1. MICROBES INVOLVED IN CONCRETE SEWER PIPE CORROSION (title)
Eulyn Pagaling1, Kun Yang1, and Tao Yan1 (Authors)
1Department of Civil and Environmental Engineering, University of Hawai’i at Mānoa (Institution)
INTRODUCTION
Concrete corrosion weakens the structure of the sewer pipe
Leads to pipe failure and release of raw sewage into the environment posing a public health risk
Costs the US millions of dollars for remediation
*Corresponding Author: Tao Yan ：
Cell：180********
Mechanism of concrete corrosion
Taken from Roberts et al (2002). Int Biodet Biodeg 49: 1 2 3 4
MECHANISM:
When sediment builds up, Sulfur-Reducing Bacteria (SRB) break down organic matter to produce H2S.
H2S volatizes into the sewer atmosphere.
Sulfur-Oxidizing Bacteria (SOB) consume the H2S to produce sulfuric acid (H2S04).
H2S04 corrodes the concrete to produce gypsum and ettringite, which weakens the structure of the pipe.
Concrete corrosion has led to complete failure of this concrete pipe
Surface of severely corroded concrete sewer pipe
AIMS: Discover the SOB involved in the final stages of concrete corrosion
Correlate bacterial community composition with environmental factors
METHODS
Sample sewer pipe crowns from 6 sites in situ at Moana Park, Honolulu, HI
Collect environmental data using a gas probe: [H2S], [02], [volatile gas]
X-ray Diffraction (XDR) on the samples to determine the extent of corrosion products
Total genomic DNA extraction followed by:
Quantitative PCR (qPCR) to measure cell density
Pyrosequencing of the 16S rRNA genes to determine the bacterial species present
Moana Park sampling area
Corroded sewer pipe at Moana Park
RESULTS
Fig. 1 X-Ray Diffraction Peaks for Corrosion Products
All samples were compared to the standard peaks for elemental sulfur and gypsum (bottom two panels).
All samples consisted mainly of either elemental sulfur or gypsum, but not both.
No ettringite was detected in the samples.
The samples that contained gypsum also contained other unidentified compounds not related to concrete corrosion.
Fig.2 Bacterial Diversity, Richness and Density varied with pH
The qPCR results showed that bacterial cell density was low in the sewer crown.
However, the density was even lower as the pH dropped further.
Bacterial diversity and richness was also low in the sewer crown, but dropped further as the pH dropped.
Log (cell density) (CCE/ml)
Diversity/Richness
Sample
[H2S] (ppm)
LEL (%)
[O2] (%)
pH of corrosion products 1A
102.8 6
18.5
0.80 1B
95.5 8
18.2
0.28 1C
222
19.0
0.58 2
76.7 19
19.1 3
29.5
19.2
0.24 4
9.2
19.4
1.45 5
11.3
20.0
1.18
5.5
19.3
1.47
Fig 3. Heat Map of the Pyrosequencing Results
The pyrosequencing results showed that Mycobacterium and Acidithiobacillus were the main SOB corroding concrete in our sewer system.
Acidithiobacillus is a well-known SOB, but mycobacteria are only just becoming known as SOB. Therefore this study confirms observations by others.
When correlated with the environmental data, Acidithiobacillus only appeared to proliferate in less acidic pHs, while Mycobacterium was dominant in all samples, but was more prolific in the highly acidic samples.
Bacteria that use methane as an energy source i.e. Methylacidiphilum infernorum also proliferated, but were not involved in concrete corrosion
Table 1. Environmental Data Collected On-site
The environmental data showed that the sewer atmospheres contained high [H2S], were depleted in 02 and some contained volatile gases.
The corrosion products were also shown to be highly acidic.
CONCLUSIONS
Species diversity and richness and bacterial cell density is low in the sewer crown. They are directly influenced by pH (lower pHs gives lower diversity, richness and density).
Mycobacterium and Acidithiobacillus are important SOB involved in concrete corrosion.
Acidithiobacillus proliferates in less acidic pH, while Mycobacterium proliferates in highly acidic pH.
ACKNOWLEDGEMENTS
The authors would like to thank Prof Henglin Cui for his assistance with sampling and Mrs Bunnie Yoneyama for her assistance in the lab. They also acknowledge the Characterization Facility at the University of Minnesota for the x-ray diffraction analysis of the corrosion products.
This work was funded by the EPA.