The formation of biofilms on heat exchange surfaces was studied using water contaminated with Pseudomonas fluorescens. The rate of deposition and the maximum amount of deposit decrease as the fluid velocity increases. The processes occurring at the interface and in the biofilm appear to govern the build-up of the deposit. together with the removal (shear stress) mechanism. Values of the attachment and biological growth rates
were calculated from the changes observed in the biofilm thermal resistances after suppressing the addition of nutrients to the flowing water. In both cases there is a decrease for higher velocities.JNICT. Portugal. through Project no. 8750

Melo, L. F.

Pinheiro, Maria Manuela

Vieira, Maria João

Universidade do Minho: RepositoriUM

BIOFILM FORMATION BY Pseudomonasjluorescens L.F. Melo, M.J. Vieira and M.M. Ptnhetro University of Minho - C.Q.P.A. 4 700 Braga - PORfUGAL ABSTRACT The formation of bioftlms on heat exchange swjaces was studied ustng water - contaminated with Pseudomonas jl.uorescens. The rate of deposition and the maximum amount of deposit decrease as the fluid velocity increases. The processes occurring at the interface and in the btofrlm appear to govern the build-up of the deposit. together with the removal (shear stress) mechanism. Values of the attachment1f;Z.nd biological growth rates were calculated from the changes observed in the biofllm thermal resistances after suppressing the addition of nutrients to the .fl.owing water. In both cases there is a decrease for higher velocities. INTRODUCTION Biofouling - the accumulation of deposits on surfaces due to the adhesion and biological activity of microorganisms (or macroorganisms) - affects different types of industrial equipment, such as heat exchangers. cooling towers, filtration membranes and sensors. As a consequence, heat transfer efficiency ts reduced, membranes are clogged. production losses are increased and corrosion may be induced on metal surfaces. In systems where cooling water ts used (its temperature often reaching 30-40°C) btofouling can potentially occur. It may also occur in the process fluid side of heat exchange units, such as in the regenerative zone of milk pasteurizing plants, where thermoreststant micro-organisms can grow (1. 2). BIOFOULING MECHANISMS The main processes involved in biofilm formation in a flow system are (3): the formation of a "conditioning film" due to the adsorption on the deposition surface of organic macro-molecules (glycoproteins, polysaccharides. . .. ) contained in the fluid ("induction period"); the transport of microorganisms and dissolved components (which may act as nutrients) to the surfaces: the attachment of the microorganisms to the surface: the biological development of the deposit due to the growth and reproduction of organisms, as well as to the extracellular polymers and other products they excrete: the removal of parts of the btofilm caused by the hydrodynamic forces of the flowing fluid. This last process opposes the other four and, as a consequence, the build-up of biological deposits ts often described by a slgmoidal curve leading to a final maximum amount of biofilm. The quantitative aspects of btofilm formation are still poorly understood and few mathematical approaches are avaiable (4, 5). although a. considerable amount of data has . . -2-already been published (5-8) reporting the effects of different variables on the extent of biofoullng. In this work, the formation of deposits caused by Pseudomonas jl.uorescens (often present in industrial cooling waters) was monitored by measuring heat transfer coefficie~ts. The effects of fluid velocity and of the presence of nutrients were studied in an attempt to quantify some of the individual processes involved in biological fouling. BIOFOULING MODEL The basic structure of the model follows Kern and Seaton's concepts (9), who postulated that fouling ts the result of a deposition process. occurrtng at a constant rate. and ·~u .. a simultaneous removal process, the rate of which increases with the thickness of the deposit. The amount of biofilm - here represented by its thermal resistance, Rr - will then increase according to the rate equation: (I) where 0d and 0r are the deposition and removal fluxes, in terms of thermal units. Upon integration. Equation 1 leads to the asymptotic model (valid after the "induction" period): Re= Rr (I-exp (- ~.t)) (2) where Rf' 1s the maximum value of Rr. and ~ 1s a parameter depending on the hydrodynamic conditions of the system and inversely proportional to the mechanical strength of th\! deposit. The following e.~ressions are directly associated to this model: (3) (4) In the case of biofoultng, deposition involves two parallel phenomena: L Fluid and interface processes (0dil - the transport of microorganisms to the deposition surface. where specific attachment mechanisms are established. 2. Fluid and biofilm processes (oct2l - the transport of nutrients to the deposit followed by the biological processes occuring in the biofilm (growth/reproduction, exopolymers production). Since processes 1. and 2. are simultaneous, the overall deposition flux will be: (5) -3-0d 1 depends on the transport and attachment rates of bacteria and ts constant with ttme (apart from the 1n1Ual "induction" period). 0d2 can also be assumed to be constant with time for the following reasons: a) the mass tr~fer rate of nutrients to the deposit does not change as long as the operating ... condlUohs are maintained: b) although the biofllm production rate could get higher on account of the increasing number of microorganisms attached. the resistance to the difiusion of nutrients throughout the deposit may lead to the establishment of a layer of active bacteria which represents a decreasing fraction of the total biofilm and compensaters the first effect. MATERIALS AND METHODS Tests were carried out in a rtg containing a fermenter, a mtxing vessel and two simulated heat exchangers (the test sections) - Figure 1. 111.lllS llATU ,. COllTtOI. ovr.UUN rt~t!rTU lOT~tTU I I \I / _l ... 1::1 Figure 1 - Biofouling rig. The fouling fluid was water at 27°C continuosly contaminated with the bacteria Pseudomonas jluorcsccns kept in a culture in the fermenter. A constant flow of nutrient (glucose, pep tone and yeast extract) was added to the fermenter and to the mixing vessel. so that. the concentration of glucose in the test cells was maintained at about 0.03 g/l. pH was controlled in the fermenter at the value of 7. no adjustments being needed in the mLxtng tank to maintain this value. . .. -.· · . .. \ -4-The test exchangers (Figure 2) were made of a perspex duct with a semi-circular cross section of 1.8 cm diameter. with an aluminium plate acting as the deposiUon sunace. This metal plate was heated by water at 60°C circulating in an adjacent perspex duct with rectangular cross section. Temperatures were measured in four positions along the exchang_~ (A. B. C. D) using thermocouples placed in the fluid tr3) and in the heat transfer wall tr1 and T2). ·· A 8 C 0 manometer \ I I l manometer t · A • i t B lOLOGi c;,L _Jl.._ ...... l.._ __ tt....._ _ __.fl.__ _ __.11.....__,l__ FLUID -a 1 u::ii n i um ::: l .i :c (.__1_. __ ,_1 __ , _, _._,_._,_._,_. __ . _, _, ._' _, _, _ •.._1_,_,_,_1_,_, _, __ pcr'.:~e~ I_ I ll1a1ei;.u:1 \ 11 · .... s ,\ ,,,, , , , .. , .. ,,; ' 1 • I T · 1 WAiER - "'-''"9 u ... :o 60 c \ A e ic 0 Figure 2 - Details of the test sections. The overall heat transfer coefficient in each position was then calculated by K..v T1 -T2 u--- Yw Ti -T3 where Yw is the distance between thermocouples Ti and T2. (6) The effects of biofilm roughness on the convective heat transfer coefficient were determined from pressure drop measurements. as described elsewhere (10). The fouling ' •11;. resistances were e,·aluated as a function of Ume using the followtng equation: (7) Se\'eral runs were penormed at dilf erent fluid velocities corresponding to Reynolds numbers between 4200 and 12000. In three of them. the nutrient addition '"·as stopped ailcr the biofilm ma."<imum thermal resistance was reached. Durin~ the tests. the concentration of cells tn the contamincd water was cktcrmincd by spn•adinA a certain amount of fluid (after diluting it SC\'cral limes) tn solid a~ar nutncnt. After tncubaun~ for '..!·l hours at 27°C. the bactena coloncs wt>rc counted: the edl concentration was around G x 107 cells/ml in every run. l I -I ' . . -5-Total carbohydrates in the circulating fluid were analysed by the phenol-sulphuric colorimetric method (11). allowing the glucose concentration to be determined. RESULTS AND DISCUSSION ~The biofilms obtained in the tests showed an irregular topography. with average thickness reaching 1 mm (or even more) in the lower range of fluid velocltles. A high density of biopolymeric material was obseived on the scanning electron microscope (see photo at the end of this text). Fouling curves Figures 3 and 4 present typical fouling cuives CRr versus time) for position B in the test heat exchangers. Similar behaviour was found in the other measuring points. As expected. the asymptotic thermal resistances decrease with increasing Reynolds numbers. on account of the stronger removal action exerted by the fluid on the deposit. Furthermore. although the transport of bacteria to the surface is enhanced. their adhesion probability tends to decrease due to the greater shear stresses. If the prevailing mechanism in 0d 1 is adhesion. then 0d 1 will tend to decrease for higher velocities. 0.006 r:------------------. 0.006 0.005 I • • 0.005 •• - 0.004 I-} 0003l 0.004 ~ -:IC N e_ I a: O.OOJ E -t. .1 .:. • • Rh 6830 a: 0 00 2 !- I _.--;c ,; o J er_- c_ c :: A •. -------0001 -1 1 ~ _ :J l • I~ ··u;. -R~" 10400 0 "'"' 0 2 4 5 8 10 12 14 I (d) Figure 3 - Fouling curves. Deposition flux (od) 0002 ~ 1 I ++ 8 Figure 4 - FouUng curves. R~:9100 10 12 14 I ( d) I3y tlllinA Equations 2 and 3 to the data of Rrversus time. values of O<l were obtained for the different tests. TI1c deposition t1ux (which includes. amon~st others. the biolo~tcal ~rowth tcm1l decreases continuously with increasing Reynolds numbers (Fl!!ure 5). This Indicates that the transport mechanisms arc not controllln~ the deposition process. since -6-mass transfer rates would increase with Re. Thus, as attachment and/ or biologtcal growth seem to be the limiting steps, an estimation of their contribution to the overall deposiUon flux was made. For that purpose, the study of the effect of nutrtent suppresslon was found to be useful. - 4 Ill J .... le N e l .., 2 .. ,, 'Q,. ,r a a ~ 11-l ~ I 0 0 2 4 6 8 10 12 14 16 R•• 103 Figure 5 - Deposition n~x versus Reynolds number. Effect of nutrient suppression Fiqures 6 to 8 show the reduction in the biofoulinq thermal resistance after removin~ the nutrtents from the flowtn~ water. for the cases of Re = 4200. Re = oS30 and Re= 13955 U1uid velocities: 0 .34 m/s. 0.54- m/s. 0.71 m/s). The effect is more pronounced for hi!!h than for low Reynolds numbers. but in e\•ery case the fouling cun·e tends to a new (lower) asymptoptlc resistance (Rr:rl (Figures 6 to 8). · ·11;. Takin~ t = tnr as the time when Rf staned to decrease. the negative ioulinq rate for t > tnr can be described by an equation that does not contain the biological term. Odz· that is: dRf (it I = Od l - o'r (SJ t>tnr where u 'r = W Rr ts the remo\·al rate in the second phase of the expertment. o'r and Wan." not nccessanl~· equal to Ur and It since stn1ctural changes could ha\·e taken place in the inner layers 1>f the deposit before l he nutrients were removed. TI1c inte~rallnn of Equation S bctwl'cn the limits (l = 0, Rr = R ;"'.i and (t. H1l results In: -7-(9) 00 and: 0d1 = W Rfnr (10) After fitting Equation 9 to the expertmental data for t > t 0 r. values of 0d 1 and 0d2 (Equation 5) were obtained and plotted in Figure 9. . . 0.006 0.005 0.004 J -:IC N 0.003 E -a: 0.002 0.001 o o 2 4 6 : R•s4200 l-;1thout ¢ nutr1•nts 8 10 12 \4 t (d) Figure 6 - Effect of nutrient suppression for Re = 4200. -l -:IC N E -a: 0.006 0.005 0.004 0.003 0.002 0.001 " 4 6 ..: R•268JO I I I I ·-: wtthout I nutrtftttS I I I I I I I 8 0 : Oo :,,,, .. -o 10 12 14 t (d) Figure 7 - Effect of nutrient suppression for Re = 6830. -8-0.006 0.005 -0.004 ~ -:.: N 0.003 e .... a:: 0.002 0.001 \ R•s8955 I I I 1_ I without I nutrients I l I I I . ··~. • • •• 6 B . : trY : • ••• 10 12 14 t (d) Figure 8 - Effect of nutrtent suppression for Re = 8955. -.. ~ -:.: E «I g .. ~ ' 2.5 2 1.5 0.5 I 0'--~~~~~~~~~~~~~~~~~~~-0 z 4 10 12 Figure 9 - 0d and od versus Reynolds number. 1 2 The relative lmport:mce of the tluld and interface processes (Od 1 l ls somewhat sufl)rtslnf.!. The mechanism of microbial attachement are rather complex since bacteria use plll and excrrled polymers for this pttfl)ose (3), creating a deposit wtth an open structure rtlkd with water. In the pn·st·nt case. due to the hl~h nutrient content In tht• mlxln!.! tank It -9-ls probable that the mlcroor~anlsms produce blopolymers to a certain extent even before they reach the surface. This means that some of the "particles" that adhere to this surface arc already complex structures. not simple bacteria. The thermal resistance added by such particles is thus higher than it would be expected from simple microorganisms. which explains the high values of 0d I· ~·· The deposition term related to the processes within the blofilm (0d2) also decreases with increasing Re. The compactness of the deposit tends to increase with fluid velocity. and this will make nutrient dlffusion more dilTicult within the biofilm. These values are only partly in accordance with other published results ( 10) where 0d2 increased with fluid velocity for low Reynolds numbers. However. the previous assumption that 0d2 was much smaller (tending to zero) than 0dl. in these range of Reynol~~}1umbers, was not confimled by the recent data obtained in the experiments where nutrient was suppressed. Bott and Miller (8) also observed a pronounced reduction in blofilm weight after removing the nutrients for a fluid velocity of 0.5 m/s. A similar result was not found by these authors when the fluid velocity was 2 m/s. which means that 0d2 (biofllm production rate) did not play an important role in these flow conditions. Accordingly. the trend shown by the present data (Figure 9) suggest that for a velocity of 2 m/ s (Re = 24 780} the value of Od2 would also be quite small. There are also other aspects that have to be taken into account. such as the fraction of "active" layers in the biofllm. The present experiments do not allow to draw any conclusions about this question. which would also require the use of a more detailed biofouling model. CONCLUSIONS The results obtained in biofouling tests perfomed at dlfferent fluid velocities showed that the processes occurring at the interface (attachment) and in the biofilm (biological activity). together with the removal action of the hydrodynamic forces. determine the overall fouling rate. Both types of processes are slowed down by high fluid velocities. In the case of attachment this tendency has been observed by several authors. Biological activity in the deposit could be e."'Cpected to show a dilTerent behaviour. but the present data point out to the importance of compactness changes in the film that may increase the difficulty of nutrient dilTusion to the Inner layers of the deposit. The analysis of this question implies a more detailed model involving the concept of "active" and "inactive" layers in the blofilm. - 10-ACKNOWLEDGMENTS The financial support of JNICT. Portugal. through Project no. 87509 is grateful acknowledged. The authors wish to thank Dr. Cecilia Leao and the staff of the Biology Department of the University of Minho for their valuable assistance. SEM picture of e.xopolymers tn the blofilm - 11 -SYMBOIS f - fricUon factor with the fouled surface f o t tnr - friction factor with the clean surface - convective heal transfer coeITicient (with the clean surface). W /m2K - thermal conductivity of the wall, W /m.k = 0.68 Pr'0•215 (parameter) - Prandtl number - Reynolds number - thermal resistance of the deposit. m2 K/W asymptotic thermal resistance of the deposit. m2 K/W - asymptotic thermal resistance reached by the deposit after removal of nutrients. m 2 K/W - Ume. s ·:u •. - Ume when Rr starts to decrease after suppression of nutrients. s - wall temperatures. K - fluid temperature. K - overall heat transfer coeficient. W /m2 .K - overall heat trasnfer coefficient With the clean surface. W /m2.K - distance between the thermocouples in the wall, m - parameter related to the strenght of the depostt. s·1 - parameter related to the strenght of the deposit after removal of nutrients. s· 1 - deposiUon flux (overall). m2K/W.s ") - deposition flux related to the transport and attachment of bactcrta. m-K/W.s - dcposiUon flux related to the transport of nutrtents and to the biolo~ic:il ,., processes 1n the film. m-K/W.s ") - removal fiu."'C.. m-I{fW.s er - removal fiux after suppression of nutrients. m2K/W.s REFERENCES (1) Lund. D. and Sandu. C .. "State-of-the art of fouling: heat transfer surfaces··. in Fnndamentals <Jnd Appllcntions nf Surface Phennmem1 AssoclQtect wtth Fm1lin!! and C'leanin:;! in Food Prnce:5~ing. 27-56. Univ. Lund. Sweden. April. 1981. (2) Wennerber~. J., "Observations on the behaviour of foulin~ and cle::min!! in industrial processes". in Fnnct:imentals and Appltcattons of S11rface Phenomrn:l Assnctated with Fm1llnq :mct Cleaning in Food Processing, 76-89. Univ. Lund. Sweden. April. 1981. (3) Kent. C.A .. "Biological Foulin~: Basic Science and models'', in Fm1l!ng Sctc-nc-t• :me\ Tt•chnoloay. 207-222. edit. by Melo. Bott and Bernardo. Kluwer Ac:idl•mic Publishers, Dordrecht. Netherlands. 1988. (4) Characklis. W.G .• Zel\•er. N .. Turakhta. M. and Nimmons. M.J.: "Fouling and heat transfer". ln Foul!n[! tn beat t'Xch;mce em1tpment. pp 1-15. ed. by Chenoweth and Impaglla Impag11a7..zo, American Soc. of Meehan. Engrs .. HTD-vol. 17. USA. 1981. .• - 12 -(5) Bryers. J.D. and Characklls, W.G.: "Processes governtng prlmary biofllm formation", Blotech. & Bloeng .• vol XXIV. 2451-2476. 1982. (6) Duddrtdge. J.E .. Kent. C.A.. Mlller. P.C .. Laws. J.F .. Pritchard. AM. and Bott. T.R; "EITects of flow on btofllm development". in Fouling of heat exchanger surfaces. pp. 717-726. ed. by Bryers & Cole. Engineering Foundation. USA. 1983. (7) Pinheiro, M.M., Melo. L.F., Bott, T.R. Pinheiro. J.D. and Leitao, L.: "Surface :'phenomena and hydrodynamic effects on the depos1t1on of Pseudomonas fluorescens", Can. J. Chem. Eng., §2, 63- 07, 1988. (8) Bott. T.R. and Miller, P.C.: "Mechanisms of blofUm formation on aluminium tubes", J. Chem. Techn Blotechnol. 33B. 177-184. 1983. (9) Kem. D. and Seaton, R. "A theoretical analysts of thermal surf ace fouling". Brit. Chem. Eng. i (5), 258, 1959. (10) Pinheiro, M.M .. Melo, L.F .• Pinheiro. J.D. and .,l!Jott. T.R.: "A model for the interpretation of blofoultng", 2nd UK National Conference on Heat Transfer. Vol l, 187-197, Glasgow. 1988. (11) Chaplin, M.F.: "Monosacchartdes". ln Carbodydrate analysts - a practical approach. 1-36. edit. by Chaplin and Kennedy, IRL Press. Oxford. U.K.. 1986. 

Biofilm formation by Pseudomonas fluorescens

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