Mass transfer coefficients within denitrifying biofilms were determined with an inert
compound (LiCl) using two different flow conditions in a membrane flow cell and feeding an easily
degradable substrate. The experiments were made until the biofilm reached steady state. The results obtained
from the biofilm mass transfer experiments show that the biofilms grown under these two different
conditions reach similar values in the steady state. However, the mass transport was higher during biofilm
formation, for the biofilm developed under higher upflow velocity.Comissão Europeia, TMR program (project BioToBio); Spanish CICYT (project AMB 98-0288)

García López, L. A.

Melo, L. F.

Nogueira, R.

Veiga, M. C.

Universidade do Minho: RepositoriUM

Paris 2000Studies of Mass Transfer Coefficients in Denitrifying BiofilmsGarcía López L. and Veiga M.C.Chemical Engineering Laboratory. Faculty of Science. University of A Coruña.Campus da Zapateira s/n. A Coruña 15071 (Spain)Nogueira R. and Melo L.F.Center of Biological Engineering-IBQF, University of Minho, 4700 Braga (Portugal)e-mail address: lourdesg@mail2.udc.esAbstract - Mass transfer coefficients within denitrifying biofilms were determined with an inertcompound (LiCl) using two different flow conditions in a membrane flow cell and feeding an easilydegradable substrate. The experiments were made until the biofilm reached steady state. The results obtainedfrom the biofilm mass transfer experiments show that the biofilms grown under these two differentconditions reach similar values in the steady state. However, the mass transport was higher during biofilmformation, for the biofilm developed under higher upflow velocity.Key words: denitrifying biofilm, diffusion, flow cell, mass transfer coefficients.INTRODUCTIONBiofilm technology is widely used in biological nitrogen removal from wastewater. In the last years,denitrifying biofilm systems are being used for nitrate removal. Mass transfer usually is limiting thedevelopment of biofilm systems (Siegriest and Gujer, 1985). It is important to understand and optimize themass transfer mechanisms to develop biofilm reactors. In natural and industrial biofilms, convective andmolecular diffusion are the predominant mechanism of transport of solutes (De Beer et al., 1996). Howevertransport between surface film and base film occurs primarily by molecular diffusion for dissolvedcomponents and by volumetric displacement for particulate components (Characklis and Marshall, 1990).To evaluate the mass transfer coefficients within biofilms, a simplifying method can be used, where amass balance with a non reactive tracer is applied to a biofilm adhered to a porous membrane (Kitsos et al.,1992, Vieira et al., 1993, Brito and Melo, 1999). The inert tracer is transported through the biofilm underdifferent flow conditions. With this method, biofilm mass transfer coefficients are studied for anaerobic andaerobic biofilm systems.The present work was carried out to study the mass transfer coefficients in a denitrifying biofilmgrown in a membrane flow cell, using two different upflow velocities, similar to the ones used indenitrifying biofilm reactors.METHODSExperimental systemA denitrifying biofilm was grown in a vertical flow cell (Figure 1) consisting of two chambers (I andII), separated by a hydrophilic membrane of cellulose esters, with a mass transfer area and pore diameter of1.6·10-3 m2 and 0.22 µm, respectively. The flow cell was made of plexiglass with a semi-circular geometry.A centrifugal pump was connected to each chamber (circuit I and circuit II) in order to recycle the liquidcontinuously. Sample ports were placed in chambers I and II.Two experiments were carried out applying two different upflow velocities to the system. An upflowrate of 0.01 m·s-1 was adjusted with a flow meter on each side of the membrane, in the first case. Adifferential manometer was connected to both sides of the membrane in order to have the same pressure andavoid the transport due to a pressure gradient across the membrane. In the second case, an upflow rate of0.04 m·s-1 was given by the centrifugal pump flow. The total volume of each circuit was about 800 mL and500 mL for the two different cases respectively.Initially, circuit I was inoculated with a denitrifying bacterial suspension and water was pumped incircuit II. After 24 h the inoculum was replaced by medium solution fed in fed–batch mode in the first case,and in a continuous mode, using a peristaltic pump connected to circuit I, working with 4 hours of hydraulicretention time, for the second case, to promote biofilm growth during the experiments.Figure 1. Mass Transfer Flow Cell DesignThe nitrate concentration in the feed of the flow cell was always 50 mg N/L, with a C/N ratio of 4and 2.25, using acetate and methanol, respectively for the two different conditions. Inorganic elements wereused as nutrients in both cases. The pH was adjusted between 7.3 and 7.5. The operation temperature was 20°C.Mass transfer measurements were performed on the membrane cell and during biofilm growth withan inert compound, lithium chloride (Vieira et al., 1993, Kitsos et al. 1992). A fixed amount of LiCl (200 mgLi+·L-1) was added to vessel I together with the medium and the mass transfer experiment started afterequilibrium conditions were reached. Samples were collected at intervals of 30 min, during 8 h, in bothcircuits. After that, LiCl was removed from the system. Lithium concentration was measured by atomicabsorption spectroscopy (Varian SpectrAA.250 plus).Biofilm characterizationAt the end of the experiments, the biofilm thickness (Lb) was determined, in the first case with amicrometer and a video camera, according to Brito and Melo (1999). In the second case, the biofilmthickness was determined with a microscope Leica Leitz DMRD at magnification of 5x0.12p and with acalibrated ocular micrometer. The biofilm was detached from the support by ultrasound treatment. The following parameters of thebiofilm were characterized. Total protein, and Total Polymers according to the methods of Lowry (Sigma kit5656) and Dubois, respectively and dry weight (TS) by Standard Methods (1995). Biomass densityrepresents the weight of biomass expressed as TS per unit volume of biofilm. Also, scanning electronmicroscopy observations were made on biofilm samples submitted to dehydration in an ethanol series(30,50,70,90 and 100%).Evaluation of Mass Transfer CoefficientsMass transfer coefficients were determined in the system with a nonreacting substance (LiCl),diffusing across the membrane and the biofilm attached to the membrane, assuming that the biofilmthickness remained constant during the period of the experiment (i.e., when LiCl is introduced).A material balance for the inert compound in the system, based on Fick´s model, can be made usingthe following equations:( ) √√↵+?−??=IIIIIIT V1V1CCAkdtdC (1)With the integration of (1) is possible to obtain the concentrations in both sides for each time duringthe experiment:InfluentC+NBiofilmMembraneCircuit I Circuit IIDiffusionChamberEffluent21 1341 Centrifugal pumps2  Peristaltic pump3,4 Valves( ) √√↵√√↵+???−−?−+= ××IIIT00II V1V1Ak)tt(expCCCC (2)( ) √√↵√√↵+???−−?−−= ××IIIT00IIII V1V1Ak)tt(expCCCC (3)with the concentration at infinite time:IIIII0III0IVVVCVCC+?+?=× (4)where C0 and C are  the lithium concentrations at t = t0 and t = t, for circuit I and II, respectively, VI and VIIthe volume for both circuits, A is the mass transfer area, kT the overall mass transfer coefficient, includingthe biofilm, the membrane and the external mass transfer resistances, and t is the time during which thelithium accumulates in circuit II.The value of kT was calculated by fitting equations (2) and (3) to the measured concentrations bynon-linear regression.The biofilm mass transfer coefficient, kb, can be calculated from the overall mass transfercoefficients, Tk , and the initial mass transfer coefficient of the membrane without the biofilm, 0Tk0TTb k1k1k1−= (5)The diffusion coefficient in the biofilm (Db) results from the biofilm mass transfer coefficient and thebiofilm thicknessbbb LkD ?= (6)RESULTS AND DISCUSSIONMass transfer studies were performed during the biofilm formation. Data were collected for 15 daysat different stages of the biofilm development. All the experiments were undertaken in duplicate. Thebiofilm was grown under two different conditions. They were called Biofilm 1 and Biofilm 2.Figure 2 shows one example of the several diffusion experiments conducted. The approach of themeasured concentrations in both circuits as well as modelled concentration curves can be seen. From each ofthese experiments values for kT and kb could be estimated.Figure 3 shows the evolution of the biofilm mass transfer coefficient kb for both biofilms. In bothcases, the biofilms reach a steady state after 10 days. The final value of kb was about 1·10-6 m·s-1. However,before steady state was achieved, the biofilm mass transfer coefficient of the lithium for the biofilm grownwith the higher flow rate was 30 to 50% higher than for the biofilm grown under low flow rate. This couldpossibly be explained by a better interaction between the biofilm and the substrate in biofilm 2, resulting in amore compact biofilm. The results obtained from the measurement of the biofilm thickness can alsocorroborate this hypothesis.Biofilm thickness was around 418 and 467 µm for biofilm 1 and 2, respectively. This results in a Dbof lithium of about 4.18 10-10 m2·s-1 and 5.83·10-10 m2·s-1, for each biofilm. These values correspond to 41%and 57% of the diffusivity of lithium in water. This agrees well with the range of relative biofilm diffusioncoefficients reported by Stewart (1998) for inorganic ions, like Li+ and NO3- (58 ± 24 % of the diffusioncoefficient in water).                   Figure 2. Example of experimental results                 Figure 3. Biofilm Mass Transfer                    of each side (biofilm of 2 days).                                   Coefficient.Previous work was also performed in a laminar flow, but with a methanogenic biofilm. The internalmass transfer coefficient approached similar values (Brito and Melo, 1999). Biofilms formed under turbulentflow with Pseudomonas fluorescens, showed the same behaviour (Vieira et al., 1993).The biofilm was characterized at the end of each experiment The specific mass of the biofilm was inthe range of 29 and 49 kg dry biofilm·m-3 wet biofilm, for Biofilm 1 and 2, respectively. Total proteincontents were 0.5 and 0.66 kg·kg-1 dry biofilm and total polysaccharides were 0.2 and 0.12 kg·kg-1 drybiofilm, respectively.Biofilm surface was irregular and not homogenous for Biofilm 1 and regular and almost homogenousin Biofilm 2. However, SEM photographs of the both biofilms showed an irregular surface and biofilmstructure of clusters and channels as described by Lewandowski et al. (1995).CONCLUSIONSA non reactive compound, LiCl, was used as tracer to estimate the biofilm mass transfer coefficientsin a denitrifying biofilm, at different states of growth. The conditions were similar to those in denitrifyingbiofilm reactors. It was found that increasing the upflow rate, lithium diffusion was better, during the biofilmformation time. The final values achieved, for the biofilm mass transfer coefficients, were in the same orderof magnitude for the all experimental studies and similar to the results previously obtained with differenttypes of biofilms under various hydrodynamic conditions. More work is needed in order to understand thisparticular pattern of mass transfer results in biofilms.Acknowledgments- This research was financially supported by the TMR program of the EuropeanCommission (project BioToBio) that funded the visit of L. García to the University of Miño and by theSpanish CICYT (project AMB 98-0288).REFERENCESBrito A. G. and Melo L.F. (1999). Mass transfer coefficients within anaerobic biofilms: effects of external liquidvelocity. Was. Res., 33 (17), 3673-3678.Characklis W.G. and Marshall K.C. (1990). In: Biofilms. Wiley series in ecological and applied microbiology. JohnWiley & Sons, Inc. 265-340.De Beer D., Stoodly P. and Lewandowski Z. (1996). Liquid flow and mass transport in heterogeneous biofilms. Wat.Res., 30 (11), 2761-2765.Lewandowski Z., Stoodly P. and Altobelli S. (1995). Experimental and conceptual studies of mass transport inbiofilms. Wat. Sci. Tech., 31, 153-162.Siegrist H. and Gujer W. (1985). Mass transfer mechanisms in a heterotrophic biofilm. Wat. Res., 19 (11), 1369-1378.Standard Methods for the Examination of Water and Wastewater (1995). 19th edn, American Public HealthAssociation/American Water Works Association/Water Environment Federation, Washington DC, USA.Stewart P.S. (1998). A review of experimental measurements of effective diffusive permeabilites and effectivediffusion coefficients in biofilms. Biotech. Bioeng., 59 (3), 261-272.Vieria M.J., Melo L.F. and Pinheiro M.M. (1993). Biofilm formation: Hydrodynamic effects on internal diffusion andstructure. Biofouling, 7,67-80.1001301601900 200 400 600Time (min) CI (mg Li + .  L-1 )04812C II (mg Li + .  L-1 )CICI Imodel0,0E+002,0E-064,0E-066,0E-068,0E-060 4 8 12 16Time (d)kb (m . s-1 ) Biofilm 1Biofilm 2

Studies of mass transfer coefficients in denitrifying biofilms

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Studies of mass transfer coefficients in denitrifying biofilms

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