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Biochemical Engineering Journal 37 (2007) 131–138

Treatment of diesel fuel contaminated soil in jet–fluidized bed

Jazia Arrar a,∗, Nadia Chekir a, Fatiha Bentahar b

a Laboratoire des Sciences et Techniques de l’Environnement (LSTE), Ecole Nationale Polytechnique,10 Avenue Hassen Badi, Belfort, El-Harrach, 16110 Alger, Algeria

b Laboratoire des Phenomenes de Transfert (LPT), Universite des Sciences et de la Technologie de Bab Ezzouar,USTHB, BP 32 EL Alia, 16311 Alger, Algeria

Received 24 July 2006; received in revised form 26 March 2007; accepted 15 April 2007

bstract

Bioremediation of diesel-oil contaminated soil (4 wt%) was studied in a jet–fluidized reactor by the stimulation of indigenous oil micro-organisms.he experiments were carried out using different rates of aeration and/or jet air flow and consequently various mixing rates to investigate their

nfluence on biodegradation and removal ratios. The diesel-oil content decreased rapidly in all experiments in the first 7 days, with differentemoval ratios. The presence of the jet favoured inter-particle exchanges, transfers between the various phases involved, and accelerated the diesel-il removal process. Also, the influence of aeration seemed negligible compared with that of the jet. Removal and biodegradation ratios ranged

rom 69% to 99%, and 54% to 84%, respectively after 15 days of treatment. The best biodegradation ratio of 84% occurred in the case of anxpanded bed with minimum fluidization, operating at a jet velocity of 37 m/s. The diesel-oil biodegradation was governed by first-order kinetics.mportant air flows enhanced the efficiency of diesel-oil removal, and abiotic loss and hence decreased the biodegradation ratio.

2007 Elsevier B.V. All rights reserved.

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eywords: Bioremediation; Biostimulation; Biodegradation; Jet–fluidized bed;

. Introduction

The release of hydrocarbons into the environment, whetherccidental or due to human activities, is the main cause of waternd soil pollution and increases the risk of groundwater pol-ution. Many of these components are toxic, mutagenic andarcinogenic [1]. It has been observed that diesel-oil presentedhigher toxicity than crude oil in all mesocosms [2]. Sev-

ral technologies successfully treat hydrocarbon-contaminatedoils, among them physical/chemical techniques such as incin-ration, combustion, extraction and soil washing but these areighly expensive and require the use of elaborate equipmentsnd large amounts of energy. Bioremediation is a more attrac-ive proposition than conventional treatments because it is simpleo maintain, cost-effective and has the ability to destroy the

ollutants completely. Bioremediation accelerates the naturallyccurring biodegradation under optimized conditions such asemperature, pH, nutrients, water content and availability of

∗ Corresponding author.E-mail addresses: jazia.arrar@enp.edu.dz (J. Arrar),

atihabentahar@yahoo.fr (F. Bentahar).

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369-703X/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.bej.2007.04.016

tion; Diesel fuel

lectron acceptors [3]. The results under aerobic conditions areonsiderably better than those under anaerobic conditions [4,5].ioremediation applications fall into two important categories:

n situ and ex situ. Ex situ techniques are faster, easier to control,nd more effective for the treatment of a wider range of contam-nants and soil types than in situ techniques. It is clear that theransfer of oxygen or other electron acceptors may be a limitingactor for such treatment technology. Therefore it is an essentialarameter for process control [6] and influences the feasibil-ty and effectiveness of bioremediation. Some authors showed aositive correlation between the strength of the gas–solid mix-ng and the degradation ratio [7,8]. Consequently, among ex situechniques, slurry reactors, where the contact between contam-nated soil, water, nutrients, oxygen and biomass is increased,re well adapted to improving the biodegradation ratio. Slurryeactor designs, including mixing-tanks, airlifts, fluidized beds,nd rotating drums, maximise the contact between the bioticnd abiotic phases [9]. The conditions in the bioreactor are con-rolled to create the optimum environment for micro-organisms

o degrade the contaminants.

Among hydrocarbon pollutants, diesel-oil is a complex mix-ure of alkanes and aromatic compounds that are frequentlyeported as soil contaminants [10]. Commercial diesel-oil is not

132 J. Arrar et al. / Biochemical Engineer

Nomenclature

dP soil particle diameter (m)d0 nozzle diameter (m)DC column diameter (m)[Diesel]0 initial diesel-oil content after 21 days of incu-

bation (g/kg)[Diesel]abio residual diesel-oil content in an abiotic system

(g/kg)[Diesel]bio residual diesel-oil content in a biotic system

(g/kg)g acceleration of gravity (m/s2)H bed height (m)kabio abiotic removal constant rate (day−1)kbio biodegradation constant rate

(day−1)/(g−1 kg day−1)krem biotic removal constant rate (day−1)Qf fluidization volumetric flow rate (L/s)Qj jet volumetric flow rate (L/s)Qt total volumetric flow rate (L/s)Qmjf volumetric flow rate at minimum jet–fluidized bed

(m3/s)[Qa(0)]mjf volumetric annulus flow rate at Qmjf (m3/s)Uf superficial fluidization velocity (m/s)Uj orifice jet velocity (m/s)Umf minimum superficial fluidization velocity (m/s)Umj minimum superficial jet velocity (m/s)Umjf minimum superficial jet–fluidized velocity (m/s)

Greek lettersε porosityρf fluid density (kg/m3)

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pmto 10−10. After inoculation, cultures were incubated at 37 ◦C for24 h and the number of colony forming units (cfu) was counted.

ρs particle density (kg/m3)

otally biodegradable. The biodegradation of diesel-oil dependsn the chemical structure of organic compounds. The intrin-ic biodegradability of diesel-oil varies either between 60%nd 73% [11] or between 70% and 90% [12]. All n-alkanes,re totally degraded whereas higher molecules are reducedt lower ratios [7]. It has been reported that between 67%nd 98% of C15 and C22 are degraded after an incubationeriod of 30 days [13]. Diesel-oil bioremediation in soil cane improved by the stimulation of indigenous micro-organisms,dding nutrients and oxygen into a contaminated system (bios-imulation), and/or introducing oil-degrading micro-organismsbioaugmentation) into the soil. Many studies showed thato significant correlation was noted between the concen-ration of oil-degrading micro-organisms and hydrocarboniodegradation potentials. However, nutrient availability washe main factor limiting oil biodegradation [14]. Therefore,iostimulation can be considered as an appropriate remedia-ion technique for diesel-oil removal in soil and requires the

valuation of both the intrinsic limiting factors and the envi-onmental extrinsic conditions in promoting kinetic degradation15].

Tsl

ing Journal 37 (2007) 131–138

Air flow and agitation rates are important factors affectinghe efficiency of biodegradation in a slurry-phase bioreactor.tirring increases the contact between the involved phases, thusnhancing the mass transfer and, as a consequence, the biodegra-ation rate. However, excessive agitation increases the lossesy volatilization, diffusivity, and convection and decreases theiodegradation ratio.

In this work, we focused on jet–fluidized reactors whereeveral flow regimes with characteristics such as a packeded, a bubbling bed, a fluctuating jet and a stable jet [16],ould be generated in the bed. Jet–fluidized beds favour theontact between gas and solids in different ways by simplyhanging the rates of jet and fluidization gas. They overcomehe individual limitations of fluidized and jet beds by super-mposing one system on another to achieve higher rates of

ixing and a better solid–fluid contact, therefore improving theass transfer. In addition, there is no restriction on the size

f particles [17]. The object of this study was to determinehe influence of aeration on diesel-oil removal and biodegra-ation ratios, under different conditions in a jet–fluidizeded.

. Material and methods

.1. Soil

The soil sample for the experiments was collected accordingo a standard procedure (AFNOR X31100) from a non-ontaminated area located in Bordj-El-Kiffan near Alger. Theoil was air-dried, homogenised and sieved (<0.8 mm). It washen artificially polluted with 4 wt% diesel-oil, amended withitrogen (NH4NO3) and phosphorus (KH2PO4) to adjust C:N:Polar ratios to 100:10:1 as recommended [18], and moist-

ned to maintain a sufficient water content. Several studiesuggest a water content of 60% of the field holding capacity19]. It is a very significant parameter and is essential to theevelopment and growth of the micro-organisms, and to thearious transfers. Then, the soil was kept in a closed vesselt room temperature for three weeks before treatment. Dur-ng this period of time, it was mixed and moistened to ensureetting of the soil matrix by the diesel-oil and stimulation of

he indigenous micro-organisms. The soil used for referenceas sterilised by autoclaving at 121 ◦C for an hour on three

uccessive days [20] before adding diesel-oil, nutrients andater.

.2. Microbial analysis

Microbial cell numbers were estimated by using the mostrobable number method: a soil suspension was prepared byixing 1 g of soil with 9 ml of distilled water and serially diluted

hree plates were inoculated for each dilution. The calculatedtandard deviation for all the results was found to be equal to oress than 25%.

ineering Journal 37 (2007) 131–138 133

2

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Fig. 1. Schematic representation of laboratory experimental set up: (1) compres-sor; (2) pressure controller; (3) carbon dioxide trapping; (4) pressure reducervalve; (5) valve; (6) diaphragm; (7) pressure indicator; (8) nozzle; (9) distrib-utor plate; (10) fluidization gas buffer; (11) fluidization column (right and leftcolumn are abiotic and biotic); (12) disengagement zone; (13) parietal taps; (14)p

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J. Arrar et al. / Biochemical Eng

.3. Diesel-oil extraction and analysis

The pollutant used in our study was a diesel fuel obtainedrom a gasoline station. Samples of soil (20 g) were mixed,rushed with anhydrous sodium sulphate and extracted witharbon tetrachloride for 6 h in Soxhlet. The concentrated extractas weighed just after solvent evaporation to determine the totalydrocarbons (TPH). This extract was then filtered in an Epsomalt column to remove the polar compounds and was analysedy gas chromatography.

The determination of the hydrocarbons content was car-ied out using a gas chromatograph (GC; HP 6890 Hewlettackard) equipped with a flame ionization detector (FID) and

methylpolysiloxane capillary column (30 m × 0.25 mm ×.25 �m). The injector and detector were maintained at 300 ◦Cnd the column temperature was programmed to rise from0 ◦C to 280 ◦C at a rate of 7 ◦C/min. Diesel-oil containslarge number of hydrocarbons (2000–4000), which cannot

e completely separated by chromatography. Only n-alkanesnd a few ramified hydrocarbons can be identified as sepa-ate compounds [11,21]. Diesel-oil contents were determinedy calculation of the total area of the chromatogram in rela-ion to the internal standard anthracene. This method was alsosed to quantify n-alkanes. Anthracene has a retention timeanging between those of n-C17 and n-C18 and good reso-ution of separation compared to those of hydrocarbons ofhe diesel-oil. The variation of the reproducibility of extrac-ion and quantification of hydrocarbons in soil samples wasetermined by duplicating sampling, and by three successivenjections of the same sample. This variation was estimatedt ±8%.

. Experimental set up and procedure

The experiments were carried out in two jet–fluidized reac-ors, one of which acted as an abiotic control system. Botheactors contained 16 kg of diesel-oil contaminated soil andperated under the same conditions, namely: fluidized reac-or (Uf, 0), jet reactor (0, Uj), jet–fluidized reactor (Uf, Uj).he schematic of the laboratory experimental set up is shown

n Fig. 1. The reactor is a cylindrical fluidization column of50 mm of diameter and 500 mm of height fitted with pressurend sampling taps. On the upper part, the fluidization columnas a disengagement zone to prevent particles being ejected fromhe column. Aeration was provided by compressed air injectedhrough a gas distributor plate (void fraction of 3%) and through

central injection nozzle of 15 mm of internal diameter and0 mm of height placed above the gas distributor plate, at theottom of the bed.

In order to investigate the influence of the aeration and thushe agitation on diesel-oil removal and biodegradation, differentow regimes were applied by varying the jet and fluidizationelocities in both the abiotic and the biotic systems. Differ-

nt aeration velocities (Uf = 0, 9.9, 13.4 and 18 cm/s) and jetelocities (Uj = 0, 20, 37 and 42 m/s) were used.

Preliminary studies were carried out to determine the minimaluidization (Umf) and minimal jet (Umj) velocities, before and

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ump; (15) storage tank of water.

fter soil pollution, and to investigate the influence of enhancederation on the moisture content for all the treatments. In ordero ensure an overall circulation of the soil particles and conse-uently, a good mixing of the solids and oxygen transfer in theet–fluidized reactor, higher jet velocities, beyond or equal to

mj were used. Thus, a stable jet channel forms between therifice and the top of the bed and the circulation flow patternonsists of an upward flow of soil particles in the jet chan-el and a downward flow of soil particles in the annulus. Theinimal velocities (Umf) and (Umj) were determined, respec-

ively by the classic methods based on the total pressure loss inhe bed and the static pressure measurements over the injec-ion nozzle for various fluidization and jet flow rates. Then,n order to maintain a constant level of moisture at 15% andompensate the water loss due to evaporation, we followed theemporal evolution of the moisture content for the above oper-ting conditions. Thus, the water flow rates required for all thereatments were determined. Increasing the air flow rate affectshe moisture content and increases the evaporation ratio. A sys-em of continuous sprinkling was used to feed the bioreactor withater.The diesel-oil concentration, microbial growth, pH, mois-

ure, temperature, nitrogen, and phosphorus concentrations inhe reactors were periodically monitored for 15 days. The soil

as sampled from the top, the middle and the bottom of theed to have an average sample and to verify the bed uniformityoncerning the investigated parameters.

1 ineering Journal 37 (2007) 131–138

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Table 1Characteristics of soil before pollution

pH 7.63Organic matter (g/g) 0.04Total organic carbon (g/g) 0.03Sand (g/g) 0.88Silt (g/g) 0.09Clay (g/g) 0.03Texture SandyUmf (m/s) 0.10Total N (mg/kg) 23.9Total P (mg/kg) 0.16Biomass (cfu/g) 2.6 × 10+6

Mean diameter (�m) 398Density (g/cm3) 2.60Porosity 0.38WU

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netIba

tTdrrespective fluidization velocities of 9.9 cm/s, 13.4 cm/s and

34 J. Arrar et al. / Biochemical Eng

The diesel-oil removal and biodegradation ratios were esti-ated from the following equations:

iesel-oil removal ratio = [Diesel]0 − [Diesel]bio

[Diesel]0(1)

iesel-oil biodegradation ratio = [Diesel]abio − [Diesel]bio

[Diesel]0(2)

he first-order and the second-order equations used to describehe biodegradation kinetic model were expressed, in an inte-rated form, by Eqs. (3) and (4), respectively:

[Diesel]abio − [Diesel]bio)t = [Diesel]0 exp(−kbiot) (3)

1

[Diesel]0− 1

([Diesel]abio − [Diesel]bio)t= kbiot (4)

. Results and discussion

The diesel-oil used in this study was characterised (Fig. 2)y the presence of hydrocarbons ranging from C6 to C30 with9.55% of alkanes between C10 and C24.

The characteristics and composition of the experimental soilere shown in Table 1. It was sandy soil with neutral pH. The

ntrinsic contents of nitrogen and phosphorus, essential to thectivity and development of micro-organisms, were very low.he moisture content was also very low (0.72%). Microbialnalysis of the soil, clearly confirmed the presence of indige-ous micro-organisms of 2.6 × 10+6 cfu/g of dry soil. Afteroil contamination with diesel-oil, this number would increaseccording to several authors [22].

As a result of the diesel-oil soil pollution, the pH increasedrom 7.63 to 8.07 and the organic matter content from 3.83% to.07%. Micro-organisms developed very well during incubationrom 2.6 × 10+6 cfu/g to 10+9 cfu/g after adding diesel-oil andutrients to the soil and before the contaminated soil entered the

eactor. The increase in the microbial population was accompa-ied by a decrease in diesel-oil and nutrient contents during theame period. Diesel-oil concentrations varied between 3.65%nd 3.80%. Before starting the experiments, the polluted soil

Fig. 2. Diesel-oil composition.

1p

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ater content (g/kg) 7.20

mj (m/s) 0.07

as again amended so as to readjust C:N:P molar ratios to00:10:1.

After soil pollution, the minimum superficial fluidizationnd the minimum superficial jet velocities increased and wereespectively equal to 13.4 cm/s and 7.2 cm/s for a moisture con-ent of 15%.

For all experiments, the bioremediation did not generate sig-ificant variations of pH, which remained constant during thentire process. The pH values (between 7.5 and 8.0) were inhe range for the normal development of a microbial population.n addition, parameters such as pH, nutrient contents, micro-ial count, and diesel-oil contents, remained practically constantlong the bed depth.

In a fluidized bed reactor, according to Fig. 3, the aera-ion generated an important reduction of the diesel-oil content.he degradation was most rapid during the first 7 days then, itecreased and seemed to stabilise after 10 days. The removalatios of diesel-oil reached 68.7%, 76.5% and 82.0% for the

8 cm/s. Diesel-oil disappearance was due to both biotic (com-lete mineralization, production of biomass and metabolites)

ig. 3. Evolution of diesel-oil removal and biodegradation ratios in a fluidizeded.

J. Arrar et al. / Biochemical Engineering Journal 37 (2007) 131–138 135

Table 2Correlation coefficients and rate biodegradation constants for first- and second-order kinetic models

Uf (cm/s) Uj (m/s) First-order kinetic model Second-order kinetic model

kbio (day−1) R2 kbio (g−1 kg day−1) R2

9.90 0 0.12 0.94 0.02 0.8813.4 0 0.14 0.99 0.03 0.9518.0 0 0.10 0.98 0.02 0.92

0 20.0 0.11 0.96 0.02 0.920 37.0 0.14 0.96 0.03 0.880 42.0 0.15 0.91 0.03 0.819.90 37.0 0.19 0.96 0.03 0.799.90 42.0 0.17 0.94 0.03 0.78

13.4 37.0 0.22 0.97 0.03 0.8213.4 42.0 0.17 0.94 0.03 0.801 0.96 0.03 0.731 0.79 0.03 0.49

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adof 76.5%. On the other hand, for higher jet velocities (37 m/s and42 m/s) the diesel-oil content decreased rapidly during the 5 firstdays and stabilized afterwards. After 15 days of treatment, the

8.0 37.0 0.158.0 42.0 0.12

nd abiotic (volatilization) processes. Both biotic and abioticrocesses were involved in the biotic systems while, in thebiotic control systems, only volatilization took place. Theecorded depletion ratios of diesel-oil in abiotic control sys-ems ranged from 12% to 21% depending on the fluidizationelocities. The loss ratio was relatively negligible for velocitiesess than or equal to Umf. Beyond the minimum fluidization,t was significantly greater than 20% after 5 days of treatment.he same tendency was observed for the biodegradation ratiohen the bed was aerated, or fluidized (≤13.4 cm/s) (Fig. 3).ith higher aeration velocity (18 cm/s), the bed was more

xpanded and the biodegradation ratio decreased. The best resultf diesel-oil biodegradation was obtained for a bed state at theinimum of fluidization (13.4 cm/s) and reached 64%. The plots

f ln([Diesel]0/[Diesel]) and 1/[Diesel] versus time for differentelocities of aeration showed a straight line of slope k. The cor-elation coefficients, determined by linear regression, indicatedhat the first-order biodegradation model was well adapted to allreatments in fluidized bed reactors (Table 2).

Several studies indicate that hydrocarbons biodegradations governed by a first-order kinetic model [23,24] although aecond-order kinetic model may give results that are statisti-ally valid [25]. The first-order biodegradation rate constantskbio) rose with higher velocities of fluidization. Beyond theinimum of fluidization, the biodegradation rate constants fellhich concurred with the results of biodegradation. At min-

mum fluidization, the biodegradation rate constant was 20%igher than that observed in an aerated bed. This rate constantas reduced to 26% when the bed was more expanded. Theresence of diesel-oil in the soil led to an increase in the totalacterial counts compared with the contaminated soil enteringhe reactor. The increase was of 306%, 530% and 493% for theystems (9.9 cm/s, 0) (13.4 cm/s, 0) (18 cm/s, 0), respectively.otal bacterial counts did not change significantly for the threeystems, i.e., enhanced aeration had no effect on the stimulationf microbial numbers in comparison with the diesel-oil removal

nd biodegradation ratios as shown in Fig. 4.

In the jet bed reactor, according to Fig. 5, the diesel-oilemoval ratios recorded after 15 days of treatment rangedetween 76% and 84% although a significant part was due to

Fig. 4. Microbial growths in fluidized, in jet, and in jet–fluidized beds.

biotic loss (17–22%). At minimum jet velocity (20 m/s), theiesel-oil content was reduced gradually to reach a removal ratio

Fig. 5. Diesel-oil removal and biodegradation ratios in a jet bed.

1 ineering Journal 37 (2007) 131–138

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36 J. Arrar et al. / Biochemical Eng

iodegradation ratios ranged between 60% and 62% (Fig. 5).his would correspond to the depletion of the nutrients and/or

he biodegradation of the most recalcitrant hydrocarbons and theresence of toxic metabolites. As for the fluidized bed, the linearegression method was used in order to determine the degrada-ion kinetic model. The biodegradation rate constants and theorrelation coefficients were determined by supposing that first-rder as well as second-order kinetics could be applied to theata and were reported in Table 2. The biodegradation reactionas also governed by a first-order kinetic. The values of corre-

ation coefficients for the linear plots using the first-order modelere higher than those obtained using the second-order model.ompared with those observed in the case of fluidized beds, the

ate constants were significantly higher for high jet velocitiesnd increased with increasing jet velocity.

No significant differences in microbial growth were observedetween the systems (0, 20 m/s), (0, 37 m/s), (0, 42 m/s) (Fig. 4).n insignificant increase in microbial population in the range of50–469% followed by a slow decrease was observed. In thisase, the total bacterial count was 8 × 10+8 to 6 × 10+9 cfu/g.

For the jet–fluidized bed, a stable jet can be achieved at jetir velocities higher than or equal to the minimum jet–fluidizedelocity (Umjf). The minimum jet–fluidized velocity was theum of the minimum jet velocity and the minimum fluid velocityequired to fluidize the annulus and was correlated [26] by theollowing equations:

mjf = Umj + Umf(1 − φ) (5)

mj = 2.44

(dP

DC

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DC

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DC

)0.5

×{

2gH

[ρs − ρf

ρf

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(6)

here

= 3.66 × 10−5 d0.1040 d−0.485

P H−0.656[Qa(0)]−1.3mjf (7)

ndeed, at jet velocities above 20 m/s, rapid percolation of soilarticles from the jet channel to the annulus was observed.

tabj

able 3omparison of bioremediation efficiencies and rate constants of contaminated soil w

f (cm/s) Uj (m/s) Qta (L/s) Diesel-oil removal Diesel-

Ratio krem (day−1) R2 Ratio

0 20.0 3.53 0.77 0.16 ± 0.03 0.99 0.170 37.0 6.54 0.82 0.23 ± 0.04 0.99 0.210 42.0 7.42 0.84 0.25 ± 0.06 0.96 0.229.90 0 4.86 0.69 0.16 ± 0.04 0.97 0.129.90 37.0 11.4 0.91 0.25 ± 0.04 0.99 0.149.90 42.0 12.3 0.95 0.28 ± 0.05 0.98 0.143.4 0 6.58 0.77 0.18 ± 0.02 0.99 0.123.4 37.0 13.1 0.97 0.32 ± 0.06 0.98 0.133.4 42.0 14.0 0.99 0.37 ± 0.06 0.99 0.248.0 0 8.84 0.82 0.17 ± 0.03 0.99 0.218.0 37.0 15.4 0.98 0.40 ± 0.05 0.99 0.358.0 42.0 16.3 0.99 0.47 ± 0.08 0.98 0.44

a Qt = Qf + Qj.

ig. 6. Diesel-oil removal and biodegradation ratios in a jet–fluidized bed.

Fig. 6 shows the influence of the air flow rates of aeration andet on diesel-oil removal and biodegradation ratios. The degra-ation of diesel-oil in the different experiments showed a similarrend but different degradation ratios. The diesel-oil contentecreased rapidly in all treatments including the abiotic controlystems. After 10 days of treatment, negligible removal of dieselccurred. As the oxygen supply was increased by enhanced aer-tion and jet air flows, the diesel-oil removal was intensified. Theest results were obtained for the highest aeration and jet air flowates (Uf = 18 cm/s, Uj = 37, 42 m/s). They reached 98–99% for5 days compared to the fluidized bed where the removal ratioas 82%. It appears clearly that the presence of the jet, favouring

he inter-particles exchanges and transfers between the differenthases, accelerated the process of elimination of the diesel-oilnd that the influence of aeration seems a priori negligible com-ared to that of the jet (Table 3). At constant jet velocity, theiotic removal ratio increased from 11% to 20% when the aera-ion velocity varied between 0 m/s and 18 m/s. On the other hand,

t constant aeration velocity (≤Umf), for example 9.9 cm/s, theiotic removal ratio was improved from 33% to 38% when theet velocity went from 0 m/s to 37 m/s and 42 m/s, respectively.

ith diesel-oil under different treatments

oil abiotic loss Diesel-oil biodegradation

kabio (day−1) R2 Ratio kbio ( day−1) R2 k′bio (day−1)

0.03 ± 0.01 0.97 0.60 0.11 ± 0.02 0.96 0.120.04 ± 0.01 0.95 0.61 0.14 ± 0.03 0.96 0.190.05 ± 0.02 0.92 0.62 0.15 ± 0.04 0.93 0.200.03 ± 0.01 0.94 0.57 0.12 ± 0.03 0.94 0.130.02 ± 0.01 0.89 0.77 0.19 ± 0.04 0.96 0.230.04 ± 0.02 0.86 0.81 0.17 ± 0.04 0.94 0.240.03 ± 0.01 0.88 0.64 0.14 ± 0.01 0.99 0.150.05 ± 0.02 0.89 0.84 0.22 ± 0.04 0.97 0.280.08 ± 0.03 0.96 0.75 0.17 ± 0.04 0.94 0.290.04 ± 0.01 0.98 0.61 0.10 ± 0.02 0.98 0.140.08 ± 0.03 0.94 0.63 0.15 ± 0.03 0.96 0.310.15 ± 0.04 0.97 0.55 0.12 ± 0.05 0.79 0.31

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J. Arrar et al. / Biochemical Eng

t higher aeration velocity (>Umf), where the bed was morexpanded, the biotic removal ratio increased by 20% with theresence of the jet.

Although, the diesel-oil fraction of low volatility was signifi-antly high [27], a considerable decrease in the diesel-oil contentas observed in abiotic control systems. The depletion ratios ofiesel-oil obtained ranged from 14% to 46% depending on theuidization and jet velocities. These results indicated that notnly volatilization but also other phenomena such as transportnd/or convection took place. Several authors [4,25] reportedhat abiotic loss of diesel-oil by volatilization could account fornly 10% and could reach 12% over an extended period. In all thexperiments, the depletion ratio significantly increased for therst 5 days with increasing total air flow rate, and then remainedelatively constant. Subsequently, the biodegradation ratios, asllustrated in Table 3, decreased.

Significant biodegradation ratios were observed in all sys-ems (Uf, Uj) for the first 7 days ranging between 50% and 70%epending on the fluidization and the jet air flow rates. After 15ays of treatment, the highest biodegradation ratio of 84% wasbtained for the system (Uf, Uj) = (13.4 cm/s, 37 m/s).

At low and high aeration velocities (9.9 cm/s and 13.4 cm/s),he presence of the jet also accelerated the process of biodegra-ation. By increasing in jet velocity to 37 m/s and to 42 m/s, theiodegradation ratios increased by 36% and 42%, respectivelyhere Uf = 9.9 cm/s, but where Uf = 13.4 cm/s, they increasedy 30% and 17% in comparison with the fluidized bed.

At still higher fluidization velocity (18 cm/s), the diesel-oiliodegradation was higher in the system (Uf, Uj) = (18 cm/s,7 m/s) than in the system (Uf, 0) = (18 cm/s, 0) although theifference between the two systems at the end of the experi-ent was not significant. For a higher jet velocity (42 m/s), the

iodegradation ratio was enhanced at the beginning to reachalues lower than those observed in the case of the fluidizeded system (18 cm/s, 0). The highest biodegradation ratios of0–83% were observed in jet–fluidized beds (Uf, Uj) operating at9.9 cm/s, 42 m/s) and (13.4 cm/s, 37 m/s), respectively. In addi-ion to the influence of the jet, these results highlighted also,he ability of the indigenous microbial population to degradeiesel-oil to a large extent and suggested that oxygen supply andow regimes were the factors limiting diesel-oil biodegradationTable 3).

The diesel-oil removal ratios increased when the total flowate rose in the course of time, while diesel-oil biodegradationatios did not necessarily increase. These biodegradation ratiosncreased when the total air flow rate increased, except when theed was expanded, at the beginning of the experiment. From thehird day, significant fall in the diesel-oil biodegradation ratiosor the high flow rates (systems (13.4 cm/s, 42 m/s) (18 cm/s,7 m/s) and (18 cm/s, 42 cm/s)) was recorded. This behaviourould not be attributed to the depletion in the source of carbon,n reference to the ratio of removal of the diesel-oil, or to theutrients but to the state of the bed and agitation rate generated

y aeration and jet flow rates.

Diesel-oil biodegradation in the jet–fluidized bed also fol-owed a first-order kinetic model (Table 2) as was the case inhe fluidized and jet bioreactors. The first-order rate constants

aard

ing Journal 37 (2007) 131–138 137

ighlighted that the most active biodegradation occurred at aera-ion and jet velocities of 13.4 cm/s and 37 m/s (k = 0.221 day−1),.9 cm/s and 37 m/s (k = 0.186 day−1) and 9.9 cm/s and 42 m/sk = 0.172 day−1) as previously noted.

In all the experiments in the jet–fluidized bed, an increase inotal bacterial count was observed during the first 7 days (Fig. 4).he highest microbial counts recorded, were observed undereration and jet velocities of 13.4 cm/s and 37 m/s which concursith the biodegradation results.On the basis of the experimental results obtained, abiotic and

iotic diesel-oil removals were governed by first-order kineticsharacterized by the rate constants kabio and krem, respectivelys shown in Table 3. The jet generated faster abiotic removalinetics compared with those observed in the fluidized bed. Inhe case of a packed bed (<13.4 cm/s), the kinetics were slowerndependently of the jet velocities. This could be attributed tohe shrinkage of the jet zone. When the bed was expanded≥13.4 cm/s), kabio increased with increasing jet velocity.

Regarding kbio, in both jet and fluidized bed reactors, thisarameter increased with increasing air flow rates. In theet–fluidized bed reactor, the variation of kbio was more complexince it depended on the total air flow rate, and the flow regime.he first-order biodegradation rates were significantly higher in

he jet–fluidized bed than in the fluidized bed. At higher flu-dization velocities, the biodegradation rate constants decreasedn spite of, the presence of the jet and the overall circulationf soil particles generated. This could be explained by the facthat the inter-particle exchanges decreased. Rate constants trans-ated the effects of different operating conditions on diesel-oilegradation.

The study of the abiotic removal, biotic removal andiodegradation kinetics showed that all followed the first-rder kinetic model. The removal kinetic model could be thenxpressed by the following equation.

d[Diesel]

dt= −krem[Diesel] = −(kabio + k′

bio)[Diesel] (8)

y comparing the biodegradation constants kbio with thosebtained from constants krem and kabio, considering their pre-ision, the results were equivalent except in the case of the morexpanded bed. In this case, the abiotic degradation ratio was veryignificant, and consequently the easily biodegradable fractionas greatly reduced.

. Conclusion

The stimulation of indigenous micro-organisms by optimiz-ng fluidization and jet air flow rates and, consequently thetirring rate, is an effective approach for the treatment of soilsolluted by hydrocarbons, provided there is favourable pH andutrient content. The degradation of the diesel-oil was achievedn three systems: a fluidized bed, a jet bed and a jet–fluidizeded. The diesel-oil and removal ratios varied between 69%

nd 99% and the biodegradation ratios varied between 54%nd 84%. The presence of the jet accelerated the process ofemoval and biodegradation of diesel-oil. The best biodegra-ation ratio was obtained with the expanded bed at minimum

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38 J. Arrar et al. / Biochemical Eng

uidization and with a relatively high jet velocity. In all cases,he diesel-oil removal was most rapid during the first 7 days, thent decreased with time, and seemed to stabilise after 10 days.

first-order biodegradation model was well adapted to all thereatments. Important air flow rates enhanced the efficiency ofiesel-oil removal but caused greater abiotic loss and decreasedhe biodegradation ratio. The biodegradation could be maxi-

ized and abiotic losses minimized by determining optimumeration and jet velocities. Considering the results obtained, itould be judicious to operate, for the first day at a high jetelocity and continue the treatment in a jet–fluidized bed.

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