Which of the following pumps is most commonly used for pumping raw sewage containing suspended solids?

Question

Which of the following pumps is most commonly used for pumping raw sewage containing suspended solids?

1. A. Reciprocating pump
2. B. Diaphragm pump
3. C. Pneumatic ejector
4. D. Centrifugal pump with open impeller (Correct answer)

Correct Answer

Option D — Centrifugal pump with open impeller

Detailed Solution & Explanation

The correct answer is Centrifugal pump with open impeller.

Key Points

• Raw sewage carries rags, grit, plastics and other suspended solids, so the governing requirement for a sewage pump is that it must pass solids without clogging.
• The centrifugal pump with an open (or non-clog) impeller meets this. Open and semi-open impellers have few or no shrouds and wide passages, allowing solids through, and the pump handles large volumes at moderate head with simple, robust construction.
• The alternatives are unsuitable for raw sewage in bulk: reciprocating and diaphragm pumps have valves and tight clearances that foul readily, and a pneumatic ejector is used only for small flows, typically from a basement or an isolated low-lying property.

Additional Information

• The reason a closed impeller is avoided is straightforward — its narrow shrouded passages are exactly where rags accumulate and bind.
• Sewage pumps are usually installed in a wet well/dry well arrangement, with the pump in a dry chamber beside the wet well, and are sized so that at least one standby unit is always available.
• Pumps must be set so the suction is under positive head wherever possible, since a pump handling sewage should never be asked to prime itself against a suction lift.
• Diaphragm pumps do retain a niche in this field — they are commonly used for pumping sludge and thick slurries, not raw sewage, and pneumatic ejectors for isolated small inflows.