As per MARPOL ANNEX - I , how oil filtering equipment are used on board ships to control the discharge of bilge oil.

Bilge oil separators control the discharge of oil overboard when bilges are being pumped out from the engine room, as required by ANNEX-1 discharge of oily water into the sea is permitted through an oil discharge monitoring and control system together with oil filtering equipment.




The oil filtering equipment ensures that the oil content, while discharging bilge water, does not exceed 15 ppm. If the oil content exceeds 15 PPM, the bilge water he is discharged into the bilge holding tank. The oil residue are collected in separate sludge oil tank and should either be incinerated or discharged into shore reception tanks in port.

OWS working process :
  • The separation of oil water in the separator takes place in two stages.
  • In the first stage, the separation is by gravity, whereas in the second stage, the separation is by coalescing filters.
  • The bilge pump delivers clean sea water to the first stage of the separator through the inlet valve.
  • The vent is kept open till all the air is removed from the separator and water issues out.
  • Oily water is delivered to the separator. As it enters through the inlet valve, oil separates by gravity and collects at the top of the separator.
  • The remaining oil sticks to the baffle plate as the water passes to the second stage. Oil droplets from the plates tend to travel upwards to the oil collection space.
  • Oil from the top of the chamber is automatically drained to the oil tank where the level reaches the bottom of the sensor as shown.
  • The oil drain valve from the top of the first stage separator is a diaphragm controlled piston valve, control air is supplied to the diaphragm through a solenoid operated pilot valve.
  • The capacitance probe census the oil quantity in the collection space and energises the solenoid through the control switch.
  • Water passes downward from the first to the second stage coalescer through a central pipe.
  • The filter in the right chamber removes solids and some oil.
  • Coalescing filters in the left chamber takes out the remainder of the oil in the form of small droplets, which coalesce to form larger drops.
  • Thes rises to the oil collecting space.
  • Oil content of the final discharge is below 15 ppm.



  • In case the discharge of water after the second stage is more than 15 ppm, the monitor senses this and gives an audible and visual alarm in the engine room.
  • At the same time, the monitor sends a signal to the three way valve on the overboard discharge line, which closes the overboard and opens to the bilge tank.
  • The sensor reverts to normal operation once the oil content drops below 15 ppm. The test cocks can be used to ascertain the levels of oil and water manually.
  • A spring loaded valve is usually fitted on the both discharges.
  • However, it is essential that a relief valve be provided on the shell or incoming mixture line to prevent over pressure and accidental discharge to a confined space or overboard under all working conditions.
  • Such a relief valve should be preferably led back to the suction side of the supply pump or to an overflow tank.
  • The usual working pressure for the separator is in the region of two bar, i,e. the pressure at which the spring loaded water a discharge valve is set.
  • The relief valve is set at about two and a half bar approximately.
Oil content monitoring system :



An oil content monitor is a device for real time sampling and measuring of the oil content in a moving stream of water.
The new oil content monitors for oily water separators perform several functions including preventing illegal " over limits" discharge that occurs when the oil content is above 15ppm, by either shutting down the separator or by putting the separator into a recirculation or recycle mode where the oily bilge water is sent back to the oily wastes holding tanks.



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