Patchy Biofilm Coverage Can Explain the Potential Advantage of

Aug 16, 2003 - patchy biofilm coverage with exposed areas of GAC develops. Adsorption and ... carrier and a GAC carrier with an atrazine degrading bio...
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Environ. Sci. Technol. 2003, 37, 4274-4280

Patchy Biofilm Coverage Can Explain the Potential Advantage of BGAC Reactors MOSHE HERZBERG, CARLOS G. DOSORETZ, SHELDON TARRE, AND MICHAL GREEN* Faculty of Civil and Environmental Engineering, Technion, Haifa 32000, Israel

An adsorbing biofilm carrier, like granular activated carbon (GAC), can be the source of an extra flux of pollutant to the biofilm in addition to the bulk liquid. This double flux can improve the performance of a biological GAC (BGAC) reactor as compared to a nonabsorbing carrier reactor but only under conditions of pollutant partial penetration in the biofilm. Pollutant partial penetration in a biofilm often occurs in treatment processes where very low effluent concentrations are required. However, under these conditions, adsorption in BGAC reactors is questionable and requires the existence of biofilm free areas on the GAC carrier. The purpose of this investigation is to prove that under normal BGAC fluidized bed reactor operational conditions patchy biofilm coverage with exposed areas of GAC develops. Adsorption and desorption through these exposed areas can explain the widely debated advantage of BGAC reactors regarding higher biofilm activity. The patchylike nature of the biofilm coverage on the GAC particles was verified using experimental and modeling tools. Comparison between a nonadsorbing granular carbon carrier and a GAC carrier with an atrazine degrading biofilm (Pseudomonas ADP) under conditions of atrazine partial penetration in the biofilm showed higher biodegradation and lower effluent atrazine concentrations in the BGAC reactor.

Introduction For the past 30 years numerous researchers have investigated the use of granulated activated carbon (GAC) as a carrier for biofilm in reactor systems. The combination of biofilm with GAC as the carrier is reported to have the following advantages: (1) The rough surface of the particles containing macropores with wide channels provides excellent shelter from fluid shear forces for colonization of microorganisms (1, 2). (2) Due to its adsorptive properties, GAC has the ability to attenuate high or toxic incident influent concentrations of pollutant while maintaining constant effluent quality (3-5). (3) Gradual desorption of shock loads at nontoxic concentrations allows for pollutant biodegradation (6). (4) Higher biofilm activity is achieved compared to inert media, due to substrate desorption from the GAC (3). While the first three advantages are well established and documented, the last one regarding the higher biofilm activity is controversial. In BGAC reactors the microorganisms comprising the biofilm have two potential sources of * Corresponding author phone: 972-4-8293479; fax: 972-48221529; e-mail: [email protected]. 4274

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substrate: the liquid phase (bulk solution) and the solid phase (desorption from GAC). The mechanism of adsorptiondesorption in GAC has been discussed by various researchers (3, 7, 8). However, their findings with regard to such a mechanism are contradictory, and no decisive conclusions can be drawn. The present paper contends that the reason for the controversy stems from the differing operational conditions of the BGAC reactor systems studied. The advantage of higher biofilm activity will only be observed under conditions of substrate partial penetration, where the deeper layers of the biofilm do not receive substrate. Under these conditions, substrate desorption from the GAC to the adjacent base biofilm layer will increase the total biofilm activity. While this line of reasoning is straightforward, the adsorption of the substrate to GAC (an obvious prerequisite to substrate desorption from GAC) under conditions of substrate partial penetration to the biofilm is not clear. Biofilms have a nonuniform structure consisting of interconnecting channels and pores (9-12). Typical flow rates prevailing in fluidized bed (FB) BGAC reactors exclude convective mass transfer through the open pores. The open pores are diffusion-limited as the biofilm itself and cannot provide the means for adsorption to the GAC under substrate partial penetration to the biofilm. GAC is characterized by a very irregular surface with many holes, ridges, and crevices. The regular removal of excess biomass from a FB reactor to ensure steady-state conditions and to prevent bioparticle washout is performed regularly by mechanical means. As a result, a patchy-like biofilm coverage develops where the biofilm resides mainly in sheltered areas. This type of coverage has been suggested by previous epifluorescent microscopic observations (13). The exposed areas can provide the sites for substrate adsorption to GAC even under conditions of substrate partial penetration into the biofilm. If indeed exposed areas on the GAC exist through which substrate adsorption under conditions of partial penetration take place, a BGAC reactor should show a higher effluent quality due to the increase in the biofilm activity when compared to a reactor with a nonadsorbing carrier. The increase in biofilm activity is due to a double flux of the pollutant to the biofilm: one from the bulk solution and the other from the GAC. The intrinsic adsorption-desorption mechanism is characterized as follows: a given pollutant is first adsorbed to the GAC in the nonbiofilm covered areas, diffuses through the GAC pores, and is subsequently desorbed to the inner biofilm layers that would otherwise not be exposed to the pollutant. This mechanism can be effective even when the exposed areas consist of only a small fraction of the GAC surface area. The purpose of this investigation was to demonstrate the existence of a patchy-like biofilm coverage on the GAC particles with exposed areas that can allow for substrate adsorption under conditions of substrate partial penetration in the biofilm. To determine the type of biofilm coverage present, the effect of different BGAC biofilm concentrations on adsorption kinetic in batch experiments was studied. A modified homogeneous surface diffusion model (HSDM) (14-17) was developed to evaluate two different biofilm coverage alternatives: (1) the GAC particles are fully covered by biofilm, with different biofilm thickness for different biofilm concentrations, and (2) the GAC particles are partially covered by the biofilm, with different coverage fraction for different biofilm concentrations in addition to changes in biofilm thickness. To demonstrate increased biofilm activity that results in higher effluent quality, a continuous fluidized 10.1021/es0210852 CCC: $25.00

 2003 American Chemical Society Published on Web 08/16/2003

FIGURE 1. Schematic diagram of a modeled BGAC slice. bed BGAC reactor was operated using Pseudomonas ADP (P.ADPsa fast atrazine degrading bacteria) with citrate as the electron donor and atrazine in low concentrations and compared to another reactor operating under identical conditions with a nonadsorbing carbon carrier (“Baker product”). Modeling. A modified HSDM model generally applied for adsorption and intraparticle diffusion in activated carbon was used to describe the BGAC system. The biofilm was taken as an additional layer adjacent to the GAC where atrazine mass transfer occurs (18). The biofilm was assumed to form either a traditional uniform layer fully covering the GAC particles (7, 18) or a nonuniform structure with a patchy-like coverage of the GAC particle (Figure 1). Model parameter values including the film layer coefficient (kf), the average GAC pore diffusion coefficient (Ds), the average biofilm diffusion coefficient (Db), biofilm average density (Fb), and biofilm coverage fraction (fc) were determined using a leastsquares fit for model and experimental results (15). Batch adsorption tests on virgin GAC (no biofilm) were first carried out in order to estimate atrazine mass transfer coefficients (film and pore diffusion coefficients). To prove that substrate adsorption occurs mainly in the regions not covered by biofilm, a baseline experiment of atrazine adsorption to fully biofilm covered GAC particles was conducted. A non-atrazine degrading biofilm was used to eliminate the interference of atrazine degradation. The biofilm concentration in this experiment was the maximal attached concentration attainable (see Materials and Methods). Assuming a full coverage (covered fraction, fc ) 1) the average biofilm diffusion coefficient and biofilm density were evaluated by the model. Adsorption kinetics under lower concentrations of the non-atrazine degrading biofilm were also studied. Using all the predetermined parameters, the fraction of the nonbiofilm covered area from total surface area of the GAC was determined for the nonuniform biofilm coverage model. This allowed for the evaluation of the effect of the biofilm coverage fraction (fc) on atrazine adsorption to the GAC. Basic Model Assumptions. The following assumptions were made for modeling mass transfer in the batch adsorption experiments: (1) The GAC particles were assumed to be spherical with homogeneous intraparticle characteristics. (2) The intraparticle diffusion of the adsorbate is governed by Fick’s law and takes place in the radial direction only. (3) The biofilm density is constant and uniform. (4) The biofilm diffusion coefficient is constant, and the adsorbate diffusion in the biofilm is governed by Fick’s law. (5) The biofilm geometry is spherical and exists only on the outer surface of the GAC. (6) Biofilm growth and decay are negligible (short-term batch experiments of several hours).

(7) The biofilm had no ability to degrade atrazine (the biofilm’s inability to degrade atrazine was experimentally confirmed). (8) Adsorption from the bulk liquid to the GAC particles occurs either via the biofilm or directly from the bulk via biofilm free areas on the GAC. The final concentration profile was based on the weighted average of the profiles obtained for the two cases and according to the biofilm coverage fraction. The profile was used to calculate the average total adsorbate concentration. It was assumed that the covered and noncovered areas of the biofilm are closely interspaced, so adsorbate transport distances (not in the radial direction) are relatively small allowing algebraic manipulation. (9) Substrate adsorption is completely reversible. (10) An instantaneous equilibrium exists between the solid adsorbed concentration on the outer surface of the GAC and any liquid interface (biofilm or bulk liquid). (11) The film layer coefficient (kf) is assumed to be the same for the both the GAC-bulk interface and the biofilmbulk interface. (12) The bioparticle radius is similar for the two different cases: covered and noncovered GAC areas. Model Equations for Batch Adsorption Experiments. Equation 1 describes the overall mass balance in a completely mixed batch adsorption experiment where Cr is the bulk concentration and qav is the average adsorbed concentration in the GAC particle, m is the total mass of the GAC particles, and V is the liquid volume.

dCr dqav ‚V ) -m dt dt

(1)

The average adsorbed atrazine concentration is represented in eq 2 where q(r,t) is the adsorbed atrazine concentration inside the particle, dp is the particle diameter, and r is the particle radial coordinate.

qav )

3 (dp/2)3

∫

dp/2

0

q(r,t)‚r2dr

(2)

The general partial differential equation that describes the adsorbed concentration q as a function of space and time, in spherical coordinates, is given in eq 3. Ds is the average pore diffusion coefficient.

(

)

∂q(r,t) ∂2q(r,t) 2 ∂q(r,t) ) D s‚ + ‚ ∂t r ∂r ∂r2

(3)

This equation was solved separately for each of the following cases: (a) external mass transfer of atrazine from the bulk liquid to the GAC through “biofilm free” areas on the GAC and (b) external mass transfer of atrazine from the bulk liquid to the GAC through the biofilm (non-atrazine degrading biofilm). For each of the above cases, the adsorbed atrazine concentration profiles q(r,t) were integrated for every time interval while taking into account the ratio of the biofilm coverage fraction. The fraction of uncovered area (1-fc) and biofilm covered area (fc) of the total GAC surface area (Atotal) are defined by eqs 4 and 5 accordingly.

( (

1 - fc ) fc )

)

Auncovered GAC Atotal

)

(4)

Acovered GAC Atotal

(5)

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overall profile at any given time interval is calculated according to eq 6:

q(r) ) fc‚qcase b + (1 - fc)‚qcase a

(6)

The general partial differential equation, which describes the atrazine concentration, Cb, in the biofilm as a function of space and time in spherical coordinates is given in eq 7, where Db is the average biofilm diffusion coefficient and rb is the biofilm radial coordinate.

(

)

∂Cb(rb,t) ∂2Cb(rb,t) 2 ∂Cb(rb,t) ) Db‚ + ‚ ∂t rb ∂rb ∂rb2

(7)

The initial adsorbed concentration for all cases is given in eq 8:

q(r, t ) 0) ) 0

(8)

The boundary condition at the center of the particle is maintained for all cases, i.e., the flux is zero at the center of the particle:

∂q(r ) 0, t) )0 ∂r

(9)

Equation 10 describes the average flux from the bulk to the GAC in case (a), at the GAC-bulk interface, where kf is film layer coefficient, Fp is the wetted particle density, and Cs is the concentration of the adsorbate in the liquid phase adjacent to the GAC surface.

∂q(r ) Dp/2,t) ) kf(Cr - Cs)‚(1 - fc) ∂r

Fp‚Ds‚

(10)

Equation 11 similarly describes the average flux to the GAC through the biofilm covered area (case b), at the biofilmbulk interface:

∂Cb ) kf(Cr - Cb)‚fc ∂rb

Db‚

(11)

An instantaneous equilibrium is assumed between the adsorbed concentration on the GAC surface and the solute concentration Cs and is expressed by the Freundlich isotherm:

q(r ) dp/2,t) ) KCsn

(12)

Similarly the following equation describes the solute concentration in the GAC-biofilm interface.

q(r ) dp/2,t) ) KCbn(r ) dp/2,t)

(13)

The biofilm depth (z) is calculated in eq 14, where Vp is the particle volume, X is the biomass concentration, and Fb is the biofilm average density.

z)

Fp‚Vp‚X π‚dp2.fc‚Fb

(14)

Parameter evaluation and sensitivity analysis were determined based on minimum least-squares function (15, 19), where k is the number of data points. k

f)

∑(C (t ) r

i experiment

- Cr(ti)model)2

(15)

i)1

Materials and Methods Chemicals. Atrazine (94% purity) was supplied by Agan Chemicals, Ashdod, Israel. GAC used was type F-400 (CalgonChemviron Co.). 4276

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TABLE 1. Biofilm Concentration and Operational Conditions for the Three BGAC FB Reactors Used as the Source for the Bioparticles in the Adsorption Batch Experiments biofilm concn [mg protein/ g GAC]

nitrate loading rate [g N/L/day]

citrate loading rate [g/L/day]

upflow velocity [m/h]

3.32 0.96 0.68

2.6 1.28 0.64

14 6.8 3.44

45 45 45

BGAC Bioparticles Production in FB Reactors for Adsorption Experiments. GAC particles covered with three different concentrations of a denitrifying biofilm were used in the batch adsorption experiments. A non-atrazine degrading biofilm was used to eliminate the interference of atrazine degradation. The bioparticles originated from continuous anoxic fluidized bed reactors using GAC (F-400, Calgon-Chemviron Co.) as the carrier operating at different loading rates. The growth conditions of the bioparticles are given in Table 1. In each case a different constant biomass concentration was maintained by daily removal of excess biomass by means of mechanical stripping of the bioparticles provided by applying high shearing forces (brushing), followed by filtering out the excess biomass and adjusting a constant bed height under a predetermined constant fluidization velocity. The maximal attained biofilm concentration was 3.32 mg protein/g GAC due to particle aggregation and washout at higher biomass concentration. A ratio of 0.11 ( 0.01 g protein/g VSS was used in the model as determined in a similarly operated reactor using a sintered glass carrier. Batch Adsorbing Columns. Four 250 mL columns filled with either washed virgin GAC (dry weight 100 g) or with GAC covered with a non-atrazine degrading biofilm were used in the batch adsorption experiments. A non-atrazine degrading biofilm was used in order to eliminate atrazine removal by biodegradation. Confirmation of the biofilm’s inability to degrade atrazine was carried out on an inert media (sintered glass beads). Virgin GAC particles were meshed between 0.8 and 1.2 mm. The columns were connected to 100 L tank containing buffered tap water with an initial atrazine concentration of 19-25 mg/L. A high recirculation rate between the feeding tank and the GAC column ensured completely mixed conditions. The atrazine concentration in the bulk solution and in the GAC was measured periodically. During the batch experiments the biofilm concentration was measured continuously in order to make sure that the biofilm concentration did not significantly change. Adsorption Isotherm for Atrazine. Different volumes (0.25-1.0 L) of buffered atrazine solution (25 mg/L, pH ) 7.2) were mixed at 25 °C with different amounts of GAC particles until equilibrium was reached (7-14 days). Freundlich isotherm coefficients of n ) 0.18 and K ) 79.4 [(L/ mg)0.18(mg/g)] were observed. These values are different from the results measured on pulverized activated carbon (PACF400) due to the different physical characteristics (14, 20, 21). FB Reactors for Biological Atrazine Degradation. Additional fluidized bed reactors with an atrazine degrading biofilm were operated for the verification of adsorption capabilities of the BGAC, under conditions of partial penetration in the biofilm. An aerobic fluidized bed reactor (2.5 L) filled with 700 g GAC (F400, Calgon-Chemviron) and the P.ADP bacterium with citrate as electron donor and atrazine as a sole nitrogen source was used. A second FB reactor with atrazine degrading biofilm but with nonadsorbing carbon particles was used for comparison purposes: particles with

FIGURE 2. Experimental results for batch adsorption tests with different biofilm concentrations on the GAC. the same physical characteristics as the GAC taken from the GAC (F400, Calgon-Chemviron) manufacturing process before the activation stage (“Baker product”). The available surface area for biofilm growth and the media porosity were identical for both types of carrier. The influent concentrations of atrazine were between 0.7 and 1 mg/L, and the retention time was 4 h. Analytical Methods. High atrazine concentrations (>0.2 ppm) were extracted and analyzed by HPLC according to Katz et al. (22). Low atrazine concentrations (0.2 ppm). Adsorbed atrazine was extracted with 20 mL of ethyl acetate, from 20 mg of dried (105 °C) and pulverized sample of BGAC. Ethyl acetate (0.4 mL) was evaporated, and atrazine precipitate was dissolved with 2 mL of acetonitrile and injected onto HPLC. Protein concentration was determined by mild hydrolysis of 1 g carrier in 10 mL of 2 M NaOH for 10 min at 60 °C. After diluting the sample 1:25 in distilled water, Bradford procedure (23) using Bio-Rad reagent was applied. Model’s Numerical Solution. A finite difference numerical solution was applied using MATLAB technical computing language (The MatWorks, Inc. version 5.2).

Results and Discussion The experimental results from the batch adsorption tests (Figure 2) show that atrazine bulk concentration decreased with time. This was due exclusively to adsorption since a non-atrazine degrading biofilm was used eliminating any possible atrazine degradation. The biofilm coverage of GAC acts as an additional mass transfer layer and should affect the kinetics of atrazine adsorption depending on biofilm concentration. On the other hand, the equilibrium adsorption capacity is only negligibly affected by the biofilm coverage. Indeed, the results in Figure 2 show that the rate of atrazine removal from the bulk solution (rate of adsorption) decreased with higher biofilm concentrations. Determination of Model Parameters. Determination of model parameters was divided into two sets of experiments. The first set included batch adsorption tests on a virgin GAC and on GAC covered with the maximal attained biofilm concentration. The results were used to evaluate the intrinsic parameters of the system (mass transfer coefficients and biofilm density). The second set of experiments was used to evaluate an intermediate condition where biofilm concentration was not maximally attained on the particles.

The adsorption results on the virgin GAC (no biofilm) were used for the determination of the film diffusion coefficient (kf) and the pore diffusion coefficient (Ds). Contours of f (eq 15) were drawn for different combinations of the two parameters (results not shown). The model results showed a minimum deviation from experimental results for the combination of kf ) 2.5‚10-3 cm/s and a Ds value above 3‚10-9 cm2/s. To compare film diffusion with pore diffusion the nondimensional Biot number was calculated (15):

Bi )

kfdpC0 2DsFpq0

where C0 is the initial liquid-phase atrazine concentration and q0 represents the equilibrium adsorbed concentration corresponding to C0 according to Freundlich isotherm. A Biot number lower than 6.4 was calculated, indicating that each of the two mass transfer mechanisms can be the rate limiting (15). Results from adsorption experiments on GAC with the maximal attained biofilm concentration (see Materials and Methods) were used to determine the average biofilm density (Fb) and biofilm diffusion coefficient (Db). The GAC was assumed to be fully covered by biofilm (fc ) 1) as suggested by epifluorescent microscopy examination. Minimum leastsquares analysis resulted in a biofilm density of 0.038 g VSS/ cm3 and a biofilm diffusion coefficient of 1.5 ‚ 10-7 cm2/s. Based on a biomass concentration of 3.32 mg protein/g GAC, the biofilm thickness was calculated to be 175 µm. The good agreement between the experimental and model adsorption results for the virgin GAC and BGAC particles with the maximal attained biofilm concentration is shown in Figure 3. The value obtained for the atrazine biofilm diffusion coefficient was relatively low. Liquid-phase diffusivities of herbicides are typically in order of 1‚10-6 cm2/s (24), and for biofilm diffusion these values are usually lower by less than an order of a magnitude. However, much lower values of up to 2 orders of magnitude for diffusion coefficients in biofilm relative to water have been reported (25). Moreover, it is known that biofilm diffusion is not uniform throughout the biofilm, which may be highly heterogeneous. The “conditioning film”, the first step in the process of biofilm formation, has been suggested as a possible barrier for substrate transport (26). In addition, biofilm roughness may add to VOL. 37, NO. 18, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Comparison between experimental (symbols) and model (lines) batch adsorption test results: virgin GAC (without biofilm) and BGAC with maximal attained biomass concentration (3.32 mg protein/g).

FIGURE 4. Minimum least-squares function (f ) values for combination of Db and fc in atrazine adsorption kinetic batch experiment, with medium biofilm concentration (0.96 mg protein/g GAC).

the flow resistance by decreasing the film diffusion coefficient, kf, thus contributing to the low value for the diffusion in the biofilm obtained from the model. Determination of Biofilm Coverage by Modeling. Two model alternatives for the description of the biofilm coverage were studied: (1) The GAC particles are fully covered by biofilm, with different biofilm thickness for different biofilm concentrations. (2) Patchy-like biofilm: the GAC particles are partially covered by the biofilm, with different coverage fraction for different biofilm concentrations in addition to changes in biofilm thickness. Batch adsorption tests on GAC particles with two biofilm concentrations (0.96 and 0.675 mg protein/g carrier) were used. In the case of the patchy-like model, the GAC-biofilm coverage fraction (fc) was calculated based on the predetermined parameters (Ds, kf, Db, Fb) using the minimal least squares function (Figures 4 and 5). When the sample with biofilm concentration of 0.96 mg protein/g carrier was used for the adsorption experiment, the results showed a biofilm coverage fraction of 0.55 (Figure 4). The second sample with 4278

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a biofilm concentration of 0.675 mg protein/g carrier showed a biofilm coverage fraction of 0.4 (Figure 5). The biofilm thickness calculated for the two samples was similar - 90 µm. In both cases the results indicated a very high sensitivity of the model to biofilm coverage fraction and low sensitivity to the biofilm diffusion coefficient (Db approximately 1‚10-7 cm2/s), which is in accordance with the hypothesis that atrazine penetration occurs mainly through the nonbiofilm covered GAC. Low sensitivity of the model to the film diffusion coefficient, kf, was also shown in a similar way (results not shown). Spietel and Zhu (27) also reported low model sensitivity to kf and Db together with high model sensitivity to biofilm concentration. Comparison between the full and patchy-like biofilm coverage model alternatives (lines) with the experimental results (symbols) is given in Figure 6 for the two biofilm concentrations of 0.675 and 0.96 mg protein/g GAC. It can be seen that assuming full biofilm coverage of the GAC particles with a correspondingly lower biofilm thickness

FIGURE 5. Minimum least-squares function (f ) values for combination of Db and fc in atrazine adsorption kinetic batch experiment, with medium biofilm concentration (0.675 mg protein/g GAC).

FIGURE 6. Suitability of the two model alternatives to the experimental results: symbols for experimental results; lines 1 and 2 for full biofilm coverage model; lines 3 and 4 for patchy biofilm coverage model with calculated fc. (calculated values of 36 µ and 51 µ for biofilm concentrations of 0.675 and 0.96 mg protein/g GAC, respectively) yielded model results which strongly deviate from the observed data (Figure 6 lines 1 and 2). In contrast, the patchy-like biofilm coverage model alternative was in good agreement with the experimental results (Figure 6 lines 3 and 4). Comparison between BGAC and Nonadsorbing Carriers in Reactor Operation. Establishing the existence of a patchylike biofilm coverage and atrazine adsorption occurring mainly in the noncovered areas of the GAC particles, it can now be assumed that adsorption to GAC occurs even under conditions of substrate partial penetration in the biofilm. Under these conditions, substrate desorption from the GAC may result in higher specific biofilm activity due to an increase in active biofilm surface area. To confirm this phenomenon, atrazine degradation by P.ADP in a FB reactor with granulated activated carbon as a carrier for the biofilm was studied in FIGURE 7. Atrazine degradation in a FB reactor utilizing BGAC and comparison to an identical reactor with nonadsorbing carbon nonadsorbing carbon as carriers for the biofilm. as the carrier (see Materials and Methods). Both reactors Figures 7 and 8 show that at 0.7-1 mg/L influent were operated at conditions of atrazine partial penetration concentrations of atrazine, simultaneous atrazine biodegin the biofilm. VOL. 37, NO. 18, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 8. Atrazine adsorbed concentration in the GAC FB reactor operation. radation and desorption from the carrier were observed. The atrazine concentration in the BGAC reactor was higher during the first 30 days since the BGAC was preadsorbed with 44 mg atrazine/g GAC. Due to biological degradation, the adsorbed atrazine concentration decreased to about 30 mg/g GAC. Following the initial transient period an effluent concentration of 0.002 mg/L was achieved in the GAC reactor versus 0.005-0.006 mg/L in the nonadsorbing carbon reactor. The lower effluent concentration in the GAC reactor, despite the same physical characteristics of the carrier and the same operational conditions, demonstrates the higher biofilm activity in the GAC reactor. This can only be explained by an additional source of atrazine from the GAC. Based on the results presented in this paper it can be concluded that under normal BGAC operational conditions a biofilm with a patchylike coverage will be developed with the consequent adsorption of substrate to the GAC through the nonbiofilm covered areas. This phenomenon can explain substrate adsorption to the GAC even under conditions of substrate partial penetration in the biofilm. Adsorption under these conditions followed by intraparticle diffusion and desorption can allow for increased biofilm activity. The results presented in this paper are important when pollutants are required to be removed to very low concentrations. Special care should be taken to develop a patchylike biofilm coverage (a prerequisite for adsorption under substrate partial penetration conditions). This can easily be attained by applying the proper shear forces during daily excess biomass removal.

Nomenclature Auncovered GAC uncovered surface area of GAC Acovered GAC

biofilm’s covered surface area of GAC

Atotal

total surface area of GAC available for biofilm growth

Cb

biofilm substrate concentration [mg/L]

Cr

bulk concentration [mg/L]

Cs

GAC surface concentration [mg/L]

Db

average biofilm diffusion coefficient [cm2/s]

Ds

average pore diffusion coefficient [cm2/s]

dp

particle’s diameter [cm]

f

minimal least-squares function

fc

biofilm coverage fraction (Acovered GAC/total GAC surface area) [-]

K

freundlich isotherm coefficient [(L/mg)1/n (mg/g)]

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k

number of experimental data [-]

kf

film layer coefficient [cm/s]

m

mass of the GAC particles [g]

n

freundlich isotherm coefficient [-]

q

adsorbed substrate concentration [mg/g]

qav

average adsorbed substrate concentration [mg/g]

r

particle’s radial coordinate [cm]

rb

biofilm radial coordinate [cm]

t

time [s]

V

liquid volume of batch adsorption system [L]

Vp

typical particle volume [cm3]

X

biomass concentration [g VSS/g GAC]

z

biofilm depth [cm]

Fb

biofilm density [g VSS/cm3]

Fp

wetted particle density [g GAC/cm3]

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Received for review December 23, 2002. Revised manuscript received June 16, 2003. Accepted June 18, 2003. ES0210852