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The influence of porous structure and biofilm on the hydrodynamicsof two types of trickle filters

Pang Haoran, Letisse Valérie, Paul Etienne, Hébrard Gilles ⇑

Université de Toulouse, INSA, UPS, INP, Laboratoire d’Ingénierie des Systèmes Biologiques et des Procédés (LISBP), 135 avenue de Rangueil, 31077 Toulouse, France

INRA, UMRA792, LISBP, 31400 Toulouse, France

CNRS, UMR5504, LISBP, 31400 Toulouse, France

h i g h l i g h t s

Concrete-Block media with/without biofilm in MSB.

 Majority of liquid held by particles with biofilm.

  Biofilm reduces dynamic fractions and delays liquid release.

  Biofilm lengthens liquid residence time and increases active liquid volume in filter.

  Biofilm improves liquid mixing in filter and reduces liquid film thickness.

a r t i c l e i n f o

 Article history:

Received 10 March 2013

Received in revised form 25 June 2013

Accepted 28 June 2013

Available online 5 July 2013

Keywords:

Hydrodynamic

Liquid holdup

Porous structure

Liquid film, RTD

a b s t r a c t

The hydrodynamic behaviors of a Multi-Section Bioreactor (MSB) and a Trickle Fixed-bed Column (TFC)

packed with porous concrete block medium were investigated in terms of liquid holdup and RTD mod-

eling in regimes with and without biofilm. It was found liquid static holdup was always higher than

the dynamic holdup due to the porous structure of medium, rather than the configuration of bioreactor.

Dynamic holdup increased when the hydraulic load increased. The presence of biofilm retarded the liquid

release through the bioreactor; however, it enhanced the mass diffusion between static and dynamicfractions. The RTD measurements and modeling showed that the presence of biofilm lengthened the

liquid residence time, and it promoted the mass diffusion between the liquid and biofilm. Liquid film

thickness estimation implied that the biofilm reduced the liquid film thickness resulting both from the

increase of surface area and reduction of dynamic holdup volume.

  2013 Elsevier B.V. All rights reserved.

1. Introduction

Trickle Filter (TF) technology is widely used in small wastewa-

ter treatment plants because it is robust and relatively uncompli-

cated with low specific energy consumption. In a TF, complex

mechanisms take place which are highly dependent on hydraulics.

To improve and operate such TF technology, it is indispensible to

understand the physical parameters which affect the hydraulics

in order to take accurately these effects into account through soft-

ware modeling such as Hydromantis GPS-X. It is important for

example, to assess the contact time between the polluted stream

and the biofilm responsible for pollutant biodegradation; this

contact time is directly dependent on liquid holdup and liquid film

thickness.

With conventional TF, the design and reliable scale-up are

highly dependent on hydrodynamic performance and on the trans-

port phenomena imposed by the type of internal packing and its

geometry [1]. The hydrodynamic behavior of certain types of med-

ium has been widely investigated by other researchers [2–7]. How-

ever, a number of natural and man-made non-spherical porous

medium with biofilm are too complicated to be well investigated

even appropriate boundary conditions were settled in some stud-

ies [8]. The effect of biofilm on hydrodynamics in a TF has seldom

been reported [5,9]; most reports have assumed the biofilm to be

homogeneous along the packing bed, but even so, they have not

quantified the biofilm.

Usually, total liquid holdup can be divided into two aspects:

static and dynamic [10,11]. Static holdup means the fraction of li-

quid that is retained within and around the particles by capillary

forces after complete draining. It depends on the flow rate and also

1385-8947/$ - see front matter     2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2013.06.115

⇑ Corresponding author at: Université de Toulouse, INSA, UPS, INP, Laboratoire

d’Ingénierie des Systèmes Biologiques et des Procédés (LISBP), 135 avenue de

Rangueil, 31077 Toulouse, France. Tel: +33 (0)5 61 55 97 89; fax: +33 (0)5 61 55 97

60.

E-mail address:  Gilles.Hebrard@insa-toulouse.fr (H. Gilles).

Chemical Engineering Journal 231 (2013) 163–171

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal

j o u r n a l h o m e p a g e :   w w w . e l s e v i e r . c o m / l o c a t e / c e j

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on the type of medium. Dynamic holdup is formed by the flowing

liquid and is highly dependent on the liquid load. For some types of 

medium, e.g. structured and non-porous medium   [1], dynamic

holdup can be considered as the predominant part of the total li-

quid holdup  [2,12]. However, due to the physical characteristics

of the porous packing employed in our study, the static holdup

fraction may have contributed more to the total liquid holdup than

the dynamic.With the ultimate aim of optimizing TF design and operation,

the main objective of this study was to characterize the hydrody-

namic behavior of two types of TF filled with the same porous

medium in order to assess the impact of the properties of the med-

ium on the overall hydrodynamics. Such impact was measured in

term of liquid holdup fractions, liquid film thickness under the re-

gimes with and without biofilm all along the column after two or-

ganic loading rates cultivation. Further objective was using RTD

experiments and modeling to investigate the changes in liquid flow

pattern and residence time due to the presence of biofilm in the

MSB. Additionally, the study attempted to verify whether the con-

figuration of the bioreactor (TFC or MSB) would affect its hydrody-

namic characteristics.

2. Material and methods

 2.1. Experimental system and medium

The experimental system was set up with two types of Trickle

Filter, a Trickle Fixed-bed Column (TFC) (Fig. 1a) and a Multi-sec-

tion Bioreactor (MSB) (Fig. 1b) linked with a balance or a conduc-

tometer connected with a data acquisition system.

The TFC had a diameter of 0.4 m and a height of 2 m and was

enclosed in a structure made of PVC. It was packed with Concrete

Block particles to a height of 1.9 m. The MSB was composed of 5

uniform open-structured frames (like baskets) stacked vertically

whose individual height and diameter were 0.4 m and 0.2 m,

respectively. The total pilot height was 2.4 m while the packingbed was 2 m high with an interval of 0.1 m between any two

consecutive sections. The geometrical characteristics of the two

bioreactors are shown in Table 1.

Non-spherical pieces of Concrete Block were used for the solid

phase. They were made up of fly ash, gypsum, and cement. A typ-

ical Concrete Block particle without biofilm is shown in the middle

of   Fig. 1  and to the right; two colonised particles obtained from

two organic regimes are shown. The physical characteristics of 

the particles and the packing bed, determined in earlier studies,are presented in Table 2.

The MSB, filled with particles of the Concrete Block medium,

was fed with the primary sludge from a WWTP-GINETOUS, after

sifting through a 1 mm-mesh sieve and mixing with a certain vol-

ume of tap water to fulfil the influent requirements for two organic

loads. The compositions of these two feed wastewaters are shown

in Table 3.

The concentration values of constituent pollutants were ascer-

tained from the mean measured values. The pilot-scale MSB was

fed with two organic loads for 120 days for each, at the same flow

rate of 0.08 m3/d. (In the following contents, the regimes with bio-

film 1 and biofilm 2 represent the regime with lower and high cul-

tivated organic loads, respectively).

 2.2. Methods

 2.2.1. Static holdup measurements

Liquid static holdup without biofilm was determined experi-

mentally by a weighing method as reported in other study   [13].

The difference between the mass of the moist packing after drain-

age and the mass of the dry packing is the static holdup mass

which represents the liquid captured in the particles.

The method for characterizing the static holdup when biofilm

was present in the MSB was slightly different from the method de-

scribed; to avoid the destruction of the biofilm, the coated particles

cannot be moved from their original positions and the biofilm

should not be completely dehydrated; therefore a constant feed

flow rate of 0.0046 m3

/d was applied to maintain bioactivity.Thanks to the easy sampling of each basket containing the biofilm

Nomenclature

hLt    total liquid holdup (m3)hLS    liquid static holdup (m3)hLd   liquid dynamic holdup (m3)ucb   apparent packing-bed void fraction (–)e   total packing-bed void fraction (–)

e0 particle porosity (–)d p   equivalent sphere diameter (cm)U   sphericity of particle (–)r2 variance of calculated RTD from experimental RTD (–)qL   liquid density (kg m3)q particle   particle density (kg m3)dL   liquid film thickness (mm)bd   liquid dynamic retention (dynamic volume/pure solid

volume) (–)bs   liquid static retention (static volume/pure solid volume)

(–)bt    total liquid retention (total liquid volume/pure solid

volume) (–)s   theoretical liquid residence time (s)h   dimensionless time (–)

hcb   height of the packing bed (m)F ad   liquid adsorption fraction of the medium (–)TFC    trickling fixed-bed columnMSB   Multi-section bioreactor

Q    volumetric flow rate (m3 h1)mDP    total dry packing mass (kg)mLS    liquid static holdup mass (kg)m fd   fast dynamic holdup mass (m3)msd   slow dynamic holdup mass (m3)

V Lt    total liquid holdup volume (m3)V LS    liquid static holdup volume (m3)V Ld   liquid dynamic holdup volume (m3)V solid   pure solid volume (m3)V  p,L   liquid volume around single particle (m3)N  particles   number of particles (–)SA total surface area of packing bed (m2)SAeff    effective surface area of packing bed (m2)S.H.L surface hydraulic loads (m h1)m   fraction of active zone in packing bed (–) f W    wetting fraction of the packing bed (–) f LSE    fraction of partial static holdup volume of tracer ex-

change (%)PF    plug flowCSTR   continuous stirred-tank reactor

LRT    liquid residence time (s)

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support, the mass of each section packed with particles coated by

moist biofilm at steady-state (no free water draining out but still

moist) was measured. The pilot was then fed at a defined higher

flow rates (0.0091, 0.0182, 0.0137 or 0.0228 m3/h) for 1hr, and

then stopped; the mass of each section packed with moist particlesand biofilm was weighed again when no further draining water

was observed. The difference between the two masses obtained

gave the liquid static holdup mass captured in both the particles

and biofilm.

The liquid static holdup volume   V LS , solid volume   V solid, total

packing volume   V  packing , the porosity of particle  e’ , the total void

fraction of packing bed  e , and the volume of column  V column  were

then taken into account to calculate the liquid static retention  bS ,

in accordance with [9]:

bS  ¼  V LS V solid

¼  V LS 

V  packing ð1 e0Þ ¼

  V LS V columnð1 eÞ

  ð1Þ

The water adsorption fraction of the medium,  F ad, can then be

calculated as the ratio of liquid static holdup mass   mLS   and drypacking mass mDP  in the following equation:

Fad  ¼ mLS 

mDP 

¼  qL

q particle  V LS V  packing 

¼ qLð1 e0Þ

q particle bS    ð2Þ

 2.2.2. Dynamic holdup measurements

The drainage method reported by Brunazzi and Viva   [13] wasadopted in our study for measuring the liquid dynamic holdup.

The procedure was as follows: the column was first stabilized by

feeding with a high hydraulic load for several minutes; after stabil-

ization at the required flow rate (constant flow measured at the

outlet), the inlet was shut and the liquid held by the packing began

to drain down into the collector placed on the scale. The real-time

drained mass of liquid collected was measured and recorded by the

acquisition system. When steady state drainage was reached (con-

stant outlet values observed), measurement was stopped.

The dynamic retention   bd   was calculated from the following

equation:

bd ¼  V Ld

V solid¼

  V Ld

V columnð1 eÞ

  ð3Þ

Fig. 1.  Diagram of the TFC (a) and the MSB (b).

 Table 2

Physical properties of Concrete Block medium and packing bed.

Particle porosity

(%)

Material density

(kg/m3)

Specific surface area

(m2/m3)

Particle density

(kg/m3)

Particle equivalent

diameter (cm)

Particle

sphericity

Total packing bed void

fraction

e0 q powder    SSA   q particle   d p   U   e61 1337 350 ± 70 586.4 2.17 ± 0.4 0.74 ± 0.07 79.1/77.4a

a For the MSB packing bed void fraction calculation, the interval void spaces between two adjacent sections were included.

 Table 1

Geometrical characteristics of the TFC and the MSB.

Bioreactor Column height (m) Diameter (m) Packing depth (m) Distributor height (m) Single section height (m) Injection spot height (m)

TFC 4 0.4 1.9 2 – 2.5

MSB 2.4 0.2 2 2.5 0.4 2.5

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where  V Ld   represents the liquid dynamic holdup volume and  V solid

the pure solid volume.

 2.2.3. Residence time distribution (RTD)

Residence time distribution experiments were carried out by

the tracer method reported in [14]. After stabilization of the flow

in the column, a salt tracer (30 ml 300 g/L aqueous solution of 

sodium chloride) was injected using a syringe from the injection

spot. The conductivity of the liquid with tracer leaving the column

was measured via a flow-through probe (XE100, RADIOMETER 

ANALYTICAL S.A.) connected to a conductometer (CDM210 Con-

ductivity Meter, RADIOMETER ANALYTICAL) and the real-time con-

ductivity was monitored by the data acquisition system in the

computer. After each experiment, rinsing was done to remove

any residual salt absorbed of the particles.

The dimensionless   E   (h) as the function of the dimensionless

time   h  derives from Eq.   (4)  and the dimensionless time   h   in Eq.

(5). The mean liquid residence time  l  derives from the following

equation:

E ðhÞ ¼ C ðt Þ

C 0ð4Þ

h ¼ t 

s  ð5Þ

l ¼

PtC ðt ÞP

C ðt Þ  ð6Þ

where   C 0 = mtracer /V liquid  gives the tracer concentration obtained if 

all the injected mass was diluted into the liquid volume;   C   (t ) is

the tracer mass held by liquid at measuring time  t ;  l  is the mean

Liquid Residence Time (LRT) and the theoretical liquid residence

time s = V liquid/Q ;  Q  is the flow rate of liquid;

 2.2.4. Liquid film thickness determination

The equivalent total solid volume calculated from the equiva-

lent diameter of spherical particles (2.17 cm) and their numberof particles (8350 pieces) is about 0.045 m3, which was close to

that of real packing bed volume (0.044 m 3) estimated from solid

mass (25.63 kg) introduced into the column and the solid density

(586.4 kg/m3) reported in Table 2. So, we assume the particles were

spherical, the thickness of the liquid layer was calculated by the

volume of dynamic liquid flowing over the wetted surface around

the particles (with or without biofilm). Assumptions for the calcu-

lation are schematized in Fig. 2.

Assuming a homogeneous distribution of biofilm in the packing

bed and a uniform coating on all particle surfaces, particles coated

with biofilm were also assumed spherical. The volume of liquid V  p,L

around a single particle can be calculated from the following

equation:

V  p;L  ¼ pðd p þ 2L f  þ 2dLÞ3 pðd p þ 2L f  Þ3

6   4pðd p=2 þ L f Þ

2dL   ð7Þ

where dL is the liquid film thickness;  L f , the mean biofilm thickness

(equal to 0 when no biofilm is present).

For the entire packing bed, the liquid dynamic volume derives

from Eq. (8). In such a case, the effective wetting surface area of 

the packing bed  SAeff  was taken into account:

V Ld  ¼  N  particles  V  p:L  ¼  N  particlesdL  SAeff   ¼ N  particlesdL   SA  f w   ð8Þ

where N  particles  is the estimated number of particles in the packing

bed; SA  represents the total surface area of packing bed;  f w denotes

the wetting factor.

The liquid film thickness was thereby calculated with the fol-

lowing equation:

dL  ¼  V Ld

N  particlesSA  f w¼

  V Ld

N  particles4pðd p=2 þ L f Þ2 f w

ð9Þ

3. Results and discussion

In order to study the hydrodynamic behavior of a pilot-scale

MSB packed with a Concrete Block medium and also the influence

of biofilm on changes in such hydrodynamic behavior, the static

and dynamic holdup were investigated by conventional methods

while RTD experiments were carried out to determine liquid film

thickness, the liquid profile and liquid residence time. A compari-

son was made with the TFC, mainly using regimes without biofilm,

to investigate the effect of filter configuration on the hydrody-

namic behavior.

 3.1. Static holdup

 3.1.1. Without biofilm

During the static holdup experiments without biofilm, it was

found that a constant water adsorption fraction   F ad   approximate

56% ± 6% at steady-state, which implies this medium, has a high

water adsorption capacity. The static retention of the two bioreac-

tors, based on the water adsorption fraction  F ad  and experimental

measurements, are shown in Table 4.

Whatever the reactor type, it was observed that no significant

differences existed in static retention values at steady-state, show-

ing that the impact of the interval and the open structure of MSB

on the static holdup can be disregarded.

Additionally, the static holdup volume V LS  was found to increase

linearly when the number of particles  N  particles  increased. This lin-

ear relation between static holdup volume and number of particles

implies a possible extrapolation of the number of particles to liquid

film thickness calculations based on Eq.  (9).

The results found accord well with what is generally reported,

i.e. the liquid static holdup depends much more on the properties

of a medium and on the porosity created than on a reactor’sconfiguration.

 3.1.2. With biofilm

The liquid static retention of both the entire pilot set-up and of 

each individual section (each basket) was calculated after long-

term biofilm development. During the biofilm development, bio-

film thickness varied along the packing bed depth and along each

section. Even on the same horizontal stratum, the distribution of 

biofilm thickness was not homogeneous. Owing to the lack of en-

ough accurate information from biofilm thickness distribution,

the values of mean biofilm thickness adopted in our study were

calculated for each section as the ratio of the volume of total dry

biomass per corresponding particle surface area. The assumption

underlying this calculation is of a homogeneous biofilm thicknessand a uniform coating of the particles by the biofilm. The liquid

 Table 3

Composition of feed wastewater for two cultivated organic loads.

Composition Concentration values Unit

Lower load High load

TSS 318 ± 95 1006 ± 115 g/m3

VSS 170 ± 64 606 ± 77 g/m3

CODt 298 ± 71 1016 ± 164 gCOD/m3

TKN 46 ± 5 161 ± 14 gN/m3

TN 46 ± 5 161 ± 14 gN/m3

Ammonia 30 ± 3 100 ± 11 gN/m3

Nitrite & nitrate 0 0 gN/m3

Organic loads 0.76 2.55 kgCOD/m2d

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static retention fractions in each section and in the entire pilot set-

up are plotted in Fig. 3 against the surface hydraulic loads.

Fig. 3 shows that the liquid static retention increased when the

hydraulic loads rose (0.29 to 0.73 m/h) in each section and in the

entire pilot set-up (from 34% to 83% in regime 1, from 24% to

79% in regime 2). The liquid static retention observed in each sec-tion of regime 1 increased when biofilm thickness increased (from

120 lm to 181 lm) which could result from lower packing-bed

voids (from 62% to 43%, a calculation based on the study by Robin

and Alfred, [15]). Throughout the whole depth of the pilot set-up,

from the top down to the lower sections, the biofilm thickness

gradually decreased. However, static retention did not decrease

in exact correspondence with the decline in thickness, i.e. when

under greater loads: the static holdup of 2nd section with a biofilm

thickness of 282 lm was lower than that of Sections 3 and 4 whose

biofilm thicknesses were 243 and 219 lm, respectively. This im-

plies that biofilm porosity and variations in thickness should be ta-

ken into account simultaneously. The trend as described for regime

1 is the same as that for regime 2. The decline of static retention

when biofilm thickness increased may be due to the reduction of biofilm porosity. Where the biofilm is thick (e.g. 294 lm in the

1st section under greater loads), it acts like a barrier to the internal

pores of the particle, limiting the outside/inside water exchange. At

a lower hydraulic load (0.29 m/h), liquid was adsorbed mainly into

the pores of the thick biofilm and thus captured; in contrast, at a

higher load (0.73 m/h), the liquid could penetrate the biofilm and

be partially adsorbed by the particles, thus leading to greater static

retention. Thanks to lower biofilm density and to a higher flow

rate, thinner biofilm (e.g. 135 lm in 3rd section) could also pro-

duce greater static retention (70%) than thick biofilm in the same

section (58% with biofilm thickness of 243 lm in 3rd section).

 3.2. Dynamic holdup

The dynamic holdup experiments in two bioreactors without

biomass and under different hydraulic loads were first carried

out using the draining method. On the basis of the real-time liquid

draining mass curve during the dynamic experiments, two por-

tions of dynamic holdup were distinguished: a linear segment, rep-

resenting the fast dynamic fraction, and the coherent curve

segment, representing the slow dynamic fraction.

 3.2.1. Without biofilm

A real-time drainage process in the TFC is plotted in  Fig. 4 for a

flow rate of 0.3 m3/h.

From Fig. 4, three standard stages of drainage can be identified:

Stage 1: stabilization of the flow; Stage 2: unbound liquid aroundthe particles’ surface leaves the packing bed; Stage 3: liquid

embedded inside the particles’ pores begins to leave the packing

bed. Because of the small pore size of the solid, it was assumed that

the majority of the fast dynamic holdup corresponded to the exter-

nal liquid film surrounding the surface of the particles (fast liquid

dynamic mass = 10.5 kg). The slow dynamic fraction corresponded

to a retained water fraction (slow liquid dynamic mass = 4.6 kg).

Repeated measurements showed the ratio between fast and slow

dynamic mass as approximately 2 at the volumetric flow rate of 

0.3 m3/h. Similarly, these calculations of fast and slow dynamic

holdup mass were thereafter carried out for MSB. However, the ra-

tio between the fast and slow dynamic holdup in the MSB was

found to have increased (from 0.6 to 1.8) when the flow rate rose,

because of the relatively stable water adsorption capacity of themedium. The fast dynamic increased gradually when the flow rate

rose, though the slow dynamic holdup remains constant.

The dynamic and static retention performances in both bioreac-

tors without biofilm are plotted in   Fig. 5   against the surface

hydraulic load.

Fig. 5 shows that the static retention values for both bioreactors

without biofilm were close at approximately 84%, even though the

Fig. 2.   Diagram of liquid layer and contact surface.

 Table 4

Static retention measurements and  F ad   of two reactors.

Reactor   mDP  (kg)   mLS  calculated (kg)   mLS  measured (kg)   V LS  (m3)   e  (%)   bS  calculated (%)   bS  measured (%)

TFC 75 42 41.8–42.2 0.042 79.1 84 84–85

MSB 25 14 13.8–14.1 0.014 77.4 84 83–84

0%

20%

40%

60%

80%

100%

120%

0 0.2 0.4 0.6 0.8

Surface hydraulic load (m/h)

   L   i  q  u   i   d  s   t  a   t   i  c  r  e   t  e  n   t   i  o  n   (   %   )

βs of entire pilot Lf=144 mmβs of section 1 Lf=181 mmβs of section 2 Lf=143 mmβs of section 3 Lf=135 mmβs of section 4 Lf=124 mmβs of section 5 Lf=120 mm

(a) low OLR cultivation

0%

20%

40%

60%

80%

100%

120%

0 0.2 0.4 0.6 0.8

Surface hydraulic load (m/h)

   L   i  q  u   i   d  s   t  a

   t   i  c  r  e   t  e  n   t   i  o  n   (   %   )

βs of entire pilot Lf=248 mmβs of section 1 Lf=294 mmβs of section 2 Lf=282 mmβs of section 3 Lf=243 mmβs of section 4 Lf=219 mmβs of section 5 Lf=202 mm

(b) high OLR cultivation

Fig. 3.  Liquid static retention of each section (Section 1 was located at the top) and

the entire pilot set-up under different hydraulic conditions.

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hydraulic loads used for the TFC was higher. Just as the discussion

from Table 4, it implies this type of media has constant adsorptive

capacity of liquid when no biofilm was present. In contrast, in thecase of MSB the dynamic retention increased (from 9% to 25%)

when the hydraulic loads rose (from 0.14 to 0.87 m/h). It indicates

that the dynamic holdup will increase when increasing the flow

rate. Even if the experiments in the TFC were carried out at a great-

er hydraulic load (2.39 m/h), its dynamic retention (25.6%) was no

much higher than the best recorded for the MSB (25%). This re-

sulted from the different mass of packing applied into the column;

however, the net volume of liquid dynamic holdup was higher than

that in the MSB.

 3.2.2. With biofilm

In the MSB, the liquid retention experiments were carried out

after biofilm development on the medium at the same flow rates

as those applied when no biofilm was present. The liquid retentionat regimes with biofilm in each section and in the entire pilot set-

up is presented in Fig. 6 in relation to the surface hydraulic loads.

From Fig. 6, it can be seen that the dynamic retention varied lit-

tle (from 5% to 10% for the entire pilot set-up) when the flow rates

rose compared to the static retention which significantly increased

(from 24% to 83%) with higher hydraulic loads (see  Fig. 3). Those

results mean that the majority of the liquid was held by the parti-

cles coated by biofilm. It was also observed that under various

operating conditions in each section, the liquid dynamic retention

fractions stayed lower, ranging between 5% and 15%. The maxima

on liquid dynamic retention observable in Fig. 6, at hydraulic loads

slightly higher than 0.40 m/h, is difficult to justify by an error mea-

surement as mass balance and timer accuracies are very good. So

we believe that it could be due to an accumulation of water inthe 2nd section, due to a thicker biofilm presence compare to other

sections where the biofilm thickness was lower. This accumulation

could justify for this condition the large dynamic liquid volume

collected on the balance after the stop time.

We also observed biofilm detachments that occurred in the 1st

and 2nd section at higher than 0.40 m/h that could sustain thisassumption.

In the regimes without biofilm, in contrast (see Fig. 5), the dy-

namic retention increased slightly (from 9% to 25%) when the flow

rate rose and the static retention kept a constant at about 84%.

It was found that the majority of the liquid was held inside the

pores of the medium’s particles coated by biofilm. The dynamic

retention in regimes with biofilm was lower than in the regimes

without biofilm. The high water absorption efficiency shown by

our porous packing was reduced by the biofilm layer that retarded

water movement from the inner to the outer parts of the particles.

However, at steady state, it was considered that the external liquid

volume (dynamic holdup) would be sufficient to ensure the requi-

site nutriment mass transport from the waste liquid to the biofilm.

Liquid film thickness with and without biofilm will be studied fur-ther later in this paper.

 3.3. Residence time distribution (RTD)

Analyzing the real RTD curves enables us to identify the varia-

tion of flow pattern in the column and also the liquid fraction in-

volved in the tracing. Furthermore, the shape of the RTD curves

and, consequently, the distribution of liquid in the packing are re-

lated to the configuration of the packing. In order to acquire the

dimensionless function E  (h), the trends of the conductivity versus

time were first established. The experimental real-time conductiv-

ity versus measuring time in the MSB without biofilm at 4 different

surface hydraulic loads is plotted in Fig. 7.

Fig. 7 shows that increasing the hydraulic load led to taller, nar-rower curves and thus smaller distribution of liquid residence

0

3

6

9

12

15

18

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Time (h)

     D    r    a     i    n     i    n    g     l     i    q    u     i     d    m    a    s    s

     (     k    g     )

flowrate=0.3m3

/h in TFC

End of drainage

Slow dynamic

Fast dynamic

Stop the influent

Stage 32

stage stage

1

Fig. 4.   Dynamic drainage process in the TFC at a flow rate of 0.3 m3/h.

0%

3%

6%

9%

12%

15%

0 0.2 0.4 0.6 0.8

Surface hydraulic load (m/h)

     L     i    q    u     i     d     d    y

    n    a    m     i    c    r    e     t    e    n     t     i    o    n     (     %     )   βd of entire pilot

βd of section 1βd of section 2βd of section 3βd of section 4βd of section 5

(a) low OLR cultivation

0%

5%

10%

15%

20%

25%

0 0.2 0.4 0.6 0.8

Surface hydraulic load (m/h)

     L     i    q    u     i     d     d    y

    n    a    m     i    c    r    e     t    e    n     t     i    o    n     (     %     )   βd of entire pilot

βd of section 1βd of section 2βd of section 3βd of section 4βd of section 5

(b) high OLR cultivation

Fig. 6.   Liquid dynamic retention in relation to the surface hydraulic load with

biofilm.

0%

20%

40%

60%

80%

100%

0 0.5 1 1.5 2 2.5

Surface hydraulic load (m/h)

     L     i    q    u     i     d    r    e     t    e    n

     t     i    o    n     (     %     )

static retention in MSB static retention in TFC

dynamic retention in MSB dynamic retention in TFC

Fig. 5.   Liquid retention in relation to surface hydraulic load in two bioreactors

without biofilm.

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time, the higher peaks implying less tracer adsorption by the sol-ids. However, the asymmetric behavior of the ‘‘tail’’ in the curves

may be due to the presence of stagnant zones. If the time   t  peak

(when the conductivity is at the maximum) is assumed as the the-

oretical liquid residence time stheo, the liquid holdup volumes V liquid

can be calculated as the  t  peak  multiplied by the flow rate Q . The li-

quid holdup represented by the RTD curve should generally corre-

spond to the dynamic holdup [16]. However, Viva and Brunazzi, [1]

reported that the liquid holdup in the RTD curves of catalytic struc-

tured packing corresponded to the total liquid holdup (the dy-

namic holdup plus the static holdup) determined by the drainage

method. In our study, the results of  V liquid can be compared with

the dynamic holdup volume  V Ld  and the total holdup volume  V Lt 

obtained from the drainage method, in order to determine which

volume represents the liquid holdup volume in the RTD curves.

Additionally, the fractions of static holdup volume   f LSE   that ex-

changes the tracer with dynamic holdup volume were determined

by the partial static holdup volume involved in V liquid over total sta-

tic holdup volume V LS . The results are shown in  Table 5.

In Table 5, calculated values of  V liquid  are basically close to the

values of  V Ld  except when biofilm is present, the measured values

of  V Ld being thus much smaller than the calculated  V liquid. Thus the

biofilm acts as a sponge that sustains and partially releases static

holdup liquid. The fraction of exchange tracing static holdup vol-

ume f LSE  decreased (e.g. 36% to 14% of regime 1) when the flow rate

rose; however, these fractions increased with biofilm development

as from regime 1 to regime 2 (e.g. from 36% to 46% at a flow rate of 

0.0091 m3/h and similarly under other flow rates). The decrease of 

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0 2000 4000 6000 8000 10000 12000 14000

Time (s)

    c    o    n     d    u

    c     t     i    v     i     t    y     (    m    s     /    c    m     )

SHL=0.29 m/h

SHL=0.44 m/h

SHL=0.58 m/h

SHL=0.73 m/h   tpeak=640 s;

tpeak=780 s;

tpeak=960 s;

tpeak=1260 s;

Fig. 7.   Conductivity versus time under different surface hydraulic loads in MSB.

0

0.5

1

1.5

2

2.5

θ

   E    (

   θ   )

   E    (

   θ   )

TFC 2.39m/hMSB 0.29m/hMSB 0.44m/hMSB 0.58m/hMSB 0.73m/h

without biofilm Surface hydraulic load

0

0.5

1

1.5

2

2.5

0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

θ

MSB 0.29m/hMSB 0.44m/hMSB 0.58m/hMSB 0.73m/h

with biofilm 2 Surface hydraulic load

(a) (b)

Fig. 8.   Dimensionless RTD curves at different flow rate in two reactors without and with biofilm.

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

θ

   E    (

   θ   )

MSB without biofilmMSB with biofilm 2

Flow rate=0.0091 m3/h

Surface hydraulic load=0.29 m/h

Fig. 9.  Experimental RTD curve at the flow rate of 0.0091 m3/h in the MSB with/

without biofilm.

 Table 5

Liquid holdup volume calculation with  t  peak  and flow rate Q .

Types   Q 

(m3/h)

t  peak

(s)

V liquid

(m3)

V Ld

(m3)

V Lt 

(m3)

 f LSE 

(%)

TFC 0.3 129 0.01075 0.01172 0.05610 0

MSB without biofilm 0.0091 1260 0.0032 0.00290 0.01710 2

0.0137 960 0.0037 0.00370 0.01790 0

0.0182 780 0.0039 0.00390 0.01810 0

0.0228 640 0.0041 0.00430 0.01850 0

MSB with biofilm 1 0.0091 1200 0.0030 0.00092 0.00672 36

0 .0137 960 0. 0037 0 .00104 0. 00 964 30

0 .0182 720 0. 0036 0 .00132 0. 0130 0 20

0 .0228 590 0. 0037 0 .00178 0. 01598 14

MSB with biofilm 2 0.0091 1080 0.0027 0.00083 0.00493 46

0 .0137 960 0. 0037 0 .000 98 0. 00 868 35

0 .0182 960 0. 0049 0 .00112 0. 01262 32

0 .0228 480 0. 0030 0 .00114 0. 01654 12

Flow rate=0.0182 m3/h

with biofilm 2

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

θ

   E    (

   θ   )

experimental curve

Calculated curve

Fig. 10.   Experimental and calculated RTD curves in the MSB with biofilm 2.

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these fractions implies that increasing the hydraulic loads will re-

duce the diffusion of tracer between the static liquid phase and the

dynamic liquid phase; whereas the diffusion will increase during

the biofilm development process under the same hydraulic condi-

tions. These changes may result from the reduction of contact time

between the liquid and solid phase when the flow rate rises.

The curves of dimensionless   E  (h) as a function of the dimen-

sionless time h  at different hydraulic loads for the bioreactors both

without and with biofilm are plotted in Fig. 8a and b. The dimen-sionless time   h   was calculated as the ratio of the time   t   to   t  peak .

The liquid holdup volume V liquid taken from Table 5 was also used

for the C 0  calculation.

In Fig. 8, comparing the RTD curves acquired at a greater flow

rate in the TFC , such RTD curves at lower flow rates in the MSB dis-

play lower peaks but a greater distribution area (hP 12) which

was assumed to be due to the sustained release of liquid in the col-

umn. The tails of the curves in the MSB may be due to the presence

of stagnant zones in the column.

Furthermore, the regimes with and without biofilm were com-

pared at each volumetric flow rate (see Fig. 8). An example for a

flow rate of 0.0091 m3/h is shown in Fig. 9.

From Fig. 9, it is clear that the presence of biofilm lengthened

the liquid residence time in the filter (h? 15) compared to timeswithout biofilm (h? 11). This could imply two time scales phe-

nomena: one was related to the dynamic flow; the other was re-

lated to the diffusive process that was promoted by the presence

of biofilm which decreased the packing bed porosity and increased

the contact time between liquid and biofilm. A sustained-release

process is also signified by the longer tail in the RTD curve. In addi-

tion, lower peak when biofilm was present implies more tracer

adsorption in the biofilm compare to the higher peak when biofilm

was absent. Furthermore, the same behavior was also observed at

other flow rates. In a bioreactor with biomass, the tracer can be

transported within the biofilm by diffusion. The biofilm slowly ex-

changes the tracer with the flow-through zones; this can signifi-

cantly increase the tailing potential and lead to a sustained

release process which was also observed and investigated by Rie-

mer et al.  [17] who proposed a ‘‘biodiffusion model’’ to fit the li-

quid residence time distribution in their study.

For the liquid distribution in a trickle bed, it is widely accepted

in many studies that a Gaussian Plug Flow (PF) model with axial

dispersion may fit the liquid profile when longer tails are present;

on the other hand, the model of n Continuous Stirred Tank Reactor(CSTR) with stagnant/dead zone reported by Levich et al.; Sant’An-

na et al.   [18,19]  has also been widely employed for investigating

the liquid profile in a Trickle Filter. Moreover, a Plug Flow reactor

can be approximated by a large number of CSTR in series. Hence

the model of n CSTR with stagnant/dead zones in series in the form

of Eq. (10) was applied to investigate the RTD in both bioreactors

with the variance derived in the following equation:

E ðhÞ ¼  1

mn

nn

ðn  1Þ!hn1e

nhm ð10Þ

r2 ¼

Pt 2C ðt Þ

l2P

C ðt Þ  1   ð11Þ

where   m   is the fraction of active zone,   n   is the number of CSTR.

When m equals 1, it means no stagnant/dead zone is present in

the packing bed.

With a variance r2 0, the liquid distribution in the column is

close to plug flow pattern, whereas when  r2 1, the liquid distri-

bution is close to ‘‘completely stirred’’ pattern.

RTD curves were calculated on the basis of Eq.   (10) and com-

pared with the experimental curves of all regimes. Fig. 10 presents

an example of the calculated RTD curve and the experimental RTD

curve in the MSB with biofilm.

In Fig. 10, theRTDcurve calculatedon the basisof Eq.(10) fitswell

with the experimental RTD curve expect for the tail segment which

is more gradual than themodel. General results from themodels for

both bioreactors are shown in Table 6. A certain discrepancy existsbetween our experimental and modeling curves (about 10%); these

errors have no consequences on the followingdiscussion, where the

flow pattern was discussed in relation to operating conditions.

In  Table 6, in the TFC without biofilm at a higher flow rate of 

0.3 m3/h,the estimatednumbern of CSTR is 23whichis greater than

that of the MSB at lower flow rates (n ranging from 7 to 13), which

implies the liquid profile in the TFC at a higher flow rate, close to

the plug flow pattern. The variance value of 0.21 in the TFC,

approaching 0, also suggeststhe liquidprofile close to theplug flow.

However, the variance values (0.61–0.83) for MSB with biofilm 2 at

lower flow rates, approaching 1, imply better tracer diffusion be-

tween liquid and biofilm. The fraction of the active zone increased

(0.55–0.72) when the biofilm was present and the number of CSTR 

decrease (2–6) compared to the cases without biofilm (7–13). Thisincrease of m corresponds well to the trend to the increased   f LSE 

 Table 6

Analysis from the model of  n  CSTR with dead zone in series.

Type Flow rate (m3/h) S.H.L (m/h) Active zone fraction No. of CSTR Variance Mean LRT (h) Error (%)

Q m n   r2 l

TFC 0.3 2.39 0.88 23 0.21 0.02 16

MSB no biofilm 0.0091 0.29 0.61 7 0.51 0.83 5

0.0137 0.44 0.46 8 0.51 0.64 7

0.0182 0.58 0.4 9 0.54 0.58 13

0.0228 0.73 0.49 13 0.45 0.45 14

MSB with biofilm 2 0.0091 0.29 0.72 2 0.61 2.71 7

0.0137 0.44 0.63 4 0.83 1.93 6

0.0182 0.58 0.6 5 0.64 1.82 7

0.0228 0.73 0.55 6 0.52 0.65 15

0.0

0.1

0.2

0.3

0.4

0.0 0.5 1.0   1.5   2.0 2.5 3.0

Surface hydraulic load (m/h)

     L     i    q    u     i     d     f     i     l    m

     t     h     i    c     k    n    e    s    s     (    m    m     )

δLδinδTFCδwith outδbiof ilm

δLδinδMSBδwithout δbiofilm

δLδinδMSB δwithδbiofilmδ1

δLδinδMSB δwithδbiofilmδ2

Nparticles=23920Nparticles=8350

Fig. 11.   Liquid film thickness estimation versus surface hydraulic loads for in twobioreactors with/without biofilm.

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where the exchange between the dynamic and static phases in-

creased when thebiofilm was present. It indicates that thepresence

of biofilm will result in enhanced mass diffusion in the filter.

Comparing the mean Liquid Residence Times l obtained for the

regime without biofilm and for the biofilm 2 regime, the presence

of biofilm indeed lengthened the residence time in the bioreac-

tor(e.g. from 0.83 to 2.71 h at a flow rate of 0.0091 m3/h).

The presence of biofilm effectively lengthened the liquid resi-dence time, improving efficiency in pollutants treatment.

 3.4. Liquid film thickness

The liquid film thickness in the different regimes was calculated

by multiplying the liquid dynamic holdup volume  V Ld   divided by

the effective surface area, assuming that stagnant zones existed

in the pilot set-up and the tracer was only exchanged in the active

fraction. When no biofilm was present,  V liquid  equaled  V Ld. In con-

trast, when biofilm was present, the values of   V Ld   from   Table 5

were introduced to Eq. (9). Additionally, during the hydrodynamic

experiments, it was observed that even the influent of liquid was

not well distributed; particles that were not initially wetted were

eventually wetted thanks to their diffusion among adjacent parti-cles after sufficient time. Hence it was assumed for the sake of sim-

plicity that the wetting factor  f w  in Eq. (9)  equals 1. Applying the

equivalent particle diameter d p  of 0.0217 m, the total surface area

of the medium was calculated by total number of particles N  particles

multiplied by the surface area of a single particle when no biofilm

was present. When calculating these values for regimes with bio-

film, the mean biofilm thickness  L f  was introduced into Eq.  (9).

Given these assumptions, the liquid film thickness for different

surface hydraulic loads of regimes with and without biofilm in

both bioreactors is plotted in Fig. 11.

As shown in Fig. 11, the liquid film thickness acquired in the TFC

without biofilm at a hydraulic load of 2.39 m/h was 0.33 mm. Un-

der lower hydraulic loads (0.29–0.73 m/h) in the MSB, the calcu-

lated liquid film thicknesses were less than those obtained in theTFC: when biofilm was absent, it was ranged from 0.23 to

0.35 mm; for biofilm 1 regime, it was ranged from (0.07 to

0.14 mm) and for biofilm 2 regime, from (0.06 to 0.09 mm).

The regimes without biofilm had obvious thicker liquid film

than the regimes with biofilm, which may be due to higher liquid

dynamic holdup volume. Furthermore, as the biofilm thickness in-

creased (from regime 1 to regime 2), liquid film thickness values

were less which resulted from the increased surface area of the

packing bed.

Itwas found that in regimes without biofilm, the thicknessof the li-

quid layer wassignificantlygreater than inregimeswithbiofilm,which

may be dueto the greater volume of dynamic holdup. However, in the

same bioreactor but at a lower flow rate, the smaller dynamic holdup

volume resulted in the reduction of liquid film thickness.The presence of biofilm led to a thinner liquid film compared to

that obtained without biofilm, a result attributable to the increase

of surface area with biofilm development and the decrease of dy-

namic draining volume (see Table 5). Such behavior will promote

the oxygen transfer rate by decreasing the limited liquid layer

and lengthening the contact time between the liquid and biofilm.

4. Conclusion

Different hydraulic experiments were carried out, essentially in

a new type of TF made up of a Multi-Section Bioreactor packed

with concrete block medium particles. The purpose of the study

was to investigate the hydrodynamic characteristics particularlyin the light of the influence of biofilm on hydrodynamic behavior.

The static experiments with biofilm indicate that most of the li-

quid will be retained by the particles coated with biofilm, increasing

static retention and, consequently, reducing dynamic retention. It

wasalso found that theliquid staticholdupmakesa greater contribu-

tionto total liquid holdup than the dynamic holdup on account of the

high adsorption potential of the Concrete Block medium resulting

from its porousstructure. However thestatic holdup is notin correla-

tion with the configuration of TF comparing with the results of TFC.RTD experiments show that at lower flow rates, the mass diffu-

sion between the liquid and biofilm was better than that at higher

flow rates. Increasing the hydraulic load will result in the flow pat-

tern approaching to the plug flow pattern in the bioreactor. The li-

quid volumes represent in the RTD agreed with the dynamic

holdup volumes obtained by the drainage method with no biofilm

is present. However, with biofilm, these liquid volumes represent

not only the dynamic holdup but also partial static holdup volume

resulting from the sustained release of partial static holdup in the

biofilm. The LRT calculated from RTD models shows that the pres-

ence of biofilm will lead to a longer liquid residence time in the fil-

ter and thus promote the distribution of liquid in the bioreactor. So,

comparing the drainage and RTD methods allow us to show that

diffusion phenomenon can occur in the biofilm, increasing the con-

tact time between liquid and biofilm.

Furthermore, the presence of biofilm will decrease the thickness

of the liquid film compared with the regimes without biofilm un-

der the same hydraulic conditions on account of the greater surface

area and a smaller dynamic holdup volume.

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