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Mitigation of biofouling using electromagnetic fieldsin tubular heat exchangers–condensers cooled byseawaterAlfredo Truebaa, Sergio Garcíaa & Félix M. Oteroa

a Department of Sciences & Techniques of Navigation and Ship Construction, University ofCantabria, Santander, SpainPublished online: 22 Nov 2013.

To cite this article: Alfredo Trueba, Sergio García & Félix M. Otero (2014) Mitigation of biofouling using electromagneticfields in tubular heat exchangers–condensers cooled by seawater, Biofouling: The Journal of Bioadhesion and BiofilmResearch, 30:1, 95-103, DOI: 10.1080/08927014.2013.847926

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Mitigation of biofouling using electromagnetic fields in tubular heat exchangers–condenserscooled by seawater

Alfredo Trueba*, Sergio García and Félix M. Otero

Department of Sciences & Techniques of Navigation and Ship Construction, University of Cantabria, Santander, Spain

(Received 24 May 2013; accepted 18 September 2013)

Electromagnetic field (EMF) treatment is presented as an alternative physical treatment for the mitigation of biofoulingadhered to the tubes of a heat exchanger–condenser cooled by seawater. During an experimental phase, a fouling biofilmwas allowed to grow until experimental variables indicated that its growth had stabilised. Subsequently, EMF treatmentwas applied to seawater to eliminate the biofilm and to maintain the achieved cleanliness. The results showed that EMFsprecipitated ions dissolved in the seawater. As a consequence of the application of EMFs, erosion altered the intermolec-ular bonding of extracellular polymers, causing the destruction of the biofilm matrix and its detachment from the innersurface of the heat exchanger–condenser tubes. This detachment led to the partial removal of a mature biofilm and a par-tial recovery of the efficiency lost in the heat transfer process by using a physical treatment that is harmless to the mar-ine environment.

Keywords: biofouling; biofilm; electromagnetic field (EMF); calcium carbonate (CaCO3); heat exchanger; seawater

Introduction

The phenomenon of biofouling (Eguía & Trueba 2007),its formation and growth processes (Rao 2012), and itsnegative impact on the performance of tubular heatexchangers–condensers (Eguía, Trueba, Río-Calonge,Girón, & Bielva 2008) pose problems for both the pri-mary designer of a facility and its subsequent mainte-nance personnel. The control of biofouling growth onthe surfaces of heat exchanging equipment is of vitalimportance to the maintenance of operating conditions.

To mitigate biofouling ‘on-line’, physical methods[ultraviolet light (López-Galindo et al. 2010), electricfields (Abou-Ghazala & Schoenbach 2000), ultrasound(Bott 2001), heat treatment (Rajagopal et al. 2012)], bio-logical methods (Dobretsova et al. 2013) and chemicalmethods [biocide oxidants (Eguía, Trueba, Río-Calonge,Girón, Amieva, et al. 2008) and non-oxidising biocides(Trueba et al. 2011)] can be used as antifouling (AF)treatments. The use of a particular method depends onthe nature of the biofouling microorganisms, on thehydraulic-thermal conditions of the process, on the costsof the treatment, on the safety regulations involved andon the environmental impact (Eguía, Trueba, Río-Calonge, Girón, & Bielva 2008).

In contrast to the toxic nature of traditional chemicalAF, physical AF methods in general, and electromagneticfields (EMFs) in particular, are environmentally harmlessand have the advantage of not being subject to strictrules regulating the disposal of effluent into the receiving

medium (EU 2006). EMFs are created by an electricalcurrent, and they are directly proportional to the flow ofthat current. EMFs cause the precipitation of calciumcarbonate (CaCO3) in fine particles that, after a crystall-ising period, are deposited in the form of sludge that iseasily removed and swept away by the shear effect ofthe circulating water itself (Lipus et al. 2011). The pro-cess of crystallising CaCO3 in water with a large concen-tration of ions (such as seawater) involves theassociation of ions and the formation of stable nuclei towhich other particles can be aggregated, thereby formingcrystals that can be deposited on the solid–liquid inter-face. Therefore, the type of deposit formed will dependon the composition and hardness of the fluid involved.Among the most commonly formed deposits are calciumcarbonate, silicates and sulphates. CaCO3 is polymor-phous (referring to its ability as a solid material to existin more than one form or crystal structure) and can existas calcite or aragonite (Xiaokai 2008). Although thechemical compositions of calcite and aragonite are thesame, their crystal structures are different. Calcite crys-tals are trigonal-rhombohedral (a semi-cubic systema = b = c with non-rectangular but equal angles) with tri-gonal hexagonal crystal lattices. Aragonite crystals arepseudohexagonal and prismatic with orthorhombic crys-tal lattices (un-equal a, b, c, with all angles rectangular).Long needles found on heated surfaces are typically ara-gonite crystals; dendrites can also occur under highlyoversaturated conditions and may form very dense layers

*Corresponding author. Email: truebaal@unican.es

© 2013 Taylor & Francis

Biofouling, 2014Vol. 30, No. 1, 95–103, http://dx.doi.org/10.1080/08927014.2013.847926

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at high temperature gradients. Aragonite is kineticallyfavourable and is usually formed from carbonically purewaters above 60 °C, whereas in freshwater, where calciteinhibitors (eg Mg2+) are present, aragonite precipitates atlower temperatures (Lipus et al. 2011). In heat exchang-ers, this threshold temperature is 35 °C (Xiaokai 2008).Below 35 °C, calcite is the main component of CaCO3

crystals and is less adherent than aragonite. It is difficultto remove crystals that grow directly from a heated sur-face (in untreated cases, this can be observed from mor-phology, as particles are oriented according to thetemperature gradient), while it is easier to remove depos-ited particles that originated from pre-precipitation duringEMF treatment (this can be observed from the randomorientations of these particles) (Shahryari & Pakshir2008; Xiaokai 2008).

EMFs have been used experimentally, for the controlof scaling in industrial and domestic systems using fresh-water or artificial water (Gabrielli et al. 2001; Shahryari &Pakshir 2008; Xiaokai 2008; Lipus et al. 2011), but inmany cases fully conclusive results were not obtained.The primary objective of this study was to reduce theamount of biofilm adhered to the tubes of a heat exchangercooled by seawater by applying EMFs and to regain lostefficiency in the heat transfer process. The specific aims tobe achieved during the experiment were as follows: (i) toallow the biofilm to grow inside the heat exchanger tubesuntil experimental variables indicated that its growth hadstabilised, (ii) to eliminate the adhered biofilm and main-tain the achieved cleanliness by treatment with EMFs and(iii) to evaluate the influence of EMFs on the physical–chemical properties of seawater.

Materials and methods

Experimental setup

Figure 1 shows a schematic diagram of the pilot plant,which comprised a single pass counterflow tubular heatexchanger–condenser pursuant to Tubular ExchangerManufacturing Association standards. Seawater circulatedas a cooling fluid through a tubular bundle of fourindependent tubes placed linearly at 90° to one another,measuring 3.163 m long (10.2 mm internal diameter and2.5 mm thick), composed of AISI316L (N5 class surfaceroughness) stainless steel. The seawater was takendirectly from the Santander Bay (43°28′N and 3°48′W),which is located in the Cantabrian Sea, the largest estu-ary on the northern Spanish coast, comprising an area of22.42 km2. Seawater was pumped to the laboratory (adistance of 250 m) by two centrifugal pumps (ITURAU-M1 1.5/10) arranged in series, where the seawaterwas subjected to a macro-filtering process and was leftto decant in a 1 m3 tank. After filtering and decanting,circulation pumps (Grundfoss CHI 4-50 AWG) pumped

the seawater, at 1.9 bar in a turbulent regime (Re = 9,400),toward the tube bundles of the heat exchangers, throughthe EMFs generating equipment, and through the controlinstruments [differential pressure meters (PTX 2170-1656) and resistance thermometers (Pt 100)]. The seawa-ter flow speed was set at a constant 1 m s−1 with a ther-mal difference of 5 °C as initial conditions.

The outer casing of the heat exchanger–condenserhad an exterior diameter of 240 mm (20 mm thick) andwas made of AISI304 stainless steel. Freshwater forheating circulated through the shell and was maintainedat 36 °C by means of a Proportional-Integral-Derivativeautomatic control system (OMRON E5CK) that operatedon a motorised three-way valve in the primary circuit,where 80 °C water from the boiler room transferred itsheat through a plate heat exchanger (Sedical UFX-26H)to the water in the secondary circuit, which was pumpedin a closed circuit into the heat exchanger at 1.8 bar bya circulation pump (Grundfos MG90SA4-24F 115IP44CLF) at a rate of 19 m3 h−1. The temperature differen-tial between the inlet and the outlet was 0.8 °C.

The plant was monitored, allowing real-time observa-tion and the maintenance of a historical record. The datamanagement system consisted of four multifunctionalconfigurable modules (STEP® model DL01-CPU) capa-ble of registering and controlling all of the parameters ofthe plant. Data were transmitted to a software program(TCS-01 version 3.0.3) via an RS232 communicationport, where they were stored on a personal computer.

Mechanisms of EMFs

The EMF unit (Figure 2) consisted of two spools placedin series (at a distance of 100 mm) rolled around a watertube (804 mm long and 50 mm in diameter) that gener-ated an EMF intensity of 15 mT at a frequency of 1 kHz(12 V) monitored by a control unit. Thus, the EMFs unitproduced a pulsing current to create time-varying EMFsinside the tube. According to Faraday’s Law (Shahryari& Pakshir 2008; Xiaokai 2008; Lipus et al. 2011), theoscillation of a magnetic field induces an oscillating elec-tric field that agitates dissolved ions (calcium and car-bonate), causing them to collide and crystallise into fineparticles that are then deposited onto the surface of theheat exchanger.

Experimental variables

The evolution of the biofilm adhered to the internal sur-face of the heat exchanger–condenser tubes was followedusing direct and indirect measurements (Eguía, Trueba,Río-Calonge, Girón, Amieva, et al. 2008).

Direct measurements determined the thickness andcomposition of the biofilm, depending on the solidmatter that was deposited on the tubes. To obtain these

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Figure 1. Schematic arrangement of the experimental plant.

Figure 2. Schematic diagram of the EMFs unit.

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measurements, a test-tube of known dimensions andweight was used. A test-tube was placed at the outlet ofeach tube using a test-tube holder system described byEguía et al. (2007). The biofilm thickness was calculatedin accordance with Equation 1 (Eguía, Trueba,Río-Calonge, Girón, Amieva, et al. 2008):

e ¼ DM2prld

�104 (1)

where, ε is the biofilm thickness (μm), ΔM is the biofilmmass (g) or the difference between the weight of the wetbiofilm and the initial unladen test-tube weight, r is thetest-tube internal radius (cm), l is the test-tube length(cm) and δ is the biofilm density (1.025 g cm−3).

After the total mass of the biofilm was determined,its organic matter content was measured as the differencebetween the total amount of solids obtained after dryingat 105 °C for 4 h and the amount of inorganic matterthat was obtained after incineration at 550 °C for 12 h.

Indirect measurements indicate the presence ofbiofilm, although they do not provide information as tothe types of deposits. Using a method based on fluidtransport properties (Characklis et al. 1990), fluidfrictional resistance and heat transfer resistance weredetermined.

The fluid frictional resistance ( f [dimensionless]) wascalculated using Equation 2 (Characklis et al. 1990).

f ¼ 2DDPLqV 2

(2)

where, D is the inside diameter of the tube (m), ΔP isthe pressure drop between the tube inlet and outlet (Pa),L is the length of the tube (m), ρ is the seawater density(1,025 kg m−3) and V is the velocity of the coolingwater (m s−1).

The heat transfer resistance (Rf [m2 K kW−1]) wascalculated using Equation 3 (Characklis et al. 1990).

Rf ¼ At

Qqcp lnTshell�Tin C=W

Tshell�Tout C=W

� � (3)

where, At is the total surface covered by biofilm depositsin the tube (m2), Q is the cooling water flow rate(m3 s−1), ρ is the seawater density (1,025 kg m−3), cp isthe specific heat at constant pressure (4.18 kJ kg−1 K−1),Tshell is the shell temperature (K), Tin C/W is the coolingwater inlet temperature (K) and Tout C/W is the coolingwater outlet temperature (K).

After the direct and indirect measurements were deter-mined, the biofilm thermal conductivity was determined,using Equation 4, to be the ratio between the thickness ofthe biofilm (direct measurement) and the heat transferresistance (indirect measurement). This variable expressesthe capability of the biofilm that was formed inside thetubes to conduct heat (López-Galindo et al. 2010).

kbiofilm ¼ eDRf

(4)

where, λbiofilm is the thermal conductivity of the biofilm(W m−1 K−1), ε is the thickness of the biofilm (m) andΔRf is the increase from the initial value of the heattransfer resistance (m2 K W−1).

Experimental procedure

During the experiment, the AF action of the EMFs inthe mitigation of biofouling material adhered to the inter-nal surface of heat exchanger–condenser tubes cooled byseawater was verified. The experiment was divided intotwo phases: the growth phase and the treatment phase.The experiment was completed in 60 days during thetime of year exhibiting maximum biological activity(May–July, 2011).

During the growth phase (40 days), the biofilm wasallowed to grow until the values of the indirect measure-ments f and Rf showed that its growth had stabilised.Once the growth phase was completed, the seawater wastreated with EMFs. In tubes 1 and 4, the evolution ofthe biofilm was followed without any AF treatment. Thetreatment phase (20 days) consisted of subjecting theseawater flowing through the treated tubes (2 and 3) tothe action of the EMFs. Tests were performed in dupli-cate and the results are expressed as the mean values ofthe data obtained from both tubes.

With the purpose of understanding the influence ofEMFs on the physical–chemical characteristics of theseawater, pH, conductivity and the concentration of dis-solved calcium (Ca2+) and magnesium (Mg2+) (ASTMD1126-12 2012) were measured daily in the influent andeffluent. At the beginning and end of the experiment,images of the inner tubes surface were taken using anindustrial borescope.

Results and discussion

Evolution of biofouling during the growth phase

At the end of the experiment, the direct measurementstaken in the untreated control tubes determined an aver-age biofilm thickness of 798 μm. The biofilm was com-posed of 80% water, 12% inorganic matter and 8%organic matter (Figure 3). The concentration of dissolvedsolids was 6.7 mg cm−2, of which 60% was inorganicmatter and 40% was organic matter (Figure 4). The indi-rect measurements recorded an average increase over theinitial values of 87% for f and 51% for Rf (Table 1). Theevolution of both indirect measurements follows a sig-moidal curve defined by Eguía et al. (2007) (Figures 5and 6). From Equation 2, it can be seen that the increasein f (Figure 5) due to the growth of biofilm with a con-stant flow of water in the tubes was the consequence of

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a higher pressure drop in the tube. This result impliesthat, to maintain constant hydraulic conditions inside thetube, the energy consumption of the circulation pumpmust be increased. The observed variation in the valuesof Rf (Figure 6) was a consequence of the thermal resis-tance provided by convection and by conduction.

According to Characklis (1981), convective heat transferdepends on f and on the properties of the fluid, whereasconductive heat transfer is a function of the thickness ofthe biofilm and of its effective thermal conductivity. Atthe end of the experiment, the thermal conductivity ofthe biofilms in the control tubes reached an averagevalue of 1.24 W m−1 K−1 (Figure 7). Knowing that ther-mal conductivity of seawater is 0.6 W m−1 K−1 (Shar-qawy et al. 2010) and that 80% of the adhered biofilmwas water, it can be concluded that both the thermal con-ductivity of the biofilm and Rf depended on the concen-tration of dissolved solids in the biofilm. The increase inRf and in the thermal conductivity of the biofilm led to areduction in the thermal efficiency of the heat transferprocess. Taking as a reference the increment in tempera-ture experienced by the cooling water, from initial condi-tions (5 °C) it was reduced by −1.4 °C (28%) at the endof the growth phase and by −1.7 °C (34%) at the end ofthe experiment.

AF action of EMFs

After the EMF treatment phase was completed, directmeasurements determined an average biofilm thicknessof 382 μm for a biofilm composed of 87% water, 7%inorganic matter and 6% organic matter (Figure 3).

Figure 3. Mean thickness and composition of the biofilm at the end of the experiment.

Figure 4. Mean concentration of dissolved solids in the bio-film at the end of the experiment.

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The concentration of dissolved solids was 3.4 mg cm−2,of which 41% was inorganic matter and 59% wasorganic matter (Figure 4). Compared to the untreatedcontrol tubes, EMFs reduced the thickness of thebiofilm by 416 μm (52%) and reduced the dissolvedsolids by 49%, while 65% of the inorganic matter and26% of the organic matter was eliminated. Figure 8shows the difference between the final condition of theinternal surface of an untreated tube sample and a tubesample treated with EMFs, after a drying process at105 °C at the end of the experiment. According toLipus et al. (2011), the reduction in the amount of

adhered inorganic matter was a consequence of theprecipitation of CaCO3 after treatment with EMFscausing ions in the seawater to crystallise and depositthem on the solid–liquid interface. Such scale adheresweakly and can be removed at low flow velocity, ie1 m s−1 (Shahryari & Pakshir 2008), causing an ero-sion effect on the biofilm that can lead to its detach-ment. Likewise, according to Eguía et al. (2008),precipitation of the dissolved ions affected the intermo-lecular bonding of extracellular polymers, causing thedestruction of the biofilm matrix and its detachmentfrom the surface.

Figure 5. Evolution of the mean values of fluid frictional resistance ( f ) during the experimental phase.

Table 1. Mean values and increases, with respect to the original mean value, of the indirect fluid frictional resistance ( f ) and heattransfer resistance (Rf ) measured at the end of each experimental phase.

Phase Clean tube (day 0) Growth phase (day 40)Treatment phase (day 60)

AF treatment Control EMFTube number 1–4 2–3 1–4 2–3 1–4 2–3

f 0.0351 0.0328 0.0658 0.0619 0.0688 0.0527Δf 0.0307 0.0291 0.0337 0.0199% Δf 87 89 96 61Rf 0.943 0.987 1.422 1.506 1.587 1.365ΔRf 0.479 0.519 0.644 0.378% ΔRf 51 53 68 38

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The reduction in the thickness of the biofilm and thereduction in the dissolved solids that partially composethe biofilm altered the trends of the indirect measurementcurves (Figures 5 and 6). However, the response to thetreatment was not immediate. The trends in the values of

both f and Rf did not change until five days after thestart of the treatment. In agreement with Xiaokai (2008),certain conditions required for deposition, such as crystalnucleation and surface conditioning, were establishedduring this period. After the sixth day of treatment, thevalues of f and Rf progressively diminished. At the endof the growth phase, EMF treatment led to a 32% reduc-tion in Δf and a 27% reduction in ΔRf. This was a conse-quence of the detachment of the biofilm. Subsequentdecreasing values of f indicate the decrease in the rough-ness of the support medium, as shown in Figure 8. Con-vective heat transfer, which depends on f, alsodiminished, as well as conductive heat transfer, which isa function of biofilm thickness, and therefore Rf. Conse-quently, the value of biofilm thermal conductivity in thetubes treated with EMFs (1.01 W m−1 K−1) was closerto that of the seawater, taken as reference, than to thatobtained from the untreated tubes (Figure 7). Thus, if atthe end of the growth phase of the biofilm in the treatedtubes, the increment in temperature decreased by−1.5 °C (32%) from the initial value (5 °C), later treat-ment allowed for a recovery of 0.5 °C, stabilising thedifference in temperature between the inlet and outlet ofthe cooling water at 4 °C. Therefore, the application of

Figure 6. Evolution of the mean values of heat transfer resistance (Rf) during the experimental phase.

Figure 7. Mean biofilm thermal conductivity at the end of theexperiment.

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EMFs as AF treatment in the mitigation of mature bio-film material adhered to the interior surfaces of the tubesof a heat exchanger–condenser cooled by seawaterallowed the recovery of 10% of the efficiency of the heattransfer process and the stabilisation of the installation’sthermal conditions.

Influence of EMFs on the physical–chemical propertiesof seawater

Seawater is slightly alkaline and can compensate forchanges in its pH due to its dissolved ion composition.Therefore, the pH undergoes few variations under normalconditions. The pH value depends on pressure, tempera-ture and the content of carbonates (CO3

−), bicarbonates(HCO3

−), borates (BO33−) and hydrated metal ions.

Hence, for specific pressure and temperature conditions,inorganic matter in general and carbonates in particularcan act as indicators and regulating mechanisms. Theaverage pH value in the influent during the experimentalphase was 8.14, with an average increase of 0.15 in theeffluent during the treatment phase. In a saline medium,pH is a determining factor in the development of a bio-film (Eguía, Trueba, Río-Calonge, Girón, Amieva, et al.2008), and pH values influence the efficiency of EMFtreatment. According to Alimi et al. (2006), when waterbecomes more alkaline, the calcocarbonic equilibrium isdisplaced toward stronger supersaturation and the proba-bility of nucleation increases. This indicates that EMFsare more effective in alkaline and hard water.

Conductivity is directly proportional to the concentra-tion of dissolved ions and to their mobility in the solu-tion. The mean value for conductivity in the influent was54.2 mS cm−1, with an average decrease of 0.8 mS cm−1

in the effluent during the treatment phase. This confirmsthat EMFs precipitated dissolved ions and caused theseions to crystallise into fine particles that were then sus-pended, not dissolved in the liquid. The process of agita-tion to which EMFs subjected the dissolved ionscompensated for the decrease in conductivity caused bythe precipitation of the dissolved ions, resulting in varia-tions that were not significant, as indicated by the final

Figure 8. Inner appearance of the tube samples at the end of the experiment: (a) untreated tubes and (b) tubes treated with EMFs.

Figure 9. Variation in ionic calcium (Ca2+) and magnesium(Mg2+) concentrations during the treatment phase.

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concentration of Ca2+ and Mg2+ ions in the effluent andtheir initial value.

Analysis of the ionic content of Ca2+ and Mg2+

allowed an understanding of the physico-chemical pro-cesses that occur in water treated with EMFs. Taking asreference an average value of 2.61 g l−1 in the influentduring the experimentation phase, the concentration ofCa2+ and Mg2+ in the untreated cooling water as it flo-wed through the exchanger during the second phase ofthe experiment decreased by an average of 8% (Figure 9).This indicates that CaCO3 precipitated naturally underthe testing conditions, becoming part of the biofilm inthe form of inorganic matter. In water treated withEMFs, the average decrease in Ca2+ and Mg2+ concen-tration during the treatment phase was 21% (Figure 9).This considerable decrease in Ca2+ and Mg2+ contentindicates that the nucleation of ionic calcium and its pre-cipitation into calcium carbonate, explained by Shahryariand Pakshir (2008), occurred when EMFs were applied.Consequently, the percentage of inorganic matter presentin the treated biofilm decreased; the percentage oforganic matter also decreased, but to a lesser extent.

Conclusions

EMFs led to nucleation, crystallisation and deposition onthe solid–liquid interface of ions dissolved in seawater.In addition to the erosion of biofilm material, precipita-tion of dissolved ions affected the intermolecular bond-ing of extracellular polymers, causing the destruction ofthe biofilm matrix and its detachment from the surface.Therefore, treating seawater with EMFs allowed the par-tial removal of a mature biofilm adhered to the internalsurfaces of the tubes of a heat exchanger–condenser.Consequently, EMFs were able to partially recover thereduced efficiency in the heat transfer process andstabilise the installation’s thermal conditions.

AcknowledgementsThis research was financed by the Spanish Ministry ofEducation and Science [project CTM2006-05471].

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