A retention and holding tank is required where no discharge of treated or untreated sewage is allowed in a port area. The sewage is pumped out to shore reception facilities or overboard when vessel is proceeding on passage at sea as per the instructions in the MARPOL 73/78, Annex – IV.
Straight holding tank for retention of sewage during the period of a ship’s stay in port where of a size large enough to contain not only the actual sewage but also the flushing water. Each flush delivered 5 liters of seawater. Passenger vessels or ferries with automatic flushing for urinals required very large holding tanks. Very little flushing water is required for zero discharge system sewage treatment plants.
Problems resulting from the retention of untreated wastes relate to its breakdown by anaerobic bacteria. Clean breakdown by aerobic organisms occurs when there is amble oxygen. In the conditions of plain retention tank where there is no oxygen, anaerobic bacteria and other organisms thrive. These cause putrefaction, probably with corrosion in the tank and production of toxic and flammable gases.
The Eslan type sewage treatment plant, also known as zero discharge system has an initial reception chamber in which separation of liquid and solid sewage takes place. Wastes drop onto a moving perforated rubber belt (driven by an electric motor) which the liquid passes through but solids travel with the belt to fall into a caustic treatment tank. Solids are then transferred by a grinder pump to he sullage or holding tank. The liquid passes via the perforated belt to treatment tanks which contain chlorine and caustic based compounds. These chemicals make the liquid effluent acceptable for use as a flushing fluid. The Pneupress arrangement which supplies liquid for flushing the toilets can deliver re-circulated fluid or, when the vessel is on passage, seawater.
Capacity of the holding tank is 2 liters per person per day. The tank is pumped out at sea, or to shore if the ship is in port for a long period. Tank size is small because liquid effluent passes mainly to the flushing system. Excess overflows to the sullage tanks.
“Marine Auxiliary Machinery” by H.D. McGeorge