Biocorrosion of Concrete Sewers in Greece: Current Practices and Challenges

Abstract

This paper is intended to review the current practices and challenges regarding the corrosion of the Greek sewer systems with an emphasis on biocorrosion and to provide recommendations to avoid it. The authors followed a holistic approach, which included survey data obtained by local authorities serving more than 50% of the total country’s population and validated the survey answers with field measurements and analyses. The exact nature and extent of concrete biocorrosion problems in Greece are presented for the first time. Moreover, the overall condition of the sewer network, the maintenance frequency, and the corrosion prevention techniques used in Greece are also presented. Results from field measurements showed the existence of H₂S in the gaseous phase (i.e., precursor of the H₂SO₄ formation in the sewer) and acidithiobacillus bacteria (i.e., biocorrosion causative agent) in the slime, which exists at the interlayer between the concrete wall and the sewage. Biocorrosion seems to mainly affect old concrete networks, and the replacement of the destroyed concrete pipes with new polyvinyl chloride (PVC) ones is currently common practice. However, in most cases, the replacement cost is high, and the authors provide some recommendations to increase the current service life of concrete pipes.

1. Introduction

Sulfide generation is a bacterially mediated process occurring in the submerged portion of sanitary sewage systems from Sulfur-Reducing Bacteria (SRB) [1]. After H₂S diffusion towards the upper part of the sewer pipe above the wastewater, due to the presence of Sulfur-Oxidizing Bacteria (SOB, e.g., Thiobacillus), H₂S can be oxidized to biogenic H₂SO₄, which rapidly corrodes the concrete in sewer pipes [1,2]. This oxidizing process can take place wherever there is an adequate supply of H₂S gas (>2 mg/L), high relative humidity, and high atmospheric oxygen content. These conditions are thought to exist in the majority of wastewater systems for at least some times during the year [1]. Figure 1 shows a section of a concrete pipe with the different phases in a typical concrete sewer pipe (adopted by Wu et al. 2018 [2]).

The root cause of biocorrosion is the formation of H₂S, which is produced from sulfates in wastewater under a reaction with sulfate-reducing bacteria located in a slime layer. The slime layer is a layer of bacteria and inert solids at the interface between the concrete wall and the sewage—the submerged portion [3,4,5]. The slime layer is typically between 0.3 and 1.0 mm thick depending on the flow velocity and solids abrasion in the sewage [6].

As shown in Figure 1, after H₂S is generated from sulfates reacting with SRB that are located in the slime layer, it diffuses through the sewage to the air where it can be oxidized to H₂SO₄ in the presence of SOB. The biogenic H₂SO₄ then deteriorates the concrete wall.

The basic conditions for the occurrence of biocorrosion are the production of H₂S in sewage and the construction of drainage networks from materials that can be corroded by the acids produced by the chemical and biological processes. Biogenic corrosion has been investigated in other European countries. In Flanders, Belgium, biogenic corrosion of sewers costs €5 million annually, representing approximately 10% of the total sewage treatment cost. [7,8].

In Greece, the use of plastic pipes have gradually become common practice since the mid-1980s, beginning with their use in the construction of new drainage systems. However, there are still cement/concrete pipes in operation, especially in the cases of large cross sections and underneath historical places. The sewer network of the two biggest cities of Greece, Athens and Thessaloniki, is made mainly of concrete, while in smaller cities such as Lamia and Komotini, it is made out of PVC.

In addition, the establishment of wastewater treatment facilities in Greece has been on the rise since the 1990s. This means that the probability of occurrence of the phenomenon has increased. The reason is the separation of urban wastewater from industrial wastewater treatment plants, which results in less concentration of heavy metals and chemicals in urban wastewater. Such substances inhibit the growth of the population of microorganisms involved in biocorrosion. The intensity and extent of the phenomenon depends on the configuration and the characteristics of each network separately. In study cases in Greece, the presence of H₂S in wastewater is mainly addressed from the point of view of odor management [9], and its treatment seems to have been investigated only with the addition of nitrates ($N{O}_3^{-}$) [10]. With regard to the contribution to scientific research of corrosion-induced concrete drainage pipes, there are publications on the development of mathematical modeling simulations [11,12]. Sulfide can be removed by chemical additives [13,14] or by additives which inhibit biological activity [15], among other methods. Based on available literature and on personal communication with local authorities, it is noted that there is no systematic monitoring and research on biocorrosion in Greece.

An ongoing national R&D project [16] focuses on the development of an innovative active product based on Mg(OH)₂ and MgO, for the coating of the inner surfaces of concrete sewer network pipes with corrosion problems. Before moving to the study for the production of the coating, a holistic approach regarding the study of the biocorrosion status in Greece needs to take place.

This paper is intended to review the current practices and challenges of the Greek sewer systems due to biocorrosion and to provide recommendations to avoid it. The authors followed a holistic approach which included survey data obtained by local authorities serving more than 50% of the total country’s population, and validation of the survey answers with field measurements and analyses. The objective of this paper is to investigate the extent of corrosion with a special focus on biocorrosion in the Greek sewer network. To do this, authors used a questionnaire as a basic research tool and also conducted field measurements in a representative town experiencing biocorrosion problems.

2. Materials and Methods

A holistic analysis took place with the methodology consisting of two parts. First, the authors wanted to investigate which Greek cities biocorrosion is a valid problem. To this end, a questionnaire, which was answered by 11 local authorities responsible for water and wastewater (i.e. Municipal Water and Sewerage Enterprises, MWSE in this paper, ΔEYA in Greek), was used. MWSEs are public utilities and one of the major distributors of drinking water in Greece. Contact with bodies and persons related to the operation of sewer networks in Greece was necessary. Second, in order to validate the findings from the questionnaire, representative samples from field measurements (i.e. Kozani) were collected for further tests (i.e., gas analysis, liquid analysis, microscopy of raw solid samples, and molecular genetic analysis of the bacterial slime).

2.1. Questionnaire Development

Based on the knowledge of the authors, no previous study related to the status of corrosion issues in Greek sewer systems existed. Therefore, the use of survey data as a basis research tool similar to the work of [17] was chosen. The survey targeted nine local authorities from different cities as well as the two public companies from the two biggest cities of Greece (EYDAP S.A. from Athens and EYATH S.A. from Thessaloniki) in which 50% of the population resides. Moreover, there are 126 small MWSEs, out of which nine replied to the questionnaire. All respondents were the directors of MSWSEs and had engineering and/or business administration background. Despite the low reply rate from MWSEs, a relatively broad geographical and socioeconomic range was covered. When incomplete or inconsistent data survey data was found, the MWSEs were contacted directly. From the various cases, the authors put special focus in Kozani, a middle-sized town, due to the fact that it showed significant biocorrosion problems based on the results from the questionnaire.

The questionnaire was split into two interconnected parts. The first part included more general questions for the purpose of drawing conclusions regarding:

• the overall state of the sewer network,
• the extent to which the corrosion of sewer pipes is generally recognized as a problem and how it is addressed,
• the extent to which different types of corrosion, especially biocorrosion, are identified as problems with different causes and how they are dealt with,
• the frequency of sewer network inspections and corrosion inhibitions measurements,
• a first estimate of the cost of network maintenance related to corrosion.

The second part of the questionnaire contained questions about specific corrosion incidents, such as the elements of the pipeline where it was found (e.g., material, age, and geometry), the type of corrosion, and repair method. The questionnaire form can be found in Figure 2.

The questionnaire results were based on personal estimations of the MWSEs directors rather than on quantitative data. Therefore, to validate the findings of the questionnaire, sample analysis was necessary for examining the existence of H₂S and the existence of biogenic sulfide corrosion bacteria in sewer pipes (Section 2.2 and Section 2.3).

2.2. Analysis of Samples

Field measurements in Kozani were carried out to quantify H₂S, CH₄, and O₂ in the gaseous phase of the sewers, as well as to take solid and liquid samples from the pipes. For the solids and liquids, sterile containers were used for the transportation of the samples to the laboratory.

H₂S, CH₄, and O₂ gas measurements of the sewer network were conducted with a validated Eurotron Rasi700 Bio automated portable analyzer. The unit was configured to measure H₂S, and with the help of the local employees, the manhole cover was opened and the gas measurement took place swiftly. The tube of the analyzer was stopped at a depth close to the top of the sewers (Figure 3a). Measurements were made under normal operating conditions (i.e., no clogging or blockage) of the sewer network. Liquid analysis of the sewage for Chemical Oxygen Demand (COD), Total Organic Carbon (TOC), total Nitrogen, and Total Phosphorus was done with the use of Merck test kits.

Raw samples of sewer pipes and the materials on them were collected (Figure 3b) and examined microscopically with a Carl Zeiss™ Stemi 2000-C Stereo Microscope. It should be noted that most samples were heavily deteriorated and degraded due to corrosion.

2.3. Molecular Genetic Analysis of Bacterial Slime

A semi-solid sample of sludge, coming from a pipe (slime layer) of the sewer network was used for bacterial community analysis. 300 mg of sample material were used for genomic DNA extraction with the ‘NucleoSpin Soil’ kit (Macherey-Nagel, Germany) following the manufacturer’s protocol. The quality and quantity of the isolated DNA were checked with a ND-2000 NanoDrop Spectrophotometer (Thermo Fisher Scientific, USA) and by electrophoresis on a 1% agarose gel, stained with Midori Green DNA stain (NIPPON Genetics Europe, Germany).

50 μl of DNA sample, with a concentration of 250 ng/μl, were sent to CeMIA S.A. (Greece) for 16S rRNA-based microbial profiling. Nowadays, 16S metagenomics is considered to be one of the most reliable methods for microbial diversity analysis of mixed samples by utilizing next generation sequencing technology. Combined with proper bioinformatic analysis, taxonomic classification of microbes is performed down to family/genus level, while in some cases, species-level resolution can be achieved. Various hypervariable regions of the bacterial 16S rRNA gene (V2, V3, V4, V6-7, V8, and V9) were amplified with two sets of primers using the Ion 16S™ Metagenomics Kit (ThermoFisher Scientific, USA). The amplified fragments were then sequenced on the Ion Torrent S5XL platform (ThermoFisher Scientific, USA) and analyzed using the Ion 16S™ metagenomics analyses module within the Ion Reporter™ software (https://ionreporter.thermofisher.com/ir/).

3. Results

3.1. Survey Results

Eleven questionnaires were returned from MWSEs. The location of the towns that replied can be found in Figure 4. On the basis of the answers provided, data was collected from the sewer networks of 12,000 km total length, serving 50% of the population of Greece. In

Table 1. Details of Sewerage Enterprises which were answered in the questionnaire.

Town	Town Number on Map	Peak Population Equivalent	Type of System	Corrosion Problems	Type of Corrosion	Corrosion Prevention Measurement	Inspection Frequency	Prevention Measures Frequency
Athens	1	5200000	Combined	Yes	mainly Mechanical	Cleaning, Ventilation	continuously	Semi-annual
Thessaloniki	2	900000	Combined	Yes	Biochemical/Mechanical	Cleaning, Ventilation	Semi-annual	Semi-annual
Ioannina	3	142000	Separate	Yes	Biochemical/Mechanical	Cleaning	trimontly	Trimontly
Serres	4	79000	Combined	Yes	Biochemical/Mechanical	Cleaning, Ventilation	montly	Montly, n.s.
Lamia	5	78200	Separate	No	n/a	Cleaning	trimontly	n/a
Komotini	6	72000	Separate	No	n/a	Cleaning	Annual, n.s.	n/a
Kozani	7	46000	Combined	Yes	Biochemical/Mechanical	Cleaning, Chemical Additives	montly	Montly
Agios Nikolaos	8	25000	Separate	Yes	Biochemical	Cleaning, Other	Annual	Annual
Florina	9	20000	Separate	Yes	Biochemical	Cleaning, Ventilation	montly	Semi-annual
Tyrnavos	10	10900	Separate	No	n/a	Cleaning	trimontly, n.s.	n/a
Chortiatis	11	4800	Combined	Yes	Mechanical	Cleaning, Other	montly	Semi-annual

, a detailed list of the cities and answers given is presented. The town numbers of Table 1 correspond to the numbers in Figure 4. The “Peak Population Equivalent” data was obtained by the monitoring database of the special secretariat for water, Ministry of Environment and Energy [18]. In Table 1, a combined system carries both surface run-off and wastewater, while a separate system carries the municipal wastewater and surface run-off separately.

Altogether, eight of the MWSEs responded that they encountered pipeline corrosion problems, while three did not. The three MWSEs citing no corrosion refers only to small provincial MWSEs (i.e., Lamia, Komotini, Tyrnavos) and is explained by the fact that these networks are relatively new (after 1990). In addition, most of the non-corroded sewer parts at these three MWSE are made from PVC.

According to the results of those who responded positively to the occurrence of corrosion in the networks they manage, the matter of which type of corrosion is most commonly encountered arose. Thus, for all respondents, the following results are shown: four biochemical and mechanical corrosion, two only biochemical corrosion, two only mechanical corrosion, and three no corrosion problems.

It seems that the corrosion phenomenon of the sewer pipelines of the Greek sewerage systems is a problem according to the replies of the majority of the respondents. In general, the use of all types of plastic pipelines in Greece began in the late-1980s. Therefore, the absence of corrosion problems has a reasonable basis in this respect.

The fact that the highest percentage (37%) of the types of corrosion found belongs to the combination of biochemical and mechanical corrosion mechanisms reflects the fact that chemical and mechanical corrosion can occur in combination.

On the question of whether corrosion prevention measures are implemented, all respondents answered that the drains were cleaned, four were ventilated, one added chemical additives to the sewage, and three have done something else. In the category “Other”, some MWSEs use the addition of microorganisms for fat removal, but no company uses coatings onto the inner surface of the pipelines.

The removal of solids is also of great importance in reducing mechanical corrosion. According to the answers given, the ventilation of the network is mainly for deodorizing purposes. However, it results in a decrease in the concentration of H₂S in the atmosphere of the pipelines, which also reduces the formation of H₂SO₄ and thus, corrosion. Again, mainly for deodorizing purposes, some MWSEs have added chemical additives to the sewage.

MWSEs were also asked about the inspection frequency of the sewer pipes under their responsibility and for the frequency of implementation of preventive measures, namely whether they are approximately monthly, trimonthly, semi-annual, or annual. Not all MWSEs inspect the sewer network monthly. Specifically, to the question of when the sewer pipes under their responsibility are inspected, four answered every month, three trimonthly, one semi-annual to annual, while two of the respondents answered that they inspect them approximately once a year. By weighting the above answers based on their frequency, it appears that in the Greek territory, the condition of sewer networks is inspected on average about six times a year and that the precautionary measures are applied approximately four times a year.

MWSEs were asked if they kept records of the inspections of the networks and what type of records they contained. They were also asked if they had been measuring and recording H₂S concentrations, pH, COD, sewage temperatures, effluents properties, and flow rates. These are all parameters that, together with the geometrical characteristics of the network, can be used for calculations of hydrogen sulfide production, risk and/or corrosion rate. Unfortunately, such records are not systematically maintained for the sewer networks. These kinds of analyses are carried out at the inputs and output of the wastewater treatment plants, and only οn some parameters of the incoming sewage and the effluent of their treatment.

It is very difficult to determine the cost of repairs for sewer pipes due to damage caused by corrosion. None of the wastewater and sanitation enterprises involved in the survey calculates this separately. The pipes are usually replaced when they are seriously deteriorated. The information provided are business estimates of the total network maintenance costs and cost per meter for pipeline replacement. The average maintenance cost of a sewer network in Greece was calculated based on the total costs and network length of each enterprise. The average cost of pipeline replacement was calculated accordingly, and the average cost of pipeline replacement presents business-to-business variations, depending on the extent to which the work is performed by the same resources or by third parties through project contracts. A more detailed determination of the costs specifically associated with corrosion was not carried out in this study. The authors’ estimation based on the answer from the questionnaire is 375 €/km as the average maintenance cost, and 200 €/m as average replacement cost (without including the salaries of external contractors).

In Thessaloniki, the sewer network is 35% combined [20]. It is noteworthy that the main part of the sewerage system of the city dates back to 1926 and was constructed in the context of the rehabilitation of the city after the devastating fire of 1917. A disadvantage of the city’s sewer network is the lack of its ventilation infrastructure which favors the occurrence of corrosion due to the presence of hydrogen sulfide. According to the local authorities, about 90% of the maintenances related to the sewer network concern concrete damages (reinforced and not).

Figure 5 shows a part of the old sewage network. The lower part of the pipe is covered with ceramic tiles. Deposits of fat and possibly sulfur compounds are observed. The upper surface shows some corrosion, although it appears to be progressing slowly with respect to the age of the pipe, which may be as old as 93 years. Potential fat deposits on the sewer pipes may enhance anaerobic conditions which favor the production of H₂S, and hence biocorrosion. Furthermore, high content of fats results in pipe blockage.

However, there are numerous cases of pipe corrosion issues. In the following diagram (Figure 6), the corrosion-related damages distribution for 2016–2018 by the decade of construction of pipelines is presented. It should be noted that the majority of damages concern concrete pipes constructed until the 1970s with 83%, while 14% concern pipes up to 40 years since construction.

In Athens, pipeline corrosion can mainly be characterized as mechanical, which in some cases is secondarily affected by H₂S chemical corrosion (based on the questionnaire answers). Corrosion of these pipes is observed in very old combine system pipes.

The sewerage network is maintained and constantly checked to avert problems that create damage to the roadway and to minimize any malfunctions. In order to locate and repair damages in the sewerage network, the responsible authority uses mobile units that inspect the network telescopically. These mobile units contain the recording, photographic, and video-recording systems that convey all the data that the camera collects from inside the pipe (such as the exact location and nature of the damage) to a computer. The cameras are used to inspect pipes ranging in diameter from 200 to 1500 mm, as well as for the inspection of individual building connections. For pipe sections of a larger diameter, which can accommodate direct inspection by technical personnel, the cameras can be adjusted to a portable system. With the use of that technology, lower maintenance costs and quicker repair time are achieved while minimizing social annoyance from unnecessary digging [21].

The problems of corrosion of sewer pipes of the MWSE of Kozani network are found in the city’s combined sewage system. The first pipes were installed in the 1950s, and in the mid-1980s, they were replaced with newer ones from the same construction material. The networks consist of pipes 1 m long and there are some problems in the connections between them. Based on personal communication, there have been numerous cases of slime and bad odor in the sewers. Some parts of the network are completely destroyed due to corrosion. Based on the microbiological results and the questionnaire, it seems that Kozani has experienced serious biocorrosion problems in the sewer network. It was reported by the local authorities that corrosion failure is found not only in the upper part of the pipeline, but also in the lower part and on the sides. In addition to that, the slime is found under the pipeline close to the connections between the pipes (due to the improper connection). In Figure 7, MWSE staff is dealing with pipeline failure, and in Figure 8, the cement pipe is completely degraded and only the slime is visible.

The strong point of the methodological approach used in this study was the combination of survey results with experimental data. Similar studies [2] for the city of Edmonton focused on hydraulic parameters and sewer system design, which were not investigated in this study.

3.2. Field Measurements Results

3.2.1. Gas Analysis

The results for H₂S, CH₄, and O₂ are presented in

Table 2. Gas measurements during the field test in Kozani.

Place	Measured Gas
	H2S (ppm)	CH4 (%)	O2 (%)
1	2	0.04	21
2	1	0.03	20.3
3	1	0	20.9
4	0	0.08	21.1
5	0	0005	21
6	1	0.16	20.9
7	1	0.03	20.9
8	2	0.03	20.5
9	2	0.2	20.6
10	1	0.09	20.8
11	1	0.04	20.4
12	1	0.03	20.2
13	1	0.02	20.9

and in Figure 9. The authors chose Kozani as a study case, because from the questionnaire results, it seemed that biocorrosion was the corrosion type. In most of the test sites, small amounts of H₂S were found, which could indicate possible biocorrosion. It should be noted that during tests, the sewerage system was under normal operating conditions and no clogs were observed. 2 ppm (mg/L) of H₂S is a sufficient concentration to lead to biocorrosion. Furthermore, H₂S concentrations ranging from 2 to 5 ppm may cause nausea and headaches, while concentrations from 100 ppm can cause coughing, throat irritation, and death.

3.2.2. Liquid Analysis

In

Table 3. Liquid Analysis Results from the sewage sample.

Sewage Chemical Parameters	mg/L
Chemical Oxygen Demand (COD)	910
Total Organic Carbon (TOC)	238,7
Total Nitrogen	49
Total Phosphorus	5,1

, average results of three samples for COD, TOC, total nitrogen, and total phosphorus from the sewage collected from Kozani are presented. As shown in Table 3, the values are matching typical values for urban wastewater [22].

3.2.3. Concrete Analysis

Images taken with the microscope are presented in Figure 10. A set of four pictures with increasing magnification are shown. What appears to have solidified onto the surface is the bacteria slime (it can be observed in the latter two images with higher magnification).

3.2.4. Slime Genetic Analysis

The microbial diversity analysis of the DNA sample revealed a wide spectrum of bacterial species that were present in the slime layer. The resolution of the microbial profiling was, in most cases, feasible down to family/genus level, and in some cases, down to species level. Bacteria belonging to various phyla were identified, i.e., Acidobacteria, Actinobacteria, Bacteroidetes, Chlamydiae, Chloroflexi, Cyanobacteria, Firmicutes, Gemmatimonadetes, Ignavibacteriae, Nitrospirae, Planctomycetes, and Proteobacteria. Among the bacterial families detected was also the Acidithiobacillaceae family (order: Acidithiobacillales, class: Gammaproteobacteria, phylum: Proteobacteria). The specific family contains a single genus, Acidithiobacillus, with Acidithiobacillus thiooxidans as the type species. Four other species of this genus are currently recognized: At. ferrooxidans, At. caldus, At. albertensis, and At. ferrivorans. The Acidithiobacillus genus is of special interest because its species include some of the most extremely acidophilic bacteria known, which tolerate extraordinarily high concentrations of some toxic metals. Acidithiobacillus thiooxidans oxidizes sulfur and produces sulfuric acid, and it has also been observed, causing biogenic sulfide corrosion of concrete sewer pipes by altering hydrogen sulfide in sewage gas into sulfuric acid [23].

4. Recommendations

Some recommendations to mitigate biocorrosion in concrete sewers are as follows:

• The regular measurement and recording of H₂S concentrations, pH, COD, sewage temperatures, effluents properties, and flow rates. These are all parameters that, together with the geometrical characteristics of the network, can be used for calculations of hydrogen sulfide production and risk.
• Use of mobile units equipped with cameras for regular inspections.
• Surface washes with water. Although flushing with high-pressure water removes the corrosion deposits from the concrete surface and increases the surface pH, the effects are short term, i.e., one month [24], or two to four months [25] and for a long-term protection, frequent flushing with high-pressure water is necessary.
• Treatment of the concrete surface so as to be less susceptible to corrosion. This can be done by using spray-on coatings, e.g., Mg(OH)₂ based coatings.
• Application of polyethylene (PE) liner.
• Inhibition of the biological activity, e.g., with biocides.

For proper concrete sewer system design, avoiding sedimentation in sewer conduits should be taken into account. Towards this direction, mathematical modeling could be beneficial [26].

Each case is different, and a life cycle costing analysis for each method could be advantageous in order to estimate the most cost-efficient biocorrosion mitigation methodology.

5. Conclusions

The results from the questionnaire showed that corrosion is present in Greece’s sewer networks and has caused the destruction of sewer pipe sections made of concrete. The replacement of the destroyed concrete pipes with new polyvinyl chloride (PVC) ones is currently common practice. Further gas and slime genetic analysis supported the findings of the questionnaire and showed that in the case of Kozani, biocorrosion is the main type of corrosion that takes place. Biocorrosion seems to affect mainly old networks, city centers, and large diameter collectors. As a next stage, since most of the concrete networks cannot be replaced easily and economically, the authors will examine the effectiveness of a protective coating based on Mg(OH)₂ and MgO that can be applied onto the concrete surfaces as a solution to control biocorrosion. For future studies, Life Cycle Cost Analysis (LCCA) can be a useful tool for the economic evaluation of various biocorrosion mitigation strategies.