8) Pergamon

PH: 50273-1223(97)00329-6

Wal. Sci. r-eh. Vol. 36. No. I. pp. 229-235. 1997.C 1997 IAWQ. PIIbli'hed by El"v'ef Selenee Ud

Pnnted in Oreat Brilain.0273-1223197 Sl7l1O +0-00

MECHANISMS OF BIOFILMDETACHMENT IN FLUIDIZED BEDREACTORS

Cristiano Nicolella, Stefania Chiarle, Renzo Di Felice andMauro Rovatti

Istituto di Ingegneria Chimica e di Processo 'G.B. Bonino', Universita degli Studi diGenova. Via Opera Pia 15. 16/45 Genova. Italy

ABSTRAcr

Biofilm detachment in liquid fluidized bed biological reactors was investigated to point out how differentmechanisms influence the process. Erosion due 10 liquid shear and abrasion due to collisions of panicleswere considered as possible mechanisms of biomass detachmenl in liquid fluidized beds. A dimensionalanalySIS technique allowed the Identlficauon of Ihe significant parameters affecting the process. 11leinfluence of Ibese parameters was established on a lab·scale reaclor. An empirical model was proposed tocorrelate the experimental data and to analyze the effcci of some characteristic quantities. such as panicleReynolds number, biomass fraction. liquid shear stress and solid concentratton. on Ibe detachment rate.Detachment rale strongly increased with flUid velocity while. owing to modifications in biofilm structure andmorphology during the biological growth. it slightly decreased with liquid shear stress. (C) 1997 IAWQ.Published by Elsevier Science Ltd

KEYWORDS

Biofilm; detachment; dimensional analysis; fluidized bed.

INTRODUcrJON

The accumulation of a biofilm is the net result of various processes such as adsorption, desorption.attachment. microbial growth and detachment (Characklis, 1990). Biofilm detachment is the entrainment ofmicrobial cells and cell products from a biofilm into the bulk liquid and is the primary process that balancescell growth in a biofilm (Peyton and Characklis, 1993). Detachment is caused by continuous anddiscontinuous processes. mainly erosion. abrasion and sloughing. Erosion refers to the continuous removalof individual cells or small groups of cells from the surface of the biofilm and is presumed to be the result ofshear forces exerted by the moving fluid in contact with the biofilm surface. Sloughing is the detachment ofrelatively large particles of biomass and is an apparently random discrete process. Abrasion is caused bycollisions of solid particles with the biofilm.

Based on a statistical analysis of experimental results from a lab-scale fluidized bed biological reactor(FBBR). this work focuses on the quantification of the effect of different mechanisms on biofilm detachmentin fluidized systems.

229

230 C. NICOLELLA .1 al.

MATERIALS AND METHODS

Detachment model

In recent work (Nicolella et al.• 1996) dimensional analysis was applied to describe biofilm detachment influidized bed biological reactors (FBBRs). The rate of biomass detachment from a fluidized particle(expressed as mass of biomass detached per unit time per unit surface area. bJ was considered to bedependent on the properties of the biomass itself attached to the inert support (such as density and porosity).abrasion due to interparticle collisions and erosion due to liquid shear stress. After assessing the dependenceof the factors affecting detachment on system physical parameters (biofilm thickness. particle size anddensity. fluid velocity. bed porosity, shear stress) and considering the correlations between these parameters.the analysis led to the conclusion that the variables to be considered for the description of biofilmdetachment in liquid fluidized beds are: liquid density (pi) and viscosity (Il). gravity accelaration (g). barecarrier size (dc) and density (Pc)' dry biomass density (Px) fluid velocity (u), biofilm-covered particle size(de) and detachment rate (bs)'

The number of experimental variables to be correlated was reduced according to the Buckingham-Pi methodand the dependence of the biofilm detachment on the considered mechanisms was expressed as a powerproduct:

(1)

where PI P2 and P3 are dimensionless groups defined as:

(2)

Relationship (I) can be used to describe the dependence of biofilm detachment on the consideredmechanisms. once the parameters K. aJ. az are quantified. In this work. K. aJo az were determined by fittingrelationship (I) with experimental estimates of PJ' pz and P3 by means of a nonlinear error minimizationroutine.

Experimental procedure

A laboratory-scale FBBR was employed to obtain the required experimental data. The device is fullydescribed elsewhere (Nicolella et al.. 1996), so only the essential features are given herein. The reactor wasmade from a 80 mm Ld. glass column 2100 mm tall. with sampling ports (210 mm apart) installed along thecolumn height to remove liquid and bioparticle samples. A 100 mm high section filled with 4 mm diameterglass beads was placed at the bottom of the column in order to obtain a uniform fluid superficial velocity inthe bed and avoid backflow of media. 0.5 kg of sand (density: 2630 kg/m3; mean diameter: 0.36 mm) wasused as an inert support.

A concentrated feeding solution was diluted with tap water and pumped into the reactor at controlled flowrate. Since neither flow rate nor composition of the feeding solution was varied, the reactor loading rate wasconstant throughout the period of the experiments.

In order to quantify the dimensionless groups P" PZ and P3 in Eq. (I). particle diameter. liquid superficialvelocity and biofilm detachment rate were determined for each experimental run and dimensionless groupsPI. pz and P3 were calculated from Eqns (2). Sets of tests were carried out at fixed particle diameter; for eachset of tests. runs were performed at different liquid superficial velocity. The detachment rate was estimatedafter the velocity was varied. in a time span sufficiently short to assume constant particle diameter duringeach set of tests.

Mechanisms or biolilm dela<:hmenl 231

Particle diameter was calculated by means of the method presented in previous work (Nicolella el al., 1995).

The rate of detachment was related to the covered particle surface area and expressed by:

(3)

where Q is the liquid volumetric flow rate, Css is the suspended solid concentration in the effluent, de is theparticle diameter and N is the number of particles in the bed.

Samples for the measurement of the suspended solid concentration were drawn from the effluent. Thesuspended solid concentration was detennined according to APHA methods (APHA, 1985) by filtration of aknown volume of effluent through filters with 0.45 j.lm diameter pores.

RESULTS AND DISCUSSION

Table 1 reports the values of the dimensionless parameters PI, P2 and P3 estimated for each experimentalrun. The empirical model used in this study (Eq. (1)] is based on the assumption that the processesresponsible for the biofilm detachment in FBBR are abrasion and erosion. This assumption is reasonable inparticular cases only, namely when sloughing is negligible. It has been observed (Peyton and Characklis.1993) that sloughing often occurs in older, thicker biofilms. Accordingly. only data corresponding to thinbiofilms (6<300 j.lm) were employed to quantify the parameters ai' a2 and Kin Eq. (1).

Table I. Experimental estimates of dimensionless parameters pI, p2 and p3

p,d.uP'=-I!-

0.780.420.560.620.410.430.810.850.490.610.650.720.770.810.410.450.65

2.012.352.352.352.402.402.402.402.502.502.542.542.542.542.592.592.59

d,.b,Pl=--;-

1.63E-094.4IE-109.19E-I07.78E-102.79E-104.72E-102.05E-092.35E-096.00E-1O6.52E-109.06E-101.26E-091.57E-091.68E-093.77E·104.53E·107.7IE-1O

The nonlinear regression of the experimental estimates of p I_ P2 and P3 by means of Eq. (I) allowed us todetermine the best value of the parameters K. a I and a2' The best description ofP3 resulted:

(4)

The computer elaboration utilized a nonlinear regression technique based on a finite difference algorithm. Acomparison of the values of P3 estimated from experimental data using Eqns (2) and those calculated

232 C. NICOLELLA tl al.

through Eq. (4) is shown in Figure 1. The empirical model of biofilm detachment described using Eq. (4)was used to investigate the dependence of the detachment rate on some quantities characteristic of thesystem under study, such as panicle Reynolds number, reactor biomass fraction and liquid shear. Figure 2shows the influence of the panicle Reynolds number (Re = pJdeulJl) on the detachment rate (bs)' It can beobserved in this figure that bs strongly increased with Re throughout the examined range of Re.

lE-08 Calculated p)

lEo09

••••

••• •

•••

1E-oll

lE-l0 "- -'

1E-l0 1E-09 1EoOeI

Experimental P1

Figure I. Comparison of e~perimentaldimensionless detachment rale with values calculated using the modelequation.

Detachment rate /mg/(mm2 s))1Eo07 r--;;..;.;;.:;.;.;;..-...;.,..e..-....:....=....'---..:.:..-----,

lE-08

/,8'

(1E·10 O'--O..!..5~------:-1.75---:-2-......,..2.-5--'3

Particle Reynolds number

Figure 2. Influence of particle Reynolds number on biofilm detachment rale. Dots represent the experimentalbehavior; lines arc model estimalion.

Figure 3 reports the dependence of bs on the reactor biomass fraction:

for different liquid superficial velocities. b. increased with the biomass mass of the panicles, but thedependence was found less strong than in the case of the panicle Reynolds number. The shear stress exerted

Mechanisms of biofilm detachment 233

by the liquid flowing around the particle surface was calculated according to Chang and coworkers (Changetal., 1991):

(5)

15

15

The dependence of the detachment rate on the liquid shear stress is reponed in Figure 4. Predicted valueshave been gathered in three series, each one corresponding to a fixed superficial velocity. As already foundby Chang and co-workers (Chang et al., 1991), predicted specific detachment rate decreased with liquidshear stress. To explain this behavior, it should be considered that, under the condition of mechanicequilibrium which is ideally realized for solid particles in fluidized beds, the liquid shear stress on thepanicle surface is given by Eq. (5). For biocoated particles, the dependence of t on de is not linear since alsothe particle density depends on de:

Detachment rate [10.smg/(mm2 s))20 ~==;;;::;==:;=~;,.:..,...:..:..:.:~-_.:.:....-,

-u=2.5 mm/s. - u=5 mmls

u=7.5mm/s

10

5

o1::==========::::::;::==~=:)02 0.25 0.3 0.35 0.4 0.45 0.5

Biomass fraction [(g biomass)/(g support))

Figure 3. Influence of reactor biomass fraction on biolilm detachment rale.

20 Detachment rate [1 O"mg/(mm2sl)-u=2.5 mm/s. u=5 mm/s. u=7.5 mm/s

10

5 . '" -- _- .

o.I::::=============-~0.28 0.30 0 32 0.3~· 0.38

Liquid shear stress [N/m 2 J

Figure 4. Influence of liquid shear stress on biolilm detachment rate.

234 c. NICOLELLA er at.

In this case, as shown by Figure 5, the function t(de), analytically represented by Eq. (5), has a minimum,whose value and position depend on support characteristics (dc, Pc) and biomass wet density (Pb)· In anycase, this minimum corresponds to high value of particle diameter, i.e. high biofilm thickness, where theinfluence of de on Pe is less important (Pe == Pb). On the other lland. thin biofilm are generally denser andmore compact than thick biofilms and probably less sensitive to liquid shear. In fact, as recently stated byLoosdrecht and coworkers (van Loosdrecht et al., 1996) higher shear forces can be balanced in two ways,either by forming a stronger biofilm or a less shear susceptible biofilm. This hypothesis could explain whythe highest biomass loss in fluidized beds is reached when the liquid shear stress is low. that is at highparticle diameter, while at high liquid shear stress. i.e. at low biofilm thickness. the detachment rate is low.The conditions for the largest biomass loss (high biofilm thickness and high liquid shear) correspond to verylight particles (Pe == Pb). In these conditions. even at very low liquid superficial velocities. the reactor wash•out is more probable than the detachment of portions of biofilm. The effect of liquid shear on biomass lossin liquid fluidized beds is almost negligible under the usual operating conditions and the detachment rate inliquid fluidized beds must be mostly influenced by particle attrition. as already reported for systems. such asFBBR (Chang et al.. 1991) and airlift biofilm reactors (Gjaltema et al.• 1995), where particles are suspendedin a mono- or two-phase flow.

,- --,25

~

..~~-_ 08 I

OJ

~., 08

m~II)

U 0.4·50-::::;

0.2

I'o .

~_ ....<=~ .....a""o 0 0

1 2 3 4 5 6 7 8 9 10

Particle diameter to support diameter ratio

Figure 5. Influence of particle diameter on liquid shear stress (continuos line) and particle density (dashed hne).

Particle shear and attrition may be influenced by either turbulence or particle concentration. During ourexperimental tests, the specific detachment rate was always increasing with increasing particle Reynoldsnumber (see Figure 2). that is with fluid superficial velocity and particle diameter. On the other hand, theRichardson and Zald law for fluidization states that particle concentration is a decreasing function of fluidsuperficial velOCity. According to the model equation and the Richardson and Zaki law, detachment rate isexpected to decrease with particle concentration (see Figure 6).

CONCLUSIONS

Continuous and discontinuous processes of biofilm detachment were investigated in an FBBR. The influenceof erosion and abrasion on the kinetics of biofilm detachment was assessed by using an empirical modeltested on experimental data obtained in a laboratory-scale FBBR.

Abrasion due to intraparticle collisions is presumed to be the key mechanism for biofilm detachment inliquid fluidized beds of particles covered by thin. compact biofilm. The results obtained are supposed to bevalid in the examined range of parameter variability. which is typical of the usual operation of the systemunder study. In these conditions the rate of biofilm detachment was found to be very low.

Mechanisms of biofilm detachment

8 Detachment rate [1 009mg/(mm2s)l

6

4

...•••••

•g3~5"-----:O"".4"-----:O:-.4""5----:0'"'5"-----'O:-!.55

Particle concentration

Figure 6. Influence of solid hold-up on biofilm detachment rate.

ACKNOWLEDGEMENTS

This research was supported by Ministero della Ricerca Scientifica e Tecnologica. Rorna (MURST 60%).

REFERENCES

235

APHA (1985). Standard methodslor the examination 01 water and wastewater. 16th edition. American Public Health Association.Washington. D.C.

Chang, H.-T., Rillmann, B. E.. Amar, D., Heim, R., Ehlinger, O. and Lesty, Y. (1991). Biofilm detachment mechanism in a liquid•fluidized-bed. Biotechnol. Bioengng.. 38, 499-506.

Charackhs, W. G. (1990). Biofilm processes. In: Biofllms, W. G. Characklis and K. C. Marshall (eds), pp. 195-231, Wiley, NewYork.

Gjaltema A., Tijhuis L.. van Loosdrecht M. C. M. and Heljnen J. J. (1995). Detachment of biomass from suspended nongrowlngspherical biofilms in airlift reactors. Biolechnol. Bioengng.• 46, 258-269.

Loosdrecht, M. C. M. van, Eikelboom, D., Gjaltema, A., Mulder, A., Tijhuis, L. and HelJnen, J. J. (1996). Biofilm structures. Wat.Sci. Tech., 32(8), 35-43.

Nicolells, C., Conveni, A., Di Felice, R. and Rovalli, M. (1995). The estimauon of the solid size and density in liquid fluidizedbed biologIcal reactors. Chem. Engng Sci.. SO, 1059-1062.

Nicolella, C., Di Felice, R. and Rovalli, M. (1996). An empirical model of biofilm detachment in liquid fluidized bed biologicalreactors. Biotechnol. Bioengng., 51, 713-719.

Peyton, B. M. and Characklis, W. G. (1993). A statistical analysis of the effect of substrate utilization and shear stress on thekinetics of biofilm detachment. Biotechnol. Bioengng., 41, 728-735.