To lessen the oily content in bilge water, which can be discharged from the ship, MARPOL has a regulation under ANNEX I which limits the oil content in the bilge water that vessel can legitimately discharge into the sea. It is now a requisite for all vessels to have an oil discharge monitoring and control system along with an oil filtering apparatus known as the Oily Water Separator (OWS).
As the name specifies, the role of oily water separator is to separate maximum amount of oil particles from the water to be discharged overboard from engine room or cargo hold bilges, oil tanks and oil contaminated spaces. As per the regulation, the oil content in the water processed from the OWS must be less than 15 parts per million of oil.
Construction and Working of OWS
OWS consists of mainly three parts:
- Separator unit
- This unit consists of catch plates which are inside a coarse separating compartment and an oil collecting chamber.
- Here the oil having a density which is lower than that of the water, which makes the former rise into the oil, collecting compartment and the rest of the non-flowing oil mixture settle down into fine settling compartment after passing between the catch plates.
- After a period of time more oil will separate and collect in the oil collecting chamber. The oil content of water which passes through this unit is around 100 parts per million of oil.
- A control valve (pneumatic or electronic) discharges the separated oil in to the designated OWS sludge tank.
- Heater may be incorporated in this unit for smooth flow and separation of oil and water.
- First stage helps in removing some physical impurities to achieve fine filtration in the later stage.
- The Filter unit
- This is a separate unit whose input comes from the discharge of the first unit.
- This unit consists of three stages – filter stage, coalescer stage and collecting chamber.
- The impurities and particles are separated by the filter and are settled at the bottom for removal.
- In second stage, coalescer induces coalescence process in which oil droplets are joined to increase the size by breaking down the surface tension between oil droplets in the mixture.
- These large oil molecules rise above the mixture in the collecting chamber and are removed when required.
- The output from this unit should be less than 15 ppm to fulfil legal discharge criteria.
- If the oil content in water is more than 15 ppm then maintenance work such as filter cleaning or renewal of filters is to be done as required.
- Oil Content Monitor and Control Unit
- This unit functions together in two parts – monitoring and controlling.
- The ppm of oil is continuously monitored by Oil Content Monitor (OCM); if the ppm is high it will give alarm and feed data to the control unit.
- The control unit continuously monitors the output signal of OCM and if alarm arises, it will not allow the oily water to go overboard by means of operating 3 way solenoid valve.
- There are normally 3 solenoid valves commanded by control unit. These are located in the first unit oil collecting chamber, second unit oil collecting chamber and one in discharge side of the oily water separator which is a 3 way valve.
- The 3 way valve inlet is from the OWS discharge, where one outlet is to overboard and second outlet is to OWS sludge tank.
- When OCM gives alarm, 3 way valve discharges oily mixture in the sludge tank.