Wastewater-Enhanced Microbial Corrosion of Concrete Sewers

Jul 8, 2016 - Microbial corrosion of concrete in sewers is known to be caused by hydrogen sulfide, although the role of wastewater in regulating the ...
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Wastewater-Enhanced Microbial Corrosion of Concrete Sewers Guangming Jiang,*,† Mi Zhou,† Tsz Ho Chiu,† Xiaoyan Sun,† Jurg Keller,† and Philip L. Bond† †

Advanced Water Management Centre, The University of Queensland, St. Lucia, Queensland 4072, Australia S Supporting Information *

ABSTRACT: Microbial corrosion of concrete in sewers is known to be caused by hydrogen sulfide, although the role of wastewater in regulating the corrosion processes is poorly understood. Flooding and splashing of wastewater in sewers periodically inoculates the concrete surface in sewer pipes. No study has systematically investigated the impacts of wastewater inoculation on the corrosion of concrete in sewers. This study investigated the development of the microbial community, sulfide uptake activity, and the change of the concrete properties for coupons subjected to periodic wastewater inoculation. The concrete coupons were exposed to different levels of hydrogen sulfide under wellcontrolled conditions in laboratory-scale corrosion chambers simulating real sewers. It was evident that the periodic inoculation induced higher corrosion losses of the concrete in comparison to noninoculated coupons. Instantaneous measurements such as surface pH did not reflect the cumulative corrosion losses caused by long-term microbial activity. Analysis of the long-term profiles of the sulfide uptake rate using a Gompertz model supported the enhanced corrosion activity and greater corrosion loss. The enhanced corrosion rate was due to the higher sulfide uptake rates induced by wastewater inoculation, although the increasing trend of sulfide uptake rates was slower with wastewater. Increased diversity in the corrosion-layer microbial communities was detected when the corrosion rates were higher. This coincided with the environmental conditions of increased levels of gaseous H2S and the concrete type.

1. INTRODUCTION Sewer networks are one of the most critical components of the urban infrastructure of modern societies. Hydrogen sulfide (H2S) is the primary cause for the shortened service life of these essential, costly installations due to microbial-induced concrete corrosion. The U.S. Environmental Protection Agency (EPA) states that severe hydrogen sulfide induced corrosion can reduce the 50 to 100 year expected life of sewer infrastructure to less than 10 years.1 It is reported that the corrosion rate of sewer pipes is between 1 and 10 mm per year.2 Corrosion causes loss of concrete mass, cracking of the sewer pipes, and, ultimately, structural collapse. Sewer systems suffering from corrosion often require premature replacement or rehabilitation of damaged pipes, manholes, and pump stations, which involves very high costs. Sewer replacement and rehabilitation is estimated to cost several billion dollars every year globally.3,4 This cost is expected to increase as the aging infrastructure inevitably continues to fail.5,6 Corrosion of concrete sewer is a complex process, involving microbial sulfide oxidation to produce sulfuric acid, chemical reactions between acids and cementitious materials, and physical changes of the concrete structure. In the anaerobic sections of the sewer, sulfate-reducing bacteria (SRB) will thrive in the sewer biofilms and sediments, and their activity generates H2S. The H2S will repartition from the liquid into the gas phase of the sewer pipes. In the presence of oxygen and through the © XXXX American Chemical Society

activity of sulfide-oxidizing bacteria (SOB) on the exposed surface of the concrete, sulfuric acid produced by the oxidation of H2S gas reacts with cement in the concrete to form gypsum and ettringite. The reactions weaken the structural integrity of the concrete and reduce the load-bearing capacity, eventually resulting in the collapse of the sewer. Our recent study, using advanced mineral analytical techniques, has established a conceptual model of the physiochemical processes for concrete sewer corrosion.7 The environmental factors impacting the initiation and corrosion rate have been investigated through long-term exposure tests in simulated and real sewers.8−10 These studies enhance the prediction of sewer service life using mathematical models based on key environmental parameters that include gaseous H2S concentrations, relative humidity, and temperature. However, microorganisms in corroding sewers are proposed drivers of concrete corrosion by catalyzing sulfuric acid production from the oxidation of hydrogen sulfide, which is ubiquitous in sewers due to its production in the anaerobic regions. Recent studies identified various microorganisms, including both bacteria and fungi, which are involved in sulfuric acid production from H2S in the sewer gas.11−14 These Received: April 27, 2016 Revised: July 1, 2016 Accepted: July 8, 2016

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DOI: 10.1021/acs.est.6b02093 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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calcium−iron−potassium aluminum silicates (e.g., amphibole, orthoclase, and plagioclase). The results indicate that the abundance of the major mineral compounds of the precorroded and fresh coupon were similar. The corrosion products, such as gypsum and ettringite, were present on the exposed surface area of the precorroded coupons. 2.2. Corrosion Chamber and Exposure Conditions. A total of two corrosion chambers were operated with gas-phase levels of 10 and 25 ppm of H2S to simulate the corrosive environment of real sewers. Details of the chamber operation and control of conditions has been described previously.9 To achieve the specified H2S gaseous concentrations in the corrosion chamber, we employed a program logic controller to trigger the corrosion-resistant solenoid pump (Bio-Chem Fluidics model 120SP2440-4TV; Boonton, NJ) for the addition of Na2S solution to acid (13% HCl), according to the monitored H2S concentrations using a OdaLog type 2 gas detector with a range between 0 and 200 ppm (App-Tek International Pty Ltd.; Brendale, Australia). The temperature and relative humidity were controlled at around 25 °C and 100%. Temperature and H2S variations were within 1 °C and 2 ppm, respectively. The environmental factors were checked regularly to ensure the correct operation of the corrosion chambers. The coupon pairs were arranged in the corrosion chambers with the original internal surface of the pipe, designated as the surface exposed to the chamber gas phase. For each chamber, five coupon pairs (one fresh and one precorroded concrete coupon) were used for different treatment purposes in this study. One coupon pair was used as the control with no wastewater inoculation. The other coupon pairs were inoculated regularly, as described in section 2.3; two were inoculated with wastewater by flooding, and the other two were inoculated by spraying. After 12 months of exposure in the corrosion chambers, the coupons were destructively sampled, with half of each coupon being used for the measurement of corrosion loss and sulfur compounds (350 day) and the other half used for analysis of the corrosion microbial communities (480 day). 2.3. Wastewater Inoculation of the Coupons. Before the inoculation and exposure tests, the coupons were washed with high-pressure water to remove the existing corrosion layer (including associated biofilm) and ensure that all coupons were starting at similar initial states. After washing, the surface pH and H2S uptake rates were measured (as described in section 2.4) to confirm the comparable status of the coupons. A total of two types of inoculation, namely flooding with diluted wastewater (50% strength) or spraying with wastewater, were used in this study. Domestic wastewater collected from a local wet well at the Robertson Park pump station (Indooroopilly, Brisbane, Australia) was used for the inoculation. The sewage typically contained sulfide (0.05%) detected in the corrosion layers of the sewer concrete coupons subjected to sewer conditions for 480 days and periodic wastewater inoculation. The labels of the columns at the bottom indicate the sample parameters, such that P or F indicates a precorroded or fresh coupon; either 10 or 25 indicates the ppm of H2S exposure level; and F, C, or S represent inoculation by flooding, no inoculum, or inoculation by spraying, respectively (e.g., the label of P25F indicates a precorroded coupon, exposed to 25 ppm of H2S, and inoculated by flooding).

indicate the growth rate of the corrosion biofilm, which might be heavily affected by the wastewater inoculation.

Table 1. Change of Microbial Abundance Induced by the Periodic Flooding and Spray-Inoculation of Concrete Coupons with Wastewater in Comparison to the Control Coupon

3. RESULTS AND DISCUSSION

inoculation

3.1. Surface pH. During microbially induced corrosion, the decrease of surface pH is mainly caused by the continuous acid production by sulfide-oxidizing bacteria and the subsequent neutralization of alkaline cement materials. Surface pH is thus a good indicator of the corrosion development. Although the starting levels of surface pH varied, the surface pH decreased clearly with time when the coupons were exposed to 10 and 25 ppm of H2S (Figure 1). It reached a level of around 2−3 after 350 days when coupons were sampled for the measurement of corrosion losses. Such a low surface pH was favorable for the colonization of acidophilic sulfide-oxidizing bacteria (SOB) and indicated an active corrosion activity.

flooding

species Acidiphilium sp At. caldus At. thiooxidans

spray

At. caldus Leptospirillum sp Mycobacterium sp

E

change of abundance 10% increase (fresh concrete; 10 and 25 ppm of H2S) 30% increase (precorroded concrete; 25 ppm of H2S) 30% decrease (precorroded concrete; 25 ppm of H2S) 20% increase (fresh concrete; 25 ppm of H2S) 10% decrease (precorroded concrete; 25 ppm of H2S) 15% increase (precorroded concrete; 25 ppm of H2S)

DOI: 10.1021/acs.est.6b02093 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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For concrete coupons subjected to wastewater inoculation, the surface pH was affected by the wastewater, which had a pH around 7.5. Therefore, wastewater sprayed on the corroding concrete surface would have some effect of neutralizing the sulfuric acid produced in the corrosion biofilm. However, flooding inoculation might remove some of the SOB biofilm all together with other corrosion products. The observed surface pH would reflect the combined effects of both biological acid production and the multiple effects of wastewater inoculation. Although the decreasing trend of surface pH is clear, a difference between the control and inoculated coupons was not observed. Consequently, the surface pH was not indicating that inoculation was contributing to more rapid levels of concrete corrosion. It should be noted that surface pH is an instantaneous measurement of the corrosion status, which reflects the conditions at that time point. Thus, it serves well as an indicator of the corrosion progression but should be assessed together with other parameters to quantitatively assess the level of corrosion. 3.2. Sulfide Uptake Rate. Sulfide uptake rate has been used as an indicator of concrete corrosion development through the measurement of the activity of the microbial biofilms in situ of the concrete corrosion layers mediating the oxidation of hydrogen sulfide.23 With the corrosion development, the SUR of all concrete coupons increased gradually with exposure time (Figure 2). The SUR of the fresh coupons showed similar SUR values for the two different H2S concentrations (Figure 2A,C), indicating that H2S is not a limiting factor for these conditions. For precorroded coupons, the SUR is slightly higher for the 25 ppm of H2S than the 10 ppm of H2S conditions (Figure 2B,D). Also, the sulfideoxidizing activity on precorroded coupons were more active than fresh coupons; this is likely due to the lower starting levels of surface pH on the precorroded coupons, which would facilitate a faster establishment of SOB biofilms. To further identify the effects of wastewater inoculation on the SUR, the Gompertz model parameters were determined and shown in Table SI-1. It is clear that the maximum SUR (rSUR,max) was increased due to wastewater inoculation for both types of concrete at the two different level of H 2 S concentrations. This implies that the actual corrosion losses and the ultimate corrosion rates would be higher on concrete coupons subject to wastewater inoculation. Also, the control coupons reached the maximum SUR at an earlier exposure time than did the inoculated concrete coupons (especially the precorroded concrete), which were still increasing at the end of 500 days (Figure 2). For another important parameter, K, which is an indicator of the growth rate of corrosion-inducing biofilm, wastewater inoculation increased the growth of biofilm on fresh concrete and exerted the opposite effects to the precorroded concrete. This is likely due to the overall balance of positive and negative effects discussed below. Wastewater inoculations might promote the biological activity (positive effects) by microbial inoculation and provide necessary nutrients and microelements. However, it may reduce the SUR (negative effects) if biofilm scouring of the loose and soft corrosion layer on concrete surface occurred with wastewater flooding. Also, the sprayed water residing in the corrosion layer could possibly temporarily reduce the SUR by blocking the mass transfer of H2S from the gas to the corrosion layer. It seems for the fresh concrete that the positive effects of wastewater inoculation exceeded the negative effects. This suggests that when the surface pH is higher (near neutrality in

Figure 5. Principle component analysis of the microbial community detected on the concrete samples exposed to different levels of H2S (A), the two types of concrete exposed to 10 ppm of H2S (B), and the two types of concrete exposed to 25 ppm of H2S (C). The arrows indicate the predominant microbial species (>0.05%) detected in the corrosion layers of the concrete coupons, subjected to sewer conditions for 480 days and periodic wastewater inoculation. F

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increased SUR and corrosion rates were detected. However, the increase in community diversity is limited and not statistically significant, likely due to the complicated effects of different factors including wastewater inoculation approaches, the level of H2S, and types of concrete. The relative abundance of the predominant microbial types (>0.05%) was detected on the coupon corrosion layers (Figure 4). The corrosion community detected on the concrete coupons is mainly composed of acidophilic SOB (in particular, those of the Acidithiobacillus genera). A recent study on microbial communities associated with corrosion fronts also identified acidophilic Acidithiobacillus spp. as the dominant sulfur oxidizer after the microbial succession from a community including heterotrophic, nitrogen-fixing, and sulfur-oxidizing genera in conjunction with decreasing pH.44 These are chemolithoautotrophic γ-proteobacteria deriving their energy from the oxidation of inorganic sulfur compounds and ferrous iron.11 At. thiooxidans was the most dominant species observed on the corroding coupons, accounting for 80−95% of the total microbial community for concrete coupons exposed to 10 ppm of H2S. Another species, At. caldus, was detected at 35−50% of abundance, comparable to that of At. thiooxidans for some coupons exposed to 25 ppm of H2S. Other Acidithiobacillus species, At. ferrooxidans and Acidithiobacillus sp., were detected at relatively high levels, i.e., 1.5−9%, on coupons exposed to 25 ppm of H2S. The dominance of Acidithiobacillus spp. in a sewer corrosion environment is consistently reported by many previous studies using both culture-dependent and molecular techniques.12,45−48 Acidiphilium species were detected on the fresh concrete coupons exposed to both H2S levels. Acidiphilium are obligate aerobic, sulfur-oxidizing heterotrophs.49 Many acidophilic SOB, including the dominant species At. thiooxidans, are inhibited by the presence of organic molecules.12,45,50 Heterotrophic microorganisms such as Acidiphilium play an important role in scavenging the organic compounds inhibitory to sulfuroxidizing autotrophs. The coexistence of many SOB and acidophilic heterotrophic bacteria such as Acidiphilium was likely important for the colonization of the fresh concrete coupons. Acidiphilium was not detected at meaningful abundancies on the precorroded coupons. In that instance, other heterotrophs might take over such an organic scavenger role in those highly developed corrosion communities. Mycobacterium spp. were also detected at notable abundancies, especially on the precorroded coupons; these were at 17% for the P25S sample (Figure 4). They were reported previously in concrete corrosion samples from real sewer environments and are known to be able to perform sulfur oxidation.14,45 Leptospirillum ferriphilum and Leptospirillum sp. were detected at 2−12% on concrete coupons exposed to 25 ppm of H2S. These iron-oxidizing bacteria normally existing in acid mine drainage have been detected at notable levels in concretecorrosion samples previously.12 Differences in the community structure of the corrosion layers were detected between the inoculated and control coupons (Table 1). The differences include the abundance of key sulfide-oxidizing bacteria (At. thiooxidans and At. caldus), the heterotrophic Acidiphilium sp., Mycobacterium sp., and Leptospirillum sp. The greatest change detected between the control and the inoculated coupons was the increased abundance of the Acidithiobacillus species (Table 1), implicating that these were important for the increased corrosion rates measured on the inoculated coupons. The change of abundance

this case), repeated inoculum aids the development of the neutrophilic and acidophilic SOB. For precorroded concrete, the corrosion biofilm growth rate K was actually reduced for inoculated coupons in comparison to the control. This might be caused by the overall higher negative effects than positive effects. It is observed that precorroded coupons with obvious corrosion layers were more susceptible to the biofilm-scouring effect (flooding) and also had higher water-retention capacity (spray) compared to the fresh coupons. Additionally, the acidophilic biofilm is likely more readily established on these coupons with a lower starting surface pH, and thus, the advantage of repeated inoculum was not evident. 3.3. Corrosion Losses. The mass loss of concrete during a certain time period is the ultimate measure of the corrosion occurring. It was found that inoculated coupons constantly showed higher corrosion losses than control coupons for both fresh and precorroded concrete (Figure 3A). The observed corrosion losses were comparable to ranges reported in other laboratory and field studies,10,41 although those studies did not report the effects of wastewater inoculation. Unlike surface pH, the SUR data clearly support this observation. This is likely due to the instantaneous nature of surface pH measurements, and thus, they are inadequate for quantifying the long-term corrosion. It was seen that the precorroded coupons had consistently higher corrosion rates than the fresh coupons. This is likely due to the different levels of microbial development on the two types of coupons. The relatively low microbial colonization on fresh coupons means its SUR and corrosion activity was limited by the abundance of microbes in the corrosion layer. It is also interesting to note that the corrosion losses of fresh coupons were similar for both 10 and 25 ppm of H2S. In contrast, the corrosion rates were about 2 mm per year greater for the inoculated precorroded coupons at the higher levels of H2S. Consequently, the SUR and corrosion activity of the wellestablished SOB biofilms on the precorroded coupons were limited by the availability of the substrate, i.e., H2S. This is supported by the different SUR detected of the precorroded concrete coupons with different levels of H2S (Figure 2) and the higher levels of sulfate detected on the corrosion layers with increased H2S (Figure SI-2). Expansion of the concrete coupons was detected, and the sprayed coupons showed a higher expansion percentage than did flooded coupons (Figure 3B). The expansion of concrete is caused by the formation of various corrosion products, mainly gypsum and ettringite, increasing the volume of the original concrete by 124% to 700%.7,42,43 The difference in expansion detected here may be due to the flooding inoculum washing off some of the loose corrosion products. 3.4. Microbial Community. With higher corrosion rates measured on concrete coupons subjected to periodic wastewater inoculation, it is highly relevant to examine the microbial communities that developed between the inoculated and control coupons. Examination of the within community diversity (richness and evenness of microbial species) showed that inoculation increased slightly the phylogenic diversity of the microbial community on the concrete coupons (Figure SI3). Other factors leading to slight increases in diversity were spraying in comparison to flooding inoculum, precorroded concrete in comparison to fresh concrete, and higher H2S concentration (Figures SI-4 and SI-5). It is apparent that increased community diversity occurred in conditions when G

DOI: 10.1021/acs.est.6b02093 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology also depended on the type of concrete and the level of H2S, thus implying that these other factors played a role on top of microbial inoculation. Using principle component analysis, we compared variation in the microbial community compositions from the coupon corrosion layers to detect clustering and relatedness among those with respect to the different H2S levels and the different types of concrete (Figure 5). Samples with similar microbial community structures tend to cluster together. It was clear that the coupon microbial communities were separated according to the level of H2S exposure (Figure 5A). It was also evident that the coupons exposed to 10 ppm of H2S had very similar microbial community structures, and those exposed to 25 ppm of H2S had more diverse populations (Figure 5A). The fast establishment of SUR activity on coupons exposed to 25 ppm of H2S means the SOB biofilm was at a more mature state than those exposed to 10 ppm of H2S. Due to the mass transfer of two important gases, i.e., H2S and oxygen, from sewer gas to the corrosion layer, high H2S levels (25 ppm) would lead to a deeper penetration depth than the low H2S level (10 ppm). It was also found that the pH increases with the depth.7 Thus, a higher H2S level lead to a wider range of pH and sulfide levels suitable to SOB proliferation, implying a higher microbial diversity in those samples exposed to 25 ppm of H2S. Figure 5B,C also show that the microbial community detected on the concrete coupons clustered separately for the two concrete types when exposed to both 10 and 25 ppm of H2S. The two types of concrete used were reported to have different ratios of silicate to calcium, acid-neutralization capacity, and pore-size distribution.7,10 These properties will directly impact the chemical composition of corrosion layer, pH, and moisture content; all of these can contribute as selection force of different microbial species. 3.5. Implications for Sewer Corrosion Management. The findings from this study support the idea that periodic wastewater inoculation promotes sewer corrosion. This may be partially due to the inoculum inducing a more-diverse microbial community in addition to other physiochemical processes enhancing corrosion. Due to climate change, extreme weather events such as floods and droughts are occurring more often. These create new challenges for the management of sewer assets, as these events would cause increased exposed-concrete surfaces (drought and reduced water use) and chances of wastewater inoculation (more floods in sewers due to high rainfall). The service life of sewers might be severely shortened due to the climate change and weather extremes, which vary from region to region. To minimize the corrosion of concrete in sewers, it would be desirable to minimize the inoculation of exposed surfaces. This study indicated that monthly flooding actually promoted corrosion. However, the maintenance of a constant water level in sewers is difficult to achieve and might lead to high corrosion activity right above the water line.8 It is also difficult to avoid the negative effects caused by wastewater splash. However, improved sewer design can be made to minimize the sharp change of slope or flow directions and sewer drops.





Figure S1: wastewater-recirculation system for flooding inoculation. Figure S2: sulfate detected on concrete surface. Figure S3−S5: rarefaction-community diversity detected on coupons exposed to sewer conditions. Table S1: parameters determined for Gompertz model based on the experimental data of sulfide uptake rates measured over 500 days. (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel: +61 7 3346 3217; fax: +61 7 3365 4726; e-mail: g.jiang@ awmc.uq.edu.au and [email protected]. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS The authors acknowledge the financial support provided by the Australian Research Council and many members of the Australian water industry through LP0882016, the Sewer Corrosion and Odor Research (SCORe) Project (www.score. org.au). Dr. Guangming Jiang is the recipient of a Queensland State Government Early Career Accelerate Fellowship.



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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b02093. H

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DOI: 10.1021/acs.est.6b02093 Environ. Sci. Technol. XXXX, XXX, XXX−XXX