In wastewater engineering, which pumping arrangement is most suitable for conveying sewage that contains appreciable suspended solids without clogging the pump or impeller?

Introduction / Context:
A common design challenge in wastewater engineering is selecting pumping equipment that can handle raw sewage containing rags, grit, and other solids without frequent clogging. This question tests knowledge of pump types and the special case where air pressure is used to move sewage without a rotating impeller in contact with solids.

Given Data / Assumptions:

• Sewage contains suspended and settleable solids that may clog narrow passages.
• Continuous, reliable operation with minimal maintenance is desired.
• Pumped medium is a mixture of liquid sewage and entrained solids.

Concept / Approach:
Many pumps (e.g., centrifugal, gear, reciprocating) have moving parts and small clearances that can become obstructed by fibrous materials and solids. A pneumatic ejector (also called an air-operated ejector) uses compressed air to displace sewage from a sealed receiver into the discharge line. Because no impeller or piston must pass solids through tight clearances, the risk of clogging is greatly reduced.

Step-by-Step Solution:
1) Identify whether the pumping method exposes solids to narrow passages or rotating elements.2) Centrifugal, reciprocating, gear, and propeller pumps all feature moving elements with finite clearances that can snag rags or plug with grit.3) A pneumatic ejector collects sewage in a sump or receiver. When a level sensor trips, compressed air is admitted.4) The compressed air increases pressure in the receiver and pushes the sewage slug into the force main, with solids riding along.5) Because solids do not encounter an impeller or check-valve passages of small size, clogging likelihood is minimal.

Verification / Alternative check:
Facilities with chronic clogging often adopt ejectors for grit-heavy or rag-laden flows, especially at small to medium capacities and where screenings are not perfect. This operational history supports the selection.

Why Other Options Are Wrong:

• Centrifugal pump: Standard sewage impellers can pass some solids but still clog with fibrous materials; grinders add complexity and maintenance.
• Reciprocating pump: Valves and small clearances are prone to blockage by rags and grit.
• Gear pump: Tight gear clearances make it unsuitable for solids-laden sewage.
• Submersible propeller pump: Intended for high-flow, low-head applications such as storm water; not ideal for mixed solids without pretreatment.

Common Pitfalls:

• Assuming “any sewage pump” will be non-clog; solids size and fibrous content matter.
• Confusing ejectors with air-lift pumps; ejectors use a sealed receiver and compressed air displacement.

Final Answer:
pneumatic ejector