*3.1. Survey Results*

wastewater and surface run-off separately.

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, 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. *Sustainability* **2020**, *12*, x FOR PEER REVIEW 6 of 15 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

**Figure 4. Figure 4.** Map with the cities which answered Map with the cities which answered the survey (map taken from [ the survey (map taken from [19]). 19]).

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 H2SO<sup>4</sup> 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 on 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).

68

68

74

74

76 based on the age of the pipe.

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). 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 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).

*Sustainability* **2020**, *12*, x FOR PEER REVIEW 9 of 15

*Sustainability* **2020**, *12*, x FOR PEER REVIEW 9 of 15

54 on the answer from the questionnaire is 375 €/km as the average maintenance cost, and 200 €/m as

54 on the answer from the questionnaire is 375 €/km as the average maintenance cost, and 200 €/m as

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. 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. 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.

69 **Figure 5.** Main sewage pipeline of Thessaloniki. **Figure 5.** Main sewage pipeline of Thessaloniki. 69 **Figure 5.** Main sewage pipeline of Thessaloniki.

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. 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. 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.

75 **Figure 6.** Pipe damage occurrence distribution in Thessaloniki's sewer system for the years 2016–2018 75 **Figure 6.** Pipe damage occurrence distribution in Thessaloniki's sewer system for the years 2016–2018 76 based on the age of the pipe. **Figure 6.** Pipe damage occurrence distribution in Thessaloniki's sewer system for the years 2016–2018 based on the age of the pipe.

**Problem Occurence %**

102 103

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. 77 In Athens, pipeline corrosion can mainly be characterized as mechanical, which in some cases is 78 secondarily affected by H2S chemical corrosion (based on the questionnaire answers). Corrosion of 79 these pipes is observed in very old combine system pipes.

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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]. 80 The sewerage network is maintained and constantly checked to avert problems that create 81 damage to the roadway and to minimize any malfunctions. In order to locate and repair damages in 82 the sewerage network, the responsible authority uses mobile units that inspect the network 83 telescopically. These mobile units contain the recording, photographic, and video-recording systems 84 that convey all the data that the camera collects from inside the pipe (such as the exact location and 85 nature of the damage) to a computer. The cameras are used to inspect pipes ranging in diameter from 86 200 to 1500 mm, as well as for the inspection of individual building connections. For pipe sections of 87 a larger diameter, which can accommodate direct inspection by technical personnel, the cameras can 88 be adjusted to a portable system. With the use of that technology, lower maintenance costs and 89 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. 90 The problems of corrosion of sewer pipes of the MWSE of Kozani network are found in the city's 91 combined sewage system. The first pipes were installed in the 1950s, and in the mid-1980s, they were 92 replaced with newer ones from the same construction material. The networks consist of pipes 1 m 93 long and there are some problems in the connections between them. Based on personal 94 communication, there have been numerous cases of slime and bad odor in the sewers. Some parts of 95 the network are completely destroyed due to corrosion. Based on the microbiological results and the 96 questionnaire, it seems that Kozani has experienced serious biocorrosion problems in the sewer 97 network. It was reported by the local authorities that corrosion failure is found not only in the upper 98 part of the pipeline, but also in the lower part and on the sides. In addition to that, the slime is found 99 under the pipeline close to the connections between the pipes (due to the improper connection). In 100 Figure 7, MWSE staff is dealing with pipeline failure, and in Figure 8, the cement pipe is completely 101 degraded and only the slime is visible.

**Figure 7. Figure 7.** Corrosion failure in Kozani's sewage pipe. Corrosion failure in Kozani's sewage pipe.

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105 **Figure 8.** Total degradation of cement pipe due to biocorrosion in Kozani. **Figure 8.** Total degradation of cement pipe due to biocorrosion in Kozani.

106 The strong point of the methodological approach used in this study was the combination of 107 survey results with experimental data. Similar studies [2] for the city of Edmonton focused on 108 hydraulic parameters and sewer system design, which were not investigated in this study. 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.

#### 109 *3.2. Field Measurements Results 3.2. Field Measurements Results*

#### 3.2.1. Gas Analysis

104

110 3.2.1. Gas Analysis 111 The results for H2S, CH4, and O2 are presented in Table 2 and in Figure 9. The authors chose 112 Kozani as a study case, because from the questionnaire results, it seemed that biocorrosion was the 113 corrosion type. In most of the test sites, small amounts of H2S were found, which could indicate 114 possible biocorrosion. It should be noted that during tests, the sewerage system was under normal 115 operating conditions and no clogs were observed. 2 ppm (mg/L) of H2S is a sufficient concentration 116 to lead to biocorrosion. Furthermore, H2S concentrations ranging from 2 to 5 ppm may cause nausea The results for H2S, CH4, and O<sup>2</sup> are presented in Table 2 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.


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

117 and headaches, while concentrations from 100 ppm can cause coughing, throat irritation, and death.


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120 **Figure 9.** Sampling points corresponding to Table 2. **Figure 9.** Sampling points corresponding to Table 2.

#### 121 3.2.2. Liquid Analysis 3.2.2. Liquid Analysis

119

126

122 In Table 3, average results of three samples for COD, TOC, total nitrogen, and total phosphorus 123 from the sewage collected from Kozani are presented. As shown in Table 3, the values are matching 124 typical values for urban wastewater [22]. In Table 3, 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].


125 **Table 3.** Liquid Analysis Results from the sewage sample. **Table 3.** Liquid Analysis Results from the sewage sample.

#### 3.2.3. Concrete Analysis *Sustainability* **2020**, *12*, x FOR PEER REVIEW 13 of 15

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). 127 3.2.3. Concrete Analysis 128 Images taken with the microscope are presented in Figure 10. A set of four pictures with 129 increasing magnification are shown. What appears to have solidified onto the surface is the bacteria

130 slime (it can be observed in the latter two images with higher magnification).

132 **Figure 10.** Microscopic images with different magnitudes of a corroded sample from Kozani. **Figure 10.** Microscopic images with different magnitudes of a corroded sample from Kozani.

#### 133 3.2.4. Slime Genetic Analysis 3.2.4. Slime Genetic Analysis

131

134 The microbial diversity analysis of the DNA sample revealed a wide spectrum of bacterial 135 species that were present in the slime layer. The resolution of the microbial profiling was, in most 136 cases, feasible down to family/genus level, and in some cases, down to species level. Bacteria 137 belonging to various phyla were identified, i.e., *Acidobacteria, Actinobacteria, Bacteroidetes, Chlamydiae,*  138 *Chloroflexi, Cyanobacteria, Firmicutes, Gemmatimonadetes, Ignavibacteriae, Nitrospirae, Planctomycetes,*  139 and *Proteobacteria.* Among the bacterial families detected was also the *Acidithiobacillaceae* family 140 (order: *Acidithiobacillales*, class: *Gammaproteobacteria*, phylum: *Proteobacteria*). The specific family 141 contains a single genus, *Acidithiobacillus*, with *Acidithiobacillus thiooxidans* as the type species. Four 142 other species of this genus are currently recognized: *At. ferrooxidans, At. caldus, At. albertensis*, and *At.*  143 *ferrivorans*. The *Acidithiobacillus* genus is of special interest because its species include some of the 144 most extremely acidophilic bacteria known, which tolerate extraordinarily high concentrations of 145 some toxic metals. *Acidithiobacillus thiooxidans* oxidizes sulfur and produces sulfuric acid, and it has 146 also been observed, causing biogenic sulfide corrosion of concrete sewer pipes by altering hydrogen 147 sulfide in sewage gas into sulfuric acid [23]. 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].

#### 148 **4. Recommendations 4. Recommendations**

149 Some recommendations to mitigate biocorrosion in concrete sewers are as follows: 150 • The regular measurement and recording of H2S concentrations, pH, COD, sewage temperatures, Some recommendations to mitigate biocorrosion in concrete sewers are as follows:

151 effluents properties, and flow rates. These are all parameters that, together with the geometrical


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)<sup>2</sup> 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.

**Author Contributions:** Conceptualization, G.F. and P.S.; Methodology, G.F., A.S., and P.S.; Software, G.F., D.L., E.P. and V.B.; Validation, G.F., D.B. and V.B.; Formal Analysis, G.F. and D.L.; Investigation, G.F. and V.B.; Resources, P.S.; Data Curation, G.F.; Writing—Original Draft Preparation, G.F., V.B.; Writing—Review and Editing, D.B., P.S.; Visualization, G.F. and E.P.; Supervision, P.S.; Project Administration, P.S.; Funding Acquisition, P.S. and A.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research has been co-financed by the European Union and Greek national funds through the Operational Program Competitiveness, Entrepreneurship and Innovation, under the call RESEARCH—CREATE—INNOVATE (project code:T1EDK-02355-title 'Novel Coating Materials for Corrosion Protection of Sewer Network Pipes'.)

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


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