## 2 Damage in Sewage Concrete Structure Due to Bio-Chemical Deteriorations  
The causes of the deterioration of a sewage concrete structure include physical erosion, such as erosion from the flow of water, however, chemical erosion is the main cause of the actual deterioration of sewage structures. The effects of various deteriorating agents interplay and in combination produce a strong deterioration mechanism; however, the bio-chemical deterioration induced by bacteria inhabiting sewage or municipal wastewater needs to be considered with priority for structure maintenance. In structures such as those of sewage systems, the corrosion of concrete is initiated by the bio-chemical activities of aerobic or anaerobic bacteria through the organic pollutants contained in streams and soil. In previous studies, the corrosion of the concrete surface and decomposition of internal hydration products were identified through the sulfate ion reducing bacteria, _Thiopacillus thioxidans_[32; 33]. The bio-chemical deterioration of concrete has been reported to being with the reduction of sulfate ions into hydrogen sulfide gas by bacteria utilizing sulfide, which involves complex biological and chemical reactions. The bio-chemical deterioration is known to consist of three stages [33], as shown in Figure 1.  
The first stage starts with the growth of sulfate-reducing bacteria in an anaerobic sewage environment and the consequent generation of hydrogen sulfide gas. The municipal wastewater flowing into the sewer pipe is deposited in the lower part of the pipe in the form of sludge [34]. Anaerobic bacteria inhabiting this deposition layer reduce sulfate ions (SO42-) to hydrogen sulfide (H2S) in the process of decomposing and consuming the organic matter required by nutrients during bacterial growth, which generates a large amount of hydrogen sulfide gas [35]. In the process, instead of oxygen (O2), oxygen bound to sulfide (S) is utilized for the purpose of protein synthesis and energy acquisition necessary for the growth of anaerobic bacteria, inducing anaerobic respiration (oxygen-free respiration) in which organic matter is oxidized. In this process, thiosulf  
Figure 1: Stages of bio-chemical corrosion in a sewage environment. and tetra-thionite ions (S4O62\({}^{-}\)) are generated as shown in Equation (1), and a hydrogen ion (H+) generated during the decomposition of organic matter present in the sewage deposits is bound to S2\({}^{-}\), which is generated during the decomposition of SO42\({}^{-}\) to produce H2S. Figure 2 shows the process of hydrogen sulfide (H2S) generation through anaerobic respiration [36; 37]:  
\[ \begin{split}\text{SO4}^{2-}+\text{ATP}\rightarrow\text{adenosyl phosphate}-\text{sulfate}(\text{APS})\rightarrow\text{SO3}^{2-}(+\text{AMP})\\ \text{S3}\text{O6}^{2-}\rightarrow\text{S2}\text{O3}^{2-}(+ \text{SO3}^{2-})\end{split}  \tag{1}\]  
The pH of the internal pore solution in concrete is maintained between approximately 12 and 13 due to the effect of calcium hydroxide [Ca(OH)2] and calcium silicate hydrate (C-S-H), which are initial cement hydrates. However, the pH of concrete is reduced to below 10 or lower due to the effect of acidic ions (H2S, S2O3\({}^{-}\), S4O62\({}^{-}\)) generated by sulfate ion-reducing bacteria and carbonation [38; 39]. In addition, lowering the pH of concrete structures induces the proliferation of sulfur oxidizing bacteria having optimal growth efficiency in a neutral environment (pH 9 or lower). The second stage of bio-chemical deterioration starts from the lowered pH of the sewage concrete, as described above, and in this stage, the deterioration level gradually increases due to the proliferation of the colony of sulfur-oxidizing bacteria growing on the concrete surface exposed to sulfate ions. Initially, due to the growth of neutro-philic sulfur-oxidizing bacteria, which can grow in a neutral pH environment, polythionic acid, a sulfur-based chemical substance, is generated, and thus the pH of the concrete surface is further lowered [21].