Process Biochemistry 40

Process Biochemistry 40 (2005) 327–335

Removal of ammonia, iron and manganese from groundwaters

of northern Croatia—pilot plant studies

Tamara Štembal, Marinko Markic, Nataša Ribi´ci.´

c, Felicita Briški, Laszlo Sipos*

Faculty of Chemical Engineering and Technology, University of Zagreb, Zagreb, Croatia

Received 21 July 2003; received in revised form 19 November 2003; accepted 3 January 2004

Abstract

The removal of iron, manganese and ammonia from groundwater originating from four different locations in northern Croatia was studied.

Four pilot plants, mainly differing in their aeration systems and operation pressures, have been used. Quartz sand, coated with a naturally

formed layer of MnO2 and a biofilm containing micro-organisms, were used as filter media. The bacteria of the genus Siderocapsa, as well as

the bacteria of the genus Nitrosomonas and Nitrobacter were identified as taking part in the removal of iron and manganese, and of ammonia,

respectively. It was demonstrated that a well-established bio-filter mass from one water treatment plant is applicable in other plants. Removal

of iron, manganese and ammonia from groundwater was achieved by single-step filtration, for which an adaptation period of 3–4 weeks

was required. The filtration rates were as high as 22–24 m/h. Under optimal operating conditions, ammonium is oxidized biologically to

nitrates and no nitrites appear in the effluent. The treatability factors k and n, which characterize the processes in packed bed bioreactors,

were determined in this pilot plant study as well. They indicated that the simultaneous removal of iron, manganese and ammonium involves

processes having different mechanisms and kinetics.

© 2004 Elsevier Ltd. All rights reserved.

Keywords: Ammonium removal; Iron removal; Manganese removal; Bio-filtration; Treatability factors; Pilot plant; Groundwater

1. Introduction

The groundwater in the alluvium of the river Sava of

northern Croatia usually contains high concentrations of

iron, manganese and ammonia, and is therefore unsuitable

for use as drinking water without appropriate treatment. A

simple and widely applicable water treatment procedure is

thus needed. There are a large number of chemical water

treatment procedures available, but biological procedures,

especially in the presence of ammonia, seem to offer certain

advantages. Nevertheless, many conventional plants still apply

chemical processes and therefore produce drinking water

of poor quality [1]. Biological processes can offer viable alternatives

to chemical processes for conventional water treatment

plants. It has been observed that various conventional

iron removal plants operate satisfactorily, even though the

raw water characteristics point to slow Fe(II) oxidation rates

[2,3]. It was obvious that in addition to the chemical process,

a secondary, biological process was occurring at some iron

*

Corresponding author.

E-mail address: [email protected] (L. Sipos).

removal plants [1]. Observing such phenomena has led to the

development of biological reactors based on the principle of

bio-filtration through a submerged granular medium [4]. The

design of such plants is much simpler, compared to conventional

plants utilizing chemical techniques [1]. Comparison

of chemical and biological oxidation of iron showed that the

existence of bacteria in the filter could dramatically improve

the filter efficiency under the same operating conditions [5].

Bio-filtration allows a combination of aerobic biodegradation

and physical retention of suspended particles by filtration

through the filter bed [4,6]. This process is enabled by

the activity of micro-organisms, which represent an integral

part of the groundwater microflora. The accumulation of a

critical mass of micro-organisms, required to bring about

the desired reactions, is the key to any biological treatment

process. Cell retention is achieved by water flow through the

filter bed where the natural attachment of cells to solid sur-

faces occurs, creating a biofilm [7]. One major advantage of

natural immobilization is the fact that cells are not permanently

trapped within the filter. Thus any micro-organisms

that die will eventually be washed out, thereby maintaining

the activity of the system at a high level [8].

0032-9592/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.procbio.2004.01.006

T. Štembal et al. / Process Biochemistry 40 (2005) 327–335

The aim of the present work was to examine the basic

characteristics and kinetics, as well as to prove the applicability

of a single stage, simultaneous biological removal

process of ammonia, iron and manganese, by using pilot

plants, from several typical groundwaters of different composition

in northern Croatia. It was also investigated whether

well-established biological filter material from one plant

could be used at other plants treating water of different quality,

with the aim of reducing the start-up period of these

new bio-filters from typically months [9], to more acceptable

times.

2. Materials and methods

2.1. The groundwaters investigated

The biological removal of iron, manganese and ammonia

was studied in four typical groundwaters at different loca

tions in northern Croatia. These were Ravnik near the city

of Popova.

ca; Sunja, east from the city of Sisak; the city

of Požega; and Cerna, near the city of Vinkovci. The concentrations

of iron, manganese and ammonia in the groundwaters

of Ravnik, Sunja and Cerna exceed drinking water

standards, whereas in Požega, only manganese is in excess.

2.2. Pilot plants

Four types of pilot plants, mainly differing in their aeration

systems and operation pressures, were used. Open air

or closed aeration systems were employed in the first treatment

step, combined in the second step with bio-filtration

units, working under different pressures. The pilot plants are

schematically presented in Fig. 1.

The pilot plant in Ravnik (Fig. 1a) contained no aeration

unit in its first treatment step. The closed aerator of the existing

water treatment plant supplied the aerated water. It was

operating at a pressure of approximately 2 bars and produced

Fig. 1. Schematic representation of pilot plants in Ravnik (a), Požega (b), Sunja (c) and Cerna (d).


Table 1

The sieve analysis of the filter material

Particle size range (mm)

Passing Retained

Mass fraction in

size range (%)

1.500

1.250

1.000

0.800

0.630

1.500

1.250

1.000

0.800

0.630

8.4

38.4

26.1

24.1

2.6

0.4

water containing oxygen at a concentration 12 +

1 mg/l O2.

The aerated water was fed from an open reservoir directly

into the bio-filter of the pilot plant using a microprocessor

controlled pump with adjustable flow rate. The cylindrical,

closed bio-filter, made of stainless steel and with a diameter

and height of 40 and 350 cm, respectively, contained filter

material of uniform structure with height of 190 cm. The biologically

active filter material used in the pilot plant was

taken from the water treatment plant in Ravnik that has been

operating for 10 years. Its basic material is quartz sand, with

particle size of 0.5–2 mm, which has, during the years of

operation, become covered with characteristic brown-black

colored manganese oxide layer. The size distribution of the

granules is given in Table 1.

The pilot plant used in Požega (Fig. 1b) was similar to

that in Ravnik, apart from its aeration unit. The groundwater

was aerated by percolating through a column open to the air,

placed above the reservoir of aerated water. The column with

20 cm diameter and 1 m in height was filled with 2.5cm ×

2.5 cm Rashing rings, producing aerated water containing at

a concentration of 8±1 mg/l O2. The second treatment step,

the bio-filter, was similar to that of the pilot plant in Ravnik.

For the experiments in Sunja, a pilot plant with a closed

aeration system, directly connected to the bio-filter described

above, was employed (Fig. 1c). Since the raw groundwater

was supplied directly from the local water supply under

pressure of 2 bars, no special feed pump had to be used. The

air for aeration was supplied by a compressor and injected, at

air/water ratio 1/4, into the raw groundwater before entering

the aerator. The concentration of the oxygen in the aerated

water obtained was 14 ±

1 mg/l O2.

The pilot plant, treating groundwater from the area of

Cerna (Fig. 1d), was similar to that of Sunja, with the exception

of the pressure of operation and system of its raw

water supply. The raw water fed by a microprocessor controlled

pump was forced at constant flow rate through the

closed aeration and bio-filtration units at a pressure of 2 bars,

controlled at the outlet of the filter. This system enabled the

production of aerated water and bio-filtration with an oxygen

concentration as high as 16–17 mg/l O2.

The pilot plants were equipped with microprocessor based

control systems providing autonomous data acquisition

functions. Automatic backwash with water and compressed

air occurred once every 24 h.

2.3. Analytical methods

The general chemical parameters of the water, such as alkalinity,

total hardness, calcium, magnesium, etc. were determined

using standard analytical methods [10]. The concentrations

of iron and manganese were determined spectrophotometrically

with 1,10-phenanthroline [10] and PAN

[11], respectively. Ammonium was determined spectrophotometrically

by the method according to Wagner [12]; while

nitrites were determined by the method with sulphanilic acid

and -naphthylamine, and nitrates by dimethylphenol [13].

A field spectrophotometer (HACH, Model DR2000) was

used for the spectrophotometric measurement and field instruments

(ISKRA, Slovenia) were employed for determination

of the values of pH (Model MA 5750), the electrical

conductivity (Model MA 5950) and the oxygen concentrations

(Model MA 5485).

2.4. Microbiological analyses

Microbiological analyses of the sand particles from the

filter were made to confirm the presence of micro-organisms

immobilized on the surface capable of oxidizing iron, manganese

and ammonia. A suspension of the bacteria was obtained

by mixing 10 g of sand particles in 100 ml of sterile

demineralized water with a magnetic stirrer. Glass tubes

containing Winogradsky liquid medium for culturing iron

bacteria [14], as well as appropriate liquid medium for culturing

manganese bacteria enriched with MnCO3 [15], were

inoculated with the suspension and incubated at 17 .C. After

seven days, flocs formed in the liquid media were examined

by light microscopy. Separate analyses were run for phase

I nitrifying bacteria, i.e. those forms which oxidize ammonium

to nitrites, and for phase II nitrifying bacteria, which

oxidize nitrites to nitrates. The previously prepared bacterial

suspension was transferred into Winogradsky liquid media

for culturing nitrifying bacteria of phases I and II [14]. Four

days after the inoculation of media I and II, the presence

of nitrite and nitrate were regularly determined by chemical

assay. Nitrites were detected in liquid cultures with the addition

of dilute sulphanilic acid and -naphthylamine. Nitrates

were detected by reaction with pyrogallol [14]. A detailed

description of these culturing and identification procedures

is given elsewhere [16]. Subsequently, one loopful of bacterial

suspension from each liquid media was transferred to

the surface of an appropriate solid media, and incubated at

17 and 25 .C for iron/manganese and nitrifying bacteria, respectively.

When surface colonies appeared on the agar media,

cultures were distinguished according to their form, and

also by optical microscopy after differential staining. Iron

and manganese bacteria were stained using a method suggested

by Meyers, and nitrifying bacteria were stained using

the Gram method [14].

To quantify the number of immobilized cells present on

the sand particles, a series of tubes [10] containing liquid

media for culturing iron and nitrifying bacteria were


inoculated with previously prepared appropriate decimal dilutions

of bacterial suspensions, and incubated at 17 and

25 .C. Based on the subsequent number of positive findings

of nitrifying and iron/manganese bacteria, their most probable

number (MPN) was calculated, and expressed as MPN/g

of sand particles.

2.5. Procedures of measurements on pilot plants

The pilot plants with biologically active filter material

were kept in continuous operation, at a filtration rate

5–10 m/h, for several weeks before detailed investigations

were performed. During this accommodation period the

operation of the pilot plants was investigated once a week.

When no change in operation efficiency was noticed in two

or three subsequent controls, detailed investigations on the

plants started.

A standard procedure for scanning the state of operation

of the pilot plants under certain conditions was adopted,

which included sampling and analysis of samples taken from

different stages in the treatment process. For the experiments

in Ravnik, Sunja and Požega, eight samples were

taken throughout, including: raw water, aerated water, water

samples at five different depths in filter bed (0.25, 0.50,

0.75, 1.0 and 1.5 m), and a sample of the effluent. For

the experiments in Cerna only samples of raw water, aerated

water and a sample of the effluent were taken. To

avoid disturbing the treatment process, samples were taken

starting from the effluent and then progressively sampling

upstream.

Samples of raw water and effluent were analyzed usually

once a week for the parameters listed in Table 2. In the other

samples, only the pH, dissolved oxygen, ammonia, nitrite,

nitrate, iron and manganese were determined.

Investigations of the treatment process with pilot plants

contained experiments conducted at different filtration rates.

During the experiments in Ravnik and Sunja filtration rates

in the range of 10–22 and of 10–12 m/h were applied, respectively.

The experiments in Požega were performed at

the filtration rate in the range of 10–24 m/h, while in Cerna

constant filtration rate of 10 m/h was applied.

3. Results and discussion

The biological removal of ammonia, iron and manganese

of four different types of groundwaters of northern Croatia

was examined. The average chemical composition of these

groundwaters is presented in Table 2. The chemical composition

of groundwater in Ravnik is typical for groundwaters

of northern Croatia, and most of the pilot plant studies were

carried out there. Also, there is a water treatment plant in

Ravnik, which has been in operation for 10 years, and whose

bio-filter material was applied to described pilot plants.

The specific feature of groundwater in Sunja is its relatively

high manganese concentration (.(Mn)

=

1.06 mg/l),

beside iron and ammonia, while the groundwater of Cerna

has high ammonia concentration (.(NH4+-N)

=

2.62 mg/l),

beside iron and manganese. On the contrary, the groundwater

of Požega contains no iron and ammonia, but has a high

concentration of manganese (.(Mn)

=

1.06 mg/l Mn).

3.1. Experiments in Ravnik

The existing water treatment plant in Ravnik was originally

designed 20 years ago for the removal of iron from

groundwater using closed quartz sand filters preceding aeration.

A filtration rate of 10 m/h was sufficient at the time.

However, increasing water consumption in the last years imposed

the necessity to increase the capacity of this treatment

plant. Today, in addition to iron, the plant successfully removes

manganese and ammonia too. Thus, it was of special

importance to identify the removal processes in operation,

and to try to estimate their maximal capacity.

It was obvious that biological processes play an important

role here. It is known that during long-term plant

operation, certain micro-organisms from the groundwater

microflora will create stable biofilms on the quartz sand

Table 2

Measured groundwater parameters in Ravnik, Sunja, Požega and Cerna

Ravnik Sunja Požega Cerna Standards,

CR [17]/EEC [18]

pH 7.48 7.00 7.29 8.09

Temperature (.C) 13.4 13.8 11.2 14.6

Free CO2 (CO2, mg/l) 25 28 30 7

Alkalinity (CaCO3, mg/l) 349 146 239 368

Total hardness (CaCO3, mg/l) 273 146 249 195

Calcium hardness (CaCO3, mg/l) 186 131 224 124

Magnesium hardness (CaCO3, mg/l) 78 15 25 71

KMnO4 oxidizability (O2, mg/l) 1.90 1.69 0.91 3.60 3/5

Iron (Fe, mg/l) 2.45 1.05 Nda 0.98 0.3/0.2

Manganese (Mn, mg/l) 0.224 1.060 1.060 0.100 0.05/0.05

Ammonium (NH4+-N, mg/l) 1.01 0.30 0.02 2.62 0.1/0.5

The data represents average values of n >10 measurements with relative standard deviations <10%.

a

Not detected.


surface, and that these films play a crucial role in the removal

of iron, ammonia and manganese [3,7,8,19].Itwas

important to identify the micro-organisms responsible for

the efficient operation of the water treatment plant in Ravnik.

Bacteria that precipitate iron hydroxides and iron and manganese

oxides belong to genera Sphaerotilus, Gallionella,

Leptothrix, Crenothrix, Clonothrix and Metallogenium.It

is also known that bacteria belonging to family Siderocapsaceae

are an environmentally important group because

their cells become nuclei of metal precipitation [15,20].

In the liquid media for culturing metal-oxidizing bacteria,

a cotton-like accumulation appeared on the bottom of

the test tubes as a result of bacterial activity. On the solid

medium, bacterial colonies with the characteristic metal

shine of iron-oxidizing bacteria appeared. Colonies also

appeared on the solid medium for culturing manganese bacteria.

In both cases, microscopic examination of unstained

living bacterial cells revealed that spherical cells were

surrounded by a rust-brown capsular material, due to the

presence of iron oxides, and with an olive capsular material

due to the presence of manganese oxides. The specific morphology

of these bacterial cells, and their ability to grow on

sand particles, together with the presence of iron/manganese

oxides within the capsules suggested that these isolated

bacteria belonged to the genus Siderocapsa [15,16].

After culturing nitrifying bacteria in liquid and solid media,

a microscopic examination of both unstained and stained

bacterial cells was carried out. Observation of bacterial cells

revealed the presence of Gram negative straight-rods belonging

to genus Nitrosomonas, and presence of Gram negative

rod-shaped and pear-shaped cells belonging to genus

Nitrobacter.

According to the quantitative analysis carried out,

immobilized on 1 g of sand particles were 2.4 ×

106

cells of iron/manganese oxidizing bacteria, 5 ×

106 cells

of ammonia-oxidizing bacteria and 3.3 ×

105 cells of

nitrite-oxidizing bacteria.

All these experiments undoubtedly confirm the conclusion

that in the water treatment plant Ravnik the bacteria of

the genus Siderocapsa, as well as the bacteria of the genera

Nitrosomonas and Nitrobacter play an important role in

removal of iron and manganese, as well as, ammonia, respectively.

In the case of manganese removal, the biological processes

are confirmed by additional arguments. Namely, the

chemical oxidation of manganese is extremely slow at pH <

8.5. The fact that the average pH value of the raw water in

Ravnik is 7.48 (Table 2) supports the assumption that the

manganese is removed by biological oxidation. Due to the

increased Mn2+

concentrations in the raw water and its oxidation

to Mn4+, the oxidized manganese is deposited as

MnO2 in the form of a black precipitate coating the sand

surface. The required period of the formation of the MnO2

layer and the biofilm usually takes 1–2 months, or even

more [2,3,9,21]. It is well known that the MnO2 layer has a

catalytic effect on Mn2+

oxidation with dissolved oxygen,

accelerating the process of manganese removal [22]. Thus,

even in spite of the great importance of biological processes,

the chemical ones cannot be completely neglected. More detailed

investigations are needed to identify the extent of each

process involved in the removal mechanism of manganese.

There are many questions concerning the maximal capacity

of the bio-filters. Filtration rates as high as 50 m/h

are sometimes mentioned [3]. The water treatment plant in

Ravnik was designed for operation at 10 m/h, thus it was

not possible to perform experiments at higher filtration rates.

This problem was solved by using the pilot plant to obtain

data at different experimental conditions (Fig. 1a), such as

operation at filtration rates up to 22 m/h. The experiments

at the pilot plant enabled also access to water samples at

different filter depths, so that the scanning of the concentration

profiles of different species of interest across the filter

medium was possible.

The characteristics of filter medium were tested by placing

it in the pilot plant, at conditions corresponding to those

of the full-scale water treatment plant in Ravnik. The pilot

plant exhibited the same high-efficiency removal capability

throughout the first month of its operation, indicating that

there was no need for any accommodation period.

Fig. 2 presents typical concentration profiles of iron, manganese

and ammonium through the filter at the filtration rate

of 22 m/h. In all three cases, concentrations decreased below

the drinking water standards at the filter depth of 1 m. Concentration

profiles of nitrite and nitrate are shown in Fig. 2,

too. Nitrite concentrations show the temporary increase in

the upper part of the filter bed and then the decrease of

its concentration as a result of nitrite oxidation to nitrates.

Nitrate concentration increased along the filter depth until

it reached the constant value. It presents characteristic behavior

of the nitrogen species during nitrification process

[23–26]. This research showed that well-established biological

filters give a high quality of water even at filtration rate

higher than 22 m/h. This corresponds to literature data [3].

To examine the possibilities of using the filter media

from an existing plant in other water treatment plants

with different water quality, pilot plants were installed and

Fig. 2. Concentration profiles of iron, manganese, ammonium, nitrite,

nitrate and oxygen in the filter, pilot plant Ravnik at filtration rate 22 m/h.


investigations were carried out at three different locations:

Sunja, Požega and Cerna.

3.2. Experiments in Sunja

The pilot plant presented in Fig. 1c, and filter medium

taken from the existing water treatment plant in Ravnik

were used for the experiments in Sunja. Raw water contained,

as shown in Table 2, increased concentrations of

iron (.(Fe)

=

1.05 mg/l) and ammonium (.(NH4+-N)

=

0.30 mg/l), but unusually high concentrations of manganese

(.(Mn)

=

1.06 mg/l). During the initial phase, experiments

were conducted at a filtration rate of 5 m/h, enabling the

micro-organisms to adapt to the new conditions. However,

this may not have been necessary, since, similarly to the experiments

performed in Ravnik, no change in the removal

efficiency was noticed even after a 2-month filter run. The

filtration rate was accordingly increased to 12 m/h. Again, all

measured parameters in the effluent continuously remained

well within the requirements for drinking water standards.

Microbiological examination of the sand particles confirmed

the presence of bacteria of the genus Siderocapsa, Nitrosomonas

and Nitrobater.

The monitoring of concentration profiles showed that iron,

manganese and ammonium concentrations fell below the

drinking water standards even at filter depth <0.8 m. Fig. 3

presents such typical concentration profiles of the species.

Nitrite concentrations along the whole filter depth were negligible

and they are not presented on Fig. 3. Obviously, the

oxidation of nitrites to nitrates was much faster than the oxidation

of ammonium to nitrites. In the upper parts of the

filter bed the expected oxygen consumption and the corresponding

nitrate concentration increase is observable. However,

nitrate concentrations reach a maximum at the filter

depth of 0.8 m and then slightly decrease unexpectedly with

increasing filter depth. In the meantime oxygen concentration

maximum at the filter depth of 1 m is observed. More

detailed study would be necessary to explain these phenomena.

A possible explanation is that biofilm contains some

heterotrophic denitrifying bacteria that lead to denitrifica

tion under aerobic conditions. Or, assimilation of ammonia,

nitrite and nitrate into cell structures occurs [27].

3.3. Experiments in Požega

The local waterworks in the city of Požega is operating

with several wells with manganese concentration below

the drinking water standards. However, one of the wells in

this area, with respectable capacity, is not in use because of

its high manganese content. The average concentrations of

manganese as high as .(Mn)

=

1.06 mg/l were measured

(Table 2), without the presence of significant amounts of

iron and ammonia. Thus, a pilot plant (Fig. 1b) was applied

to investigate the applicability of biological manganese removal

process for treatment of groundwater of such a specific

composition.

The pilot biological filter for removal of iron, manganese

and ammonia, previously used in the experiments in Ravnik,

was applied in the experiments in Požega for the removal of

manganese only.

Monitoring of the pilot plant in Požega showed excellent

manganese removal efficiency during the whole period of 8

months of continuous operation (including the accommodation

time), confirming the easy adaptability of bio-filters to

groundwaters of quite different composition.

The efficiency of the bio-filter is shown in Fig. 4.Atthe

beginning of a filter run (30 min after the backwash), the

manganese concentration decreases far below the upper limit

set for drinking water standards in the upper part of the filter

bed (curve A). This characteristic manganese concentration

profile remains practically unchanged even after 10 and 21 h

of filter operation (curves B and C, respectively), showing

no tendency for filter saturation or breakthrough.

Filter run monitoring at different filtration rates (12, 18, 21

and 24 m/h) showed that, even at the highest tested filtration

rate, the water quality is satisfactory (Fig. 5). Manganese

concentration decreased under the drinking water standard

at the filter depth of less than 1 m. Slight shift of the concentration

profiles was noticed with increasing filtration rate.

Fig. 4. Concentration profiles of manganese in the filter, pilot plant Požega

Fig. 3. Concentration profiles of iron, manganese, ammonium, nitrate and at filtration rate 11 m/h: (A) 30 min after the backwashing; (B) 10 h after

oxygen in the filter, pilot plant Sunja at filtration rate 12 m/h. the backwashing; (C) 21 h after the backwashing.


Fig. 5. Concentration profiles of manganese in the filter, pilot plant Požega

at different filtration rates (12–24 m/h).

Bacteria, prevailed on the sand surface, belonged to the

genus Siderocapsa, which is responsible for manganese oxidation.

Since groundwater in Požega does not contain ammonium,

there is not substrate for nitrifying bacteria, so their

number and activity was negligible.

3.4. Experiments in Cerna

After 8 months of continuous operation, the pilot plant

in Požega was adapted and installed (Fig. 1d) for treating

groundwater of Cerna which has moderate iron (.(Fe)

=

0.98 mg/l) and manganese (.(Mn)

=

0.100 mg/l), but unusually

high ammonia (.(NH4+-N)

=

2.62 mg/l) concentrations

(Table 2).

The efficiency of the pilot plant in Cerna, regarding the

removal of iron, manganese and ammonia, during the first 3

weeks of operation is presented in Fig. 6. Evidently, iron and

manganese are removed from the groundwater efficiently,

beginning immediately after the start-up. However, ammonia

was not removed from the beginning. Since the bio-filter

used in Požega was not adapted for the removal of ammonia,

this process began only 10 days after the start-up. This

process was followed by the appearance of high nitrite concentrations

in the effluent (.(NO2--N)

=0.36 mg/l), which

slowly disappeared during the next 10 days of plant opera-

Fig. 6. Concentrations of iron, manganese, ammonium, nitrate and nitrite

during the start-up of biological filter in Cerna.

tion. Simultaneously, nitrates, with concentrations of up to

.(NO3--N)

=2.2 mg/l appeared in the effluent.

All this experiments clearly illustrate the ability and the

flexibility of the bio-filter to accommodate to new operation

conditions, demonstrating at the same time the applicability

and advantages of the simultaneous biological removal of

iron, manganese and ammonia from groundwaters of different

composition.

3.5. The kinetics of the ammonia, iron and manganese

removal

In the next stage of the study we were challenged to estimate

the optimal dimensions of a bio-filter for simultaneous

removal of iron, manganese and ammonia in a groundwater

treatment plant. The first question to be answered

is whether the processes involved are reaction limited, or

mass-transfer limited. The increasing amounts of the species

removed with increasing flow rates, estimated on the basis

of the concentration profiles for unit volume of the bio-filter,

supports the conclusion that these processes are, most probably,

mass-transfer limited. Thus, the equation for concentration

profile for mass-transfer limited reaction in packed

bed reactors [28] can be applied:

CA kca

=exp -

L

(1)

CA0 U

where CA is the concentration of the species A, CA0 the initial

concentration of the species A, kc the mass-transfer coefficient

(m/s), a the surface area of the packed bed (m2/m3),

U the flow velocity ((m3/s)/m2 =

m/s), L the filter depth

(m).

However, the mass-transfer coefficient, kc, is dependent on

the flow velocity U, as well as on the filtration medium characteristics.

Consequently, in the case of packed bed bioreactors,

such as trickling filters, bio-sand filters, etc., a more

general equation is frequently used [29]

CA k

=exp -

L

(2)

Un

CA0

where k is the treatability factor (h-1), n the factor relating

to the medium characteristics, U the flow velocity (m/h).

The factors k and n in Eq. (2) must be determined experimentally

using pilot plants. In certain conditions, the theoretical

value of n

=0.5 can be taken without significant

error.

To determine the values of the factors k and n for iron,

manganese and ammonium removal processes, the values

of the term (k/Un)of Eq. (2) were, at first, evaluated by a

least-squares fit of the straight line:

ln .

CA

CA0 .

=-

.

k

Un

.

L

(3)

Figs. 2, 3 and 5 represent several typical concentration profiles

of iron, manganese and ammonium, used for the evaluation

of the CA/CA0 and L data pairs. Only the first three


Table 3

Review of the experimental data (k/Un) as well as the values of k and n

evaluated

Flow velocity, U (m/h) Iron Manganese Ammonium

Experimental values of k/Un

(m-1)

11 5.09 4.93

12 6.53 5.06 5.51

14 5.20 3.91 6.20

18 3.71

20 3.06 3.29 4.32

21 3.14

22 2.55 2.50 3.14

24 3.30

Calculated values of k and n

k (h-1) 294

n 1.53

r2 0.998

30.2

0.74

0.823

27.9

0.65

0.592

CA/CA0 and L values of the concentration profiles, measured

under the surface of the bio-filters, were used for the

least-squares fit. No significant difference in values of k/Un

obtained at different locations, thus, at different compositions

of groundwater (Ravnik, Sunja and Požega) was noticed.

Therefore, all these values obtained are summarized

in Table 3, together with values of k and n for iron, manganese

and ammonium, evaluated by a least-squares fit of

the straight line:

k

log =

n

log U

+

log k

(4)

Un

It is evident that the kinetics of the simultaneous manganese

and ammonium removal process in bio-filters are practically

identical (nearly identical values of k and n), but differ significantly

from that of iron. The main difference is due to

their sensitivity to flow rate, as shown in Fig. 7. Generally,

the expected filter depth for a given removal efficiency (i.e.

99%), calculated using the k and n values in Eq. (2), in-

creases with increasing flow rate. However, it increases most

rapidly for iron. Consequently, while at flow velocities below

18 m/h the breakthrough of Mn is expected first, for velocities

above 18 m/h, iron is expected to breakthrough the

filter first.

Fig. 7. Expected filter depths at 99% removal efficiency of iron, manganese

and ammonium, calculated using the k and n values presented in Table 3.

These results illustrate the importance of reported kinetic

parameters, especially when evaluating the optimal dimensions

and operating conditions of bio-filters as a function of

the composition of the groundwater.

4. Conclusions

The results reported here demonstrate the application of

pilot water treatment plants for the removal of iron, manganese

and ammonia from typical groundwaters of different

composition in northern Croatia.

Quartz sand, coated with a naturally formed layer of

MnO2 and a biofilm were used as filter media. Microbial

analysis showed that bacteria present in the biofilm belonged

to the genera Siderocapsa, Nitrosomonas and Nitrobacter,

appearing with 2.4×106,5×106, and 3.3×105 MPN cells/g,

respectively.

A well-established filter mass from one water treatment

plant could be used in other plants, after an accommodation

time of 3–4 weeks, thus the usual few-month start-up period

could be avoided.

It was proved experimentally that filtration rates as high

as 22–24 m/h, may be efficiently applied for removal of

iron, manganese and ammonia from groundwater in a single

bio-filtration step.

Under proper operating conditions, and with sufficient

oxygen content in the aerated water, no nitrites appear in

the effluent, even in groundwater having high ammonium

concentrations. The presence of nitrites is detected only in

the middle part of the bio-filter, as a product of microbial

oxidation of ammonia. However, it disappears completely,

followed by the appearance of nitrates, due to the activity

of the nitrite-oxidizing bacteria.

The treatability factors, k and n, determined on the basis

of pilot plant studies, indicate that differences exist between

both the mechanisms and kinetics of the iron, manganese

and ammonium removal processes. Thus, the set of k

and n values reported, enables evaluation of the optimal dimensions

and operating conditions of bio-filters for treating

groundwater of given composition.

Acknowledgements

The authors wish to express their appreciation to Dr.

Sc. Ivana Steinberg for useful suggestions in preparation

of the manuscript and Mr. Nikša Zokic for technical assistance.

This work was supported by Ministry of Science and

Technology of the Republic of Croatia through Grant No.

0125017.

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