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Influence of carrier type on nitrification in the
moving-bed biofilm process
Article in Water Science & Technology · February 2009
DOI: 10.2166/wst.2009.037 · Source: PubMed
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2. Influence of carrier type on nitrification in the moving-bed
biofilm process
M. Levstek and I. Plazl
ABSTRACT
M. Levstek
JP CCN Domzale-Kamnik d.o.o.
(Domzale-Kamnik WWTP),
Studljanska 91,
Domzale 1230,
Slovenia
E-mail: levstek@ccn-domzale.si
I. Plazl
Department of Chemical Engineering,
University of Ljubljana,
Askerceva 5,
Ljubljana 1001,
Slovenia
E-mail: igor.plazl@fkkt.uni-lj.si
Two different types of carriers differing fundamentally in size, shape and structure were evaluated
in parallel testing for nitrification potential using the moving-bed biofilm reactor (MBBR)
technology. One of the carriers used was a cylindrical high-density polyethylene ring shaped
carrier (AnoxKaldnes, K1 carrier) and the other was a spherical polyvinyl alcohol (PVA) gel bead
shaped carrier (Kuraray, PVA-gel carrier). For each MBBR process, using artificial wastewater
under autotrophic conditions, high maximal nitrification rates at 208C were obtained. For the K1
carrier up to 27 mgNH4-N/L.h (at 37% filling fraction) was found, corresponding to 49 mgNH4-N/L.h
at the recommended maximum filling fraction of 67%. This corresponds to a nitrification area rate
of 3.5 gNH4-N/m2
.d for the K1 carrier at 208C. For the PVA-gel carrier up to 32 mgNH4-N/L.h (at
9.6% filling fraction) was found, corresponding to 50.0 mg NH4-N/L.h at the recommended
maximum filling fraction of 15%. At the recommended filling fractions, the two carriers therefore
required about the same reactor volume to reach the maximum observed nitrification rate. This
presumption allowed us to estimate the effective specific surface area for the PVA gel carrier up
to 2,500 m2
/m3
versus 1,000 m2
/m3
when only the outer surface is considered.
Key words | biocarrier, biofilm, MBBR, nitrification rate, nutrient removal, PVA-gel
INTRODUCTION
Attached-growth (biofilm) processes have demonstrated
greater efficiency and stability than suspended-growth
processes, especially at low temperatures, in the presence
of inhibitory substances and under high or variable loading
conditions. Full-scale and lab-scale applications using
different types of biocarriers for treating various kinds of
wastewater have repeatedly demonstrated enhanced per-
formances in comparison with that of the traditional
activated sludge process. In the moving-bed biofilm reactor
(MBBR) process, the carriers are suspended and moving in
the entire water volume of the reactor and retained by a
sieve placed at the reactor outlet (Ødegaard 2006).
Biomass grows attached to the surfaces of the carriers,
while excess sludge detaches from the carrier and is
separated from the water downstream of the MBBR without
any return of biomass (as in the activated sludge process) to
the bioreactor. Carriers can differ from each other in
material composition, shape, specific surface area and
treatment capabilities. In the literature, different types of
carriers for the moving bed processes are described
(Ødegaard et al. 1994; Bengtsson et al. 2008). In this
paper, a comparative study on nitrification between the
earlier, well studied K1 carrier (AnoxKaldnes, Norway)
and the less studied PVA-gel carrier (Kuraray, Japan) is
carried out.
The K1 carriers made of high-density polyethylene are
slightly lighter than water (S.G., 0.95) and shaped like a
cylinder with a cross inside and fins on the outside. Biomass
is demonstrated to grow mainly on the protected area inside
the cylinder even though some biomass also grows between
the fins on the outside (Ødegaard et al. 1994). The effective
specific surface area of the carrier is determined to be
doi: 10.2166/wst.2009.037
875 Q IWA Publishing 2009 Water Science & Technology—WST | 59.5 | 2009

3. 500 m2
/m3
and the maximum recommended filling fraction
is 67%, though up to 70% has been used. The effective
specific surface area at 67% filling fraction is 335 m2
/m3
(Ødegaard 2006). In the literature there are many reports
concerning the MBBR technology utilizing the K1 carrier
for nitrification. The rate of the nitrification process is
primarily limited by oxygen concentration, ammonium
concentration and organic loading (Ødegaard et al. 1994).
Typical maximum nitrification rates reported from full-scale
plants (10–158C) are between 0.7 and 1.2 gNOx-N/m2
.d
(Ødegaard 2006). Using the K1 carrier for treating different
kinds of wastewater has shown its advantages for upgrading
existing plants (Rusten et al. 1999). Ødegaard et al. (2000)
demonstrated that the design should be based on the
effective surface area and that the rates (per effective biofilm
area) are the same for different carrier shapes. Furthermore,
comparison of different shapes of the ring-type carriers for
nitrification has shown that shape can be a significant factor
due to variation of the effective surface area (Bengtsson et al.
2008). Rusten et al. (1995), Christensson & Welander (2004)
and Germain et al. (2007) reported that the temperature
coefficient (F) for nitrification is in the range of 1.04–1.09
(Equation 1). The relatively weak influence of the tempera-
ture on nitrification favors the biofilm process as a nitrogen
conversion method in cold climates. Parameters kT1 and kT2
represent nitrification rates at temperatures T1 and T2.
kT2 ¼ kT1ÁFðT22T1Þ
ð1Þ
The PVA-gel carriers made from polyvinyl alcohol are
slightly heavier than water (S.G., 1.025). The gel beads
consist of 4 mm diameter spheres that are hydrophilic in
nature and have a very porous structure with only 10%
solids and a continuum of passages from 10 to 20 mm in
diameter tunnelling throughout each bead (Rouse et al.
2004; Kuraray 2005). They are normally used at a filling
fraction of around 10% with a recommended maximum of
15% to ensure effective mixing in the whole reactor volume
(though an optimal maximum has not been determined).
The actual surface area of the PVA-gel beads is not known
because bacteria are cultivated inside the core of the beads
and consequently it is difficult to give the rate in terms of
biofilm area. Fluorescence In Situ Hybridization (FISH)
analyses have shown, however, that bacteria can grow well
into the core of the beads (Kuraray 2005). It is claimed that
bacteria growing inside the beads do not slough off in
clusters and are protected from predation in the micro-scale
pores of the PVA-gel. On the other hand, one might
speculate that in practice mixed heterotrophic and auto-
trophic biomass growth on the outer surface of the beads
would prevent substrate and oxygen from diffusing into the
core of the beads and thus the interior surface will not be
effective after some time of operation. The goal of this study
was to determine the effective specific surface areas of both
carriers in order to define the mass transport limitations.
The theoretical effective surface area based on the assump-
tion of the ideal sphere was compared with the experimen-
tally confirmed rough effective surface of PVA-gel beds
estimated from the nitrification process.
Results from using the PVA-gel beads as a biocarrier
were comparable to those of other carriers. The maximum
nitrification rate obtained in a pilot-plant test treating
wastewater following primary mechanical treatment was
15 mgNH4-N/L.h (Rouse et al. 2007).
THEORY
The theoretical description of biofilm systems is consider-
ably more complex than that of dispersed cultures, in part
due to the reaction processes occurring within the biofilm
region where substrate diffusion is of concern (Kornaros
et al. 2006). Biofilm thickness on the carriers depends on
organic loading, shear forces, temperature and oxygen
concentration. Biofilm thickness larger than 100 mm
(Ødegaard 2006) allows for only partial penetration of
ammonia into the biofilm. From the mathematical point of
view the biofilm in the K1 carrier is assumed to be planar
and homogeneous (Kornaros et al. 2006). The spherical
shape of the whole surface of the PVA-gel carrier alone
would lead to different assumptions of biofilm dimensions.
It could be questioned whether or not the biofilm on the
outside of the beads would allow penetration of ammonia
and oxygen into the core of the gel (further than about
100 mm) to sustain nitrifying bacteria there. For a spherical
carrier, constituent transport into the center is defined with
spherical coordinates and when biofilm thickness on the
carrier surface is low (, 100 mm), we can assume that the
876 M. Levstek and I. Plazl | Influence of carrier type on nitrification in MBBR Water Science & Technology—WST | 59.5 | 2009

4. biofilm is fully penetrated. Combining the mass balance for
each steady-state condition we can calculate the concen-
tration of the autotrophic biomass (Equation 2).:
XA ¼
FÁðSNHv 2 SNHÞ
V
Á
1
^mAÁ iNx þ 1
YA
Á
ðKNH þ SNHÞ
SNH
ð2Þ
where V is the volume of the reactor; SNHv and SNH are the
ammonium concentration in the influent and effluent,
respectively; F is the flow to the reactor; KNH is the
ammonia half-saturation coefficient for autotrophic bio-
mass (1 mg/L); iNx is the nitrogen content in the biomass;
YA is the yield of autotrophic biomass (0.24 mgCOD/mgN);
and mA is the maximum specific growth rate for autotrophic
biomass (0.79/d).
METHODS
Two separate lab-scale CSTR reactors were filled with two
different kinds of carriers and continuously aerated. One
reactor had a volume 7.3 L and was filled with 2.7 L of the
K1 carrier (37 vol%, 185 m2
/m3
) and the other had a
volume 3.54 L and was filled with 0.34 L (9.6 vol%) of the
PVA-gel bead carrier. Both carriers had previously been
enriched with heterotrophic and autotrophic biomass. The
K1 carriers were taken from an oxic reactor of an industrial-
scale (500 m3
) pilot plant used for nitrogen removal. The
PVA-gel beads were taken from an oxic reactor of a semi-
industrial-scale (200 L) pilot plant used for nitrogen
removal. Both of the source reactors had been fed for
more than one year with wastewater following the primary
mechanical stage of the Domzale-Kamnik wastewater
treatment plant. The filling ratio with both types of carriers
used in this study was lower than recommended by
manufacturers in order to achieve good mixing conditions
for proper distribution of substrates to the biofilm in the
small lab-scale reactor used in this study.
Both reactors were operated in the same manner and
continuously fed with synthetic wastewater containing only
ammonium ((NH4)2SO4), phosphate (KH2PO4) and growth
minerals (Nitritox monitor, Growth Powder, Art. 704751;
LAR Germany). The average concentrations in the syn-
thetic wastewater was 90.2 ^ 3.0 mg NH4-N/L for K1
test, 85.6 ^ 3.8 mg NH4-N/L for PVA-gel test, 0.7 ^ 0.1 mg
PO4-P/L, 8.2 ^ 0.3 mg NOx-N/L, 12.5 ^ 1.5 mgCOD/L
and some trace compounds. The nitrification processes
were automatically regulated to pH 7.5 ^ 0.1 using a buffer
solution (Na2CO3). Kindaichi et al. (2004) reported that the
autotrophic nitrifying biofilm was composed of 50%
nitrifying bacteria (ammonia-oxidizing bacteria [AOB] and
nitrite-oxidizing bacteria [NOB]) and 50% heterotrophic
bacteria using FISH analysis. Although the influent con-
sisted of only 12.5 mgCOD/L we detected some hetero-
trophic microorganisms that are still present in the
community fed only with mineral medium by the clone
library analytical method. Since both pilot plants were fed
and operated at the same process conditions it was
presumed that the heterotrophs would have the same
influence on the performance in both systems.
The adaptation period lasted for six months. During this
period of selective feeding, nitrification rate was regularly
checked and ammonium loading was increased stepwise to
maintain at least 1 mgNH4-N/L in the effluent of both
reactors. The reactors were operated at a temperature of
20 ^ 18C and oxygen was maintained at 8.0 ^ 0.5 mg/L.
The inner walls of the lab-scale reactors were cleaned
weekly to reduce bacterial wall-growth effects. Influent and
effluent samples were analyzed for ammonium (SNH),
nitrate and nitrite nitrogen (SNO) and Kjeldahl nitrogen
(SNH þ XND þ SND þ SNDI) according to ISO stan-
dards. The influent and effluent values were based on
daily spot samples. At the end of the test, a mixer was
used to remove biofilm from the carriers to analyze the
biomass composition. The COD concentration of the
biomass was 1.2 mgCOD/mgVSS and the nitrogen content
0.034 mgN/mgCOD.
At regular intervals the amount of biofilm attached to
the biocarriers was quantified so as to estimate the total
concentration of biomass in the reactor. For the K1 carriers,
this was done by determining the weight of 10 carriers after
drying at 508C. Then the biofilm was removed from the
same carriers by washing with a Cr-H2SO4 (chromic acid)
solution and then the dry weight was determined again,
from which the organic component (biomass) was deter-
mined by calculation. For the PVA-gel carriers, the biofilm
was quantified by obtaining the weight of 50 used carriers
after drying at 508C. Then the dry weight of 50 new (unused)
877 M. Levstek and I. Plazl | Influence of carrier type on nitrification in MBBR Water Science Technology—WST | 59.5 | 2009

5. carriers was also determined and the biomass component
was estimated by calculation.
RESULTS AND DISCUSSION
K1 carrier reactor
As shown in Figure 1, the nitrification process was
influenced by the ammonium loading. At the 37% volu-
metric filling fraction used for the K1 carrier, the reactor was
operated at five HRTs corresponding to ammonium load-
ings in the range of 2.1–3.8 gNH4-N/m2
.d. Nitrification was
close to complete at all these loadings. Even at the highest
loading (corresponding to the shortest HRT of 3.1 h), 93%
nitrification efficiency was achieved. In addition, at the
same HRT and 208C, the highest nitrification rate of
26.9 mgNH4-N/L.h (3.5 mgNH4-N/m2
.h) occurred. The
maximum achievable rate was not definitively demonstrated
in these experiments, though it seems that the maximum
rate was approached in the final run. The determined
nitrification rate is about twice as high as in the results of
other rates determined in our lab with real wastewater, also
utilizing K1 carriers at 208C. Ødegaard (2006) reported a
maximal nitrification rate up to 1.2 gNH4-N/m2
.d at 118C
(2.6 gNH4-N/m2
.d at 208C–Equation 1). The high rates
obtained in this study could be explained, in part, by the
constant control at ideal levels of nutrient loading, pH,
temperature and dissolved oxygen, and especially the
absence of heterotrophic activity.
The average total biomass concentration in the reactor
was 1.12 ^ 0.14 gTS/L. The biofilm was thin and was
estimated with a confocal laser microscope (Carl Zeiss
LSM510) to be 60–100 mm in thickness. The water displaced
by the K1 carrier was 0.18 m3
/m3
(at 37 vol% filling). A
volume of 100 mL can hold approximately 100 carriers.
PVA-gel beads reactor
At the 9.6% volumetric filling ratio used for the PVA-gel
beads, the reactor was operated at five different HRTs,
where even at the shortest HRT of 2.3 h, an 86.5%
nitrification rate was achieved (Figure 2). At that HRT,
the highest nitrification rate of 32.0 mgNH4-N/L.h
occurred. Treating real municipal wastewater with
competing organic loading at 158C, using the PVA-gel
carrier with a filling fraction of 15%, Rouse et al. (2007)
demonstrated a nitrification rate of 15 mgNH4-N/L.h
(0.36 kgNH4-N/m3
.d). The lower nitrification rate in that
case (compared to the present study) was probably due to
treating real wastewater at greatly variable loadings, where
Figure 1 | Time courses of the effluent components in the lab-scale test with K1 carriers.
878 M. Levstek and I. Plazl | Influence of carrier type on nitrification in MBBR Water Science Technology—WST | 59.5 | 2009

6. the biofilm in the carrier consisted of mixed populations of
heterotrophic and autotrophic microorganisms, and due to
lower temperature.
The average total biomass concentration in the reactor
with the PVA-gel carrier was 0.83 ^ 0.36 gTS/L. With
fixation (Carnoy fixative), dehydration (xylene, paraplast),
slicing and coloring (haematoxylin and eosin) of the PVA-
gel carrier, using light microscopy, a biofilm thickness of
100 mm was estimated. This finding is not in agreement with
previous observations using FISH analyses, where the
growth of microorganisms appeared to be only within the
core of the carrier beads (Kuraray 2005). The water
displaced by PVA-gel beads carriers was 0.08 m3
/m3
(at 9.6 vol% filling). A volume of 100 mL can hold approxi-
mately 2000 carriers.
Comparison between the carriers
It is difficult to compare the efficiency of the carriers
because they are used at different filling fractions and in
different reactor volumes, and also because it is claimed that
nitrification in the PVA-gel beads also takes place inside the
bead and therefore that the effective area of the beads is not
known.
Performance comparison between the uses of two
different carriers under the same process conditions
Figure 2 | Time courses of effluent components in the lab-scale reactor filled with the PVA-gel carrier.
Figure 3 | Performance comparison of the K1 carrier and the PVA-gel carrier under the same process conditions expressed in unit mgNH4-N/L.h; a) at the tested volumetric filling
fraction; b) calculated to the recommended maximum filling fraction.
879 M. Levstek and I. Plazl | Influence of carrier type on nitrification in MBBR Water Science Technology—WST | 59.5 | 2009

7. depends on the methods used for expressing the nitrifica-
tion rates. When compared on reactor volume basis (see
Figure 3a), the performance seems to be similar. However,
the volumetric comparison is not relevant because the
recommended filling fractions were not used.
An alternative is therefore to compare the two carriers
on the basis of the recommended maximum filling fractions,
i.e 67% for the K1 carrier and 15% for the PVA-gel carrier
(Figure 3b), where performance appears to be very similar.
We considered the ratio linearly up to the maximal
recommended filling fraction, according to the guidelines
of the carrier producers.
The volumetric rates in both runs were 26.9 mgN-
NH4/L.h for the K1 carrier (at 37% filling fraction) and
32.0 mgN-NH4/L.h for the PVA-gel carrier (at 9.6% filling
fraction). The different maximal nitrification rates between
both processes result from different volumetric fillings of the
carriers. At the recommended maximum filling fraction for
the two carriers, these rates would be 49 mg NH4-N/L.h
for the K1 carrier (at 67% filling fraction) and 50 mg
NH4-N/L.h for the PVA-gel carrier (at 15% filling fraction).
This means that the two carriers would give us about the
same performance in a practical situation.
This finding can now be used to analyze whether or not
nitrification takes place in the inner part of the PVA-gel
carrier and to estimate the effective specific area of this
carrier.
Estimation of the effective specific surface area of the
PVA-gel carrier
Since the maximal nitrification performances expressed in
mgNH4-N/L.h at the recommended maximal filling
fractions were about the same (Table 1) for both the K1
and the PVA-gel carriers, this also indicates that the rates
expressed in g NH4-N/m2
.d should be about the same.
Taking this presumption into account we estimated that the
effective specific surface area at the recommended volu-
metric filling (15%) for PVA gel is 380 m2
/m3
. The
theoretical specific surface area at 100% filling (per unit
volume of the carrier) calculated only from the outer surface
of the spherical PVA-gel beds is lower (1,000 m2
/m3
)
than that estimated from nitrification rate (2,534 m2
/m3
)
(Table 1). The difference between the estimated and the
Table1|Thenitrificationrates,theeffectivespecificareaforK1carrierandPVA-gelbeadsatdifferentfillingfractions
ParameterUnitK1carrier;Vreactor¼7.3LPVAgel;Vreactor¼3.54L
Fillingfractionwiththecarriersvol%100p
theo.100†
effect.
37ourpilot
plant67recommended100p
theo.100†
effect.
9,6ourpilot
plant15recommended
MaxnitrificationratemgNH4-N/L.h26.948.732.050.0
specificaream2
/m3
8405001853351,0002,534243380
MaxnitrificationrategNH4-N/m2
.d3.53.1
p
Theoreticalarea(calculationbasedonashapeofthecarrier).
†
effectivearea(calculationbasedon100%fillingwiththecarrierinthereactor).
880 M. Levstek and I. Plazl | Influence of carrier type on nitrification in MBBR Water Science Technology—WST | 59.5 | 2009

8. calculated areas suggests an area contribution in the interior
of the beads. For both carriers at the recommended filling
fractions, the effective specific surface areas (335 m2
/m3
for
K1 and 380 m2
/m3
for PVA gel) and maximal nitrification
rates are about the same, i.e. 3.5 g NH4-N/m2
.d for K1
carrier and 3.1 g NH4-N/m2
.d (Table 1).
Active fraction of nitrification biomass
Comparing the measured concentration of biomass with the
calculated active concentration according to Equation 2, it
appears (by calculation) that the nitrifying biomass in the
PVA-gel beads of 276 mgCOD/L was only 32.8% of the
measured lab value (730 mgVSS/L ¼ 843 mg COD/L).
For the K1 carrier, it appears the active component of
the nitrifying biomass was 244 mgCOD/L, 23.5% of the
measured value (790 mgVSS/L ¼ 1,039 mgCOD/L).
When we calculate the concentration of active nitrifying
biomass at the recommended filling fraction (67 vol% for
K1 carrier and 15 vol% for PVA-gel) the active concen-
tration was about the same, 435 mgCOD/L. In the PVA-gel
beads, a higher concentration of active biomass was present
per volume of the carrier (2,873 mgCOD/Lcarrier) than in
the case of K1 carrier (660 mgCOD/Lcarrier).
CONCLUSIONS
The results of parallel testing with two biofilm reactors
containing structurally different carriers, K1 carrier and
PVA-gel beads, revealed about the same maximal nitrifica-
tion, i.e. for K1 carrier up to 3.5 gNH4-N/m2
.d and for
PVA-gel beads up to 3.1 gNH4-N/m2
.d at 208C. The process
with the PVA-gel beads, however, had a lower carrier filling
ratio (9.7%) than that of the K1 (37%). The reason for this
appears to be the higher effective specific surface area of
about 2,534 m2
/m3
for PVA-gel beads versus the effective
surface area of about 500 m2
/m3
for the K1 carrier. The
theoretical effective specific surface area for PVA-gel beads,
calculated to the outer surface of the sphere (1,000 m2
/m3
)
is lower than the value (2,534 m2
/m3
) estimated from the
result of nitrification performance obtained in the exper-
iments due to the very rough surface of the outer shell of
spherical beads with thickness of around 100 mm. It was
shown that the effective specific surface area for PVA-gel
beads did not change during the experiment.
At the recommended filling ratios the concentration of
the autotrophic biomass and nitrification rate in an
attached-growth process was thus shown to be unaffected
by the type of carrier used. In the case of PVA gel, more
biomass can be grown per volumetric filling of the carrier;
furthermore, using a higher volumetric filling of the PVA-gel
carrier than 15 vol%, which is not optimal, could yield even
higher nitrification rates compared to the K1 carrier.
ACKNOWLEDGEMENTS
The authors wish to thank the Domzale-Kamnik WWTP for
the financial support and help with the pilot plant
operation. The authors also wish to thank the Kuraray
Company, Japan and AnoxKaldnes, Norway for their
constructive suggestions and comments.
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