Influence of Chlorination and Choice of Materials on Fouling in Cooling Water System under Brackish Seawater Conditions

Abstract

Cooling systems remove heat from components and industrial equipment. Water cooling, employing natural waters, is typically used for cooling large industrial facilities, such as power plants, factories or refineries. Due to moderate temperatures, cooling water cycles are susceptible to biofouling, inorganic fouling and scaling, which may reduce heat transfer and enhance corrosion. Hypochlorite treatment or antifouling coatings are used to prevent biological fouling in these systems. In this research, we examine biofouling and materials' degradation in a brackish seawater environment using a range of test materials, both uncoated and coated. The fouling and corrosion resistance of titanium alloy (Ti-6Al-4V), super austenitic stainless steel (254SMO) and epoxy-coated carbon steel (Intershield Inerta160) were studied in the absence and presence of hypochlorite. Our results demonstrate that biological fouling is intensive in cooling systems using brackish seawater in sub-arctic areas. The microfouling comprised a vast diversity of bacteria, archaea, fungi, algae and protozoa. Chlorination was effective against biological fouling: up to a 10-1000-fold decrease in bacterial and archaeal numbers was detected. Chlorination also changed the diversity of the biofilm-forming community. Nevertheless, our results also suggest that chlorination enhances cracking of the epoxy coating.

Keywords: Baltic Sea; biofilm; biofouling; materials science; microbial influenced corrosion.

Figure 1

Photographs, showing the specimen surfaces after the three-month test. (A,B) 254SMO; (C,D) Ti alloy; (E,F) Inerta160-coated steel. Non-chlorinated: (A,C,E); chlorinated: (B,D,F).

Figure 2

Quantity of (a) bacteria; (b) archaea (c) and fungi, on the coupon surfaces, determined using quantitative PCR. Bars show standard deviations.

Figure 3

SEM images, showing the surfaces of coupons from the non-chlorinated system after three months of exposure. (A,B) 254SMO; (C,D) Ti alloy; (E,F) Inerta160.

Figure 4

SEM images, showing the surface of coupons from the chlorinated system after three months of exposure. (A,B) SMO254; (C,D) Ti alloy; (E,F) Inerta160.

Figure 5

The number of observed Operational Taxonomic Units (OTUs) (A–C); the number of estimated OTUs according to the Chao1 richness estimators (D–F); and the Shannon diversity index (G–I) of the coupon samples. The columns show mean values of three samples based on the normalized number of sequences read per sample, and the error bars indicate the standard deviation. Values calculated for the bacterial, archaeal and fungal data are presented in the left, center and right columns, respectively. Grey columns present non-chlorinated and black columns chlorinated samples. Statistically significantly different sample pairs (non-chlorinated vs. chlorinated) are indicated with stars: * p < 0.05, ** p < 0.005, *** p < 0.0005.

Figure 6

The bacterial community composition of (A) the non-chlorinated and (B) chlorinated samples; (C) a PCA plot based on the bacterial community (family level). In (C), open symbols indicate chlorinated and solid symbols non-chlorinated samples. Component 1 explains 39.2% and Component 2 explains 26.2% of the variance, with Actinobacteria, Chloroplasts, Alphaproteobacteria and Epsilonproteobacteria dominating the axis loadings.

Figure 7

The archaeal community composition of (A) the non-chlorinated and (B) chlorinated samples; (C) a PCA plot based on the archaeal community (family level). In (C), open symbols indicate chlorinated and solid symbols non-chlorinated samples. Component 1 explains 67.8% and Component 2 explains 35.1% of the variance, with Thaumarchaeota, the Miscellaneous Crenarchaeotal Group (MCG) and Parvarchaea dominating the axis loadings.

Figure 8

The fungal community composition of (A) the non-chlorinated and (B) chlorinated samples; (C) a PCA plot based on the fungal community (family level). In (C), open symbols indicate chlorinated and solid symbols non-chlorinated samples. Component 1 explains 36.7% and Component 2 explains 22.6% of the variance of the variance with Monoblepharidomycetes, unidentified Chytridiomycota and unidentified Fungi determining the loadings of PC1 and unidentified Ascomycota determining the loadings of PC2.