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MEO Class II: MEKG MEKM MEP MET SC&S SSEP 📊 Trends
Q8 (16 Marks) General 🔥 Repeated 12x in exams
MEKG • Written Exam

With reference to Vacuum Sewage Systems:

(i) Sketch & Describe a Vacuum sewage system.

(ii) State the advantage of Vacuum sewage system.

(iii) State the different causes of dropping vacuum.

Appeared In: Aug 2025Apr 2024Mar 2020Jan 2020Oct 2019Sep 2019Aug 2019Jun 2019Mar 2019Jan 2019Oct 2018Sep 2018

✓ Verified Model Answer (Text Solution)

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Part (a)

The system uses vacuum to transport sewage from toilets and urinals to collecting units. There is a vacuum only in the piping network and the toilets, urinals etc. remain under atmospheric pressure unless when the flush button is pushed. Each toilet is connected to the vacuum piping. The connection is shut all times, except during the toilet flushing. When the toilet is flushed, its discharge valve opens the connection to the vacuum piping network for a pre-set seconds and the contents of the bowl will be evacuated. When the vacuum tank is full, the contents is automatically pumped into larger storage tanks that are maintained under normal atmospheric pressure.

(b) Advantages of a Vacuum Sewage System:

  • The vacuum sewage system uses 85–90% less water for flushing compared to conventional systems, requiring very little flushing water.
  • Toilets can be positioned more flexibly, including below the level of the holding tank, which is not feasible with gravity-fed systems.
  • The system uses smaller diameter piping, reducing material and space requirements.
  • The reduced water usage contributes to overall water conservation, making the system environmentally friendly.
Part (c)

Causes of Dropping Vacuum in a Vacuum Sewage System:

  • If the pump is pumping foam instead of liquid, this will be evident due to severe vibration. Add water to the tank and try again. If adding water does not help, reduce the level of foam by pouring antifoam agent into the tank (1 cup per 2 cubic metres of foam and sewage).
  • Check that shut-off valves are fully open and not clogged.
  • If the direction of rotation of the pump is wrong, change wiring accordingly.
  • Close the valves that isolate the collecting unit from the piping system and start the pump again. If vacuum now builds up, there must be a leak in the piping system.
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