface water and crops. It hasbeen observed that the level of organic micropollutants ismuch higher in anaerobically digested sludge than in aero-bically stabilized sludge. This indicates that the organicmicropollutants in question can be degraded at least in partunder aerobic conditions, but not under anaerobic condi-tions. These observations suggest an attractive starting pointfor the development of a post-aeration process for biologi-cal degradation of organic micropollutants in anaerobicallydigested sludge with the aim of enabling continued reuse ofthe sludge for agricultural purposes.


492 Env Sci Pollut Res 1414141414 (7) 2007

2 International Conventions and Agreements

Since the North Sea Conference on Co-operation in dealingwith pollution of the North Sea by oil and other harmfulsubstances (Bonn Agreement 1983), public awareness inEurope has been introduced to the topic of water quality.Still, it took almost a decade until the Hague Declaration onthe future Community ground water policy was agreed onat the EC Ministerial Meeting on Nov. 26–27, 1991. An-other decade went by until the Agenda 21 formulated theidea that quantitative and qualitative discharge standardsfor municipal and industrial effluents should be establishedand applied by the year 2000. Connected to this recommen-dation was the proposal to revise Directive 76/464/EEC(Dangerous Substances in Water) and the Directive No. 96/61 EC on Integrated Pollution Prevention and Control (IPPC1996), as well as Directive 93/793/EEC on environmentalrisk from chemicals (testing the ecotoxicity of listed prioritychemicals). Updated European framework legislation pro-motes the reduction of micropollutants. Substantial politi-cal concern exists that water pollutants have to be moni-tored and removed. However, our knowledge of xenobioticscontrol or degradation has hardly gone beyond scratchingthe surface and confirming the importance of the problem.In particular, it is not known to which extent the treatmentof waste water in a municipal waste water treatment plantis feasible with regard to environmental effects and costs.

3 Operation of Wastewater Treatment: Current state of the Art

The state-of-the-art in the design of wastewater treatmentplants has been improved steadily since the middle of thelast century. Their performance is strongly related to the rel-evant legal framework, i.e. the compounds for which effluentstandards are relevant. According to the EU Urban Wastewa-ter Directive 91/271/EEC, the relevant parameters for designare organic matter (expressed as BOD5, COD and TOC) andnutrients like nitrogen and phosphorous (albeit only in sensi-tive areas). The increased removal of organic matter and nu-

trients has already resulted in a significant improvement inthe quality of the receiving waters. However, none of theemerging organic micropollutants (e.g. pharmaceuticals,personal care compounds, endocrine disrupting substances)is targeted by conventional treatment plant design.

Kreuzinger et al. (2005) describe an approach to determinecomparable removal rates of endocrine disruptors (EDs) andpharmaceutically active compounds (PhACs) for differentactivated sludge systems, based on mass balance and sludgeretention time, in order to allow comparison and evaluationof the removal efficiency of different layouts and conceptsin wastewater treatment. Presented results from differentWWTPs (waste water treatment plants) show a close corre-lation of removal of EDs and PhACs to the sludge retentiontime. However, this experience has not been taken into ac-count into design guidelines up to now.

Hence, another important issue concerns the treatment plantsize. Directive 91/271/EEC only focuses on larger treatmentunits, i.e. facilities for more than 2,000 people. However, avast majority of WWTPs is of a very small size. In other words,most single treatment plants, taken alone, contribute only verylittle to environmental pollution. However, the large total num-ber of small WWTPs in operation results in a big overall bur-den to the environment. Here, solutions that will be adaptablefor different scales, from the very small units to significantsizes of 10,000 to 50,000 person units, should therefore rep-resent a major objective in contemporary plant design.

State-of-the art WWTPs combine physical, chemical andbiological treatment steps to remove solids and nutrients,perform flocculation and sedimentation, and precipitatephosphates to reduce the danger of eutrophication of sur-face waters. Eighty percent of European waste waters passthrough such a treatment plant before they are dischargedinto the environment (O'Brien and Dietrich 2004). Conven-tional sewage treatment plants throughout Europe adopt asystem of 3 different steps, a primary clarifier, an activatedsludge basin, and a secondary clarifier (Fig. 1). In the pri-

Effluent containing micropollutants

Activated sludge tank

anaerobic anoxic nitrification

Secondary clarifier

Fig. 1: State of the art of wastewater treatment (modified after Siegrist et al. 2004). From left to right: Primary clarifier for sedimentation of solids; Activatedsludge basin for microbial decomposition of nutrients and pollutants; Secondary clarifier for removal of fines and chemicals by absorption processes.Sludges from clarifiers and from the activated sludge system are treated separately and finally disposed



mary clarifier, suspended solids are sedimented and buildup a first sludge fraction. In the activated sludge basin, nu-trients and pollutants are in part decomposed by nitrifica-tion / denitrification, biological mineralization, and strip-ping by aeration. Furthermore, phosphate is retained bypolyphosphate-accumulating bacteria. The secondary clari-fier removes fines and chemicals by absorption processesbased on hydrophobic interactions, or by adsorption due toelectrostatic interactions of positively charged groups ofchemicals with the negatively charged surfaces of microor-ganisms. Sludge from the primary clarifier and the surplussludge from the activated sludge system are treated in sepa-rate units and finally disposed (Siegrist et al. 2004).

Despite these developments, wastewater treatment does notcomply with the same standards throughout the EU. The di-verse technological standards existing in different countries ofthe EU aside, legislation of individual countries has addressedpollution reduction in different ways. Furthermore, factorsgoverning the selection of municipal and industrial wastewa-ter treatment technologies are today subject to major changes:Increasing expectations in European society for clean waterand a healthy environment are now manifested in stricter EUlegislation and control. This clearly calls for new developmentsin advanced treatment technologies and is a unique oppor-tunity to bring new and 'green' processes into focus.

The removal of micropollutants and potential applicationsof novel membrane techniques (e.g. Clara et al. 2005) areamongst the aspects of wastewater treatment that generatethe greatest interest at present. Due to their relatively highcosts and maintenance requirements, they will, however, onlybe adopted in larger WWTPs of wealthier communities.

4 Ongoing Research Activities in EU-Funded Programs

As the concern about water pollution has reached a EuropeanDimension, the EU has fostered research on the topic of watertreatment during FP5 and FP6. So far, research on conven-tional sewage treatment, on advanced oxidation procedures,and on membrane technologies have been financed. Success-ful projects deliver important data to the EU, which can beretrieved at several websites. Some examples are given here:

http://here.alfalaval.com/http://www.aquabase.comhttp://www.europa.int/comm/research/endocrine/pdf/env4-ct98-0798.pdfhttp://www.cranfield.ac.uk/ecochemistry/eravmishttp://www.cdcs.unina.it/-rmarottahttp://edenresearch.infohttp://www.eu-poseidon.comhttp://www.iwaponline.com/wio/2002/03/wio200203021.htm

These projects and organizations have achieved significantprogress in wastewater treatment, and their results onmicropollutants indicate removal rates in the range of 70 to90% (O'Brian and Dietrich 2004). Especially approacheswith membrane technologies, ozonation or with urine sepa-ration technologies (http://www.novaquatis.eawag.ch) seempromising with respect to high effluent quality. One of thereasons for the residual pollution load in effluents seems tobe the short residence time in the system and the inadequate

retention of problem compounds in the sludge or the bioticcompartment of wastewater treatment plants. Moreover,most sewage treatment plants release more than the origi-nally proposed effluent to surface water in a ratio of 1:10into accepting water bodies.

Hence, as promising as these technologies might be, they doeither require an incredibly high standard of wastewater pre-treatment, inadequately low throughputs, or high input ofenergy and resources. Ozonation would, for example, guar-antee a 90% reduction of micropollutants in sewage treat-ment plants, although the costs of the installation and of theoperation make this system unattractive for most communi-ties. Urine separation, on the other hand, would require acomplete alteration in the sewage system of a community.This seems unrealistic.

All presently available technologies have failed to alleviatethe load of pollutants from our waters. Treatment facilitiesacross Europe urgently need upgrading to fulfill recentlyupgraded water standards, and to keep the end-users healthy.

With view to the ongoing enlargement of the EU, soundtechnologies would have to be developed that are sustain-able and affordable for border countries and adaptable toexisting treatment technologies.

5 Upcoming Solutions

Across Europe, with its climatic gradients, specific pollut-ant situations and demands for a supply of clean water, anintegration of technologies will be needed. Research has tobegin at end-of pipe-problems, i.e. at the effluent tubes ofsewage treatment plants currently in operation. Here, mix-tures of recalcitrant pollutants occur in relatively clean wa-ter. In such an oligotrophic system, a potent microflora ca-pable of degrading stable pollutants can only exist whennutrients are added. Amendments of carbon and nitrogensources can, of course, be made use of by adding mineralfertilizer to the system. A much more elegant way to supplythe microflora can be reached by plant canopies in artificialwetlands. Furthermore, if plants with high transpiration ratesare selected in such a canopy, they will be able to take or-ganic micropollutants up and distribute them in their tis-sues, where further metabolism will occur (Coleman et al.2001, Schröder 1997, 2001, 2003, 2004, Schröder andCollins 2003, Schröder et al. 2005, Golan-Goldhirsh et al.2004). This is also true for sludge treatment and drying byreed beds, as applied in Denmark (Nielsen 2003, 2005). It isobvious that such plant-microbe associations will have tobe designed specifically for specific environmental condi-tions, and for the specific pollution / climate interactions ofthe region of interest.

Compared to engineering-based technological approaches,these green bioremediation (phytoremediation) techniquescurrently being developed and applied in constructed wet-lands and barrier systems seem rather poor. However, theyhave been demonstrated to be very effective in numerouscases and especially in small systems, although they mightappear somewhat primitive. Especially in small systems, theywill guarantee a stable effluent quality with low nutrientcontent, thus affording high hygienic levels (Vanek and



Schwitzguebel 2003). Several techniques can be distinguishedamongst phytoremediation technologies: phyto-extraction,phyto-degradation, phyto-volatilization; rhizosphere degra-dation, and constructed wetlands (Schröder and Hartmann2003). The green technologies proposed in the followingparagraphs have several key features in common; they areof high sustainability, require a low input in energy andmanpower, and offer possibilities of carbon sequestration inbiomass, as well as the recycling of materials and matter.

Constructed wetlands. Conventional treatment systems thatare based on submerged biomass are not as robust regard-ing shock loads compared to near-natural treatment systems,such as constructed wetlands (CWs). CWs are (semi-)artifi-cial wetlands designed to improve water quality. They areeffective in treating organic matter, nutrients and pathogensand are used worldwide to treat different qualities of water.Compared to conventional technical solutions for watertreatment, CWs are relatively easy to maintain and to oper-ate, resulting in low operating costs (Kadlec et al. 2000,Langergraber and Haberl 2001, Haberl et al. 2003, Langer-graber and Haberl 2004). The pioneering work of COSTaction 837 has led to the identification of the most promis-ing helophytes for constructed wetlands, amongst themPhragmites, Typha and Brassica species, but also fast grow-ing trees (lbewww.epfl.ch/COST837).

Already through the use of simple horizontal flow CWs, thepollution load can be reduced significantly due to their highefficiencies for pollutant removal (e.g. Kadlec et al. 2000).The very low energy requirement of CWs (Brix 1999) savesenergy resources. CWs perform quite favorably with othertreatment technologies according to their sustainability in alife-cycle assessment (Dixon et al. 2003, Steer et al. 2003).Besides water quality improvement and energy savings, CWshave other features related to the environmental protectionsuch as promoting biodiversity, providing habitat for wet-land organisms and wildlife (e.g. birds and reptiles in largesystems), and serving climatic (e.g. less CO2 production,Dixon et al. 2003) and hydrological functions (Brix 1999)and heavy metal bioaccumulation and biomethylation(Azaizeh et al. 1997, 2003). CW technology is emerging rap-idly, and drawbacks will probably be minimized during fur-ther development.

Vertical flow beds. The behavior in the environment of se-lected organic compounds and emerging organic micro-pollutants has been widely researched in the context of con-ventional water treatment during the last years (e.g. Burschet al. 2004, Chaudry et al. 2001, Fürhacker et al. 2003a,2003b, 2004, Lenz et al. 2005a, 2005b, Mahnik et al. 2004).CWs have clearly been shown to be effective in treatingwastewaters containing a large number of organic com-pounds (Haberl et al. 2003). Only recently, the first pioneer-ing studies on the behavior of organic micropollutants inthe context of CWs have been published (e.g. Kästner et al.2003, Masi et al. 2004, Matamoros et al. 2005). These stud-ies show that CWs are generally amenable to remove or-ganic micropollutants such as endocrine disrupting chemi-cals, as well as pharmaceuticals and personal care products(PPCPs), but the degradation of the pollutants dependsstrongly on the chemistry within the rhizosphere and the

retention time in the CW. Here, the use of twin-shaped, con-structed wetlands consisting of one or more vertical flowchamber and reverse vertical flow chambers seems to offerthe highest removal efficiencies of xenobiotics from pollutedwater (Cheng et al. 2002, Schröder et al. 2005a). In connec-tion with selected plant species that improve the oxygen sup-ply to the rhizosphere, these systems offer habitats forrhizobacteria with different requirements and capabilitiesof pollutant degradation. Furthermore, they allow for in-tense contact of the pollutant with the root surfaces.

Hydroponics. Being characterized by their extraordinary rootgrowth, several helophytes can also be grown in hydroponicsystems without soil. Such specific CWs might be useful wheninteractions between pollutants and the soil matrix have tobe excluded due to pollutant chemistry (e.g. high log Kow),or when the plant material as a whole has to be harvestedand removed after the pollutant has been accumulated inthe tissue. Specifically Phragmites, Iris, Juncus, but also Men-yanthes and Panicum seem to form extensive reeds undersuch growth conditions (Wissing 2003). To stabilize the plantcanopy, porous rubber or woven plastic mats can be used toprovide support and shelter to the growing roots. Periodicalaeration seems to stimulate root growth and biomass devel-opment. Hydroponic systems underpin the role of the plantand its metabolism for the uptake and degradation of thepollutant under consideration. They are highly sustainable,and might also contribute to diminishing greenhouse gasemissions (Dixon et al 2003).

Hybrid systems. Depending on the possibilities of the WWTPand its demands, it might be useful to add phytoremediationmodules that are mixtures of the above examples. Novelideas might be the operation of flooded horizontal beds withfloating species (Eichhornia, Pistia, Lemna), or the inclu-sion of helophytes or terrestrial plant species with differentrooting depths in vertical flow beds. Root surface chemis-try and aeration of roots might also be of concern, andhave so far not been studied with respect to rhizofiltration.Mixed stands of plants will generally be more stress resis-tant than monocultures, and establish a higher diversity ofrhizobacteria. Furthermore, the combination of plants ableto degrade specific pollutants will increase the efficiency.For example, Phragmites seem to be a good candidate forthe removal of pesticides (Schröder et al. 2005b), but it willbe subject of further studies to test its ability to degradepharmaceuticals. Successful remediation of pollutants hasalso been demonstrated in flooded soil systems planted withtrees. Here, the plant's role might be confined to the sup-port of rhizosphere bacteria, the evaporation of excess wa-ter and the potential volatilization of pollutant metabolites.Several wetland trees (poplar, birch, willow) and other plantspecies have been described to have a good potential forrhizostabilization of pollutants for further microbial degra-dation and treatment of the sludge. Also the volatilizationof xenobiotics has been demonstrated as a possible way todiminish the pollutant burden in the water body (Burkenand Schnoor 1999, Ma and Burken 2003). In any case, theselection of suitable soil/sediment systems and residencetimes seems crucial for the operation in a WWTP and hasto be tested thoroughly.



Fenton's Oxidation. Another set of novel but chemistry-basedapproaches involves advanced oxidation techniques (AOP)relying on the generation of hydroxyl radicals through vari-ous techniques, such as vacuum-UV radiation (V-UVR), elec-trochemical oxidation, use of ozone and/or hydrogen per-oxide in combination with UV-radiation (Lenz et al. 2005a).

The reactivity of metals and hydrogen peroxide to yieldhighly reactive hydroxyl radicals was first reported by Fentonin the late nineteenth century. In the presence of a metalcatalyst (usually iron(II) chloride or sulphate), a hydrogenperoxide solution forms hydroxyl radicals (OH). The mecha-nism was studied in detail by Haber-Weiss and it is referredto as the Haber-Weiss reaction, where super oxide ion andhydrogen peroxide (often in the presence of Fe(III) as cata-lyst) react to yield a hydroxyl radical, oxygen and a hydroxylion. They can oxidize most organic substances into CO2 andH2O. If there are not enough radicals, organic compoundsare decomposed to lower organic acids. Addition of a re-ducing agent, such as ascorbate, leads to a cycle (Haber-Weiss cycle) which increases damage to organic and bio-logical molecules. It was shown that copper and ascorbicacid can preferentially break down histidine in proteins in aHaber-Weiss type reaction (Shinar et al. 1983, Golan-Gold-hirsh et al. 1992), which opens a potential for a more tar-geted approach in organic compound breakdown based onthis reaction. In the use of this reaction in the context ofwastewater treatment, organic substances would decom-posed in a separate WWTP-module by mixing waste waterwith hydrogen peroxide and an iron catalyst, before the wastewater is neutralized. As a practical advantage, the iron cata-lyst precipitates as iron hydroxide. Fenton's reagent can beapplied in a batch or continuous process and guarantee deg-radation of residual pollutants.

6 The European Dimension:Objectives and needs for the future

Directive 91/271/EEC on urban wastewater treatment andDirective 96/61/EC on Integrated Pollution Prevention Con-trol illustrate the current and future EU policy to encouragedevelopment of processes and standards to prevent negativeeffects on water, using best available technologies. The lim-iting biodegradative capacity of natural microbial associa-tions necessitates the development of more integrated watertreatment and management. Research is needed (1) to searchfor biotechnological processes capable of removing suchchemicals through engineering of biochemical pathways inplants and microbial associations, and (2) to find reliablebiosensors able to generate information on residual micro-pollutants. Recent trends to exploit improved plant cano-pies and for accurate process control are of major signifi-cance in this context. However, clear-cut scientific andpolitical endorsement of the necessity to use reclaimed waste-water is of prime importance for evolving more sustainablewater management. Protection of the quality and supply offreshwater, thus, needs integrated approaches to the devel-opment, management and protection of water resources.

The European wastewater problem will have to be tackledsoon, since the mentioned EU directives require urgent ac-tion. In order to recommend currently promising technolo-gies such as phytoremediation, ozonation, membrane filtra-

tion, or the Fenton reaction in existing wastewater treat-ment facilities, it will be necessary to study in more detailthe prevailing interactions between pollutants and the plant/bacterial consortia in this context. We propose evaluatingsuch novel treatment modules by grafting them onto exist-ing sewage treatment facilities of different types (Fig. 2),and also along a climatic gradient from humid north Euro-pean through Mediterranean to harsh desert climates, inorder to gain an insight into the underlying biochemistryand biology, and to evaluate the resulting effluent quality.This will allow us to gain a better understanding and evalu-ation of the specific needs of different types of treatmentplants in different geographical situations, and to betterimplement cost-efficient, tailor-made adjustments to specificpollution problems on a case by case basis. Only thoroughtesting of phytoremediation technologies will enable the EUto set regional conditions for stable effluent quality and con-sumer security, in concert with high sustainability.

7 Perspectives

The main aim of applied environmental sciences in the fieldof wastewater treatment has to be the amelioration of the ef-fluent quality from WWTPs and the enforcement of reliablestandards of regenerated waters in contact with ground waterresources. Only hereby will Europe be able to increase thesustainability of drinking water resources and contribute in amodest way to decrease effects of global change by loweringenergy usage, CO2 emission and waste production duringwastewater treatment. It will be necessary to demonstrate costeffectiveness, reliability, long-term sustainability, resilience andreasonable input of resources, especially for border coun-tries, before local decision-makers can accept such a changein the water treatment procedure for their region.

Contrary to most technical solutions, the implementationof phytoremediation would address these demands. Here,cost effectiveness is achieved by recycling, using energy sav-ing biological processes, and by producing biomass, poten-tially biologically active compounds for medicinal use, andother non-food products for energy production, green ma-nure and building materials. Furthermore, constructed wet-lands are low-cost maintenance systems.

Compared to traditional sewage treatment methods, it canbe stated that 'green technologies' are more appropriate forwater clean up for the following reasons:• they decompose organic pollutants to non-toxic low

molecular substances easily degraded by microorganisms,• they do not introduce additional chemical substances into

the environment (solvents, alkali, PEG),• they are relatively easy to manage and they can be easily

adopted to the local needs,• they do not require large investment to be practically

introduced,• they are able to remove several pollutants in combination,• they can be applied at a small as well as at a large scale.

Of special importance is their functionality in a modulardesign, i.e. relatively small containments that would becoupled to existing WWTPs corresponding to the specificday-to-day requirements of changing wastewater qualities.Such systems will be reliable because the functioning of the



single modules has been demonstrated in several existingpilot studies. A combination of these eco-techniques is a novelapproach that will further improve the reliability. Long-termsustainability is achieved by integration of biological pro-cesses and the use of environmentally friendly materials andagents to the process.

Hence, such systems will operate with reasonable input ofresources, as they represent microcosms that stabilize them-selves. Plant harvest, maintenance, and de-clogging are lowinput activities that require no specifically educated person-nel. Public acceptance of green technologies is generally higherthan that of industrial processes. The expected, excellent wa-ter quality will lead to additional consumer satisfaction,sustainability for future generations, contribute to recreationand ecoesthetics, and it will contribute to the protection of thevulnerable parts of society, women, children, and the elderly,from pharmaceuticals and dangerous micropollutants.

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Fig. 2: Optimizing existing waste water treatment plants. Innovative green or AOP technologies are used to upgrade existing wastewater treatment plantsthat have increasing problems of removing recalcitrant micropollutants. The additional modules (e.g. oxygenation, constructed wetlands, aquatic macro-phytes, hydroponics/reed beds, swamp system with trees, hybrid systems with mixed stands, AOP) are introduced facultatively into the system. Thoroughtesting for water quality is performed after each module to ensure water quality standards. Polluted water is retained or pumped back into the module

Effluent containing micropollutants

Activated sludge tank

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Secondary clarifier

Test y/n Test y/n Test y/n Test y/n Test y/n Clean effluent

Optional phytoremediation and AOP modules



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Received: August 16th, 2006Accepted: December 21st, 2006

OnlineFirst: December 22nd, 2006

using phytoremediation technologies to upgrade waste water treatment in europe

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