Preliminary treatment aims to quickly remove solids that could clog or damage equipment early in the process. Screening traps large debris, grit removal settles out sand and heavy inorganic material, and oil and grease removal targets fats and oils that can cause blockages or odor issues in subsequent steps. Biological digestion, on the other hand, is a secondary treatment process that biologically breaks down organic matter in sludge, typically in a digestion tank after the initial physical treatment. It is not part of preliminary treatment, which is why it is the correct choice.

A depressed sewer is a section where the invert (the bottom inside of the pipe) is intentionally lowered below the elevations of the adjacent sewer sections so the line can pass beneath an obstacle. In gravity sewer systems, maintaining a steady slope is crucial for sewage to flow by gravity. When an obstacle—such as a road, railway, or other structure—would hinder continuing at the same grade, a depressed section lets the pipe drop to a lower level, then continue at the proper slope after clearing the obstacle. This approach avoids awkward rerouting or pumping and keeps the system flowing smoothly. It’s different from other terms like a relief sewer, which provides an alternate path during overloads; an outfall sewer, which carries effluent to a downstream receiving water; and a storm overflow sewer, which handles excess stormwater. The essential idea here is the pipe is deliberately lowered compared with adjacent sections to pass beneath an obstacle while preserving gravity flow.

Branch sewers collect wastewater from a relatively small area, such as a few streets or a single block, and discharge into a sub-main. This setup is what allows local areas to feed into a larger conduit that carries the combined flow toward the main sewer. In the hierarchy, a sub-main then receives from several branches and feeds the main sewer, the main sewer collects from sub-mains and transports toward the treatment system, and an interceptor is the largest line that carries wastewater over long distances to treatment. Because the scenario specifies a line serving a small area that feeds into a sub-main, the branch sewer is the best fit.

Maintenance strategies determine when and how to service equipment to prevent failures and manage risk and cost. The four commonly recognized types are preventive maintenance, corrective maintenance, predictive maintenance, and Reliability Centered Maintenance. Preventive maintenance involves scheduled tasks designed to prevent failures before they happen. Corrective maintenance is performed after a failure to restore service. Predictive maintenance uses data from monitoring equipment condition to estimate remaining useful life and schedule maintenance just in time. Reliability Centered Maintenance applies a systematic analysis of functions, failure modes, and consequences to decide the most effective maintenance tasks, often guiding and optimizing the use of the other strategies. The other options swap in reactive, risk-based, or proactive ideas, which aren’t the standard fourth type alongside preventive, corrective, and predictive maintenance in this framework.