Sewer slime

Corrosion damage of hot-dip galvanised components in oxygen-poor waste waters with pH values between approximately 3.5 and 5.0 is reported in [47]. The reason for the damage is attributed to the low oxygen content under the layers of dirt and sewer slime. Because the oxygen content is too low, the conditions for the formation of protective layers of basic zinc carbonates deteriorate. 

All types of sewer biofilms are produced at surfaces exposed to the water phase and also, to some extent, at the sewer air surfaces where aerosols are present and the humidity is high. Biofilms in sewers are often referred to as slimes and consist mainly of microorganisms, extracellular polymeric substances (EPS) produced by the microorganisms and adsorbed organic and inorganic compounds from the wastewater. 

In order to improve the resistance of sewers the use of concrete with limestone aggregate is recommended [71]. The oxidation of strlphides with chlorine and whitewash, to raise the pH value over 10, is another remedial method [222]. The systematic removal of slime and mud to accelerate the flow, but with maintaining its laminar character, is recommended too [222]. Surface sewer walls treatment, particularly with gaseous SiF can be also used [222]. 

Concrete structures can be attacked by biologically formed sulfuric acid, which may occur in sewer pipelines. This acid formation is greatest where hydrogen sulfide is readily available and where aerobic conditions and moisture are present to oxidize the sulfides to sulfuric acid. These conditions exist where biological slimes are present on the pipe walls and aerobic conditions and moisture exist above the wastewater flow. 

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