**Biocorrosion of Concrete Sewers in Greece: Current Practices and Challenges**

**Georgios Fytianos 1,\*, Vasilis Baltikas <sup>2</sup> , Dimitrios Loukovitis 3,4 , Dimitra Banti <sup>1</sup> , Athanasios Sfikas <sup>2</sup> , Efthimios Papastergiadis <sup>1</sup> and Petros Samaras <sup>1</sup>**


Received: 10 March 2020; Accepted: 23 March 2020; Published: 26 March 2020

**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 H2SO<sup>4</sup> 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.

**Keywords:** sewer corrosion; biocorrosion; concrete sewers

#### **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 [3–5]. 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]. *Sustainability* **2020**, *12*, x FOR PEER REVIEW 2 of 15

**Figure 1.** Cross-section representation of a concrete sewer pipe (based on [2]). **Figure 1.** Cross-section representation of a concrete sewer pipe (based on [2]).

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. 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 H2SO<sup>4</sup> in the presence of SOB. The biogenic H2SO<sup>4</sup> 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]. 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 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.

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 (ଷ ି) [10]. With regard to the contribution to scientific research of corrosioninduced 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 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 (*NO*− 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)<sup>2</sup> 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, ∆EYA 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:


*2.2. Analysis of Samples* 

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. *Sustainability* **2020**, *12*, x FOR PEER REVIEW 4 of 15


#### **Figure 2.** Questionnaire form of the survey. **Figure 2.** Questionnaire form of the survey.

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 (Sections 2.2 and 2.3). 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 (Sections 2.2 and 2.3).

liquids, sterile containers were used for the transportation of the samples to the laboratory.

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

### *2.2. Analysis of Samples*

Field measurements in Kozani were carried out to quantify H2S, CH4, and O<sup>2</sup> 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. *Sustainability* **2020**, *12*, x FOR PEER REVIEW 5 of 15 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

H2S, CH4, and O<sup>2</sup> 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. 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.

**Figure 3.** Field test in Kozani (**a**) Measurement for Η2S, CH4, Ο2 gases; (**b**) Sample collection from **Figure 3.** Field test in Kozani (**a**) Measurement for H2S, CH<sup>4</sup> , O<sup>2</sup> gases; (**b**) Sample collection from sewer.

sewer. *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 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.

#### for bacterial community analysis. 300 mg of sample material were used for genomic DNA extraction *2.3. Molecular Genetic Analysis of Bacterial Slime*

*3.1. Survey Results* 

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 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).

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**  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/).

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

Eleven questionnaires were returned from MWSEs. The location of the towns that replied can
