Question

With reference to shipboard sewage systems,

(a) Describe the principle of operation of EACH of the following types:

(i) Flow through system;

(ii) Collection / Holding / transfer system;

(iii) Zero discharge system.

(b) Explain why sewage systems involving aerobic action are to be preferred to those with anaerobic action.

Answer 1

With reference to shipboard sewage systems:

Part (a)

Describe the principle of operation of EACH of the following types: (i) Flow through system
(ii) Collection / Holding / transfer system
(iii) Zero discharge system.

Part (b)

Explain why sewage systems involving aerobic action are to be preferred to those with anaerobic action.

Part (a)

(i) Flow-through system: A flow-through (or "flow-through treatment") sewage system treats the sewage continuously and discharges the treated effluent overboard. The sewage is collected and passed through a treatment plant (typically a biological/aerobic treatment unit) where it is broken down by bacteria, then disinfected (e.g. by chlorination or UV) and the treated effluent is discharged overboard (subject to MARPOL Annex IV - the discharge must meet the effluent standards and be at the required distance from land, or the ship must be in an area where discharge is permitted). The system operates continuously while the ship is at sea, treating the sewage as it is produced.

Part (a)

(ii) Collection/Holding/Transfer system: A collection/holding/transfer system collects and holds the sewage in a holding tank (a sewage retention tank) without treatment, and transfers it to a shore reception facility or to a treatment plant when appropriate. The sewage is retained on board (in the holding tank) and is not discharged overboard (except possibly after treatment or in permitted areas); when the ship is in port or in a discharge-prohibited area, the sewage is held and later transferred to a reception facility or treated. This system is used where discharge is not permitted (e.g. in port, in special areas) and provides a means to retain the sewage until it can be disposed of properly.

Part (a)

(iii) Zero discharge system: A zero discharge system is designed so that no sewage is discharged overboard at all - the sewage is treated and the treated effluent is either reused (e.g. for flushing, or as process water) or the solids/effluent are retained and disposed of ashore. The system may use a vacuum or gravity collection, a treatment plant (biological/aerobic) that produces a clean effluent which is recycled for flushing (a "closed-loop" system), and the residual sludge is retained and discharged to a reception facility. This system is used in environmentally sensitive areas or where discharge is prohibited, ensuring no sewage is released to the sea.

Part (b)

Why aerobic systems are preferred to anaerobic:

Aerobic sewage treatment (using oxygen-loving bacteria) is preferred because:

It produces a cleaner, more stable effluent: the aerobic bacteria break down the organic matter more completely, reducing the BOD (biochemical oxygen demand) and the suspended solids, so the treated effluent is of a higher quality and meets the MARPOL Annex IV discharge standards.
It is faster and more efficient: aerobic digestion is quicker than anaerobic, so the treatment plant can handle the sewage in a smaller volume and in less time.
It produces less odour: aerobic treatment is essentially odourless, whereas anaerobic decomposition produces foul-smelling gases (hydrogen sulphide, ammonia, methane).
It is safer: anaerobic digestion produces flammable/toxic gases (methane, hydrogen sulphide) which are hazardous; aerobic treatment avoids this.
It produces less sludge: aerobic treatment converts more of the organic matter to CO2 and water, leaving less residual sludge to be disposed of.
The effluent is suitable for disinfection and discharge, meeting the environmental standards.

Anaerobic systems are slower, produce odours and hazardous gases, and leave a poorer-quality effluent, so they are not preferred for shipboard sewage treatment.

✓ Verified Model Answer (Text Solution)

With reference to shipboard sewage systems:

Describe the principle of operation of EACH of the following types: (i) Flow through system(ii) Collection / Holding / transfer system(iii) Zero discharge system.

Explain why sewage systems involving aerobic action are to be preferred to those with anaerobic action.

Why aerobic systems are preferred to anaerobic:

Describe the principle of operation of EACH of the following types: (i) Flow through system
(ii) Collection / Holding / transfer system
(iii) Zero discharge system.