~ Pergamon

0273-1223(95)00160-3

Wat. ScL T~ch. Vol. 31, No. I, pp. 117-128, 1995." C0i>yrigbt 4) 1995 lAwQ

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BIODEGRADATION RATES OFAROMATIC CONTAMINANTS INBIOFILM REACTORS

Jean-Pierre Arcangeli and Erik Arvin

Institute ofEnvironmental Science and Engineering. The Technical University ofDenmark, Building liS. DK-2800 Lynglly, Denmark

ABSTRACT

This study has shown that microorganisms can adapt to degrade mixtures of aromatic pollutants at relativelyhigh rates in the I1g/l concentration range. The biodegradation rates of the following compounds wereinvestigated in biofllm systems: aromatic hydrocarbons. phenol, methylphenols, cbloropbenols, niuophenol.cblorobenzenes and aromatic nitrogen-. sulphur- or oxygen-containing heterocyclic compounds (NSO•compounds). Furthermore. a comparison with degradation rates observed for easily degradable organics isalso presented. At concentrations below 20-100 Ilg/l the degradatioo of the aromatic compounds wastypically controlled by first order kinetics. The flJ'St-order surface removal rate constants were surprisinglysimilar, ranging from 2 to 4 mid. It appears that NSO-compounds inhibit the degradation of aromatichydrocarbons, even at vay low concentrations of NSO-conlpoWids. Under niuate-reducing conditions,toluene was easily biodegraded. The xylenes and etbylbenzene were degraded cometabolicaUy if toluene wasused as a primary carbon source; their removal was influenced by competitive inhibition with toluene. Theseinteraction phenomena are discussed in this paper and a kinetic model taking into account cometabolism andcompetitive inhibition is proposed.

KEYWORDS

Kinetics; biofllm; cometabolism; inhibition; aromatic hydrocarbons; phenols; heterocyclic compounds.

INfRODUcnON

The rapid advances of industrial technology and the development of petrochemical-, oil-, solvent-, and otherchemical industries have given rise to a huge consumption of man-made chemicals. Aromatic compoundsare among the 50 largest-volume industrial chemicals produced. with annual production figures in themillions of tons (Bouwer et al. 1992). These compounds are widely used as fuels and industrial solvents forpainting and degreasing. Furthermore, their chemical structures provide a starting material for the productionof pharmaceuticals, agrochemicals. polymers, explosives etc. Thus, it is not surprising that they arecommonly found in soils and groundwater. Contamination of groundwater also arises from spillage ofgasoline or other petroleum-based fuels. from leakage of gasoline, and from underground storage tanks.Another source of contamination, and probably one of the oldest, are creosote waste sites.

The design of a flXed-fllm bioreactor for the biorestoration of contaminated aquifers requires informationabout the mechanisms and kinetics of degradation in biofilm systems. Furthermore, substrate interactions arealso of interest since most polluted aquifers contain a whole spectrum of pollutants. Many reports have beenpublished on biodegradation of these aromatic compounds. However. most of the research has been carried

117

118 I.-P. ARCANGELI and E. ARVIN

out with suspended cultures, in aquifer columns, or in the field. It is interesting to note how little informationis available on biodegradation of aromatic compounds in biofilm systems. This is surprising since manytransformations in nature and in water- and wastewater treatment plants are carried out in biofilm systems.The purpose of this paper is to give an overview of the kinetics of biodegradation of aromatic compounds inbiofilm systems, including interaction phenomena.

KINETIC CONSTANTS

Kinetic data presented in this review were determined in a fixed film bioreactor. Details of the experimentalprocedures can be found in Kristensen and Jansen (1980). Other studies refered to (Bouwer et al., 1992;Rittmann et al., 1988) were performed with continuous-flow biof1lm columns containing glass beads.

The theoretical background for reactions in biofllms has been dealt with by several researchers. The theoryput forward by Harremoes (1978) is applied here because of its simplicity. The nomenclature and thecalculation method are given in the Appendix.

TABLE 1. Degradation kinetics of aromatic hydrocarbons in biof1lm systems under aerobic conditions

kl.a k\;.• k..r kl,f kx Ks kxlKs T References(mid) (g~ m'~ d") (kg m" d") (10' d'l) (mg, gi' d·l) (pgIL) (L mg"x <1") "C

Individual compoundBenzene 2.08 0.6 4.89 4.4 18 Pedersen (1992)

Toluene 2.5 I.l7 7.3 7.3 440 100 4.4 20 Arcangeli and Arvin (1993)9.5-13 2.48-4.19 27.9-79.9 84.6-160 1540-4770 30-80 29-95.4 25

Naphthalene 3.6 1.46 39.8 48.1 2154 82.71 26 10 Arv,n and Arcangeli (1994) (TO)3.5 1.78 58.9 45.5 129.5 10 ArvIn and Arcangeli (1994)

MixtureBenzene 2.33 0.44 2.68 14.9 99.1 18 5.51 10 Arvin and Arcangeli (1994) (TI)

Toluene 2.8 0.61 5.84 25.4 217 23 9.43 10 Arvin and Arcangeli (1994) (TI)0.085 0.0125 0.025 22.5 Bouwer et aI .• (1992)

Ethylbenzene 3.05 17.5 35 25 Bouwer and McCany, (1985)

Trimethylbenzene 0,5 2 0.99 8 3.4 8 0.43 10 Arvin and Arcangeli (1994)

Styrene 3.67 25 50 25 Bouwe, and McCany. (1985)

o-Xylene 2.8 0.65 7.24 29.8 267 24.3 11 10 Arvin and Arcangeli (1994) (Til0.051 0.005 om 25 Bouwe, el aI., (1992)

Naphthalene 3.46 0.84 12.8 48.6 473 26.3 17.9 10 Arvin and Arcangeli (1994) (TI)

3.19 20 40 25 Bouwe, and McCany. (1985)

0.35 0.25 0.5 22.5 Bouwer et aI .• (1992)

CHI-naphthalene 4.09 54 29.2 10 Arvin and Arcangeli (1994)

<;H,-naphthalene 3.2 35.4 19.1 10 Arvin and Areangeli (1994)

(CHJ2-naphthalene 2.4 0.4 3.12 22.7 116 13.7 8.42 10 Arvin and Arcangeli (1994) (Tt)

0.153 0.05 0.1 22.5 Bouwe, el aI., (1992)

Biphenyl 3.4 0,7 10.2 50.6 378 20.2 18.7 10 Arvin and Arcangeli (1994) (TI)

Phenanthrene 2.6 0.44 4.44 31 164 14.3 1l.5 10 Arvin and Areangeh (1994) (Til

Total H-C 2.57 1.36 32.6 24.18 1206 135 13.9 10 Arvin and Arcangeli (1994) (TI)

Xylenes 3.62 2.9 124 38.7 9500 320 29.7 25 Debu, and Wanner (1992)

TI: refer to an identical kinetic experiment.

Aromatic hydrocarbons under aerobic conditions

Most aromatic compounds can be degraded under aerobic conditions, and a vast amount of data has beenpublished on this subject. However, information is scarce on biofilm systems. Kinetic parameters related toaromatic hydrocarbons in biof1lm systems are compiled in Table 1. The degradation was ftrst-order at lowsubstrate concentrations. Most of the ftrst-order rate constants, kt,a' show a small variability, between 2 and4 m d-I. However, the calculated kl,a values reported by Bouwer et al. (1992) are very low. This could be

Aromatic contaminants in biofl1m reactors 119

the result of liquid ftlm diffusion due to the bioftlm reactor they used in their study. Figure I displays thesurface removal rate of a mixture of seven aromatic hydrocarbons (refered to as Tl in Table 1). Thedegradation was nrst-order for concentrations below 20 Ilgll. The observed kl,a values ranged between 2.3and 3.5 m d-I. The half-order rate constants, k ll2,a' also show a small variability, between 0.44 and 0.84 glflm-Ifl d- I (at 10 OC). Furthermore, it appears that the occurrence of some aromatic compounds inhibit thedegradation of others. As an example, the kinetics of naphthalene biodegradation as a sole carbon source canbe compared with its biodegradation under circumstances where other aromatic hydrocarbons are present(exp. TO and TI, Table I). This comparison is possible because of the same operating temperature and thesame source of inoculum. The naphthalene degradation rate was first-order for concentration below 200Ilgll. However, in the presence of other aromatic hydrocarbons this concentration range decreased to 20 Ilgll.Furthermore, the half·order rate constant, k lJ2,a' and the maximum substrate utilisation rate, kx, werereduced compared to the situation with naphthalene degraded as a sole carbon source. This suggests aninhibition phenomenon from the other aromatic hydrocarbons toward naphthalene. The k loa value, however,remained unchanged.

It should be noted that the composite parameter, "total-biodegradable hydrocarbons" (Exp. TI), behaved likea single compound; Le. the plot of the degradation rate of the "total-biodegradable hydrocarbons" versus theconcentration in the reactor was similar to the one observed for a defmed compound. Again, the nrst-orderremoval was observed at low substrate concentration in the range up to 200 j.tgll. This is similar to the valueencountered for the degradation of individual compounds as sole carbon sources (100-200 Ilgll). The valuesreported by Debus and Wanner (1992) are related to "technical xylene" which is a mixture of xylene isomersand ethylbenzene. Again, kinetic data suggest that this mixture behaved like a single compound (Table I).

1800- 160 0 0 0 0'to

"0"0.. ..

'liS 140 ..00

co120

o ~..~ •• 0

!,• ~_ 0

• •0:0 ifJ -0_

H 100 11. -:6- • • • •£l..0 .... -. •~

80 ...- .. . 0.0

~~ . • • ••~

60 6, • 0 -• - •ca

~40 t~· -20 6

~ ••00 100 200 300 400 500

lBulnc concentration ijB.gIlL)Fig. I. Removal rate of a mixtlD'e of seven aromatic hydrocarbons vs concentration in reactor. (.) Benzene;(al toluene; (e) o-xylene; (0) naphthalene; (.) dimethyl-naphthalene; (A) biphenyl; (~) phenanthrene.

Aromatic hydrocarbons under anoxic conditions

For heavily polluted environments, aerobic biorestoration is often limited due to low oxygen solubility andpoor oxygen mass-transfer. Therefore, alternative electron acceptors are of interest Denitrifying conditionshave received much attention since the addition of nitrate to a contaminated site would be a feasible in situtechnique due to the low cost and the high solubility of this electron acceptor.

120 l .•P. ARCANGELI and E. ARVIN

TABLE 2. Degradation kinetics of aromatic hydrocarbons in biofilm systems under denitrifying conditions.

kl .. k.... k..r klJ kx Ks kxlKs Exp.(mid) (g. m·· <1"') (kg m" d") (10' <1"') (mg, gi' d") (~gIL) (L mg". d")

Individual compound(llToluene 1.18·1.33 1.2·3.84 62.9·81.2 1.9·13.8 !3SQ..1820 58Q..1660 0.81-3.13MixtureToluene 1.75 1.03 17.1 9.85 536 173 3.1 T2

2.01 13 3.67 T3Ethylbenzene 0.05 0.009 0.0028 T2

0.029 0.003 0.00085 T3mlp-Xylene 0.028 0.003 0.00089 T2

0.045 0.007 0.00206 T3o-Xylene 0.23 0.23 0.94 0.2 29.5 465 0.059 T2

0.5 0.88 0.251 T3Total-HC 0.32 I 17.4 0.34 546 5100 0.106 T2Total-HC I 0.6 6.17 3.48 174 177 0.983 T3

(I) From Arcangeli and Arvin (1993). 1'2 and T3 are two independent kinetic experiments (unpublished results). Temperature 20 "C.

With our nitrate-reducing culture, toluene was not only the most biodegradable compound, but also the onlyaromatic hydrocarbon able to support efficient microbial growth. As can been seen from Table 2. the kineticparameters for toluene biodegradation under denitrifying conditions are similar to those encountered underaerobic conditions. However, the other aromatic hydrocarbons are more difficult to degrade biologically.Arcangeli and Arvin (1994a) reported that 80% of the total hydrocarbons biodegraded was due to the tolueneremoval. 12% to o-xylene. 5% to m1p-xylene and 3% to ethylbenzene. Benzene and naphthalene wererecalcitrant. The kinetic parameters summarized in Table 2 were obtained under circumstances where BTEX(benzene, toluene, ethylbenzene and xylenes) were the sole carbon sources. Ethylbenzene and xyleneisomers were degraded according to a cometabolic pathway. The evidence of cometabolism of the xylenesand ethylbenzene is illustrated in Figure 2 demonstrating that their removal were strongly dependent ontoluene in the reactor. Therefore, the first-order rate constants listed in Table 2 related to xylenes andethylbenzene are in fact pseudo first-order rate constants.

0.25.t:'"' r:-----"0

l 0.20 Toluene stopped

-;~ 0.15 Toluene added again~ a

i 0.10 a0

i 0.05t.?)

00 100 200 300 400 500 600 700

TIme (mm)

Fig. 2. Evidence of cometabolism of (0) o-xylene; (el) mlp-xylene; (II) ethylbenzene. NB: When toluene wasadded again. the ethylbenzene could not be quantified due to an analytical problem. From Arcangeli and Arvin

(19941).

Phenolic compounds

Aromatic contaminants in bioftlm reactors 121

Phenols ace among the most common organic chemicals in industrial and municipal wastewater and inpolluted groundwater from chemical waste sites and former gas works. Again, a large amount of informationis available with respect to degradability, toxiCity, and metabolic pathways. However, little is known of theirkinetics of degradation in biofilm systems.

~. Phenol is an easily biodegradable compound. As can be seen from Table 3, the kinetic parametersfor phenol biodegradation as a sole carbon source ace within the same order of magnitude as those found fordomestic wastewater or easily degradable organics such as acetate or benzoate (Table 6).

TABLE 3. Degradation kinetics of phenolic compounds in biofilm systems

kl .. ~ k"J klJ kx Ks kxIKs T References(mid) (I~ m'" It') (kl m" d") (10' It') (mg, Ii' It') (JJs!L) (Lmr'x lt ') re)

Individual compoundPhenol 20.8 1.86 27.84 700 480 4 120 13.8 Rittmann et aI., (1986)

3.07 17.32 334 115 16700 2900 5.76 30 Livingstone (1992)3.89 25.82 344.4 15.65 10435 22000 0.474 Chang and Rittmann (1987)4.63 3.21 88.23 36.8 13700 240 57.1 20 Bhamidimarri and a_field (1990)

p-cresol 0.034 O.QlI 0.008 20 Gantzer et aI., (1988)Mixtures of phenol and methylpbenolsPhenol 3.8 6.36 16 120 39 3.08 10 Arvin et aI., (1991a)o-Cresol 6 6.89 36 130 19 6.84 10p-Clesol 3.9 4.56 16.7 86 28 3.07 102.4-Dimethylphenol 1.7 3.34 4.7 63 71 0.89 103,S-Dimethylphenol 1.2 2.23 3.1 42 78 0.54 10TB-phenols 2.3 20.14 6.9 380 281 1.35 10Mixtures of chloropbenols, nltropbenol and peptone2,4-Dichlorophenol 4 19.46 3.37 30 Arvin et al.. (199Ib)2,4,6-Trichlorophenol 4 21.08 3.63 304-Nitrophenol 4 18.82 3.24 30

Mjxmre of phenol and methylphenols. A detailed description of the results is given by Arvin et al. (1991 a).Phenol, o-cresol and p-cresol were degraded with a relatively high rate. As expected. the removal was first•order at very low phenol concentrations, below 20-50 J.lgll-I. At higher concentrations the surface removalrate leveled off and maximum reaction rate (zero-order) was achieved above 140 J.lgll-I. A significantlysmaller reaction rate was found for the dimethylphenols. Figure 3a shows the removal rate for p-cresol and3,5-dimethylphenol. Again. the removal was first-order at very low concentrations, ranging from 20 to 50J.lgll-I. However, inhibition of 3.5-dimethylphenol degradation was observed above concentrations of 300•400 J.lgll-I. The same picture was found for 2,4-dimethylphenol. The inhibition of dimethylphenolsdegradation was probably not a result of a toxic influence by the compounds. The removal rate of "total•biodegradable phenols", (TB-phenols), increased to a maximum level, i.e. there was no sign of inhibition ofthe overall process (Fig. 3b). It is therefore hypothesized that the decrease in reaction rate ofdimethylphenols at relatively high substrate concentrations is due to preferential degradation of the mosteasily degradable compounds, phenol and the monomethylphenols when plenty of substrate is available forthe bacteria and when they operate near their maximum capacity (Arvin et aI. 1991a).

Altogether. the phenols first-order rate constants. kl,a' are of the same order of magnitude as those compiledin Table 1. However, the maximum substrate utilization rates, kx, are lower than those found for aromatichydrocarbons.

122 l.-P. ARCANGELI and E. ARVIN

2500

(lb)

20001500

........................

1000

.' '.1000

"-. 100

!I 600 ..1400 .;

I 200 ..~

ro0

0 soo100'lOG

(a)

500 600400

..' ~

.'

250,--------------.....,(;-I 200

.II 150 .•

1..

100 .•.... lIS ••••••••••

I 50 ;'.=:' •o~~.-_-_-+___-__+_-......- .....-''-~-I

o 100 200 300

Fig. 3. Removal rate ofp-cresol (.) and 3.5-dimetbylpbenol (0) vs. concentration in reactor (a). Removal rate of"IOtal-biodegradable phenols" vs. concentration in reaclOr (b). (From Arvin et al.• 1991a)

Mixtures Qf chlQrophenQls and nitrophenQl. The degradatiQn of 2,4-dicblorophenol, 2.4.6-trichlorophenoland 4-nitrophenol was studied at bulk water concentrations from 3-45 Ilg/l of each phenolic compound. Thebiofilm growth was supported by peptone as the dominant carbon source. Only 1% of the total organicmatter in the influent to the biofilm was phenolic compQunds. The removal rates of the three phenols werethe same at a bioftlm thickness of 750 Ilm. The removal was fust-order at concentrations below 20 1lg/l-1

with a ftrst-order rate constant, kl,a' of 4 m d- I for the three phenols. This rate was similar to those found forphenol and the methylphenols (Table 3) despite the very different operating situations when the latter wereadded as sole carbon sources.

Mixture Qf chlorinated benzenes

Bouwer and McCarty (1985) investigated the biodegradation of trace concentrations of chlorinated benzenesin an aerQbic biofilm. The grQwth was supported by acetate as the dominating carbon source. Kinetic dataare compiled in Table 4. It is shown that the chlorobenzenes have about the same degradation rate constantsas phenol and the methylphenols shown in Table 3. Furthermore, it appears from this study that the removal•rate of chlorobenzenes tended to increase with time but not to the extent of the primary substrate, acetate.

TABLE 4. Degradation kinetics of chlorinated benzene in an acetate-supported bioftlm. (Bouwer andMcCarty, 1985)

(·)kl.. (·)kl.l kxlKs T

(mid) (10' d") (Lm,·'.ct') ('C)

PrImary substrateAcetate 0.46-5.11 0.24-30 0.48-60 20Secondary substrateChl0r0benzene 0.52-1.41 0.45-3.35 0.91-6.7 201,2-Dichlorobenzene 0.21-2.98 0.085-16.5 0.17-33 201.4-Dichlorobenzene 0.51-3.29 0.48-20 0.95-40 201.2,4-Trichlorobenzene 0.13-2.5 0.035-12.5 0.07-25 20

(.) Calcualted based on information in the paper.

Mixtures Qf aromatic hydrocarbons and NSQ-cQrnpQunds

Creosote-contaminated groundwater contains a complex mixture Qf phenols. aromatic hydrocarbons andaromatic nitrogen-. sulphur- or oxygen-eontaining heterocyclic compounds (NSO-eompounds). Thereforethe kinetics of biodegradation of individual compounds in such a mixture is of practical interest for thedesign of treatment processes and biQremediation schemes of creQsote-cQntaminated groundwater. For thatpurpose an artificially creosote-contaminated groundwater was supplied to a biofilm reactor. Theconcentration of each of the compounds was below 300 Ilg/l-I.

Aromatic contaminants in biofilm reactors IZ3

There was a clear effect from the NSO-compounds since they reduced the degradation rate of the aromatichydrocarbons (Table 5). Both the maximum substrate degradation rates, kx, and the half-order rateconstants, kl/Z•a• were reduced compared to the situation with pure aromatic hydrocarbons <Table 1).However, the fIrst-order rate constant, kt,a. remained unchanged when compared with those listed in Table1. This constant seems to be unaffected by the presence of NSO-compounds (except for toluene for whichthe kl.a-value decreased from 2.8 to 1.13 m d-I). This suggests that for a given aromatic hydrocarbon theinhibitory effect from NSO-compounds occurred in the half- and zero-order concentration range; i.e., above20 Ilg/l- I. As an example. the inhibition of naphthalene degradation by NSO-compounds is shown in Figure4. At low naphthalene concentrations (below 20 Ilg/l-I) no clear inhibition from NSO-eompounds occurred,whereas at higher concentrations inhibition from NSO-eompounds was observed.

TABLE 5. Degradation kinetics of mixture of aromatic hydrocarbons and NSO-compounds in biofilmsystems. From Arvin and Arcangeli (1994). Temperature 10 OC.

k,.. k,... k.J k,.r kx Ks kxlKsmid g\ltm·lo\[fl (kg m" d") (10' d") (mg, gi' d") (~gIL) (L mg". d")

Toluene 1.13 0.35 1.87 7.64 66.8 24.5 2.73o-Xylene 2.Z7 0.36 2.24 19.7 80 11.5 6.96Naphthalene 3.2 0.47 4.06 39.8 145 10.2 14.2(CH3)2-Naphthalene 2 0.28 1.55 16.2 55.4 9.6 5.77Biphenyl 4.7 0.31 1.99 93.3 71.04 2.13 33.4Indene 5.4 0.63 7.13 110.6 254.6 6.44 39.5Phenanthrene 2.92 0.3 2.06 39 73.7 5.3 13.9Phenol 3.87 0.59 4.52 39.1 161.5 11.5 14o-Cresol 2.86 0.43 2.67 24.3 95.4 II 8.67Quinoline 4 0.63 7.41 59.3 264.6 12.5 21.2Dibenzofuran 2.73 0.26 1.47 31.7 52.5 4.6 11.4F1ourenonc 3.6 0.40 3.6 58.7 128.7 6.1 21.1Dibenzothiophene 2.8 0.32 2.17 34.3 77.6 6.3 12.3Total HC 0.77 0.84 13.4 3.86 478.4 346.9 1.38TotalNSO 3.21 1.02 18.6 37.1 664.8 50.2 13.2Total HC + NSO I 1.2 26.5 9.89 947.5 268.4 3.53

200

7; •....a 160 •.... .......... ............ ii· li ····•···......•.

l ...... •120 ~.' •! ·iI..

1 :.80 .:

.:: .··o···?··o................................ 'd ..•... ·0····· . .C! •••••••••

• ~ ... o 0 0@

40~

•. 0 0.~

rfliI:l

00 50 100 150 200 250

Naphthalene bulk concentration ijLgIL)

Fig. 4. Influence of NSO-compounds on naphthalene removal. (_) Naphthalene with aromatic hydrocarbons. (0)Naphthalene with NSO-compounds.

124 J.·P. ARCANGELI and E. ARVIN

With regard to the phenolic compounds contained in the mixture. namely phenol and o-cresol. interactionphenomena were noticed. The o-cresol degradation rate decreased compared to the situation with purephenolic compounds (Table 3). Phenol degradation. however. did not seem to be inhibited by the othercompounds.

Modelljn~ of substrate interactions in biofilm systems

The biodegradation of aromatic compounds is influenced by various factors. such as pollutant concentration,active biomass concentration, temperature. pH. availability of organic nutrients. electron acceptors,inhibitors etc. In addition. certain individual aromatic compounds have the ability to stimulate or inhibit thebiodegradation of other aromatic compounds. These substrate interactions are primarily inhibition orcometabolism. The kinetic parameters listed in the previous tables have been calculated on a simpletheoretical background by Harremoes (1978). Neither inhibition phenomena nor cometabolic reactions weretaken into account. The understanding of these substrate interactions as well as their detailed quantitativeinterpretation are of practical interest for soil and groundwater pollution since an environmentalcontamination with a single compound is unusual.

For that purpose. cometabolic degradation was carried out in a biofJ.1m reactor with two model substratesunder denitrifying conditions. Toluene was the primary substrate and a-xylene was the co-substrate. Detailsof the procedure are presented by Arcangeli and Arvin (1994b). A decreasing toluene degradation with anincreasing concentration of o-xylene was observed (Fig.5a). Furthermore. it was found that lack of theprimary carbon source, toluene, prevented a-xylene transformation (Fig. 2). On the other hand, if provided inexcess, toluene inhibited a-xylene transformation (Fig. 5b). These interactions may be explained bycompetitive inhibition between the two substrates if it is assumed that o-xylene and toluene are degraded bythe same enzyme and that o-xylene has a lower affmity than toluene.

t 6 t 0.6

0.("' :r. (a) ~ 0.5• • •• • • • t~0.41i: • .. ". .A 0.3..

• J 0.2

J 1 0.1i<12 0 ~ 0

0 5 10 15 20 25 30 35o-Xylene COncentraliOD in reactor (mall)

~.

Ii.~"" .-

o 20 40 60 80Toluene concentration In reactor (mlVl)

Fig. 5. InteraCtion pbenomena between toluene and o-xylene. lnfluellCe of o-xylene collCeDlration on the tolueneremoval rate (a). Influence of toluene coDcenl1ll1ion on the o-xylene removal rate (b). Arcangeli and Arvin (1994b).

The reaction rate of the primary substrate. which supports the biomass growth. can be described by equation(1), assuming competitive inhibition (Bailey and Ollis, 1977). The nomenclature is given in the Appendix.

Sid + KS(I

(I)S

l+~KS(1d)

(2)

The co-substrate cannot serve as a carbon source for cell synthesis, but its transformation can be enhancedwhen the primary substrate is supplied in the reactor. Furthermore, it is hypothesized that the enzyme thatinitiates the transformation of the co-substrate is synthesized in the presence of the primary substrate only.Enzyme production is described by a Michaelis-Menten type of equation (Lehninger. 1972). Co-substratedegradation is described by equation (2), taking into account the competitive inhibition and the stimulating

Aromatic contaminants in biofdm reactors 125

effect of the primary substrate. Further information about this modelling is given by Arcangeli and Arvin(l994c). The cometabolic transformation of a-xylene, and the substrate interactions with the primarysubstrate, toluene, could be modelled very well by using the mathematical expressions shown above. Forthree independent kinetic experiments, the toluene and a-xylene half-saturation constants were maintained atthe same value: 0.11 and 0.35 mg/l-1, respectively.

ComDarisoD with easily deuadable compounds

Table 6 gives a summary of the parameters found for a variety of organic compounds. In general. themaximum degradation rates, kx. are higher than those observed for aromatic compounds. Again, the flrst•order rate constants are of the same order of magnitude as those listed in the previous Tables for aromaticcompounds. However, the higher half-saturation constants observed for those compounds suggest that theirdegradation was flest-order for concentrations up to 5-20 mg/l. Under anoxic conditions, higher removalrates for denitrification were achieved with polar compounds as carbon sources compared to hydrocarbons.For example. data reported by Kissel et aI. (1984) show that the benzoate degradation rate is higher than therates observed for toluene (Table 1). This suggests that polar aromatic compounds are more biodegradablethan the corresponding hydrocarbons. This is supported by J0rgensen (1992). who showed that the a-methyl•benzoic-acid was degradable under nitrate-reducing conditions. whereas the a-xylene was recalcitrant unlessa primary carbon source was provided.

TABLE 6. Degradation kinetics of specific organic compounds in biofllm systems

k'A k.u k"., kl.l kx Ks kxlKs T References(mid) (J~ m'" d") O<g ",., cr') (10' cr') (mg, gi' d") (mgIL) (l. mg". cr') ("C)

Aerobic conditionsAcetate 1.11 3.15 46 U5 33000 4 8.25 20 Namkung et aI.• (1983)

1.16 3.3 SO 125 ooסס2 4 5 20 Rittmann and McCarty (1980)7.07 13.8 2214 58.6 33100 3.78 8.73 22 Namkung et aI .• (1983)

Galactose 0.78 S.32 246 1.07 15000 23 0.65 20 Ritttnann and Brunner (1984)1.27 9.14 916.5 3.55 13700 25.8 0.53 22 Namkung et aI.• (1983)

Benzoate 1.27 6.26 200 1.67 ooסס2 12 1.67 20 Kissel et aI., (1984)Glucose 3.2 578.9 11 Jansen and Harrernoes (1984)Unspec. COD 2.4 3.16 60 1.2 12000 5 2.4 20 Wanner and Gujer (1986)DenitrificationAcetate 2.29 2.9 30 37.5 2000 0.8 2.5 20 Rittmann et aI., (1988)Benzoate 0.57 2.8 40 0.33 4000 12 0.33 20 Kissel et aI., (1984)Methanol 3.64 10.45 420 2.8 10500 IS 0.7 20 Ritttnann and McCarty (1980)

2.94 55.3 22 Jansen and Harremoes (1984)

CONCLUSIONS

1. This study has shown that several commonly found aromatic pollutants can be biodegraded with relativelyhigh rates even at very low concentrations, i.e. in the Jlg/l.l concentration range.

2. At concentrations below 20-100 Jlg/l-1, the degradation is typically controlled by flCSt-order kinetics. Theflest-order rate constants, k1•a• for the different aromatic compounds investigated are surprisingly similar.Many of the kl.a-values reported in this paper range between 2 and 4 m d·l.

3. Under aerobic conditions, aromatic compounds are generally degradable as sole carbon sources. However.the presence of heterocyclic NSQ-compounds. even at very low concentrations (in the range of 0.1 mg/l- I).decreased the degradation rate of the aromatics hydrocarbons. This is of interest since NSo-compounds are afamily of compounds widely found in creosote contaminated groundwaters.

126 J.-P. ARCANGELI and E. ARVIN

4. Under nitrate-reducing conditions, the toluene was easily biodegraded. However, the ~ylenes andethylbenzene were degraded cometabolically using toluene as the primary carbon source. A detaIled study ofthe cometabolic transformation of o-xylene with toluene as the primary substrate revealed a cross-inhibitionphenomenon. Toluene promotes the degradation of o-xylene. However, an excess of toluene inhibits 0•

xylene degradation. Furthermore, o-xylene inhibits the toluene degradation. These substrate interactionswere modelled based on a concept of competitive inhibition, which assumed that toluene and o-xylenecompete for the same enzyme.

ACKNOWLEDGEMENTS

The authors would like to thank the Groundwater Centre at the Technical University of Denmark: for itsfrnancial support.

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APPENDIX

Nomenclature

S: substrate concentration (mg/l-I)Stol: toluene concentration (mg/l-I)So-xyl : o-xylene concentration (mg/l-I)1'1 a' r1/2,a and 1'0 a: fIrst-, half and zero order surface removal rates (gm-2d- I)k; a' k1/2.a and ka,a: flI'St-, half and zero order rate constants. (md-I, glflm- l12d-1 and gm-2d- l)kl'f and ko,r: fIrst and zero order inUinsic reaction rate constants in the biofdm. (d-I and gm-3d-l)k~: maximum substrate utilization rate (mgs gx-l dol)kX(o-xy\): maximum o-xylene utilization rate, mg o-xylene/(g biomass day).kXCtol): maximum toluene utilization rate, mg toluene/(g biomass day).Ks: Monod constant (half-saturation constant) (mg/l-I)KSCtoO: toluene half-saturation constant (mg/l-I)KSCo-xy\): a-xylene half-saturation constant (mg/l-l)Xf: biofllrn dry weight density (gm-3)D: diffusion coefficient of the substrate in the biofIlm (m2/d)L: biofllm thickness (m)a: biofllm constant (-)£: flI'St order efficiency (-)

EQuations

First-order removal rate: rl,a =kl,a x SHalf-order removal rate: rl/2,a =kl/2,a x SII2Zero-order removal rate: rO,8 = kO,1

with:

k. Xk.... ku x L x £ • K x , x L x £

•

~ .. J2 x D x koJ

k.... k.xX,xL.koJxL

(1)(2)(3)

(4)

(5)

(6)

128

and

£ = Tanh aa

l.-P. ARCANGELI and E. ARVIN

(7)

where a = Jkl..xL (8)exD

Reaction rate constant are measure graphically (Le., in Fig. 4). Xc and L were determined experimentally.The diffusion coefficient of the organic was estimated according to the Wilke and Chang method (Perry andGreen 1987). The relative diffusivity of the substrate was estimated as 0.47 (Fan et aI.. 1990).

a is found through iteration from equation (7) and (8). For a greater than 2, I; '" 1/a. The Monod constant.Ks' is estimated from eq. (9). St is the substrate bulk concentration at the transition from fIrst- to half-order.

SK = I52 (9)