Biological activity in long, sluggish-flow, flat grade sewer lines will likely cause which problems?

• A. Concrete and metal corrosion
• B. Oxygen deficiency in the air in manholes, sewers, or wet wells
• C. Toxic gas production
• D. All of the above

Answer: (D)

In long, sluggish-flow sewer lines with a flat grade, the flow is not energetic enough to keep sediments suspended or to thoroughly aerate the wastewater. That creates pockets where microbes thrive on the organic matter and consume the available oxygen, producing anaerobic conditions. In these environments, hydrogen sulfide gas can form and build up, and as it escapes into the air in manholes and wet wells it becomes hazardous.

The presence of hydrogen sulfide and other byproducts also accelerates deterioration of infrastructure. When hydrogen sulfide is released and then oxidized on concrete surfaces, it forms sulfuric acid, which attacks the concrete and can also corrode metal components over time. So you get corrosion of concrete and metal, a drop in the oxygen level in the air around manholes and wet wells, and the production of toxic gases.

Because all of these issues can arise in those conditions, the best answer is that all of the above problems are likely.