What can biological activity in long, sluggish-flow, flat grade sewer lines cause?

Biological activity in long, sluggish-flow, flat grade sewer lines can lead to a variety of issues due to the conditions created by stagnant wastewater. In these environments, bacteria and other microorganisms thrive and can cause several adverse effects.

One significant consequence is the production of toxic gases, such as hydrogen sulfide, which can arise from the anaerobic decomposition of organic materials. This gas is not only hazardous to human health but can also contribute to corrosion of the sewer infrastructure, particularly when it interacts with water and concrete or metal, leading to deterioration.

Additionally, because of the slow flow and high levels of decomposition, oxygen levels in the water can become depleted. This not only affects aquatic life but can also result in oxygen deficiencies in the air, particularly in enclosed spaces like manholes, posing health risks for maintenance personnel.

Thus, the biological activity in these sewer lines indeed encompasses all these issues—corrosion, oxygen deficiency, and toxic gas production—making the most comprehensive answer appropriate.