Step 1: Understand crown corrosion. Crown corrosion in reinforced cement concrete sewers occurs due to the conversion of hydrogen sulfide gas ( ) to sulfuric acid ( ). This reaction is facilitated by sulfur oxidizing bacteria like Thiobacillus.
Step 2: Role of sulfur oxidizing bacteria. In sewer systems, anaerobic bacteria produce in the wastewater. The gas rises to the crown (top) of the sewer and reacts with oxygen in the presence of sulfur oxidizing bacteria, producing . The sulfuric acid corrodes the concrete at the crown of the sewer.
Step 3: Analyze the options. (1) Sulfur oxidizing bacteria: Correct, as they produce , causing crown corrosion. (2) Iron oxidizing bacteria: Incorrect, as they primarily oxidize iron and are not involved in this process. (3) Denitrifying bacteria: Incorrect, as they convert nitrates to nitrogen gas, unrelated to corrosion. (4) Fermentative bacteria: Incorrect, as they are involved in anaerobic fermentation processes, not corrosion.