sustainability Article Biocorrosion of Concrete Sewers in Greece: Current Practices and Challenges Georgios Fytianos 1,*, Vasilis Baltikas 2, Dimitrios Loukovitis 3,4 , Dimitra Banti 1 , Athanasios Sfikas 2, Efthimios Papastergiadis 1 and Petros Samaras 1 1 Department of Food Science and Technology, International Hellenic University, Sindos, GR-57400 Thessaloniki, Greece; [email protected] (D.B.); [email protected] (E.P.); [email protected] (P.S.) 2 DECUS Consultants and Engineers, Ethnikis Antistaseos 4, 55133 Kalamaria, Thessaloniki, Greece; [email protected] (V.B.); asfi[email protected] (A.S.) 3 Department of Agriculture, International Hellenic University, Sindos, GR-57400 Thessaloniki, Greece; [email protected] 4 Research Institute of Animal Science, ELGO Demeter, 58100 Paralimni, Giannitsa, Greece * Correspondence: [email protected]; Tel.: +30-2310013355 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 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. 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]. Figure1 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 Sustainability 2020, 12, 2638; doi:10.3390/su12072638 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 2638 2 of 15 submerged portion [3–5]. The slime layer is typically between 0.3 and 1.0 mm thick depending on the flow velocitySustainability and 2020 solids, 12, x FOR abrasion PEER REVIEW in the sewage [6]. 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 Figure1, after H S is generated from sulfates reacting with SRB that are located As shown in Figure 1, after H2 2S is generated from sulfates reacting with SRB that are located in in the slime layer, it diffuses through the sewage to the air where it can be oxidized to H SO in the the slime layer, it diffuses through the sewage to the air where it can be oxidized to H2SO2 4 in4 the presencepresence of SOB. of SOB. The The biogenic biogenic H2 SOH2SO4 then4 then deteriorates deteriorates thethe concrete wall. wall. The basicThe basic conditions conditions for for the the occurrence occurrence of of biocorrosion biocorrosion areare the production of of H H2S2 inS insewage sewage and and the constructionthe construction of drainage of drainage networks networks from from materials materials that that cancan be corroded corroded by by the the acids acids produced produced by by the chemicalthe chemical and and biological biological processes. processes. Biogenic Biogenic corrosion corrosion hashas been investigated investigated in in other other European European countries.countries. In Flanders, In Flanders, Belgium, Belgium, biogenic biogenic corrosion corrosion of of sewerssewers costs €5¿5 million million annually, annually, representing representing approximatelyapproximately 10% 10% of the of the total total sewage sewage treatment treatment cost. cost. [ 7[7,8].,8]. In Greece,In Greece, the usethe use of plasticof plastic pipes pipes have have gradually gradually becomebecome common practice practice since since the the mid-1980s, mid-1980s, beginning with their use in the construction of new drainage systems. However, there are still 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 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 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. made mainlyIn addition, of concrete, the establishment while in smaller of wastewater cities such trea astment Lamia facilities and Komotini, in Greece it has ismade been on out the of rise PVC. Insince addition, the 1990s. the This establishment means that the of probability wastewater oftreatment occurrence facilities of the phenomenon in Greece has increased. been on the The rise sincereason the 1990s. is the This separation means thatof urban the probability wastewater offrom occurrence industrial of wastewater the phenomenon treatment has plants, increased. which The reasonresults is the in separationless concentration of urban of heavy wastewater metals fromand chemicals industrial in wastewaterurban wastewater. treatment Such plants, substances which resultsinhibit in less the concentration growth of the populati of heavyon metalsof microorganisms and chemicals involved in urban in biocorrosion. wastewater. The Such intensity substances and inhibitextent the growthof the phenomenon of the population depends of microorganismson the configuration involved and the in characteristics biocorrosion. of The each intensity network and extentseparately. of the phenomenon In study cases depends in Greece, on the the presence configuration of H2S in wastewater and the characteristics is mainly addressed of each from network the point of view of odor management [9], and its treatment seems to have been investigated only with separately. In study cases in Greece, the presence of H2S in wastewater is mainly addressed from the pointthe addition of view of of nitrates odor management( ) [10]. With [9 ],regard and itsto the treatment contribution seems to scientific to have beenresearch investigated of corrosion- only induced concrete drainage pipes, there are publications on the development of mathematical with the addition of nitrates (NO )[10]. With regard to the contribution to scientific research of modeling simulations [11,12]. Sulfide3− can be removed by chemical additives [13,14] or by additives Sustainability 2020, 12, 2638 3 of 15 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