This paper presents a review of Microbial Induced Corrosion (MIC) of concrete sewer pipes. The focus of this review is to understand the processes that lead to the corrosion of concrete pipes in the sewer environment and to report on the current materials and methods in use to prevent this type of deterioration. Microbial induced corrosion involves a sequence of microbial, chemical and physical processes, a simplified summary of which includes; the reduction of sulphates by bacteria under anaerobic conditions in sewer effluent to produce sulphides in solution, the liberation hydrogen sulphide gas, the growth of Sulphur oxidizing bacteria on the exposed surface of the sewer pipe, the oxidation of hydrogen sulphide by sulphur oxidizing bacteria to produce Sulphuric acid and finally the corrosion of concrete by acid attack. In response to this severe form of degradation scientists and engineers have developed solutions which are effective at different stages of this cycle.
For civil engineers, the most common intervention is in the pipe material selection, where usually, concrete pipe is selected based on its resistance to sulphuric acid formed in the MIC process and its inhibition of the growth of Sulphur reducing bacteria.
Introduction
Acid attack in sewers, commonly referred to as Microbial Induced Corrosion or H2S induced biogenic corrosion, is a process of sewer deterioration that is a serious concern worldwide. Developed and developing countries face problems with regards to aging or decaying sewer infrastructure. The nature of this decay, especially where concrete pipes are concerned, is in many instances associated with Microbial Induced Corrosion.
Saucier (2016) reports that the average of age of sewer networks in European countries is 50 years, 10% of sewer networks are over the age of 60 and that most of these sewer networks were designed for a life-span of 60-80 years.
The American Society for Civil Engineers (2017) scored the US’s sanitation infrastructure a D, for the state of its 1.2 million kilometres of Sewer and its 14 748 sewage treatment plants. It is estimated that a sum of $271 billion is required to meet current and future demands over the next 10 years (ASCE, 2017).
In South Africa, the South African Institute of Civil Engineers (2017) scored Sewer and Sanitation infrastructure a C-, indicating that this infrastructure is nearing as state duress and will require investment in the short to medium term. Furthermore, Goyns (2010) states that many South African sewer-lines, installed over 40 years ago, have deteriorated, are semi functional, have collapsed or are about to collapse.
There is therefore a need to ensure that new and rehabilitation sewer infrastructure projects of are implemented in a way that responds adequately to this type corrosion and degradation.
Causes of MIC
Prevention of Microbial induced corrosion requires an understanding of the underlying biological, chemical and physical process that lead up to Sulphuric acid based deterioration concrete in pipes.
The processes that lead to Microbial induced corrosion may be broadly categorized to include:
Sulphur compounds in sewage and the proliferation of Sulphide Reducing Bacteria (SRB).
Sulphur compounds are naturally occurring in human in human waste, some researchers have pointed out that it especially prevalent in urine (Parker and Zhang, 2008).
Sulphur Reducing bacteria (SRB) are organisms that exist in the biofilm that grows on the walls of sewer pipes (Guiterrez et al, 2016). These bacteria typically oxidize sulphur compounds (Sulphates) in the presence of dissolved oxygen in sewer effluent. Under anaerobic sewer conditions (no dissolved oxygen), sulphate (SO42-) becomes the most thermodynamically efficient electron acceptor for Sulphate reducing bacteria to use in place of dissolved oxygen.
SRB obtain energy by oxidizing organic compounds and/or molecular hydrogen (H2) while reducing sulphate (SO4-2) to hydrogen sulphide (H2S-HS-) during its metabolism. Desulfovibrio and Desulfotomaculum are the dominant genera of SRB (Guiterrez et al, 2016).
SO42- + 2 H+ + 4 H2 → H2S↑ + 4 H2O
Eq 1: Reduction of Sulphates by Sulphur Reducing Bacteria (Cwalina, 2008).
The production of sulphides (HS-) by sulphur reducing bacteria has been shown to be more acute in rising sections of the sewer network, or in sections where the sewer runs full (Guiterrez, 2016, Goyns, 2008).
The liberation of H2S gas from Sulphides (HS-) in solution occurs mainly in partially filled sections of the pipe, and is due in an imbalance of Sulphides in solution and H2S in the sewer atmosphere. In theory, H2S will be liberated until equilibrium is reached between H2S gas in the sewer atmosphere and HS- ¬in solution (Goyns, 2008).
The stripping of H2S from solution to the sewer atmosphere is a function of the sewer flow regime. Turbulent or super critical flows, which may occur where effluent velocities are high or where abrupt changes to the sewer geometric profile occur cause significant increases in the liberation of hydrogen sulphide gas when compared to subcritical flows (Goyns, 2008).
pH reduction of concrete Pipe and colonisation by Sulphur Oxidizing Bacteria
Concrete is by far one of the most affordable and available materials on earth. It therefore follows that bulk infrastructure such as dams, paving works, sewer networks and other infrastructure a built using this material. Therefore, this part of the study will focus on the growth of Sulphur Oxidizing Bacteria on this material.
New concrete is a hostile host to most organisms because of its high pH (12-13). However, this ideal state doesn’t last long as concrete is susceptible to forms of degradation that reduce this value over time. Carbonation is one such form of degradation and it occurs when calcium hydroxide (CH), which is a product of cement’s hydration reaction reacts with carbon dioxide in the presence of water to form calcium carbonate. Carbonation is a slow process and occurs in a front. Figure 1 adapted from Chang and Chen (2006) shows the relationship between the degree of carbonation and the pH of concrete. Figure 2 shows the intensifying effect of cracks on the carbonation front.
Furthermore, the H2S gas in the sewer atmosphere also reduces the pH as it reacts with concrete hydration products such as calcium hydroxide (Guiterrez et al, 2016).
Figure 1 : Relationship between pH value and carbonation (Chang and Chen, 2006)
Figure 2: Carbonation occurs in a front, however it is exacerbated by cracks in the concrete (Xypex, 2018).
Of interest to the durability of concrete is a set of bacteria referred to as Sulphur Oxidizing Bacteria. Much like Sulphur Reducing bacteria discussed earlier, these bacteria use a sulphur compound, H2S as an input in their metabolic processes.
It should be noted that the sewer is a complex environment, and is a host to many bacteria, fungi and other micro-organisms with complex interrelationships. Okabe et al (2006) found that over 50% of the corrosion biofilm community was heterotrophic bacteria other than SOB.
As the pH of concrete reduces, successive species of Sulphur oxidizing bacteria (acid producing bacteria) which thrive at varying pH levels begin to colonise the pipe. The bacterial species with the most intense effect on the pipe is Acidithiobacillus Thiooxidans, which exists from pH 4 to 1. It should be noted that the sewer ecosystem is highly complex, and it plays host to many bacteria and fungi with complex inter-relationships. A few Sulphur oxidizing bacteria studied by Saucier and Herrison (2016) are shown in figure 3.
Figure 3: The successive growth of 4 species of Sulphur Reducing Bacteria in the sewer environment, adapted from Saucier and Herrison (2016)
Bacteria such Acidithiobacillus Thiooxidans convert H2S gas in the presence of water to form Sulphuric acid.
H2S + 2O2 → H2SO4
Eq 2: Oxidation of H2S by Sulphur Oxidizing bacteria, (Cwalina, 2008).
Corrosion of concrete by Sulphuric acid
The reaction between cement hydration products and sulphuric acid is fairly well understood.
Ca(OH)2 + H2SO4 → CaSO4.2 H2O
Eq 3: Reaction between Calcium hydroxide and Sulphuric acid in the sewer, (Cwalina, 2008).
Apart from the corrosive action of the acid, the product of the reaction is chemically identical to gypsum, part of which further reacts with the cements gel’s Tricalcium Aluminate hydrate (C¬3A) to form ettringite.
3CaSO4 + 3CaO · Al2O3· 6H2O + 25H2O → 3CaO · Al2O3 · 3CaSO4 ·31H2O (ettringite)
Eq 3: The formation of ettringite (Cwalina, 2008).
Gypsum plays a retarding role in the initial setting behaviour of cement and is a reactant in the formation of ettringite, a cement hydration product. Gypsum is credited for preventing cement from flash setting, however ettringite is an expansive cement hydration product which isn’t problematic at cements early age. When concrete is not in its early setting phases anymore, the formation of ettringite is problematic. Where the matrix is hardened and rigid ettringite forms long and slender crystals which push hardened cement paste, leading to internal stresses in the concrete and ultimately cracking (Alexander, Bentur and Mindness, 2017).
Figure 4: Sulphur Reducing and Sulphur Oxidizing bacteria, corrosion of the pipe and ettringite induced cracking.
Prevention and Mitigation
Control of Sulphides and Sulphur Reducing bacteria
With the world population becoming more conscious of the scarcity of water and natural resources, and the drive towards water conservation and sustainability, it is probable that human intervention will cause the proportion of sulphates in sewage effluent to change. Studies to recover useful nutrients such as phosphorous, ammonium and potassium, which are present in human urine, has prompted the idea of urine separation.
Inhibiting the growth Sulphur Reducing Bacteria
In the United States of America, studies on the concentration of H2S show a marked increase in its levels from the late 1970’s onwards. In the decade 1970-1980, average H2S levels ranged from 2 to 10 ppm of sewer atmosphere. However recently concentrations have peaked at 50 ppm, especially during warm seasons. One proposed reason for this increase relates to the implementation of legislation limiting the allowable levels of heavy metal ions discharged by industrial plants. Heavy metal ions are thought to be responsible for inhibiting the growth of bacteria, including Sulphur reducing bacteria (Guiterrez et al, 2016).
Minimizing Release of H2S into the sewer atmosphere
This is one aspect that sewer design engineers and planners can exercise a degree of control. Goyns (2008) recommends that hydraulic design of sewers ensure that the flow regime is subcritical where high concentrations of H2S are present in the effluent.
The design and detailing considerations that may be used in minimizing the release of H2S include (Goyns, 2008):
• Minimizing sewage retention times, this involves making sewer sections as short as practicable.
• Designing for minimum effluent velocities which will ensure that anaerobic conditions are not easily realized.
• Design the sewer to flow full, thus ensuring no release of H2S gas. This is achieved by using the smallest practical pipe diameter.
Inhibiting the growth Sulphur Oxidizing Bacteria
Inhibiting, or stifling the growth of Sulphur Oxidizing bacteria is still the subject of much research. Bacteriostatic properties of the pipe material are now considered to be an important parameter in the selection of sewage pipes. Saucier and Herrison (2016) report that Calcium Aluminate Concrete pipes have superior bacteria inhibiting properties when compared to Portland cement concrete as a result of its chemical composition. Between pH 3 to pH 4, alumina gel (a product of Calcium alumina cement hydration), becomes unstable causing the liberation of aluminium ions in the biofilm of the sewer pipe. Saucier and Herrison (2015) state that once the concentration of Aluminium ions reaches 300-500 ppm level within the biofilm on the surface of the pipe, a bacteriostatic effect is observed on the bacteria’s metabolism, effectively stopping the activity of Sulphur reducing bacteria (SRB).
There are also numerous commercially available concrete additives such as Conshield which are also claimed to have a bacteriostatic effect on Sulphur Oxidizing bacteria, however this research study found very little verification by independent scientific studies to back up Conshield’s claims.
Using acid resistant materials for sewer pipes
The use of acid resistant material such as organic polymer based materials in sewage is an obvious solution for to avoid acid based deterioration of sewer infrastructure. Acid resistant pipe such as High density poly ethylene (HDPE) pipe are available as pipe structures and as pipe linings. However, there are limitations with respect to structural capacity and cost which causes them to be less favourable than concrete pipes.
Buckling of HDPE pipe used as linings in concrete pipes has been shown to be a problem when they were used as rehabilitation solution for leaking sewers. Kierczak and Kuiczkowska (2013) report that the bonding inadequacy between the HDPE lining and the host concrete sewer pipe caused the early failure of a rehabilitated sewer line after only one year of operation. In contrast the deterioration of concrete takes much longer, structural capacity is seldom an issue, concrete is very economical and it’s available in most places where human beings have settled.
Concrete is a composite material, thus its behaviour under the action of acids is a function of the how its component parts are effected by acid attack. It is known that siliceous aggregates highly resistant to acid attack.
An emerging area of research is in using alternative cementitious technologies, such as Alkali activated cements (or Geopolymers) instead of Portland cement. Geopolymers are a class of binder that uses alumina silicate precursor materials (Davidovits, 2013) Researchers have found that the resistance of alkali activated cements is superior to that of Portland cements under inorganic acid test procedures. Figure 4 shows the difference in Geopolymer concrete and Portland cement concrete after immersion in sulphuric acid for a period of 18 months.
Figure 5: Visual appearance of Portland Cement concrete (left) and Geopolymer cement concrete (right) after long term immersion in Sulphuric acid (Ariffin et al, 2013)
Unfortunately long term test results on Geopolymer durability are not yet available with respect to biogenic acid attack. Research on this subject is on-going.
Mitigating concrete damage by using cements and aggregates with close/similar acid solubility properties
A common approach by pipe manufacturers to mitigate the effects of acid attack is by using calcareous aggregate. The use of calcareous aggregate, such as dolomite as a sacrificial aggregate in concrete subjected to acid attack was influenced by the observed effects of using aggregates that are acid resistant. It was found that sewer pipes with siliceous aggregate suffered rapid and severe disintegration because the corrosive reaction was mainly between sulphuric acid and the cement paste, thus causing the siliceous aggregate to be dislodged from the matrix.
Calcareous aggregates have acid solubility comparable to Portland cement paste. Corrosion is thus layered and gradual, thereby increasing the service life of the structure. High alumina cement is often used instead of Portland cement because of its bacteriostatic properties, however because of its high cost, high alumina cements are used as internal pipe lining.
Long term data has been collected on the durability of various concrete mix combinations with the Virginia sewer experiment (Goyns, 2016). The Virginia sewer experiment is a live sewer experiment, where sections of pipe and specimens of concrete are exposed to biogenic corrosion. Specimens which included Calcium Aluminate cements, Portland Cements, Fly Ash and GGBS blends of Portland cement were used with both siliceous and calcareous aggregate. The data collected in the study could be used by design engineers to estimate the material factor (Material loss factor) of various concrete mixes. This enables engineers to make informed analysis, such as cost to benefit ratios of pipes with varying mixes.
Conclusions
The chain of processes that result in the corrosion of concrete by acids is well understood. To make sewers last longer requires the identification feasible interventions at each stage of this process will stop the process form reaching an undesirable state. The most effective solutions to the problem of MIC arise from modifications to the pipe material, as this is the most practical to control. The durability of concrete pipe has been shown to be influenced by two chief properties, a) its ability to stifle the growth of Sulphur oxidizing bacteria and b) its ability to resist acid (acid solubility). Theoretically the ideal solution would require a material that has both properties. It is thus the subject of a significant amount of scientific research world-wide.
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