MICROBIAL CORROSION OF CONCRETE SEWER PIPES, H2S PRODUCTION FROM SEDIMENTS AND DETERMINATION OF CORROSION RATE

Little slime layer was found to exist on the walls of sewer pipes in an anaerobic condition when the sewage flow rate was higher than approximately 30 cm/sec. Therefore, H2S is not produced from slime layers but from sediments which existed in the main trunk or the pressure main. The H2S production rates ranged from 5.5 to 64-mu-g-H2S/g-solid.h depending on the contents of organic compounds and the cell numbers of sulfate reducing bacteria (SRB). The limiting nutrients are assumed to be fatty acids by the spike test. The pipe wall in the area just above the sewage level corroded severely, but the crown of the pipe was not heavily corroded. The corrosion rate of the concrete sewer pipe at the Ohmuta treatment plant was 4.3 to 4.7 mm/y in the area just above the sewage level and 1.9 mm/y at the crown. The experiments using mortar as a test specimen showed that the corrosion rate usually ranged from 3.7 to 7.7 mm per year. But an extremely high corrosion rate was observed when the mortar was placed on the stage of the manhole of the pipe at Ohmuta Treatment Plant at temperatures of 25 to 30-degrees-C and H2S concentrations of 3 to 400 ppm. Corroded materials found on the surface of the pipe wall and mortar specimens were gypsum at low pH. Ettringite appeared within cracks of the mortar specimens at high pH.