optimization of biofilm control using an on-line monitor ... · 722 powerplant chemistry 2010,...

722 PowerPlant Chemistry 2010, 12(12)

PPChem Optimization of Biofilm Control Using an On-Line Monitor: Case Studies from around the World

INTRODUCTION

The control of biofilm represents one of the greatest chal-lenges facing cooling water system owners and operators.The presence of biofilm can lead to severe plant problemssuch as reduction in heat transfer and microbiologicallyinfluenced corrosion (MIC). MIC is a localized corrosionphenomenon produced as a result of the chemical andelectrochemical differences that are created and that canbe maintained by the presence or metabolic activity ofmicrobes. Biofilm also promotes the deposition of sus-pended solids and the settlement of macroorganisms,effects which can significantly impair process perfor-mance and efficiency and result in plant damage.Moreover, biofilm is a primary concern for cooling towersystem health and safety, due to its tendency to harborand proliferate the potentially lethal bacterium Legionella.

Given that more than 99 % of all viable bacteria in a cool-ing water system reside on system surfaces, the ultimateaim of biofilm control should be the control of sessilemicroorganisms. If the asset owner can monitor the activ-ity of sessile bacteria – a biofilm – on-line and in real time,then the opportunity truly exists to optimize an efficientwater treatment control strategy, one that accords withplant performance indicators, health and safety targets,and environmental discharge regulations.

An electrochemical biofilm activity monitoring system withintegrated data acquisition and data analysis capabilitieshas been available to the marketplace for several yearsand has found application in cooling water facilitiesaround the world. The system has provided cooling waterusers with a unique tool to help control biofilm, andthereby ensure clean, efficient, and safe plant operation.

Experiences with the use of this on-line, real time technol-ogy for biofilm control in a variety of water systems andplant types are described herein.

EFFECTS OF MICROBIOLOGICAL FOULING

If allowed to develop in an uncontrolled manner in a watersystem, microbial populations can result in the prolifera-tion of biofilm on system surfaces [1]. A biofilm is a complex community of synergistically interacting micro -organisms, consisting of an aggregation of microbial cellsand their extracellular polymeric substances (EPS). It isimportant and significant to note that more than 99 % ofviable microorganisms in any water system will residewithin a surface biofilm [2]. One can say of any free-float-ing planktonic bacteria introduced into the water system

Optimization of Biofilm Control Using an On-Line Monitor:Case Studies from around the World

ABSTRACT

The presence of biofilm in water systems can contribute to severe plant problems, such as microbiologically influ-enced corrosion (MIC) and under-deposit corrosion, reduction in heat transfer efficiency, increased chemical costs,and health and safety risks. While it is standard practice to apply treatment chemicals to a water system to controlbiofilm formation and growth, it is often unclear how these chemicals are performing or will perform in the future – thelimited information available to the operator (usually from bacterial plate counts from the bulk water) reflects a system that has since moved several hours or even days into the future, and does not actually tell the operator any-thing about the sessile bacteria populations on system surfaces.

However, on-line and real time monitoring of the water system condition is available that can provide early warning ofunsatisfactory biocide performance as well as allow optimization of the chemical dosing regime. Process problemsrelating to biofilm formation, such as MIC, heat transfer losses and chemical efficacy, can be known and counteredbefore they develop. This performance-based approach to biocide dosing is not only more environmentally sustain-able, but costs less and puts less stress on plant materials.

The electrochemical biofilm activity monitoring system is described, and experiences with the use of this on-line, realtime technology for biofilm control in a variety of water systems and plant types are presented.

Hugh Fallon and George J. Licina

© 2010 by Waesseri GmbH. All rights reserved.

Heft2010-12:Innenseiten 12.12.10 11:04 Seite 722

723PowerPlant Chemistry 2010, 12(12)

PPChemOptimization of Biofilm Control Using an On-Line Monitor: Case Studies from around the World

that it is their "goal" to find a surface upon which to resideand multiply. Surface adhesion is integral to EPS produc-tion and the gene transfer and gene derepression thatcharacterizes the phenotypically distinct microorganismsthat reside in biofilm [3]. Such microbes are termed ses-sile, i.e., "surface dwelling." The accumulation of sessilemicroorganisms at equipment surfaces contributes to avariety of phenomena that reduce the performance and/orlifetime of industrial plants and materials [4].

The water treatment triangle (Figure 1) shows the fourproblem areas for the asset owner [5]. The arms of the triangle encompass the microbiological processes in thecenter; the presence or otherwise of microbiological foul-ing can be central to the initiation, development and over-all severity of the plant fouling processes. Biofilm con-tributes to microbiologically influenced corrosion andunder-deposit corrosion. Biofilm also provides possiblenucleation sites for mineral scale growth, and the stickyEPS can aid the accumulation of suspended solids andgeneral fouling deposits. The negative effects of micro -biological fouling in water systems are discussed furtherbelow.

Microbiologically Influenced Corrosion

As soon as a metallic surface such as steel is exposed towater, two processes occur simultaneously: corrosion ofthe metal starts immediately, and, unless the water iscompletely sterile, biofilm begins to form. The localizedenvironments within biofilm can be very aggressive, andthereby can accelerate the "natural" corrosion processesby any or all of the following mechanisms:

• They can excrete substances that accelerate corrosion(e.g., sulfides, organic and mineral acids, ammonia);

• They can fix the corrosion potential of metals at levelsthat promote accelerated corrosion, i.e., they can resultin ennoblement;

• They can actively participate in anodic- and cathodic-based electrochemical corrosion reactions;

• They can effectively accelerate steps in the metal dis-solution process that would tend to slow corrosiondown; and,

• They can simply glue together silt, sand, scale, and cor-rosion products to produce occlusion of part of themetal surface, which leads to the development of a cor-rosion concentration cell, with rapid metal loss from theoccluded anode.

Four main kinds of bacteria have been linked to acceler-ated corrosion in water systems:

• Sulfate reducers, e.g., Desulfovibrio, DesulfomonasThese bacteria are anaerobic and occur in most naturalwaters. They survive in bulk water but generally onlyflourish beneath deaerated sludge or silt deposits andthe deoxygenated slime layer/metal surface interfacethat is indicative of mature biofilm.

• Acid producers, e.g., Clostridium, Thiobacillus thiooxi-dansThese types of bacteria can be aerobic or anaerobicand they produce acids, lowering the pH and thusaccelerating corrosion attack. Thiobacillus can exist ina synergistic relationship with sulfate reducers, whileClostridia produce short-chained organic acids that canbe quite aggressive to steel.

• Metal depositors, e.g., GallionellaSuch metal-depositing bacteria oxidize ferrous iron toferric iron, with ferric hydroxide being the result. Sincethis hydroxide is more voluminous than the originalmaterial, a mound of deposit called a tubercle results.Pitting of stainless steels in association with metaldepositors is likely to be the result of the bacteria's indi-rect contribution to under-deposit corrosion.

• Slime formers, e.g., PseudomonasMost slime formers are aerobic, although some, suchas the commonly referenced Pseudomonas, are facul-tative, being able to grow in either oxygen-rich or oxy-gen-poor environments. Since most slime formers haveaerobic-based metabolisms, they consume oxygen inthe region around and beneath the slime deposit, contributing significantly to the formation of oxygenconcentration cells, and resultant concentration cellcorrosion. Slime formers can contribute indirectly tocorrosion in another way: since the slime-to-metalboundary tends to be deoxygenated, the slime sup-ports the existence of the anaerobic bacteria and theassociated corrosion processes thereof.

Reduction in Heat Transfer

The thermal conductivity of a biofilm (0.6 W · m–1 · K–1) iscomparable to that of water [6], which is not surprisinggiven that a biofilm is 95 % water. The thermal conductiv-ity of stainless steel is around 16 W ·m–1 · K–1 at 20 °C and

Co

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r

Scale

Fouling

Microbiological

Figure 1:

The fouling triangle.



PPChem

that of titanium is around 21 W ·m–1 · K–1. Biofilm is evenmore insulating than calcium carbonate scale, which has athermal conductivity in the range 0.8–2.0 W · m–1 · K–1.Siemens Kraftwerk Union calculated that the formation ofa 26 µm thick biofilm in the main condenser of a 740 MWpower plant resulted in the loss of 2 MW due to the influ-ence on steam turbine back pressure [7]. As stated pre -viously, a biofilm is mostly water, which is a poor conduc-tor of heat. An insulating and stagnant layer of biofilm prevents the efficient transfer of heat from the steam tothe cooling water, via the condenser tubes. The steam isthus not condensed to the lowest achievable temperature,and since the steam turbine efficiency and output is afunction of condensation temperature, the higher the con-densation temperature, the lower the efficiency and poweroutput.

Increased Chemical Costs

Bacteria within a biofilm have increased resistance to anti -microbial agents. Bacteria in biofilm can be 150 to 3 000times more resistant to HOCl and up to 100 times moreresistant to NH2Cl than their planktonic counterparts [8].

The presence of biofilm can exert a large biocide demand.The response of bacteria to a chemical (toxic) shock isoften to produce more and more EPS, which is preferen-tially attacked by the biocide, thereby consuming it andleaving at least some of the bacteria in the biofilmuntouched. EPS can therefore act as a general physicalbarrier, excluding or limiting the penetration of biocidalagents. The further risk is that the depleted biocide resid-ual is unable to efficiently kill any remaining bulk waterbacteria, which can then be added to the biofilm, or candevelop new biofilm. The chemical treatment of biofilmcan often be such that a mature biofilm consists of chemi-cally resistant organisms.

Increased Health & Safety Risks

Established biofilms offer an environment conducive tothe growth and multiplication of numerous pathogenicmicroorganisms. The operating conditions of open recir-culating cooling water systems, which have ideal tem-perature, nutrient, pH and oxygen profiles for the rapiddevelopment of bacteria and biofilm, can increase thepopulations and activity of pathogenic organisms to theextent that there is a significant health risk to staff, con-tractors and the wider public. Biofilm sloughing is a natu-ral part of biofilm evolution. Should pathogen-ladenbiofilm enter a cooling tower, or slough off from coolingtower surfaces, opportunity exists for the transmission ofthese organisms as a fine aerosol across a wide area,where susceptible people can become infected. Humanpathogens often associated with cooling towers areNaegleria spp., Acanthamoeba spp., and Legionella pneu-mophila [9].

There are thousands of cases of Legionnaires' diseasereported worldwide each year; the majority of these arecaused by cooling towers and air-conditioning units. Thehistoric compensation claims for these cases have runinto the hundreds of millions of dollars.

THE REAR-VIEW MIRROR EFFECT

The rear-view mirror effect (Figure 2) can be defined as anyattempt to know the future based solely on an examina-tion of events in the past. It is akin to trying to drive anautomobile by only looking through the rear-view mirror.The driver must make sense out of road markings behindhim to try and see where the bends in the road occur.There is no opportunity to know what might be lying inwait. Progress is inevitably slow and fraught with risk.

Operating under the rear-view mirror effect (RVME) leadsto many problems including:

• Examination of data after the fact. Waiting for the roadto reveal itself is time consuming and often confusingand contradictory. Knowing past data allows one to seewith hindsight how one should have reacted then.Actual reactions may have been different. Certainly,future reactions based on past data are tantamount toguesswork.

• Responding to a constantly changing reality based onpast events. Tail chasing means the RVME operatorfails to identify and anticipate impending urgent eventsand so cannot take preemptive, proactive action tolessen the severity of those events.

• Focus restriction. Attention is paid only to the last pieceof road navigated. No time can be put into assessingthe condition of the road, the number of other driverson the road, the impact of actions on those other roadusers, etc. The ability to act with speed and efficiency istotally lost.

Figure 2:

The rear-view mirror effect.

Optimization of Biofilm Control Using an On-Line Monitor: Case Studies from around the World


The majority of biofilm management practices are akin tothe rear-view mirror effect. While it is standard practice toapply treatment chemicals to a water system, there isoften little knowledge of how these chemicals are per-forming in the present, and usually no knowledge of howthey will perform in the future. This is because, at best, anybacteria monitoring is of the bulk water only. Not onlydoes this information tell us nothing about sessile bacteriapopulations, but the time delay in determining bacterialplate counts means that we only know a limited amount ofinformation on a system that has since moved severalhours or even days into the future. For Legionella testing,we might be given information on extant populations fromtwo weeks in the past. It is difficult to imagine how theasset owner can adjudge his risk management ofLegionellosis to be current in light of this knowledge timelag.

If surface swabs are taken for sessile bacteria determina-tion, the subsequent reports can again take several daysto be received. As before, the dilemma is to decide whatto do with the information. Has my chemical treatmentstrategy removed biofilm in the interim? If not, has morebiofilm grown? If so, how would I know? Take anothersample and wait another week? Then what?!

It is obvious that no competent water treater or assetowner would willingly indulge in RVME practices inrespect of water system biofilm minimization. Yet mostasset owners are not aware of the sophisticated and cost-

efficient solution to the problem: on-line, real time biofilmmonitoring. With this type of monitoring one could apply amicrobiological control strategy in response to a knownrisk of biofilm development.

Operation of the water system would thus be efficient andcost-effective because it would be performance-based.With the performance-based approach, control of the sys-tem is based on fact and not guesswork. When the assetowner can monitor for biofilm he can ensure the rightstrategy for its control, and thus its minimization. And withno biofilm in the system, the asset owner knows in realtime the significance of this information: no host for path-ogenic organisms, no opportunity for MIC, no heat trans-fer losses, and no wasted chemical.

AN ON-LINE, REAL TIME BIOFILM ACTIVITYMONITORING SYSTEM

The BIoGEORGE™ probe (Figure 3) is an electrochemicalbiofilm activity monitoring system, which has been devel-oped to provide on-line monitoring of biofilm activity onthe probe surfaces, either in real time or with a lead factor,dependent upon user setup. The technology has beendescribed previously [10–13].

The electrochemical biofilm activity monitoring system issimple, sensitive, and robust, consisting of a 2-electrodeprobe, a cable and a controller. Each electrode set is com-


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prised of a series of titanium or stainless steel disks. Oneelectrode is polarized relative to the other for a set period(and set potential) each day. The polarization cycle pro-duces slightly modified environments on the electrodesthat are conducive to biofilm formation, with differentmicrobiological colonies becoming established on thepositive and the negative electrodes [14], as has beenobserved by Nekoksa & Gutherman [15] and by Guezen -nec et al. [16]. Tracking the daily applied current and not-ing significant changes in that current provides onemethod to detect the onset of biofilm formation.

The generated current, which is the current that flowsbetween the two nominally identical electrodes when noexternal potential is applied, provides an additional, andoften more sensitive, indicator of biofilm activity. In gen-eral, the onset of biofilm formation is signaled by anabrupt change in the generated current from the typicalbaseline value, which is always near zero. Like the appliedcurrent, the magnitude of the deviation of the generatedcurrent from the baseline value provides a qualitativemeasure of the activity of the biofilm. The sign of the gen-erated current can be either positive or negative [14,17].

The probe actively encourages biofilm formation on itssurfaces; biofilm plates out on the sensor surfaces beforeit does so on the general non-polarized surfaces of thewater system. Not only does this provide an indication ofthe activity of sessile bacterial communities, but it canalso provide the user with a lead factor, whereby futureplant performance can be assessed, and changes madeto dosing programs, before biocontrol in the system islost. In general, if the control system can maintain a cleanprobe, then it can be assumed that the non-polarized sys-tem surfaces are similarly free of biofilm.

The electrochemical biofilm activity monitoring systemhas been used in numerous facilities throughout the world;some interesting case studies are presented in the follow-ing.

CASE STUDY #1 – COMBINED CYCLE GASTURBINE POWER STATION

A 400 MW combined cycle power station with a heatrecovery steam generator (HRSG) has used the electro-chemical biofilm activity monitoring system since 2002 aspart of an in-house total water treatment managementpackage for cooling water chemistry control optimization[18].

The application of a stabilized bromine product to an estu-arine water cooling tower system maintained extremelyclean plant conditions, but at considerable cost. The bio-cide optimization program involved using the electro-chemical biofilm activity monitoring system to monitor thedevelopment of surface biofilm in the system as theamount of stabilized bromine biocide was reduced. So asnot to jeopardize the existing system cleanliness duringthe optimization program, the strategy was to place theelectrochemical biofilm activity monitoring system probein a dead leg of cooling water and allow some biofilm todevelop. Process cooling water, which contained thenominal concentration of stabilized bromine biocide at thenominal dose frequency, was then allowed to flow past theprobe, thereby killing the biofilm. This was seen via thecurrent responses from the probe. The dead leg wasreestablished and the frequency of dosing (but not theconcentration) was slightly reduced. When the subse-quent exposure of the probe showed that this new dosing

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Eapp applied potential




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regime was successful at keeping the probe biofilm-free,this constituted the first optimization iteration (Figure 4).Verification was achieved through the use of a secondprobe, which was set up in an adjacent location underidentical test conditions.

A differential microbial analysis (DMA) of the two probes atthe end of the first iteration (Table 1) confirmed some inter-esting facts about the performance of the electrochemicalbiofilm activity monitoring system:

• Bacterial counts on probe electrodes were higher thanon non-polarized surfaces, confirming that the probeactively promotes biofilm formation;

• Given that some sessile bacteria could develop on theprobe electrodes while biocide was dosed, it seems

reasonable to assume that the probe would be a validindicator of biofilm activity during times when biocidelevels might be insufficient; and,

• The results for the two different probes are very similar,showing repeatability and consistency of the method.

Subsequent iterations allowed further reduction in theoverall dosed amounts of stabilized bromine chemical, tothe extent that a 2 h on/2 h off dosing regime could beimplemented. The probe current outputs confirmed thatthis strategy was sufficient to maintain cooling systemsurface cleanliness for a range of plant operating condi-tions (Figure 5). This constituted an almost 50 % reductionin the amount of dosed stabilized bromine biocide.

Sludge + + ++++ ++

Siliceous material – – + +

Protozoa – – –

Nematodes – – – –

Diatoms + + ++++ ++

Unicellular algae + + ++ ++

Filamentous algae + + ++ ++

Unicellular bacteria

Filamentous bacteria

Amphipods (larva, nymphs, etc.)

Yeast (filamentous fungi)

Microscopic evaluation (1 000 x magnification)

Culturing (CFU per mL)

Yeasts ND ND ND ND

Moulds 100 100 500 ND

TVC at 25 °C 14 000 13 000 7 000 1 100

Anaerobic SRB ND ND ND 200

Total anaerobic bacteria

Pseudomonas ND 2 000 100 ND

Clostridium ND ND ND ND

Heterotrophic iron precipitating bacteria 100 ND 100 ND

Nitrifying bacteria ND ND ND

Denitrifying bacteria ND ND ND

Table 1:

DMA of two probe electrodes and two non-polarized surfaces.

CFU colony-forming units; TVC total viable count; SRB sulfate reducing bacteria; ND not detectable

On-Line TestSlide Corrosion

17 September 2002 Biofouling BiofoulingBioBox Coupon

Monitor Monitor




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At each stage of the biocide optimization, bulk water andsurface swabs were taken for routine analysis. The resultsconfirmed the trends of the electrochemical biofilm activ-ity monitoring system, i.e., that the same process effi-ciency was being maintained with less application ofchemical biocide.

The ultimate aim of the program was to substitute theexpensive but highly effective stabilized bromine with therelatively inexpensive oxidizing biocide sodium hypochlo-rite. This latter biocide would kill planktonic bacteria andwould counter some biofilm formation risk. Where thatbiofilm formation risk exceeded a threshold level as indi-cated by the on-line electrochemical biofilm activity moni-toring system, the stabilized bromine would be dosed tocounter the threat.

Figure 6 illustrates this performance-based strategy inaction. When a significant microbiological loading enteredthe cooling tower system – as confirmed by high hetero -trophic plate counts – the electrochemical biofilm activitymonitoring system duly indicated a rising current trend,indicative of a significant increase in sessile bacteria activ-ity. The biocide dosing and control system countered thisthreat by applying two additional shock doses of stabi-lized bromine biocide within two hours of each other. Thebiocide addition successfully subverted the developingbiofilm and returned the system to normal, as indicated bythe decreasing current trend from the biofilm monitoringprobe. As before, the qualitative assessments derivedfrom these current trends were substantiated via hetero -trophic plate counts.

Figure 5:

Using less biocide did notnegatively impact onsystem biofilm control.

Figure 6:

Performance-basedoptimization.

ATP adenosine triphosphat

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CASE STUDY #2 – MUNICIPAL WASTE DISPOSALFACILITY

A large municipal waste disposal service operates anincinerator plant cooled by brackish water in a once-through cooling water system. The plant successfullyapplies sodium hypochlorite for control of macrofoulingand microfouling [9].

During the summer months, the focus of the biocide dos-ing is control of macrofouling. Biocide is typically appliedat short, frequent intervals throughout each day. Duringwinter months the focus shifts to microfouling control andat this time the dose frequency of biocide is less than forthe summer months, being around 75 min per day, thoughthe target dose concentration remains the same. The elec-trochemical biofilm activity monitoring system was added

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Biofilm activity monitoring during biocide optimization trials.

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Biofilm activity trends using initial dosing regime of 75 min biocide dosing per day.




PPChem

to the system to evaluate biocide effectiveness and as atool for biocide optimization. The probe was installeddirectly in the cooling water flow at the outlet of the con-denser. Baseline biofilm monitoring involved trending theprobe current outputs during the normal (i.e., non-opti-mized) frequency of winter dosing. As shown in Figure 7both the applied and generated currents from the proberemained low during this dosing regime, as expected.

Following this baseline biofilm monitoring, the biocidedosing schedule was stopped for a period of 10 days.During this time, both the applied and generated currentsexhibited definite increasing trends, indicative of biofilmactivity (Figure 8). A series of optimization schedules weretrialed whereby biocide dosing was implemented forbetween 15 and 75 min per day, for a number of days perweek. Results of the trials showed that effective biofilmcontrol could be assured using a dosing regime of only30 min per day, twice per week. This schedule was suc-cessful at controlling biofilm up to water temperatures of18 °C and represented an 89 % reduction in sodiumhypochlorite use for the winter dose schedule.

CASE STUDY #3 – NUCLEAR POWER STATIONFACILITIES

The electrochemical biofilm activity monitoring systemhas been used in the cooling water systems of severalnuclear power stations.

In one facility, a test and a control probe were used totrend the biofilm formation potential in an untreated fireprotection water system. The test probe was placed in aflow sample of system process water and the controlprobe was placed in a sterilized sample of the same sys-tem water. As Figure 9 shows, the test probe indicated astrong response to biofilm development, which was veri-fied via off-line microbiological analysis, both from sam-ples of the fire protection system water and swabs fromthe probe electrodes [19].

Another probe was installed in the essential equipmentcooling water (EECW) system of the same facility. Whilethere is a biocide treatment schedule for the coolingwater, it is non-frequent, though successful, and is basedaround control of macrofouling, not biofilm. The probecurrent trends (Figure 10) clearly demonstrate that therewas no biofilm growth in the cooling water system duringthe early winter months, but from late winter through tospring biofilm activity began to increase steadily. Anabrupt decrease in biofilm activity (around April) coincidedexactly with the commencement of the facility's springbiocide treatment schedule. Probe trends showed that thebiofilm could recover reasonably quickly after this routinedosing was terminated, but was again successfully coun-tered by the next chlorination. However, the probe current

trends also conclusively demonstrated that subsequentchlorinations, during summer months, were not effectiveat controlling biofilm. This information on biocide effec-tiveness was very useful for the facility [19].

At another nuclear facility, a probe system was used inalliance with two model heat exchange units (Figure 11).The output signals from the electrochemical biofilm activ-

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Biofilm formation and destruction followed cooling water systemchlorination schedules.




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ity monitoring system and the model heat exchangers alltracked biofilm formation on-line and in a similar manner,even though the operating methods for the two methods(electrochemical versus heat transfer resistance) and thematerials used (stainless steel versus 90-10 copper/nickel) were very different [17]. The advantage of the elec-trochemical biofilm activity monitoring system is that it isspecific to biofilm development; this technology can thusbe used to help determine the nature of a trend increase ina model heat exchanger, i.e., whether that trend increaseis due to scale or biofilm.

CASE STUDY #4 – COAL-FIRED POWER STATIONFACILITY #1

A fresh water cooled coal-fired power plant used the elec-trochemical biofilm activity monitoring system to predictmicrobiological effects on condenser tube surfaces and tocontrol MIC [17]. The condenser tubes are fitted with abrush-and-basket system (Figure 12) that is operated dailyto remove loosely adhered deposits from the tube sur-faces. The biocide of choice for the cooling system issodium hypochlorite, which is fed 5 times a week duringsummer and 3 times a week during winter. Despite thesecleaning measures, and the fact that the condenser tubemetallurgy was upgraded from Type 304 stainless steel toType 316 stainless steel, several condenser tubes hadfailed via small through-wall pits that had been attributedto MIC. Failures had usually occurred in tubes with failedbrushes.

The electrochemical biofilm activity monitoring systemprobe was inserted into the condenser return waterbox.The probe current trends (Figure 13) showed that theplant's winter dosing schedule was effective at maintain-ing biofilm control. However, the current trends alsoclearly showed that biofilm activity began to increase earlier than the scheduled change from a three timesweekly biocide addition to the summer schedule of fivetimes per week. Continuous monitoring of biofilm activityalso showed that the summer dosing schedule could bedecreased from 60 min five times per week to 45 min fivetimes per week, with no loss of biofilm control.

The electrochemical biofilm activity monitoring systemhas proved very successful for this plant. Not only has itallowed the plant to apply the right dosing strategy at theright time to meet the biofilm development risk, but it hasallowed the plant to follow the performance-basedapproach. The plant now uses less chemical than before,with superior biofilm control and no further history of MIC-related tube failures. In addition, since the chlorinatedcooling water must be dechlorinated before discharge, theoverall reduction in biocide usage has seen a commensu-rate reduction in the use of dechlorinating agent.


A large coal-fired power station uses river water as coolingwater in once-through condensers. Due to environmentalrestrictions on river heating, the plant often had to reducegeneration load during peak summer months. To counter

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The biofilm activity monitoring system and model heatexchangers showed similar trends.

Figure 12:

Condenser brush-and-basket tube cleaning system.




PPChem

this, the plant constructed a large once-through coolingtower to reject heat from any one of the four generatingunits' hot water return prior to discharge into the river.

Again, due to the environmental sensitivity of the coolingwater discharge zone, the site was barred from using con-ventional cooling water biocides such as hypochlorite ornon-oxidizers in the new cooling tower. Instead, the siteran a research and feasibility study on the use of a propri-etary stabilized peracetic acid-based oxidizing biocidethat is promoted as being highly efficient as acontroller of micro- and macrofouling organ-isms, but is uniquely low-corrosive and verylow in residual toxicity at use concentrations.

The site implemented a detailed study on thedosing requirements for this proprietary bio-cide product, which looked at aspects ofdose efficacy and materials compatibility. Theelectrochemical biofilm activity monitoringsystem became an integral part of the study.The site was motivated to use a suitable bio-cide in the cooling water circuit not onlybecause it would keep the cooling tower sys-tem clean and promote good control ofpotentially pathogenic organisms such asLegionella, but also to keep the waterside ofthe steam turbine once-through condensersfree of biofilm. Heat transfer loss due towaterside biofilm fouling was a known prob-lem on site and could only be countered viaoff-line mechanical cleaning. Some MIC-related condenser tube damage was alsonoted during outage inspections of the con-densers.

Using actual untreated process cooling water in a modelcooling tower system, trials were conducted to ascertainthe most efficient combination of biocide dose concentra-tion, dose frequency and dose timing. Again, the electro-chemical biofilm activity monitoring system was instru-mental in allowing this performance-based concept to bedeveloped. The site was easily able to determine from theprobe trends the best combination of concentration andfrequency that would counter biofilm development in thecooling water system.

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Figure 14:

Biofilm activity monitoring to determine "maintenance" dose for a proprietaryoxidizing biocide in a once-through cooling water system.




PPChem

As shown in Figure 14, a typical trial runinvolved setting up the probe in a contin-uous flow of untreated water to allow somebiofilm to develop on the electrode surfaces.Various combinations of biocide dose con-centration, duration and frequency werethen tested to determine the most efficientregime. Although effective at counteringbiofilm activity, most of these "maintenance"doses allowed some biofilm activity to re-establish once dosing was stopped. The sitewas happy with this approach, as it alloweda minimization of microbial fouling for theleast amount of chemical in a clean system.As discussed previously, this is the core ofthe performance-based approach, which isnot only more environmentally sustainable,but costs less and puts less stress on plantmaterials.

The site also wanted to know what dose ratewould be required to kill mature biofilm thathad built up in-between the off-line tubecleaning routines. Figure 15 shows that fol-lowing the application of 200 mg · L–1 of bio-cide for a period of 30 min there was animmediate sharp decline in generated (and applied) cur-rent output from the probe. This is fully consistent with thetotal deactivation of any mature biofilm on the probe elec-trodes. If the probe was maintained in a clean conditionthereafter – as monitored via the probe current trends –then the system surfaces would be kept similarly clean.

At all stages of the testing program, the current outputsfrom the probe were validated via microbial analysis ofbulk water and surface swab samples.


A river water cooled twin unit coal-fired power station hasin its history experienced microbiological fouling, includ-ing silt accumulation on some high efficiency fill in thecooling tower, and corrosion of steel and copper alloys[20]. Water treatments have included the use of bleach,dispersant, and a proprietary oxidizing biocide. In 2005,the plant installed an electrochemical biofilm activity mon-itoring system in the cooling loop of each unit to monitorbiofilm activity on the probe's metallic surfaces. The con-tinuous monitoring of microbiological activity on surfaceshas permitted the plant to closely control treatments andto track their effectiveness so that the cleanliness of sys-tem surfaces is maintained. Since the probes wereinstalled, the station has kept the cooling systems cleanerthan before (Figure 16), with an associated direct positiveimpact on overall system thermal performance, and has a

continuous indication that the system is clean (and knowsimmediately when it is not), all with the added benefit thatsignificantly less chemical is now being used. For exam-ple, bleach usage has been decreased and the proprietaryoxidizing biocide has been completely eliminated from thetreatment. In addition, the reduced bleach additions havedramatically reduced the corrosion of copper alloys. Thetotal annual savings are a large multiple of the total costfor purchase and installation of the electrochemical biofilmactivity monitoring systems.

Based on the outstanding success in the cooling watersystems, the same site installed another electrochemicalbiofilm activity monitoring system immediately upstreamof their reverse osmosis (RO) plant to monitor the forma-tion of biofilm on membrane surfaces, thereby allowingthe station to proactively manage their membrane clean-in-place (CIP) routine [21]. Prior to the installation of theelectrochemical biofilm activity monitoring system, the ROplant had been subject to major fouling by organic andmicrobiological material, necessitating frequent cleaning,thus impacting the availability of the entire ultrapure waterproduction plant. Following the introduction of a numberof optimization measures, including the use of the electro-chemical biofilm activity monitoring system, RO mem-brane cleaning cycles have decreased from 31 times in2005 to 6 times in 2009. The improved microbiologicalmonitoring project demonstrated that the increased atten-tion to microbiological activity and its control did increasethe time between cleanings and improved the overalltreatment plant performance (Figure 17).

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Figure 15:

Biofilm activity monitoring to determine "cleanliness" dose for a proprietaryoxidizing biocide in a once-through cooling water system.




PPChem

CONCLUSIONS

An electrochemical biofilm activity monitoring system hasfound widespread use in industrial facilities around theworld as a sophisticated monitoring tool for biofilm activityin water systems.

The system is simple, sensitive, accurate and robust, withno moving parts, no reagents, and little or no maintenancerequirements. Data collection and treatment is software-driven and the entire system requires very little user input.

On-line and real time monitoring of the water system con-dition provides early warning of unsatisfactory perfor-mance. If process problems relating to biofilm formation,such as MIC, heat transfer losses and chemical efficacy,can be known and countered before they develop, thenthe asset owner has the opportunity to add real value tohis system operation: less downtime, less maintenance,less chemical, improved environmental performance, andimproved health and safety.

The performance-based approach is the best way to man-age bacteria control in water systems. On-line, real timemonitoring of biofilm activity should be integral to thisapproach.

REFERENCES

[1] Schaefer, W. P., Pilsits, J. P., Proc., Industrial WaterConference, 1997.

[2] Costerton, J. W., Cheng, K. J., Geesey, G. G., Ladd,P. I., Nickel, J. C., Dasgupta, M., Marrie, T. J., AnnualReview of Microbiology 1987, 41, 435.

[3] Bai, U., Mandic-Mulec, I., Smith, I., Genes Dev. 1993,7, 139.

[4] Biofouling and Biocorrosion in Industrial WaterSystems (Eds.: G. G. Geesey, Z. Lewandowski, H.-C.Flemming), 1994. CRC Press Inc., Lewis Publishers,Boca Raton, FL, U.S.A.

[5] Brominated Biocides: Chemistry Monitoring andControl, 1997. Nalco Chemical Company, Naperville,IL, U.S.A., TECHNIFAX TF-181.

[6] Flemming, H.-C., Werkstoffe und Korrosion 1994, 45,29.

[7] Czolkoss, W., 2nd STI Symposium "BeneluxKorrosiedagen, Koelwater in de Industrie", 1991(Antwerp, Belgium).

[8] Bremer, P. J., Webster, B. J., Water & Wastes in NewZealand 2001, 121, 32.

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/07

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/07

/07

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/07

/07

01

/06

/08

Date [mm/dd/yy]




Baseline

Figure 17:

Use of on-line biofilm monitoring to improve RO plant performance.

Figure 16:

Continuous improvement in Unit 1 fill cleanliness,between 2005 (above) and 2007 (below).




PPChem

[9] Bruijs, M. C. M., Venhuis, L. P., Jenner, H. A., Licina,G. J., Daniels, D., PowerPlant Chemistry 2001, 3(7),400.

[10] Garrett, W. E., Licina, G. J., Eighth EPRI ServiceWater System Reliability Improvement Seminar, 1995(Charlotte, NC, U.S.A.). Electric Power ResearchInstitute, Palo Alto, CA, U.S.A.

[11] Licina, G. J., Nekoksa, G., Howard, R. L., Micro -biologically Influenced Corrosion Testing (Eds.: J. R.Kearns, B. J. Little), 1993. ASTM International, WestConshohocken, PA, U.S.A., STP 1232.

[12] Licina, G. J., Willertz, L. E., Swoyer, B. M., Tom -baugh, R. S., The Ninth EPRI Service Water SystemReliability Improvement Seminar, 1996. ElectricPower Research Institute, Palo Alto, CA, U.S.A.

[13] Licina, G. J., Proc., CORROSION/2001, 2001 (SanDiego, CA, U.S.A.). NACE International, Houston, TX,U.S.A., Paper #01442.

[14] Dorsey, M. H., Licina, G. J., Saldanha, B. J., Eber -sole, R. C., PowerPlant Chemistry 2002, 4(12), 721.

[15] Nekoksa, G., Gutherman, B., Cathodic ProtectionCriteria for Controlling Microbially Influenced Cor -rosion in Power Plants, 1991. Electric PowerResearch Institute, Palo Alto, CA, U.S.A., NP-7312.

[16] Guezennec, J., Dowling, N. J., Conte, M., Antoine,E., Fiksdal, L., Microbially Influenced Corrosion andBiodeterioration (Eds.: N. J. Dowling, M. W. Mittle -man, J. C. Danko), 1991. University of TennesseePress, Knoxville, TN, U.S.A.

[17] Licina, G. J., CORROSION/2004, 2004 (NewOrleans, LA, U.S.A.). NACE International, Houston,TX, U.S.A., Paper #04582.

[18] Fallon, H. P., PowerPlant Chemistry, 2004, 6(4), 203.

[19] Licina, G. J., Nekoksa, G., Proc., International Con -ference on Microbiologically Influenced Corrosion,1995 (New Orleans, LA, U.S.A.). American WeldingSociety, Miami, FL, U.S.A. and NACE International,Houston, TX, U.S.A.

[20] Licina, G. J., Brumfield, D., The 29th Annual ElectricUtility Chemistry Workshop, 2009 (Champaign, IL,U.S.A.).

[21] Brumfield, D., Licina, G. J., Ultrapure Water 2009,26(10), 21.

THE AUTHORS

Hugh Fallon (B.Eng., Chemical & Process Engineering,University College Dublin, Ireland) is a senior consultantwith Structural Integrity Associates (SI), specializing in theoptimization of treatment programs for the minimization ofcorrosion, scaling and fouling in cooling and service watersystems, with special emphasis on the use of on-linemonitoring techniques for the control of surface biofilm.He has instituted programs for the assessment andbenchmarking of facility-wide chemistry, and the imple-mentation of expert programs for the evaluation of chem-istry performance and long-term asset reliability.

Hugh Fallon has been involved in the power industry since1997, when he joined Siemens Power Generation as aproject chemical engineer, working on a number of powerprojects around the world. He also spent a number ofyears as the group chemist for Contact Energy Limited,New Zealand's largest electricity generator and retailer.Prior to joining SI, he was an independent consultant, pro-viding expertise in all facets of industrial and power gener-ation chemistry to international clients.

George J. Licina (B.S., Metallurgical Engineering, Uni -versity of Illinois, Urbana-Champaign, IL, U.S.A.) hasworked in the power industry since 1972. He is the chiefmaterials consultant with Structural Integrity Associates,and has dealt primarily with failure analysis and predictingthe degradation and environmental compatibility of powerplant materials under a variety of operating conditions.These degradation mechanisms include corrosion andenvironmentally assisted cracking in BWR, PWR, and var-ious raw water environments, and embrittlement of pres-sure vessel steels and high performance alloys. GrorgeLicina is a recognized authority on microbiologically influ-enced corrosion and has authored reference documentson this topic for the Electric Power Research Institute andnumerous utilities. He has developed an on-line monitor-ing tool, the BIoGEORGE™ system, to assist plants withtracking biofilm activity on metallic surfaces for MIC con-trol and biocide optimization. George Licina has authoredmore than seventy-five publications in the open literatureand is the author of four patents.

CONTACT

Structural Integrity Associates, Inc.5215 Hellyer AvenueSuite 210San Jose, CA 95138U.S.A.

E-mail: [email protected]@structint.com



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