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Desalination 210 (2007) 225235

0011-9164/07/$ See front matter 2007 Elsevier B.V. All rights reserved

Presented at the 9th Environmental Science and Technology Symposium, September 13, 2005, Rhodes, Greece.

Organized by the Global NEST organization and prepared with the editorial help of the University of Aegean,

Mytilene, Greece and the University of Salerno, Fisciano (SA), Italy.

*Corresponding author.

Ammonia, iron and manganese removal from potable waterusing trickling filters

A.G. Tekerlekopoulou, D.V. Vayenas*

Department of Environmental and Natural Resources Management, University of Ioannina,Seferi 2, 30100 Agrinio, Greece

Tel.+30 26410 74117; Fax++30 26410 39576; email : [email protected]

Received 10 November 2005; revised 9 March 2006; accepted 11 May 2006

Abstract

Pilot scale trickling filters were constructed and tested in order to study biological removal of ammonia, ironand manganese from potable water. The effect of the size of the support material on nitrification performance wasstudied extensively. The mean size of the gravel and hence, the specific surface area was found to be critical foroptimal nitrification operation. A steady-state model developed in previous work was used to predict filtersperformance. The model was very accurate only for the gravel size for which maximum nitrification rates wereobserved. The effect of the operational conditions on the physico-chemical and combined physico-chemical andbiological iron oxidation was also studied. It was found that the contribution of biological oxidation is significant,increasing filters efficiency by about 6% and reducing the required filter depth by about 40%. Manganese biologicalremoval was studied using gravel with small mean diameter, thus providing high specific surface area. Feedconcentrations up to 4.0 mg/l were treated sufficiently. Finally, experiments were performed to investigate thesimultaneous removal of ammonia, iron and manganese. Experimental results showed that the combined, as well asthe simultaneous removal of the aforementioned pollutants, can be achieved by single-step filtration.

Keywords: Ammonium removal; Iron removal; Manganese removal; Potable water; Trickling filter

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226 A.G. Tekerlekopoulou, D.V. Vayenas / Desalination 210 (2007) 225235

1. Introduction

Ammonia, in the form of ammonium, is animportant pollutant of surface and subsurface po-table water deposits. The increasing and very of-ten uncontrollable use of fertilizers has led to in-creased amounts of ammonia in potable water,which often exceed the upper permitted limit of0.5 mg NH4

+-N/l [1]. The impact of ammonia pres-ence in water systems reflects on oxygen deple-tion, eutrophication of surface water and toxicity

for fish [2].When iron and manganese are present in awater supply at concentrations exceeding the per-mitted limits of 0.2 and 0.05 mg/l [1], respectively,they are objectionable for the following reasons:(a) their precipitation gives water a reddish andbrown-black color, respectively, when exposed toair, (b) iron and manganese give water an unpleas-ant metallic taste, (c) home softeners becomeclogged by iron and manganese precipitates andthus their softening efficiency is reduced and (d)deposits of iron and manganese precipitate in the

distribution system, reduce the pipe diameter andeventually clog the pipe. In addition, manganesehas been found to affect the central nervous sys-tem [3].

Ammonia, iron and manganese are often en-countered in water deposits at concentration lev-els above the upper permitted limits for humanconsumption. Redox is the key parameter whichdetermines the sequence of the oxidation of thesepollutants. Iron is oxidized at low redox values(< 200 mV), ammonia at higher values (200400 mV) while manganese needs even higher re-dox values (> 400 mV) for neutral pH [4]. Thusthe simultaneous removal of these pollutants,when present in a water deposit, requires severalstep processes or spatial redox variation.

The above pollutants may be removed chemi-cally or biologically from a water supply. Bio-logical removal is preferable, since there is no needfor the addition of extra chemicals and the vol-ume of the regenerated sludge is appreciably

smaller and hence easier to handle. Trickling fil-ters, in contrast with activated sludge processes,provide a support medium for biofilm growth.Bacteria remain in the filter for long periods oftime, thus making high hydraulic and pollutantloadings possible. The filter medium is of greatimportance for the function of biofilters and sev-eral experiments have been carried out both tostudy and compare different filter media types[5,6]. The support materials used in trickling fil-ters are either granulated or fixed media. Among

the selection criteria for filter media in tricklingfilters are: void ratio, specific surface area, ho-mogeneous water flow and economics.

An adequate flow of air is of fundamental im-portance to the successful operation of a tricklingfilter. The principal factors responsible for air flowin an open top filter is the natural draft. The driv-ing force for airflow is the temperature differencebetween the ambient air and the air inside thepores. Thus, the use of a trickling filter has theadvantage of not requiring an external air supplyor an aeration system. If, in addition, the mean

diameter of the filter media is sufficiently small(up to 5 mm), then complete aeration and verygood filtration may be affected at the same time[7].

The aim of the present work was to study am-monia, iron and manganese removal from potablewater using pilot-scale trickling filters. The in-fluence of the specific surface area on nitrifica-tion of potable water was examined, and the va-lidity of the steady-state model of Vayenas andLyberatos [8] for various gravel sizes used wastested. Experiments were also performed in orderto investigate the contribution of biological andphysicochemical oxidation on total iron oxidation,using a pilot-scale trickling filter under variousoperating conditions. Manganese oxidation is amuch more difficult and slow process than ironand ammonia oxidation for natural pH values.Therefore, in order to increase significantly man-ganese oxidation rates or the specific surface areaof the support material, a pilot scale trickling fil-

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A.G. Tekerlekopoulou, D.V. Vayenas / Desalination 210 (2007) 225235 227

ter with gravel of small diameter was used to studymanganese removal from potable water. Finally,experiments were performed to investigate thesingle stage biological removal of ammonia-ironand ammonia-iron-manganese from potable wa-ter.

2. Materials and methods

All pilot-scale trickling filters consisted ofPlexiglas tubes, 160 cm high and 9 cm internal

diameter. The water feeding was downwards(gravity flow) and the beds were always wet butnot flooded. There were 10 sampling ports, alongthe filters depth, for ammonia-, nitrite-nitrogen,nitrate-nitrogen, iron and manganese concentra-tion measurements in the bulk liquid. The feedsolution resulted from mixing concentrated solu-tions of ammonia (in the form of NH

4Cl), iron (in

the form of FeSO47H2O) and manganese (in theform of MnSO4H2O), also containing phospho-rous, with water from the water supply networkin mixing chambers located at the top of the fil-

ters. The concentrated solutions were always keptin a refrigerator located close to the filters, in or-der to avoid undesirable bacterial growth in thefeed medium. For the start-up of the filters in-oculums were used from the wastewater treatmentplant of the city of Agrinio. An underdrain sys-tem collected the treated water and any biologi-cal solids that would detach from the media. Aera-tion was taking place through natural draft andno external mechanical aeration source was usedin the filters while pH, temperature and total am-monia, iron and manganese concentrations weremeasured on a daily basis according to the Stan-dard Methods for the Examination of Water andWastewater [9]. The pH and dissolved oxygenmeasurements were made using the Hanna pH211pH meter, and the Hanna HI9143 dissolved oxy-gen meter, respectively.

2.1. Ammonia removal

Three pilot-scale trickling filters with differ-

ent calcitic gravel sizes were used for the experi-ments. The mean diameters of the gravels were2.4 (small-sized), 3.9 (medium-sized) and 5.5 mm(large-sized), with specific surface areas of 2124,1372 and 1059 m2/m3 respectively. The depth ofthe support media was 143 cm in all cases andfilters porosity was 0.36, 0.38, 0.4, respectively.The dissolved oxygen concentration in the bulkliquid at the effluent and the interior of the trick-ling filters was always between 7.0 and 8.0 mg/l,thus suggesting no oxygen limitation [10]. Also

throughout the experiments the temperature andthe pH were fairly constant at about 20C and 7.0respectively. Experiments lasted for severalmonths, ensuring that long term processes weretaking place, that filters had enough time to ad-

just to the feed ammonia concentration and thatbiofilm thickness had reached a steady state. Be-fore sampling, the filters were operated in a con-tinuous mode and influent conditions were keptconstant for at least six hours to ensure pseudo-steady-state conditions with respect to ammo-nium-, nitrite- and nitrate-nitrogen concentrations.

2.2. Iron removal

A pilot-scale trickling filter was used for ironremoval (Fig. 1). The support material was silicicgravel with a mean diameter of 3.9 mm, and spe-cific surface area of 1372 m2/m3, while the depthof the support media was 143 cm and the filterporosity 0.38. The pilot-scale filter was kept incontinuous operation for four months to ensurethat a steady state was achieved. Throughout allexperiments, water temperature was fairly con-

stant at about 141C, while ambient tempera-ture was constant at about 20C (room tempera-ture). The pH in the liquid phasethroughout thefilter depth ranged from 7.0 to 7.5. In this rangeof pH ferrous iron was the only form of dissolvediron in the liquid phase. The precipitated form ofiron was detained on the support material of thefilter. The concentration of the dissolved oxygenin the liquid phasethroughout the filter depth wasalways between 7 and 8 mg/l. Under these oper-

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ating conditions, and since the redox potential

ranged from 0.3 to 0.4 V (as measured with a re-dox potential electrode), according to Hem [11],both physico-chemical and biological iron oxi-dation were taking place simultaneously. In orderto assess the extent of physico-chemical iron oxi-dation, the filter was disinfected using commer-cial chlorine bleach (NaOCl 5%) every 2 days,for 2 h, and then passing network water throughthe filter before starting to feed with the iron con-centrated solution. Under these conditions, onlyphysico-chemical iron oxidation was possible.

2.3. Manganese removal

A pilot-scale trickling filter was also used formanganese removal. The support material wassilicic gravel with a mean diameter of 1.9 mm,and specific surface area of 3105 m2/m3, whilethe depth of the support media was 143 cm andthe filter porosity 0.39. The pilot-scale filter plantwas kept in continuous operation for more thanten months before the experimental series took

Water supply

network

Effluent

Sampling

valve

Distributor

Drainage

system

D=3.9 mm

Fe

Refrigerator with

feed tank

Dosometric

pump

Fig. 1. Schematic drawing of the pilot-scale trickling filters arrangement.

place, in order to ensure that a steady state was

achieved. Throughout all experiments, water tem-perature was fairly constant at about 25C. ThepH in the liquid phaseranged from 7.07.3 at theinlet of the filter to, 8.08.3 at the outlet of thefilter. In this range of pH, homogenous manga-nese oxidation by oxygen is very slow. The pre-cipitated form of manganese was detained on thesupport material of the filter. The concentrationof the dissolved oxygen in the liquid phasethroughout the filters depth was always between7.0 and 8.0 mg/l. The redox potential throughoutthe filter depth ranged from 0.3 to 0.5 V (as mea-

sured with a redox potential electrode).

2.4. Simultaneous removal of iron-ammonia and

iron-ammonia-manganese

Two pilot-scale trickling filters were con-structed and tested for combined and simultaneousremoval of ammonia-iron and ammonia-iron-manganese, respectively. The support material forcombined NH3 and Fe removal was silicic gravel

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with a mean diameter of 3.9 mm. The lower partof the filter (72 cm) was filled with gravel withimmobilized ammonia oxidizing bacteria, whilethe upper part was filled with the same gravel withiron oxidizing bacteria. The pilot plant was keptin continuous operation for eight months to en-sure that steady state was achieved. The feed so-lution was the result of mixing a concentrated ironand ammonia solution with water from the watersupply network (that contained no measurable ironand ammonia amounts). The temperature of wa-

ter in the filter was maintained at 22C, while thepH was at the range of 7.58.0. The dissolvedoxygen in the liquid phase throughout the filterdepth was always between 7.0 and 8.0 mg/l.

For simultaneous ammonia, iron and manga-nese removal the filter consisted of two differentsupport materials. The upper part of the filter(70 cm) was filled with silicic gravel of mean di-ameter 3.9 mm, specific surface area (As) of1385 m2/m3 and porosity 0.38. This support ma-terial came from an already established filterwhich was responsible for ammonia and iron re-

moval from potable water, so immobilized am-monia and iron oxidizing bacteria were on thesurface of the gravel. The lower part of the filter(73 cm) was loaded with gravel of mean diameter1.9 mm, with immobilized manganese bacteria.The use of gravels with immobilized acclimatedbacteria led directly to continuous operation, thusavoiding a start-up period. The pilot-scale filterwas kept under continuous operation for sevenmonths to ensure steady-state conditions. The feedsolution was the result of mixing a concentratedsolution of ammonia, iron and manganese (withsmall quantities of nitrogen and phosphorous) andwater from the water supply network (containingnegligible ammonia, iron and manganeseamounts). The temperature of the water in the fil-ter was at 22oC, while the pH throughout the fil-ter depth was at the range of 6.87.8. The dis-solved oxygen in the liquid phase throughout thefilters depth was always between 7.0 and8.0 mg/l.

3. Results and discussion

3.1. Ammonia removal

In order to design reliable trickling filters andassess nitrification performance, appropriatemathematical models that adequately describe thekey physicochemical and biological processes arenecessary. This work is based on the steady-statemodel of Vayenas and Lyberatos [8]. This modelconsiders nitrification as a one-step process, thusresulting in analytical recursive equations. The

model also predicts the steady state profiles ofammonia and oxygen concentration as well as thebiofilm thickness along the filter depth. Based onthis model, Tekerlekopoulou and Vayenas [12]constructed the operating diagram of a nitrifyingtrickling filter, which defines the operating con-ditions for complete and safe nitrification.

In an attached growth process, the supportmaterial plays a key role on the biodegradationof the pollutants that takes place on its surface,and may affect biodegradation in several ways.The choice of the size of the support material is

of great importance. Since nitrifying bacteria areautotrophic microorganisms, their growth rate isat least one order of magnitude less than thegrowth rate of heterotrophic ones. Thus, adequatesurface per volume unit of the reactor should beprovided in order to achieve satisfactory nitrifi-cation rates. Small-sized gravel provides a highspecific surface area while at the same time thevoid space becomes smaller and pore clogging,high local flow velocities, shear stress, and masstransfer limitations become substantial. In orderto study the effect of support material three dif-ferent calcitic gravel sizes were used, namelysmall-sized, medium-sized and large-sized. Fig. 2shows experimental (symbols) ammonia concen-tration profiles along the filter depth and the cor-responding steady-state model (lines) predictionsfor the three different gravels. This Figure showsclearly the negative effect of the extreme in-crease of specific surface area on biological nitri-fication in a fixed bed filter in comparison with

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medium-sized gravel. On the other hand, the useof large-sized gravel may lead to easier substrateand oxygen transfer and avoiding pore cloggingbut it reduces the specific surface area and hencethe filters efficiency. Thus, there is an appropri-ate support material size (medium sized gravel)to achieve optimal nitrification performance.

For example, Fig. 2 shows that for mediumsized gravel about 70 cm are needed in order toreduce ammonia concentration below the maxi-mum permitted limit of 0.5 mg/l, while for thelarge sized gravel the required filter depth in-creases from 75 to 110 cm. This was expectedbecause as the specific surface area is reduced sodoes the filter efficiency. It would have been ex-pected that the use of small-sized gravel wouldhave reduced the required filter depth. Neverthe-less, experimental data did not demonstrate theexpected behavior. Small-sized gravel may pro-vide a higher specific surface area but at the same

Fig. 2. Effect of gravel size in nitrification compari-son of experimental data (symbols) and model predic-tion (lines) for feed ammonia concentration 2.0 mg N/land volumetric flow rate 1000 ml/min for the three dif-ferent support materials.

0 20 40 60 80 100 120 1400.0

0.5

1.0

1.5

2.0Large-sized gravel

Medium-sized gravel

Small-sized gravel

Q=1000 ml/min

Ammoniaconcentration(mgN/l)

Filter depth (cm)

time void space becomes smaller, resulting ineasier pore clogging.

The results of the experiments performed re-vealed that the mathematical model of Vayenasand Lyberatos [8] could predict filter performancevery well for medium-sized gravel, and in somecases (low flow rates and ammonia load) for large-sized gravel, while for the small-sized gravelmodel predictions were inaccurate. In the case oflarge-sized gravel and high ammonia load themodel seems to underestimate the filter perfor-

mance. The development of the above model wasbased on the implicit hypothesis that flow condi-tions do not affect biofilm growth and that thereare no limitations by mass transport and shearstress.

Fig. 3 presents experimental and predictedammonia concentration profiles along the filters

0 20 40 60 80 100 120 1400.0

0.5

1.0

1.5

2.0

2000 ml/min

1000 ml/min

model

experimental

Ammoniaconcentration(mgN/l)

Filter depth (cm)

Fig. 3. Comparison between experimental (symbols)ammonia concentration profiles along the filter depth andthe corresponding steady-state model predictions (lines)for volumetric flow rates Q = 1000 (circles and dottedlines) and 2000 (triangles and solid lines) ml/min, feedammonia concentration 1.0 and 2.0 mg N/l, and supportmaterial: medium-sized calcitic gravel.

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depth (with the medium-sized gravel) for variousammonia feed concentrations and volumetric flowrates. Very good agreement between experimen-tal data and model prediction was observed. Fig. 3shows that an increase of the volumetric flow ratereduces filter efficiency. Particularly, for ammo-nia feed concentration of 1.0 and 2.0 mg/l an in-crease of the volumetric flow from 1000 ml/min(226 m3/m2d) to 2000 ml/min (453 m3/m2d) re-duces filter efficiency from 100% to 81% and77.3% respectively.

3.2. Iron removal

Two series of experiments were performed inorder to investigate the effect of feed iron con-centration and the volumetric flow rate on filtersperformance for physicochemical and combinedphysicochemical and biological oxidation.

Fig. 4 shows iron concentration profiles alongthe filter depth for volumetric flow rate 1000 ml/minand iron feed concentration 1.0 and 3.0 mg/l un-der physicochemical and combined physico-

chemical and biological oxidation. The combinediron oxidation leads to better filter performanceand increases iron removal efficiency.

Fig. 5 shows a similar diagram for a highervolumetric flow rate of 2000 ml/min and iron feedconcentration 1.0 and 2.0 mg/l. We observe that,as in the previous case, the contribution of bio-logical oxidation results in increased iron oxida-tion rates. Outlet iron concentration remains be-low the upper permitted limit of 0.2 mg/l (dottedline) for both cases.

According to experimental data, the effect ofbio-oxidation improves filter efficiency by about5 to 6% for the particular experimental conditions(nutrients, pH, and temperature). Although bio-logical oxidation increases filter efficiency onlyabout 5%, the corresponding filter depth toachieve outlet iron concentration below the up-per permitted limit decreases drastically. For ex-ample, Fig. 5 shows that for inlet iron concentra-tion 1.0 mg/l, the effect of biological oxidation

0 20 40 60 80 100 120 1400.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Physicochemical and Biological oxidation

Physicochemical oxidation

Ironconcentration(mg/l)

Filter Depth (cm)

Fig. 4. Effect of physico-chemical and combined physico-chemical and biological oxidation on total iron oxidationfor volumetric flow 1000 ml/min and feed iron concen-trations 1.0 and 3.0 mg/l.

0 20 40 60 80 100 120 1400.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

Physicochemical and Biological oxidation

Physicochemical oxidation

Ironconcentration(mg/l)

Filter Depth (cm)

Fig. 5. Effect of physico-chemical and combined physico-chemical and biological oxidation on total iron oxidationfor volumetric flow 2000 ml/min and feed iron concen-trations 1.0 and 2.0 mg/l.

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increases filter efficiency by about 5% while re-duces the required depth by about 40%.

3.3. Manganese removal

The oxidation of Mn (II) to Mn (IV) by aera-tion alone is a slow process unless the pH is raisedabove neutrality [1315]. Therefore, manganesecannot be removed by simple aeration and pre-cipitation in neutral waters. It is well establishedthat microorganisms are responsible for manga-

nese oxidation in neutral environment [16,17]. Thestart up period of biological filters for the removalof manganese generally takes 36 months [18].Since biological manganese removal rate is lowerthan ammonia and iron, gravel with small meandiameter (1.9 mm) was used as support materialin order to increase the specific surface area. Fig. 6shows typical manganese concentration profilesalong the filters depth for volumetric flow rate

0 13 26 39 52 65 78 91 104 117 130 143

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

1.0 mg/l2.1 mg/l

3.2 mg/l

4.2 mg/l

5.2 mg/l

Manganeseconcentration(mg/l)

Filter depht (cm)

Fig. 6. Manganese concentrations profiles along the filter depth for volumetric flow rate 500 ml/min and feed manganeseconcentrations of about 1.0, 2.0, 3.0, 4.0 and 5.0 mg/l.

500 ml/min (113 m3/m2d) and various feed con-centration (1.05.0 mg/l). It is obvious that man-ganese concentration at the outlet of the filter isunder the maximum permitted limit of 0.05 mg/l(dotted line), even for high feed concentrations(up to 4.0 mg/l).

3.4. Simultaneous removal of iron, ammonia and

manganese

The simultaneous biological removal of thethree elements is very complicated issue mainly

due to the different redox potential values neededfor their oxidation [4]. According to the literature[4,18,19] when raw water contains ammonia, bio-logical manganese removal can only take placeafter complete nitrification because of the neces-sary evolution of the redox potential. Howeverthere are few references which prove the applica-bility of a single stage, simultaneous biologicalremoval process of ammonia, iron and manganese,

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by using pilot plants, from different ground wa-ters [2022].

A pilot-scale trickling filter was used to studythe simultaneous iron and ammonia removal us-ing medium sized diameter gravel as support ma-terial, while another one was used to study theremoval of the three pollutants (iron, ammoniaand manganese). Fig. 7 presents iron and ammo-nia concentration profiles along the filters depth,for a flow rate of 1000 ml/min. Both pollutantsare reduced below the upper permitted limits at

the upper compartments of the filter while redoxincreases from 140 to about 400 mV. The addi-tion of manganese (Fig. 8) under the same volu-metric flow rate reduced filter efficiency concern-ing ammonia and iron removal, while redox in-creased to 550 mV at the outlet of the filter. There-fore, lower feed concentrations of ammonia andiron can be treated under the presence of manga-nese. However, Fig. 8 shows that the simultaneousremoval of ammonia, iron and manganese is pos-

0 20 40 60 80 100 120 140

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

120

160

200

240

280

320

360

400

440ORPFe

NH3

Fe,NH

3

concentration(mg/l)

Filter Depth (cm)

ORP(mV)

Fig. 7. Concentrations profiles of iron and ammonia in the filter for volumetric flow rate 1000 ml/min and feed concen-trations 1.1 and 1.9 mg/l, respectively.

sible in one stage processes. Since manganese isthe rate-limiting pollutant, it determines the ap-propriate hydraulic and pollutant loading for thesimultaneous removal of ammonia, iron and man-ganese. The feed concentrations used in Fig. 8are typical for groundwater in Greece.

4. Conclusions

Biological ammonia, iron and manganese re-moval using trickling filters, offers an alternative

method to conventional water treatment plants.The main conclusions of this work are: The high retention time of biomass in the fil-

ters enables high hydraulic and pollutant load-ings.

The natural aeration of the filters minimizesthe operational cost while achieving at thesame time high removal efficiencies.

The size of the support material plays a keyrole for optimal ammonia removal rates.

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Biological oxidation of iron enhances total ironoxidation while drastically reduces the re-quired filter depth.

Simultaneous ammonia and iron removal isfeasible in one stage process even for high hy-draulic and pollutants loadings.

Even if manganese is the rate limiting pollut-ant and different redox values are needed, si-multaneous ammonia, iron and manganese re-moval is also feasible in one stage processunder satisfactory hydraulic and pollutant load-

ings.

Acknowlegments

This research was funded by the programHeraklitos of the Operational Program for Edu-cation and Initial Vocational Training of the Hel-lenic Ministry of Education under the 3rd Com-munity Support Framework and the EuropeanSocial Fund.

0 20 40 60 80 100 120

0.0

0.2

0.4

0.6

0.8

1.0

Fe

NH3

Mn

Fe,N

H3,

Mnconcentration(mg/l)

Filter Depth (cm)

150

200

250

300

350

400

450

500

550

ORP(mV)

ORP

Fig. 8. Concentrations profiles of iron, ammonia and manganese in the filter for volumetric flow rate 1000 ml/min andfeed concentrations 0.97, 0.61 and 0.45 mg/l, respectively.

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