Metallic iron: dawn of a new era of drinking water treatment research? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 Noubactep Chicgoua*(a,c), Schöner Angelika(b) (a) Angewandte Geologie, Universität Göttingen, Goldschmidtstraße 3, D - 37077 Göttingen, Germany. (b) Institut für Geowissenschaften, Friedrich-Schiller-Universität Jena, Burgweg 11, D–07749 Jena, Germany. (c) Kultur und Nachhaltige Entwicklung CDD e.V., Postfach 1502, D - 37005 Göttingen, Germany. (*) corresponding author: cnoubac@gwdg.de; Tel. +49 551 39 3191, Fax: +49 551 399379 (Accepted 9 Jun 2010: Fresen. Environ. Bull.) Abstract The goal of water treatment in water works is safe drinking water production. Various technological options are available to this end. However, many conventional water treatment technologies are too expensive for extensive deployment in rural communities worldwide. Research over the past two decades has demonstrated the efficiency of metallic iron (Fe0) for the aqueous removal of a wide range of chemical and microbial contaminants (e.g. bacteria, chlorinated organics, dyes, emerging contaminants, heavy metals, radionuclides, viruses). The prevailing concept considers that the mechanism of Fe0 remediation varies depending on the contaminant of interest. This concept was recently revisited and Fe0 was proven an universal material for water treatment. As a consequence Fe0 filtration beds are proposed in this communication to replace ultra-filtration, nano-filtration, and disinfection units in water works. It is anticipated that the success of Fe0 filtration beds in producing safe drinking water in large scale will depend on the ability of researchers to produce adequate reactive materials. Target experimental work is needed to confirm and extend the applicability of this affordable method. Key words: Emerging contaminants; Micro-pollutants; Safe drinking water; Zerovalent iron. Capsule: Filtration on Fe0 beds could replace ultra-filtration, nano-filtration, and disinfection units in water treatment plants. 1 1 Introduction 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 The goal of water treatment is the production of safe drinking water for proper distribution. Conventional water treatments with coagulation, rapid sand filtration, granular activated carbon filtration, and disinfection (chlorination, ozonation or ultraviolet radiation) have been proven inefficient for the quantitative removal of several micro-pollutants from surface waters [1-3]. The situation is exacerbated by the occurrence of so-called emerging contaminants of unknown property and toxicity [4-7]. Emerging contaminants are mostly pharmaceuticals and personal care products. Approximately 3,000 different pharmaceutical ingredients are used in the EU today, including painkillers, antibiotics, antidiabetics, beta-blockers, contraceptives, lipid regulators, antidepressants, antineoplastics, tranquilizers, impotence drugs and cytostatic agents [4]. As a consequence alternative and innovative water treatment concepts are under development [1,6]. For example, a new treatment four-stages-concept was recently proposed in the Netherlands: (i) fluidized ion exchange (FIEX), (ii) ultrafiltration (UF), (iii) nanofiltration (NF), (iv) granular activated carbon filtration (GAC) [2]. The FIEX process removed calcium and other divalent cations; the UF membrane removed particles and micro- organisms; and the NF membrane and GAC removed natural organic matter (NOM) and micro-pollutants. The results of a pilot study showed successful removal of most micro- pollutants. However, very polar substances with a molecular weight lower than 100 Daltons could not be quantitative removed. These substances are too small to be rejected by the NF (size exclusion), and too polar to be quantitatively adsorbed by the GAC. Therefore, a process is needed to quantitatively remove both small and polar substances from water. Water filtration on metallic iron is a serious candidate as will be shown later. The technology of water filtration on metallic iron will first be presented. 2. Water filtration on metallic iron In early 1990 metallic iron (Fe0) was introduced as reducing agent for groundwater remediation in permeable reactive barriers (iron walls). Fe0 was proved particularly efficient 2 for the decontamination of halogenated organic compounds [8-10]. Subsequent studies have confirmed the efficiency of Fe 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 0 for quantitative removal of several substances including nitrate, bromate, chlorate, nitro aromatics compounds, pathogens, pesticides, arsenic, chromium, copper, lead, triazoles, uranium, and zinc [9-14]. Although successful removal of reducible (e.g. CrVI, lindane) and non reducible (e.g. ZnII, triazoles) contaminants was reported, the initial premise of reductive transformation is still prevailing. However, there is clear evidence that contaminants are basically removed by an unspecific mechanism [12,15,16]. The unspecific nature of the processes yielding aqueous contaminant removal by Fe0 is confirmed by reports on successful removal of more than 20 different species (including bacteria and viruses) in Fe0-based filters (3-Kolshi and SONO filters) designed for arsenic removal at the household level in South East Asia [17-19]. The qualitative aspect of the efficiency of Fe0 materials for contaminant removal in iron walls and household filters is the motivation for this communication. The most important output is that Fe0 is an efficient filter material to quantitatively remove all contaminants including small size and polar species which have been shown difficult to remove in conventional water treatment plants. 3. Suitability of Fe0 bed for water treatment plants The voluminous literature on "remediation with corroding iron" is characterized by the overwhelming number of parameters which have been shown to affect the process of aqueous contaminant removal in the presence Fe0 [20-25]. These parameters include the nature of the contaminant, the pH of the solution, the nature of Fe0 (e.g. carbon steel, cast iron, direct reduced iron), the size of Fe0 (mm, μm, nm), the temperature, the water flow velocity, the water salinity, the presence of oxidizing agents (e.g. O2), the character of the oxide scales on Fe0. The importance of all these factors was traceably demonstrated from isolated sets of experiments. However, due to lack of a standard experimental protocol, available results could be collectively regarded as qualitative as they are not comparable to each other [22-25]. 3 Taken together, results from Fe0-based filters (field walls and household filters) demonstrate the suitability of Fe 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 0 beds for the removal of all possible contaminants from water. This statement is supported by the fact that some parameters (e.g. bed depth, Fe0 type, flow velocity) could be adjusted for performance optimization. Another argument to support this view is that field Fe0 walls have quantitatively removed species (e.g. 1,2-dichloroethane and dichloromethane), which were proven to be not treatable by Fe0 in batch studies [26]. Accordingly, the mechanism of contaminant removal in Fe0 beds is different from that which is investigated in batch studies. 4 Mechanism of contaminant removal in Fe0 beds Despite two decades of intensive research, there is no agreement on the fundamental mechanisms of aqueous contaminant removal in the presence of reactive Fe0. The prevailing concept considering Fe0 as a reducing agent was shown inconsistent with many experimental observations [12,15]. The new more consistent concept considers adsorption and co- precipitation as the fundamental mechanism of aqueous contaminant removal in the presence of Fe0 (adsorption/co-precipitation concept) [12,15]. The adsorption/co-precipitation concept is not yet accepted by the scientific community as recently discussed [25,27]. The new concept was recently validated by Ghauch et al. [16] while investigating aqueous clofibric acid removal in the presence of Fe0. Nevertheless, even newer works explicitly disprove the adsorption/co-precipitation concept without any convincing argument [28,29]. Moreover, the new concept is even falsified. As an example, the authors of ref. [28] considered that the validity of the adsorption/co-precipitation concept means that “degradation of chlorinated organics is unimportant because some metals are removed mainly by sequestration”. However, the adsorption/co-precipitation concept considers that chlorinated organics (RCl) are certainly adsorbed and co-precipitated. RCl reduction certainly occurs to some unknown extent and reduction is not likely to be mediated by electrons from of Fe0 as adsorbed FeII and adsorbed H are more accessible than Fe0 and partly more powerful than Fe0 [12,15]. 4 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 Although the adsorption/co-precipitation concept is not yet univocally accepted, it is considered as actual state-of-the-art knowledge and used for the further presentation. For the sake of clarity the process of aqueous iron corrosion will be briefly recalled. 4.1 Aqueous iron corrosion Aqueous iron oxidative dissolution in water (aqueous iron corrosion) is an electrochemical process. Aqueous iron corrosion is a heterogeneous reaction characterized by the spontaneous dissolution of iron (yielding dissolved FeII or FeIII – Tab. 1). Iron oxidation is driven by any oxidative species which standard electrode potential (E0) is higher than –0.44 V (E0 of the couple FeII/Fe0). Table 1 shows clearly, that H+ (E0 = 0.00 V), Fe3+ (E0 = 0.77 V) and dissolved O2 (E0 = 0.81 V) are oxidizing agents for Fe0 (and H2 is a reducing agent for FeIII). Accordingly, in a contaminant-free aqueous solution, Fe0 is oxidized. Resulting FeII and FeIII are hydrolysed, and precipitated (Eq. 8-10). The kinetic of iron corrosion is controlled mainly by diffusion-convection of oxidizing agents from the bulk solution towards the metal surface (and reciprocally). At pH > 4.5, the Fe0 surface is always covered by an oxide scale. 4.2 Contaminant removal in batch systems It was found that mixing operations have a negative effect on identifying the real mechanism of contaminant removal by Fe0 [16,22,23,25,30,31]. In fact, mixing disturbs the formation of the universal oxide scale in the vicinity of Fe0. As state above, the universal oxide scale serves as a diffusion barrier for contaminants. Accordingly, if a contaminant exhibits a poor adsorptive affinity to the oxide scale, it will not readily diffuses into the oxide scale where it could be enmeshed or further transformed (oxidized or reduced, if applicable). However, mixing batch systems disturbs the spatial disposition of the oxide scale relative to Fe0. In particular, the Fe0 surface is rendered accessible to all contaminants, regardless their affinity to the iron corrosion products (“fluidized cells”). These conditions, however, are not reproduced in nature. 5 Under stagnant and slow-mixing conditions, dissolved species (including contaminants) are adsorbed or/and enmeshed during the process of the oxide scale formation. Adsorbed and enmeshed species could be further chemically transformed as discussed above. It is certain that the kinetics of aqueous removal of various species will depend on their relative affinities to the oxide scale. However, all species will be removed as they are in trace quantity in a domain of precipitating iron [32,33]. This is the first argument suggesting Fe 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 0 as universal materials for save water production. It is very important to note that contaminants are neither exclusively removed by Fe0 nor the oxide scale. Contaminants are quantitatively removed during the dynamic process of oxidative dissolution yielding a transforming oxide scale [27]. Table 2 gives an overview on contaminant classes that have been successfully removed from aqueous solutions by Fe0. In a column system size exclusion is added to the described dynamic process. 4.3 Contaminant removal in column systems The mechanism of contaminant removal in Fe0 beds was recently elucidated [27,43]. A Fe0 packed bed initially contents an inter-granular volume (pore volume) which ideally, should never been totally filled by in situ generated iron corrosion products (long-term permeability). A Fe0 bed in its initial stage of implementation is comparable to a sand filter for the removal of suspended solids by size exclusion (media filtration). The filtration efficiency depends on the sand particle size. The suitability of Fe0 as reactive medium for drinking water filters relies on two essential characteristics: (i) the interactions of corroding iron with contaminants (adsorption, co-precipitation/enmeshment as for batch systems), and (ii) the improved size exclusion by virtue of the expansive nature of iron corrosion. The improved size exclusion is due to the fact that generated iron corrosion products are at least 2.1 times larger in volume than Fe0 in the metallic lattice [27,43,44]. An essential feature of corroding iron for contaminant removal in Fe0 beds are the expansion/contraction cycles accompanying iron oxide formation from Fe0. In fact, each Fe0 6 (SSA < 2 m2/g) is first oxidized to nebulous hydrated iron hydroxides having specific surface areas (SSA) larger than 500 m 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 2/g. The hydrated hydroxides are then progressively transformed to amorphous and crystalline oxides (Fe(OH)3, Fe2O3, Fe3O4, FeOOH) (SSA < 40 m2/g) [45]. To illustrate the transformations yielding to contaminant sequestration/trapping, the evolution of three iron atoms from the Fe0 material will be discussed. The space between the individual atoms is neglected for simplifications. Three atoms Fe0 will be first oxidized to 3 FeII species and may further be oxidized to 3 FeIII species. Then, the hydrolysed species will be transformed to colloidal species (SSA > 500 m2/g) before they aggregate and crystallize to one Fe3O4, 1.5 Fe2O3 or 3 Fe(OH)2, Fe(OH)3 or FeOOH (Tab. 1). Calculations (not presented here) clearly showed volumetric expansion relative to 3 Fe0 for all iron oxides except magnetite for which a volume reduction of 30 % was noticed [27]. However, even magnetite is a final stage of a transformation going through voluminous colloidal, amorphous and highly adsorptive species. Considering for simplifications only Fe0 and the crystalline forms of the oxides, the coefficient of volumetric expansion (Voxide/Viron) varies from 2.1 for Fe3O4 (magnetite) to 4.2 for Fe(OH)3 (bernalite) [27,44]. As stated above, dissolved Fe first experienced volumetric expansion, and then contraction in the inter-granular space of a filter. The transformation sequence: Fe0 – hydroxides – amorphous oxides – crystalline oxides probably occurs in the presence of trace amounts of contaminants. In particular, the "nebulous" hydroxide can be considered as an instantaneous spider-like web which traps contaminants in the inter-granular space in the filter. Sequestrated contaminants could be further chemically transformed. This presented mechanism does not consider the nature of the contaminant and in particular its reactivity with Fe0. Contaminants are transported to the bed and within the bed by gravity (or other hydrodynamic forces) and are primarily removed from the aqueous phase by adsorption and/or co-precipitation. This explains why non reducible and reducible contaminants are removed (Tab. 2) and suggests that upon proper design Fe0 beds will 7 produce safe drinking water at several scales (households, small communities, large cities). It is very important to note that in this scheme no energy is needed, apart from the energy to pump water from wells into the Fe 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 0 beds. This makes Fe0 bed filtration ideal for developing countries where shortages in water production are sometimes due to lack of energy power [46]. The complete absence of chemicals (including disinfecting agents) is a further key advantage. It could be anticipated that Fe0 beds is primarily for small water treatment plants. However, several small urban quarters drinking water production units using Fe0 beds could produce enough water for large cities. 5 Innovative iron filters An inherent problem of Fe0 filter beds is porosity/permeability loss due to the expansive nature of iron corrosion [44]. The volumetric expansion of corroding iron is presented in the present communication as a useful tool to improve (slow) sand filtration efficiency. In other words, Fe0 filters are regarded as Fe0 assisted sand filtration [47]. Accordingly, calculated amounts of Fe0 are added to sand to warrant quantitative (abiotic) contaminant removal while keeping acceptable water flow velocity. Calculations (not presented here) assuming Fe3O4 as sole corrosion products demonstrated that, regardless from the filter size, a 51:49 (vol:vol) Fe0/quartz mixture is clogged upon Fe0 depletion. Accordingly, a long-term efficient Fe0 filter should contain less than 51 vol-% Fe0. Considering that the oldest Fe0 reactive barrier initially contained only 8.8 vol-% (22 wt-%) Fe0 [8] and efficiently removed the target contaminant for more than 10 years, it could be anticipated that Fe0 volumetric ratios of 30 to 40 % (60 to 70 vol-% sand) might satisfactorily treat water for several months. It is certain that the actual used Fe0 proportion will depend on its intrinsic reactivity and the extent of the contamination. The presentation above confirms that mixing sand to iron is a prerequisite for long-term efficient Fe0 filters. Previous efforts [48,49] to sustain Fe0 reactivity by mixture with sand were not really rationalized as expressing the proportion of Fe0 by a weight percent is not consistent with the fact that pore volume availability is discussed (expansive nature of iron 8 corrosion filling the pore space in the filter). Moreover, despite Fe0:sand mixtures the porosity of the Fe 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 0 filters was sustained by the application of a source of vibratory energy and/or an auger system [50]. It should be acknowledged that some few authors have properly rationalized the used Fe0/sand volumetric ratios [47,51-53]. However, the discussion was not based on the available volume for in-situ generated corrosion products. Provided proper material selection, Fe0 filters could satisfactorily render raw waters safe for drinking without any addition of chemicals. The three major operating parameters to be addressed on a site specific basis are: (i) Fe0 selection, (ii) bed depth, (iii) water flow velocity. In other words, pilot studies should focus at determining Fe0 type (e.g. particle size), bed’s length, and water flow velocity. 5.1 Iron filter design Innovative Fe0 filters for drinking water production at community level are very flexible in their design. The simplest design is a single bed containing layers of gravel, pebble, sand, “sand + Fe0”, and fine sand. The actual Fe0 bed size depends on the size of the population to be deserved or the volume of water to be produced daily. Single beds are necessarily good for very small communities (e.g. up to 100 people). For larger communities the device could/should comprise separate beds of gravel, pebble, and sand for media filtration, and Fe0 beds (containing one or more layers of “Fe0 + sand”) for the removal of dissolved substances. Here, the system may comprise several Fe0 beds in series (Fig. 1) and each bed may contain: (i) the same Fe0 material in a certain Fe0:sand ratio, (ii) various layers of the same Fe0 material with different Fe0:sand ratios, or (iii) various Fe0 materials (e.g. differing in their particle size). 6 Discussion Generally, save drinking water is obtained either from surface water or groundwater after several treatment steps (Tab. 3). The goal is to remove pathogens, dissolved, and suspended substances [2,6,46]. The treatment processes may include (i) flocculation, (ii) sedimentation, 9 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 (iii) media filtration, (iv) ion exchange, (v) carbon adsorption, (vi) membrane filtration, and (vii) disinfection. The first three named treatment processes remove colloidal and suspended solids and the subsequent processes remove dissolved substances. Water disinfection is achieved by chlorination, ozonation, or ultraviolet radiation. The actual drinking water treatment scheme depends on the source water characteristics. Generally, surface water is characterized by high contents of suspended solids and organic matter. Accordingly, pre-sedimentation, coagulation or coarse filtration processes are generally used for water treatment followed by disinfection. On the other side, groundwater is characterized by high contents of salts, organics, or gasses. Therefore, groundwater is aerated to remove dissolved gasses followed by softening to remove dissolved salts prior to disinfection. Regardless of the water source, there could be a need to further remove dissolved organics. In this case air stripping, ion exchange, carbon adsorption, or membrane filtration processes may be used [6]. The presentation above has demonstrated the suitability of corroding iron in a filter to remove contaminants which are conventionally removed in several treatment steps: ion exchange, carbon adsorption, membrane filtration and disinfection [43,47]. Accordingly, after the removal of colloidal and suspended solids (media filtration), filtration on a properly designed Fe0 system will free water from dissolved substances and pathogens. In other words, processes achieve by ion exchange, carbon adsorption, membrane filtration, and disinfection could be achieved in Fe0 beds (Tab. 3). Moreover, emerging contaminants which are not quantitatively removed in conventional water treatment plants are successfully removed in Fe0 beds (Tab. 2). Therefore, Filtration through Fe0 beds is a promising technology for safe drinking water production in water works. The technology is necessarily limited to raw waters having a pH value larger that 4.5. For lower pH values, iron solubility is increased and dissolved iron is transported out of the column. The consequence is increased porosity/permeability with time and thus decreased bed efficiency. Remember that size 10 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 exclusion at pH > 4.5 is improved by precipitating iron oxides progressively filling the intra- particular space in the filter (pore volume). 7 Concluding remarks The present communication has presented a new concept for water treatment in beds of metallic iron (Fe0). An efficient removal of particles, natural organic matter, pathogens and micro-pollutants is possible using only sand filtration and filtration on Fe0 beds (Fig. 1). The concept is inspired by (ii) recent reports on efficient removal of contaminants from several classes by Fe0 in iron walls [10] and in household filters [19], and (ii) the profound understanding of the mechanisms of contaminant removal in Fe0 filters [12,15,16,27]. Provided the use of appropriated Fe0 materials and proper bed design, all classes of substances will be removed in Fe0 beds. It can be anticipated that event nourishing element (e.g. Ca, Mg) will be removed from water such that the contaminant free water could need addition of selected nutrients to be healthy. A clear advantage of Fe0 beds is that no chemical is added and iron oxides is the sole wastes which could be recycled to produce new Fe0 materials. Actually, Fe0 beds (household filters and field reactive barriers) have been successfully used under conditions of slowly corroding iron. Under these conditions, microbial mechanisms assist the process of contaminant removal and the kinetics of iron corrosion is sufficient for quantitative contaminant removal. To produce water in small municipal drinking water utilities, it will be necessary that water flows at an increased velocity. Therefore, several beds of more reactive materials could be necessary. Known tools for more reactive materials should be tested. These tools include using smaller Fe0 particles (small granules and powders) [48], using bimetallic systems (e.g. Fe0/Cu0) [10,13,14], and mixing Fe0 with a reactive oxide as MnO2 [16,17]. Intensive research is needed at several fronts to transform the concept of Fe0 beds into a viable technology for safe drinking production in water works. The research could/should 11 start by numerical modelling followed by laboratory studies and pilot scale installations to determine the practicality of several designs from numerical modelling. It is emphasized that Fe 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 0 intrinsic reactivity, and water flow velocity will be the most important design parameters determining for example the thickness of the filter (or the number of beds). In particular Fe0 materials of different size [42] could be used in individual beds or Fe0-based composites could be developed. In this regard the porous composite iron matrix which has been successfully used in SONO arsenic filters [17] could be used or at least serves as model. Fe0 beds could be a very efficient technology at several stages: from small communities up to mega cities. This affordable technology further fulfils the key requirements of (i) minimum electricity needs, and (ii) environmental friendliness [14]. It could be a precious instrument to help to achieve the water Millennium Development Goal of “halving by 2015, the proportion of people without sustainable access to safe drinking water”. Although cost issues are not discussed here, it is expected that the Fe0 bed technology will be affordable. Material recycling is a point that should be considered from the beginning on. Several technologies are available to transform iron oxides to Fe0 at several scales [54,55]. Researchers are encouraged to perform target experimental work to confirm the efficiency of this affordable technology and identify its possible limits. In this effort characterizing the removal of molybdenum which is known for its low adsorption efficiency onto iron oxides [56] will be a good (negative) reference. Acknowledgments Maggy N.B. 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Fe0 ⇒ Fe2+ + 2 e- (0) 2 Fe0 + 2 Fe3+ ⇒ 2 Fe2+ (1) Fe0 + 2 H+ ⇒ Fe2+ + H2 (2) 2 Fe0 + O2 + 4 H+ ⇒ 2 Fe2+ + 2 H2O (3) 2 Fe0 + 6 H+ ⇒ 2 Fe3+ + 3 H2 (4) 4 Fe0 + 3 O2 + 12 H+ ⇒ 4 Fe3+ + 6 H2O (5) 2 Fe2+ + 2 H+ ⇒ 2 Fe3+ + H2 (6) 4 Fe2+ + 4 H+ + O2 ⇒ 4 Fe3+ + 6 H2O (7) Fe2+ + 2 OH- ⇒ Fe(OH)2 (8) Fe3+ + 3 OH- ⇒ Fe(OH)3 (9) Fe(OH)2, Fe(OH)3 ⇒ FeOOH, Fe2O3, Fe3O4 (10) 448 449 450 19 Table 2: Selected references supporting the suitability of metallic iron beds for safe drinking water production. Emerging chemicals are chemicals which are not covered by existing water quality legislation. Relatively little information is available on their environmental behavior and toxicological properties [34]. 450 451 452 453 454 Contaminant group Examples References Chlorinated solvents chlorinated hydrocarbons [35], [36] DNAPL carbon disulfide (CS2) [37] Dyes methylene blue, azo dyes [38] Emerging chemicals pharmaceuticals, personal care products, [16], [39] drugs-of-abuse, endocrine disruptors, nanochemicals Inorganic ions ammonium, nitrate, nitrite perchlorates [3] Metals and metalloids copper, chromium, arsenic, selenium [40], [41] Pathogens viruses, bacteria [11], [13], [17], [18] Radionuclides technetium, uranium. [30], [31], [42] 455 456 20 Table 3: Overview on the processes required for the removal of suspended solids and dissolved contaminants from raw waters to produce save drinking water in a conventional water plant and by the innovative treatment concept. It is evident that no chemicals are needed in the new treatment concept. 456 457 458 459 460 Contaminant Conventional water plant New concept Colloidal solids flocculation, sedimentation, media filtration sand beds Suspended solids flocculation, sedimentation, media filtration sand beds Dissolved species air stripping, carbon adsorption, ion exchange, iron beds membrane filtration, reverse osmosis Pathogens disinfection (chlorination, ozonation, UV) iron beds 461 462 463 464 21 Figure 1 464 465 466 467 468 469 Figure 1: Comparison of the processes of groundwater treatment in a conventional treatment plant and by metallic iron (Fe0) beds. Modified after ref. [6]. 22