True or False: Biological activity in long, sluggish-flow, flat-grade sewer lines is likely to cause odors.

The statement is true because biological activity in sewer lines, particularly those that are long, sluggish-flow, and flat-grade, indeed leads to the production of odors. In these types of sewer conditions, the flow of wastewater is slow, allowing solids to settle and facilitating the growth of anaerobic bacteria. These bacteria decompose organic matter, producing gases such as hydrogen sulfide, methane, and ammonia, which are known for their foul odors. The lack of sufficient flow or turbulence limits the ability to dissipate these gases, thereby allowing odors to accumulate and become more noticeable.

The other options suggest variability in the occurrence of odors based on certain conditions, but the continuous nature of biological processes in stagnant conditions generally leads to odor generation, irrespective of environmental factors or seasonal changes. The presence of biological activity in poorly flowing sewer lines consistently contributes to the production of odors.