ETO ETO Exam papers 📅 Dec 2025

Exam Question

With reference to MARPOL Annex IV

(a) Draw a biological sewage treatment plant and explain the principal of operation. (8)

(b) Periodical maintenance and checks and tests required to be done to verify the effectiveness of the above system. (8)

Reference Answer

The basic principle of working of a biological treatment plant is decomposition of the raw sewage. This process is done by aerating the sewage chamber with fresh air. The aerobic bacteria survive on this fresh air and decompose the raw sewage which can be disposed of in the sea.
A typical aerobic STP consists of 4 chambers
Primary: The raw sewage enters the primary chamber via a coarse mesh filter where large solids are broken down
Aeration: From primary chamber sewage enters the aeration compartment where it is digested by aerobic bacteria and microorganisms in the presence of oxygen.

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