Sewage treatment systems on ship


Sewage means:

(a) Drainage and other wastes from any form of toilets, urinals, and WC scuppers;

(b) Drainage from medical premises (dispensary, sick bay, etc.) via wash basins, wash tubs and scuppers located in such premises;

(c) Drainage from spaces containing living animals; or

(d) Other waste waters when mixed with the drainages defined above.

Sewage mixed with water is also termed as Black Water.

Why sewage is prohibited from discharging in sea near coast?

As per the Annex IV of MARPOL 73/78, merchant ships are prohibited from discharging raw sewage in ports and estuaries within certain distance from nearest land. There are two reasons for this namely;

1. The bacteria in the sewage require large quantities of oxygen for their metabolism to digest sewage used by them as food, which is taken from the dissolved oxygen in water leaving less or no oxygen for other living organisms and causing unbalance in eco system.

2. There could be disease causing bacteria / virus / pathogen in the sewage discharged resulting in health problems for humans and other sea creatures.

How is this problem tackled?

The merchant ships can have any one of the three options to fulfill above requirements as per the MARPOL regulations namely;

1. Ships can have large retention tank depending on the length of voyage and the number of persons working onboard the ship. The contents of the retention tank could be discharged into open sea at least 12 nautical miles from the nearest land when the ship is enroute and moving at the speed not less than 4 knots.

2. Ships can have comminuting and disinfecting equipment along with the small retention tank depending on the number of persons onboard and the possible duration of port stay. The contents of this small retention tank could be discharged into sea not less than 3 nautical miles from the nearest land when the ship is enroute and moving at the speed not less than 4 knots.

3. Ships can have a Sewage Treatment Plant (STP) of the type approved by the Administration. Effluent water from this equipment could be discharged anywhere into sea subjected to the conditions that it should not contain any visible solids, excess chlorine used for disinfections and cause discoloration of water.

Equipment for Sewage Treatment.

For second option, the equipment consists of a grinder cum filter in which causes the sewage to break into smaller particles and separates solids from liquid. While the sludge containing solids is disinfected using chemicals and retained in the small retention tank, the liquid effluent is disinfected using chemicals filtered again using fine filters. The effluent thus free from solids and any bacteria could be re-used for flushing of the toilets.

For third option, there are several types of equipment available, approved by the administration. Some types use chemicals to disinfect the effluent water prior to discharging whereas, other types use membrane technology to separate solids and the bacteria so as to avoid use of chemicals for disinfecting the effluents. The effluent water from membrane types is 99.5% clean and could be re-cycled for any use onboard except for drinking purposes.

Does any one check the quality of effluent water discharged from sewage treatment plants?

Normally, no one checks the sewage effluent from treatment plants. The port health officer can come onboard at any port, if they have any doubts about the quality of the effluent being discharged and, ask for sample of the effluent from the sewage treatment plant installed onboard for analysis to find any objectionable contents in the effluent. If excess amounts of bacteria or chlorine etc. are found on testing in the laboratory, they may impose penalty on the ship.