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Journal of Failure Analysis and Prevention

, Volume 10, Issue 5, pp 416–426 | Cite as

Scale Formation by Calcium-Precipitating Bacteria in Cooling Water System

  • S. Maruthamuthu
  • P. Dhandapani
  • S. Ponmariappan
  • S. Sathiyanarayanan
  • S. Muthukrishnan
  • N. Palaniswamy
Technical Article---Peer-Reviewed

Abstract

Scale formation in heat exchanger tube reduces heat transfer efficiency and enhances corrosion. Scale formation in cooling water is due to many factors including pH, temperature, salt etc. In this study, microbiological aspects of scale formation and their role on corrosion are presented. The calcium precipitating bacteria (CPB) were isolated from the scales collected from heat exchanger tube in a gas turbine power station using B4 medium. The dominant CPB was isolated and identified using 16s rRNA sequencing, and the phylogenetic analysis reveals that the predominant bacteria were Serratia sp. (FJ973548), Enterobacter sp. (FJ973549, FJ973550), and Enterococcus sp. (FJ973551). The nature of crystal deposits of bacteria has been explained. The corrosion behavior of CPB on mild steel was studied by the electrochemical method (polarization and impedance), and the biogenic calcium scale formations in CPB were analyzed by XRD method. The scale formation by bacteria reduced the cathodic corrosion current, where resistance was lower in the presence of bacteria. It is claimed that the CPB is one of the causative factor for scale formation and corrosion in cooling water system.

Keywords

Cooling water system Heat exchangers tube Scale formation Calcium precipitating bacteria 

Introduction

Calcium carbonate (CaCO3) precipitation is a common phenomenon found in environment such as marine water, fresh water, and soils [1, 2]. It is a rather straight forward chemical process governed by four key factors: viz calcium (Ca2+) concentration, the concentration of dissolved inorganic carbon (DIC), pH, and the availability of nucleation sites [3]. Some microorganisms accumulate inorganic compounds such as phosphorites, carbonates, silicates, iron, and manganese oxides [4, 5, 6, 7, 8]. It is also well known that the kidney stone formation and scale deposition in oral cavity were due to bacterial activity [9]. Besides, calcium accumulation was also noticed in hot spring [10]. Mixed fouling of microorganism and other minerals existing in various water systems take place when the water flows through equipment and pipes. It decreases the performance of equipment; increases pressure drop in water circuits, introduces thermal resistance in heat exchangers and induces corrosion of surface materials. The mixed fouling consisting of microbes and calcium or magnesium salts is, in particular, the major part of the fouling in real water systems [11]. The primary role of bacteria in the precipitation process was ascribed to their ability to create an alkaline environment (high pH) through various physiological activities [12, 13]. Although microbiological calcium precipitation has been investigated extensively both in natural environments and under a defined laboratory condition, the exact mechanism of precipitation and the function of this process within the microbial ecology of the precipitating microorganism remains unresolved [14, 15]. Stocks-Fischer et al. [16] suggested that microbial plugging could be greatly enhanced using microorganisms with urease enzyme activities indirectly involved in CaCO3 consolidation. Hammes and Verstraete [17] reviewed the key roles of pH and calcium metabolism in microbial carbon precipitation. Moreover, bacteria-induced biomineralization in the scale formation was investigated by various investigators from various sources viz., soil, caves, and teeth pulp stone [18, 19, 20, 21]. Recently, Chen et al. [19] investigated bacteria-mediated (Proteus mirabilis) synthesis of metal carbonate minerals with unusual morphologies and structures. To the best of our knowledge, there are almost no reports on biomineralization in scale formation of heat exchanger tubes. As the identification of calcium precipitating bacteria (CPB) in heat exchangers of cooling water system appears lacking, this study would be useful for the selection of suitable biocides/inhibitors in the control of CPB on scale in cooling water system. In the present investigation, scale from heat exchanger tubes was collected from a gas turbine power station, Tirumakottai, Tamilnadu Electricity Board to find the diversity of calcium precipitating reason for the scale formation and its role on corrosion.

Experimental Materials and Methods

Sample Collection (Scale and Cooling Water)

The samples of scales were collected from gas turbine power station, Tamilnadu Electricity Board (TNEB), Tirumakottai, Thiruvarur (Dt), India (Fig. 1). Scale samples were scrapped from IA/PA mild steel heat exchanger tubes employing sterile surgical knife and then further used for the microbial characterization. The cooling water samples were collected from the cooling tower for microbiological corrosion studies. The collected samples were placed in an ice box and transported to microbiological laboratory, CECRI, Karaikudi, India for further analysis.
Fig. 1

Photographs of mild steel tubes collected from Tirumakottai gas power station, TNEB

Bacteriological Analysis of Scale

Isolation of Bacteria

One gram of scale was removed aseptically and transferred to 9 ml of sterile 1% peptone water and stirred for 1 h at 37 °C. After 1-h stirring, the samples were subjected to serial dilution and plated on B4 medium (g/l): Calcium acetate 2.5: Yeast extracts 4.0, and Glucose, 10) by the pour plate method as described previously by Baskar et al. [18]. The plates were incubated at 37 °C for 2–3 weeks. The total viable bacterial counts were enumerated, and the bacterial population was expressed as colony forming units per gram (CFU/gm).

Partial Biochemical Characterization of the Isolates

Morphologically dissimilar dominating isolated colonies were selected randomly, further streaked on B4M agar plates and purified. The pure cultures were maintained in B4M agar slants at 4 °C to keep the microbial strain viable. The dissimilar aerobic bacteria isolated from medium were identified according to Bergey’s Manual of Determinative Bacteriology [22]. The isolated bacterial cultures were identified up to genus level by their morphological and partial biochemical characterization using the following tests: (1) Gram staining, (2) Catalase test, (3) oxidase test, (4) pigment production, (5) nitrate reduction test, (6) indole production, (7) methyl red test, (8) Voges–Proskauer test, (9) citrate utilization test, (10) McConkey test, (11) Urease activity, (12) starch hydrolysis test, and (13) carbohydrate fermentation test [22]. In addition, citrate agar was used to detect the iron-reducing activity of the isolates. Biochemical characterization of the isolates was carried out employing Himedia biochemical test kit (Mumbai) according to the manufacturer’s instructions.

16s rRNA Gene Sequencing and Phylogenetic Analysis

The cultures were grown overnight in B4M broth at 37 °C. The cultures were centrifuged at 5000×g for 15 min. The bacterial pellets were used for the genomic DNA extraction. Genomic DNA was extracted according to the method described by Murmur [23], and the small subunit rRNA gene was amplified using universal primers 16S1 (5′-GAGTTTGATCCTGGCTCA-3′) and 16S2 (5′-CGGCTACCTTGTTACGACTT-3′). The PCR products were visualized in 1.5% agarose gel and gel purified using QIA quick gel extraction kit (Qiagen, Germany). The purified PCR product, approximately 1.5 kb in length, was sequenced using five forward and one reverse primers as described earlier [24]. The deducted sequence was subjected to blast search for the closest match in the database. The 16s rRNA gene sequence of CPB were submitted in Genbank. The phylogenetic analysis were carried out using pairwise evolutionary distances and computed using the DNA DIST program with the Kimura 2 parameter model as developed by Kimura [25]. The phylogenetic tree was constructed using four tree-making algorithms, the UPGMA, KITSCH, FITCH, and DNAPARS of the PHYLIP package [26]. The stability among clads of a phylogenetic tree was assessed by taking 1000 replicates of the data set and was analyzed using the programs SEQBOOT, DNADIST, UPGMA and CONSENSE of the PHYLIP package.

pH Measurements

The calcium precipitating bacteria (CPB) was grown in B4 medium. The pH was also measured with time using a digital pH meter.

Characterization of Cooling Water and Scale

Atomic Adsorption Spectroscopy (AAS) Analysis

The quantities of calcium, magnesium, sodium, and iron were estimated in the scale collected from IA/PA heat exchanger tube mild steel employing AAS. Approximately, 0.1 gram of scale sample was collected from the mild steel (IA/PA heat exchanger tube) inner part and was dissolved in 9-ml sterile distilled water with 1 ml of 0.1 N HCL and used for the AAS analysis. The cooling water characterization was done by standard chemical analysis method.

X-Ray Diffraction (XRD) and FTIR Analyses

The CPB was inoculated into B4 medium and incubated for 2–3 weeks at 37 °C. The cultures were centrifuged at 6000 rpm for 30 min, which form slime layer sediment (pellet) in the bottom (white color) and allowed to air dry. These sediment samples were characterized employing XRD and FTIR. The scale samples collected from IA/PA mild steel heat exchanger tube were powdered and analyzed by XRD and FTIR. The XRD model of X’pert PRO PAN analyzed X-ray diffractometer with Syn Master 793 software to identify the nature of scale product. The XRD pattern was recorded using the computer-controlled XRD-system. The “peak search” and “search match” program built-in software (syn master 7935) was used to identify the peak table and ultimately the identification of the XRD peak. The FITR spectrum was taken in the mid IR region of 400–4000 cm−1. The spectrum was recorded using ATR (attenuated total reflectance) technique. The sample was directly placed in the zinc selenide crystal and the spectrum was recorded in the transmittance mode.

Scanning Electron Microscope (SEM) Studies

Stainless steel coupons of size 1 × 2 cm were mechanically polished to mirror finish and immersed in 10% nitric acid at 60 °C for 5 min in a water bath followed by gentle cleaning with trichloroethylene. These coupons were autoclaved at 121 °C at 15 psi for 15 min. The autoclaved coupons were immersed in calcium precipitating bacterial culture system described above over a period of 2–3 weeks for the formation of biofilm. The coupons were removed from the medium and then rinsed with sterile, distilled water to remove the unattached bacteria. The biofilm was fixed for 12 h at room temperature with 3% glutaraldehyde in 0.1 M phosphate buffer solution (pH 7.2) and then immersed in 0.1 M sodium cacodylate buffer (pH 7.2). The coupons were dehydrated using different concentration of ethanol solution 25%, 50%, 75%, and 100% concentration. The air-dried coupons were sputter coated with gold using ion sputters. The coupons were examined at different magnifications [2.0, 5.0, 6.0, and 10 K] by the scanning electron microscope. The biogenic scales were also examined employing SEM.

Electrochemical Studies

Polarization Measurements

The elemental composition of mild steel is as follows (%): carbon, 0.1–0.2; manganese, 0.1–0.2; phosphorous, 0.40–0.50; sulphur, 0.02–0.03; and iron remainder. The steel coupons of 1 × 1 cm dimension with an extended stem of 15-cm length were used for polarization and impedance studies. Specimens were polished to mirror finish using emery paper 1/0 down to 4/5. The specimens were also finally degreased with trichloroethylene, followed by deionized water. Three specimens were immersed in separate 100-ml conical flasks that contained B4 medium. The were inoculated in a B4 medium and used as the experimental system, while the uninoculated specimen was used as the control system.

Polarization measurements were carried out potentiodynamically employing a potentiostat with voltamaster-1 software. Mild steel coupon of size 1 cm2 as working electrode a Saturated Calomel Electrode (SCE) as a reference electrode and a large platinum electrode were employed for polarization study. The system was allowed to attain a steady potential value for 10 min. The steady-state polarization was carried out from OCP to −200 mV SCE and +200 mV SCE from the OCP using separate electrodes at a scan rate of 1800 mV/h. The polarization study was done on the 16th day of the immersion period.

Impedance Studies

The electrodes of the same specification that employed for the polarization studies were also used for the impedance studies. Impedance studies were carried out using a computer control system. After a steady state was attained, an AC signal of 10 mV amplitude was applied, and the impedance values were measured for frequencies ranging from 0.1 Hz to 100 kHz. The values of R t were obtained from the Nyquist plot. Impedance measurements were also taken on the 16th day of the immersion period.

Results and Discussion

The water quality data are presented in Table 1. The gas power station cooling water contains 740 mg/l of chloride, 160 mg/l of calcium, 60 mg/l of magnesium, and the total hardness was around 220 mg/l with a total dissolved solid of 1330 mg/l. The pH of the water was 7.5. The concentrations of sodium, calcium, magnesium, and iron in the scale and water samples are presented in Table 2. The concentrations of sodium, calcium, magnesium, and iron in the water were 58, 24, 360, and 0.3 mg/l, respectively. The scale collected from mild steel heat exchanger had 2120 mg/kg of calcium, 1800 mg/kg of magnesium, and 200 mg/kg of sodium levels, whereas iron level was 2470 mg/kg (Table 2).
Table 1

Characteristics of cooling water collected from gas turbine power station

S. No.

Parameters

Water sample

1

Conductivity, μs/cm

2222

2

pH

7.5

3

Total hardness, ppm

220

4

Calcium, ppm

160

5

Magnesium, ppm

60

6

Chloride, ppm

740

7

Sulphate, ppm

134

8

Silica, ppm

33

9

Sodium, ppm

1043

10

Total dissolved solids, ppm

1333

Table 2

Chemical analysis of cooling water and scale collected from IA/PA mild steel heat exchanger tubes

S. No.

Sample

Elements analysis, mg/l

Calcium

Magnesium

Sodium

Iron

1

Cooling water

58 ± 5

24 ± 2

360 ± 5

0.3 ± 1

2

IA/PA mild steel heat exchanger scale

2120 ± 10

1800 ± 15

200 ± 5

2470 ± 15

The calcium precipitating bacterial count was 5.8 × 103 CFU/gm in the scale collected from the industry. Preliminary identification of the bacteria by biochemical test indicated that the isolates belonged to the genera Serratia, Enterobacter and Enterococcus. The phenotypic profiles of the scale strains are shown in Table 3.
Table 3

Partial biochemical characterization of isolates from scale

Characteristics

CPB1

CPB2

CPB3

CPB4

Cell morphology

 Gram stain

−ve

−ve

−ve

+ve

 Shape

Rod

Rod

Rod

Cocci

 Catalase

+ve

−ve

−ve

−ve

 Oxidase

−ve

−ve

−ve

−ve

 Pigment production

−ve

−ve

−ve

−ve

 Nitrate reduction

+ve

+ve

+ve

+ve

 Indole

−ve

−ve

−ve

−ve

 Methyl red

+ve

+ve

+ve

+ve

 VP

+ve

+ve

+ve

+ve

 Citrate

+ve

+ve

+ve

+ve

 McConkey

+ve

−ve

+ve

−ve

 Urease activity

+ve

+ve

+ve

+ve

 Starch hydrolysis

−ve

−ve

−ve

+ve

Carbohydrate fermentation test

 Sucrose

+ve

+ve

+ve

+ve

 Arabinose

−ve

−ve

−ve

−ve

 Lactose

−ve

+ve

+ve

+ve

 Fructose

+ve

+ve

+ve

−ve

 Maltose

+ve

−ve

−ve

−ve

 Galactose

+ve

+ve

+ve

+ve

 Xylose

+ve

−ve

+ve

−ve

 Mannitol

−ve

+ve

+ve

+ve

CPB, calcium precipitating bacteria; CPB1, Serratia sp.; CPB2, Enterobacter sp.; CPB3, Enterobacter sp.; CPB4, Enterococcus sp.

The sequences obtained were submitted to a BLAST search to retrieve the corresponding phylogenetic relatives. The phylogenetic affiliations (Firmicutes and Gamma proteo bacteria) were confirmed by the analyses of all the related species recognized by the taxonomic and classification hierarchy done using the NCBI Taxonomy Homepage and Ribosomal Database Project-II Release 10. The phylogenetic tree was constructed for the Enterococcus and Enterobacter (Fig. 2) to analyze the relationship among the sequences of the ribosomal library and related organisms from the GenBank database. In the class of Enterococcaceae, one isolate of the genus Enterococcus exhibited high sequence similarity with Enterococcus sp. (99%). In the class of Enterobacteriaceae, among the three isolates of the genus, two Enterobacter were similar to Enterobacter sp. (99%), and one isolate was Serratia genus (Serratia sp.).
Fig. 2

Neighbor-joining tree based on 16S rRNAgene sequences, showing the phylogenetic position of calcium precipitating bacteria Enterococcus sp., Enterobacter sp. and Serratia sp. strains. Bootstrap values are given at the nods

The pH curves of CPB in B4 medium were plotted in Fig. 3. The initial pH of the B4 medium was 5.96. In the control system, the pH of the B4 medium was 4.82 at end of the 16th day. In presence of bacteria, the pH (4.32) slightly decreased similar to the control system and gradually increased with time (8.3). It indicates that the bacterial metabolic activity increases the pH of B4 medium.
Fig. 3

pH curve of calcium precipitating bacteria collected from scales on mild steel heat exchanger tube in B4 medium

Figure 4(a) and (b) shows the SEM images from the laboratory experiments containing mixed cultures of bacteria within 1 week. The presence of long and short rods could be noticed. The sample consists of exclusively angular crystals. These are generally either aggregates of planar, elongated crystals, or cubic-shaped crystals. It can be seen that bacterial cells are embedded with each other, and form as crystals (Fig. 5a, b) within 2–3 weeks. This study confirms that the scale formation in cooling water system is due to bacteria where the living cells of bacteria Serratia sp., Enterobacter sp. and Enterococcus sp. induced precipitates of calcite.
Fig. 4

SEM studies for calcium precipitating bacteria collected from IA/PA mild steel heat exchanger scale

Fig. 5

SEM studies for biogenic calcium crystal formation collected from B4 medium in the presence of calcium precipitating bacteria

FTIR spectrum was recorded for the scale sample collected from IA/PA mild steel heat exchanger tube and two-week old bacterial pellets (laboratory cultured bacteria) and are presented in Fig. 6(a) and (b). Figure 6(a) shows the FTIR spectrum of the scale collected from cooling water. A peak at 3417 cm−1 indicates the stretching vibrations of primary and secondary amines. The corresponding bending vibration was seen at 2927 cm−1 which can be assigned to the C–H stretching vibrations of alkane compounds. The two peaks observed at 2517 cm−1 were assigned to the C=C stretching vibrations of aromatic. Another peak at 2361 cm−1 indicates the presence of P=O stretching vibration. A peak observed at 1800 cm−1 was identified as (CO4)2− stretching vibrations of metallic carbonates. A peak at 1432 cm−1 was assigned to the Ca(CO4)2− stretching vibrations of metallic carbonates. The other two peaks observed at 1133 cm−1 were assigned as SO 4 2− stretching vibrations of aromatic amine, and a peak at 875 cm−1 indicates the presence of calcium carbonate. A peak noticed at 709 cm−1 can be assigned to the (CO4)2− stretching vibrations of metallic carbonates. Figure 6(b) shows the FTIR spectrum for calcium precipitating—laboratory-cultured bacterial pellets isolated from heat exchanger scale. A peak at 3450 cm−1 can be assigned to the stretching vibrations of primary and secondary amine. The corresponding bending vibration was seen at 2927 cm−1 and assigned to the C–H stretching vibrations of alkane compounds. A peak observed at 2368 cm−1 was assigned to the C=C stretching vibrations of alkyne compounds. Another peak was observed at 1646 cm−1 indicated the (CO4)2− stretching vibrations of metallic carbonates. A peak noticed at 1410 cm−1 indicated the C=O, where the corresponding bending vibration was noticed at 1082 cm−1, which indicates the calcium carbonate. FTIR reveals the adsorption of carbonate by bacteria. The presence of organic content indicates the role of bacteria on carbonate accumulation.
Fig. 6

Fourier-transform infrared spectrum of (a) scale collected from IA/PA mild steel heat exchanger tube and calcium precipitating bacteria cultured (b) in laboratory by using isolates collected from IA/PA mild steel heat exchanger tube

Figure 7 presents the details of XRD data corresponding to the phases present in the corrosion product of scale samples collected from mild steel heat exchanger tube. Calcium carbonate (CaCO3), Tri calcium phosphate (Ca3(PO4)2), and Iron oxide (Fe2O3) were observed in the IA/PA mild steel heat exchanger scale (Fig. 7a). Calcium phosphate carbonate (Ca10(PO4)CO3), magnesium carbonate (Mg2CO3), calcium carbonate (CaCO3), and tri calcium phosphate (Ca3(PO4)2) were noticed in the calcium-precipitating cultured bacterial pellets isolated from IA/PA mild steel heat exchanger tube (Fig. 7b).
Fig. 7

XRD analysis of scales collected on mild steel heat exchanger tube in cooling water system (a) and calcium precipitating bacteria cultured in laboratory by using isolates collected from IA/PA mild steel heat exchanger tube (b)

The polarization data (anodic and cathodic) for mild steel in the presence and the absence of mixed CPB in cooling water system are presented in Fig. 8(a) and (b). Polarization studies reveal that accumulation of calcium carbonate inhibited cathodic reaction from −750 to −850 mV, whereas the formation of biogenic calcium carbonates can act as good cathodic inhibitor. However, there is no significant change in anodic current between control and bacterial inoculated system.
Fig. 8

Polarization behavior for mild steel in the absence and the presence of calcium precipitating bacteria (anodic and cathodic curve) in cooling water system

Impedance spectroscopy analysis is presented in Fig. 9 and Table 4. In the control system, the R t value was 2.33 kΩ/cm2, whereas the R t value was 0.80 kΩ/cm2 in the bacterial system, It indicates that the biogenic accumulated calcium may be penetrated by chloride and decreased the resistivity of mild steel. This result contradicts with polarization result where calcium carbonate reduces cathodic current in the presence of bacteria.
Fig. 9

Impedance behavior for mild steel in the absence and the presence of calcium precipitating bacteria using cooling water system

Table 4

Impedance data for the calcium precipitating bacteria in cooling water system

S. No.

System

Rt values,

Ω cm2

Rs values,

Ω cm2

Rct values,

Ω cm2

1

Control

2331

22

2309

2

Experiment

800

19

781

Microbial associations with natural carbonate deposits have been described for seawater [27] saline lakes [28], and soils [29]. The occurrence of human kidney stone development [30], as well as the deposits on the Martian meteorite [31] and the phenomenon of nanoforms were hypothesized to be in association with microbial calcification [32]. In this study, the occurrence of scale formation in cooling water system has been reported. The identified isolates showed positive for urease activity. The major genera viz., Serratia sp., Enterobacter sp. and Enterococcus sp. identified are members of the proteo bacteria and Firmicutes bacteria. The result supports the observation made by previous investigators [33, 34], who noticed urease positive in CPB. The 16s rRNA gene sequence of the CPB isolates were blasted against Genbank sequences, and the phylogenic tree was constructed with the closely related genera. Hammes and Verstraete [17] also noticed that microbial carbonate precipitation by Synechococcus spp., Nannochloris atomus, Bacillus spp., Pseudomonas spp., Vibrio spp., Flavobacterium spp., and Acinetobacter spp. Photosynthetic-induced calcification is regarded as the most common form of microbial calcium precipitate (MCP), and is associated with algae or cyanobacteria in primarily aqueous environments such as marine and/or freshwater [35]. The calcium-accumulating process is based on metabolic utilization of organic acids as their role as source of carbon and energy. Such acids include acetate, citrate, oxalate, glyoxylate, succinate, and malate, and consumption of these acids results in a pH increase for the system, and thereby leading to precipitation in the presence of calcium ions [33, 34]. In this study, it is possible that the identified species may consume calcium acetate and produce organic acids which results in a reduction of pH. The low pH shifted to higher side as per the following reaction. In the presence of bacterial system, there is no significant gradual reduction of the pH was noticed. At the end of the 16th day, the pH was as high as 8.38. It clearly indicates that the presence of calcium acetate may reduce the pH initially which encourages the formation of free calcium ions. The calcium ions will be accumulated by bacteria as calcium carbonate and calcium phosphate crystals by the cell wall due to high pH. At the end of the 16 days of the experiment, the pH in all the cultures had reached about 8.3. It may be due to the OH production due to bacterial activity in calcium acetate.

In the presence of bacteria, the formations of CO2 and OH ions determine the pH of the solution. The continuous formation of OH enhances the pH and probability of calcium accumulation on the metal surface. Further, the reaction would induce a shift in the bicarbonate equilibrium and a subsequent pH rise in the bulk medium.
$$ {\text{CH}}_{ 3} {\text{COO}}^{ - } + 2 {\text{O}}_{ 2} \to {\text{CO}}_{ 2} + {\text{H}}_{ 2} {\text{O}} + {\text{OH}}^{ - } $$
(1)
$$ 2 {\text{CO}}_{{ 2 { }}} + {\text{OH}}^{ - } \leftrightarrow {\text{CO}}_{ 2} + {\text{HCO}}{_{3}^{ - }} $$
(2)
$$ {\text{CO}}_{ 2} + {\text{H}}_{ 2} {\text{O}} \to {\text{CH}}_{ 2} {\text{O}} + {\text{O}}_{ 2} $$
(3)
$$ {\text{CO}}{_{3}^{ 2- }} + {\text{H}}_{ 2} {\text{O}} \leftrightarrow {\text{HCO}}{_{3}^{ - }} + {\text{OH}}^{ - } $$
(4)
$$ 2 {\text{HCO}}{_{3}^{ - }} + {\text{Ca}}^{ 2+ } \to {\text{CaCO}}_{ 3} + {\text{CO}}_{ 2} + {\text{H}}_{ 2} {\text{O}} $$
(5)

The presence of CaCO3 and calcium phosphate in XRD peaks indicate the role of bacteria on calcium precipitation. The presence of metallic carbonate and phosphates noticed by FTIR indicates the role of bacteria on calcium accumulation. Boquet et al. [28] showed that most soil bacteria are able to precipitate crystals of calcium carbonate when tested in a medium containing calcium acetate and that calcite production by bacteria is just a function of the media composition. In this study, CPB from scales collected from heat exchanger tubes were observed. The growth of calcium carbonate crystal by various sizes and shapes depend on the environmental condition. Stocks-Fischer et al. [16] have examined the physical and biochemical properties of CaCO3 precipitation in sand and found that only rhombohedral calcite crystals were produced. They also found that the bacteria were in the middle of calcite crystals acting as nucleation sites. SEM observation (Fig. 5b) indicates the occurrence of both bacteria and caclite where the crystals provide substrate for the bacteria to develop colonies and the colonization either promotes or retards the further growth of the crystal faces [36, 37].

It can be assumed that in the cooling water system, dissimilatory sulphate reduction is also encouraged by sulphate-reducing bacteria (SRB). The reaction typically occurs in anaerobic environments rich in sulphate and organic matter. Under these circumstances, organic matter can be consumed by SRB carrying out sulphate reduction, sulphide and metabolic CO2 is released.
$$ {\text{CaSO}}_{ 4} \cdot 2 {\text{H}}_{ 2} {\text{O}} \leftrightarrow {\text{Ca}}^{ 2+ } + {\text{SO}}{_{4}^{2 - }} + 2 {\text{H}}_{ 2} {\text{O}} $$
(6)
$$ 2 {\text{CH}}_{ 2} {\text{O}} + {\text{SO}}{_{4}^{2 - }} \to {\text{H}}_{ 2} {\text{S}} + 2 {\text{HCO}}{_{3}^{ - }} $$
(7)

In biological system, many calcareous organisms couple calcification to their metabolic assimilation processes to scavenge protons [35]. In urease-based reactions, NH3 also released by the enzymatic hydrolysis of urea uses the protons generated from calcite precipitate to produce NH4 + [32]. It can be claimed that the calcium adsorption was encouraged by creation of low pH by bacteria, where the formation of ammonia by urease activity may encourage the pH and enhance the formation of calcium carbonate continuously. Chen et al. [19] also noticed that urease-generated bacterium, P. mirabilis, is able to convert urea to ammonia and CO2 and thereby leading to the precipitation of metal carbonates. The present investigation claims that the presence of minor quantities of acetate may involve in the calcium crystal formation in the cooling water system. The polarization study indicates the decreasing of cathodic current by the formation of cathodic inhibitors, viz calcite, and subsequently, due to the penetration of chloride, calcite formation may reduce the resistance of the steel.

In this study, the chloride and sulphate contents were 740 and 134 ppm, respectively. The total hardness value of the cooling water was 220 ppm. Hence, it is assumed that the formation of calcium crystal in heat exchanger tubes enhances corrosion due to penetration of chloride and sulphate (Fig. 10).
Fig. 10

A model for accumulation of calcium carbonate deposition on mild steel

Conclusions

  1. 1.

    This study claims that the scale formation is due to CPB in cooling water system.

     
  2. 2.

    The formation of low pH by bacteria on the heat exchanger tubes encourages the distribution of Ca2+ ions, where the formation of OH may encourage the pH and enhance the formation of calcium carbonate continuously.

     
  3. 3.

    The calcium crystal formation by bacteria inhibits corrosion. At the same time, the porosity of calcium carbonate enhances pitting corrosion on heat exchanger tubes.

     

Recommendations

  1. 1.

    In general, iron-oxidizing, manganese-oxidizing, and acids-producing bacteria are the major contributors for corrosion. This article claims that CPB also encourage the adverse effects due to the formation of scales in heat exchanger tubes.

     
  2. 2.

    Addition of antiscalants, viz., ATMP (Amino Tri Methylene Phosphonic acid and HEDP (1-Hydroxy Ethylidene-1,1-Diphosphonic Acid) reduces the calcium/magnesium content in the cooling water. This study also indicates that the addition of good biocide is also needed along with antiscalant. The bacterial and calcium levels in cooling water should be reduced by good practice of water treatment.

     
  3. 3.

    The cooling water should be monitored continually by enumeration of bacteria and estimation of calcium and magnesium.

     

Notes

Acknowledgments

The authors wish to thank Mr. R. Ravishanker and Miss. S. Krithika of Instrumentation Division, CECRI, for their assistance in the utilization of SEM and XRD facility.

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Copyright information

© ASM International 2010

Authors and Affiliations

  • S. Maruthamuthu
    • 1
  • P. Dhandapani
    • 1
  • S. Ponmariappan
    • 2
  • S. Sathiyanarayanan
    • 1
  • S. Muthukrishnan
    • 1
  • N. Palaniswamy
    • 1
  1. 1.Council of Scientific & Industrial Research (CSIR)Central Electrochemical Research InstituteKaraikudiIndia
  2. 2.Biotechnology DivisionDefence R&D EstablishmentGwaliorIndia

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