Oily water separator (OWS), bilge water separator is a device used to separate oil from oily water mixtures and from the emulsion. Bilge separators are necessary aboard vessels to prevent discharge of oil overboard while pumping out bilges or while cleaning oil tanks. If the set limit of the oil content is exceeded, the effluent is automatically recirculated to the collecting tank, or the separator is stopped.

OWS consists of mainly three segments:

1. 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) releases the separated oil into the designated OWS sludge tank. The heater may be incorporated in this unit for smooth flow and separation of oil and water. A heater may be incorporated in this unit either on the middle or sometimes in the bottom part of the unit (depending upon the area of operation and capacity of the separator equipment) for smooth flow and separation of oil and water. The first stage helps in removing some physical impurities to achieve fine filtration in the later stage.

2. 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 the 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. A freshwater inlet connection is also provided to the filter unit to clean and flush the filter. This is usually done before and after the operation of an oil separator unit.

3. 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 an 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 the control unit. These are located in the first unit oil collecting chamber, second unit oil collecting chamber and one in the 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 the second outlet is to OWS sludge tank. When OCM gives an alarm, 3-way valve discharges oily mixture in the sludge tank.

The complete unit is first filled with clean water; the oily water mixture is then pumped through the separator inlet pipe into the coarse separating compartment. Here some oil, as a result of its lower density, will separate and rise into the oil collection space. The remaining oil and water mixture now flow down into the fine separating compartment and moves slowly between the catch plates. More oil will separate out onto the underside of these plates and travel outwards until it is free to rise into the oil collecting space. The almost oil-free water passes into the central pipe and leaves the separator unit. The purity at this point will be 100 parts per million or less. An automatically controlled valve releases the separated oil to a storage tank. Air is released from the unit by a vent valve. Steam or electric heating coils are provided in the upper and sometimes the lower parts of the separator, depending upon the type of oil to be separated. Where greater purity is required, the almost oil-free water passes to a filter unit. The water flows in turn through two filter stages and the oil removed passes to oil collecting spaces.

The first-stage filter removes physical impurities present and promotes some fine separation. The second-stage filter uses coalescer inserts to achieve the final de-oiling. Coalescence is the breakdown of surface tension between oil droplets in an oil and water mixture which causes them to join and increase in size. The oil from the collecting spaces is drained away manually, as required, usually about once a week. The filter inserts will require changing, the period of useful life depending upon the operating conditions. Current legislation requires the use of a monitoring unit which continuously records and gives an alarm when levels of discharge in excess of 15 parts per million occur.

The oily water separator (OWS) is a very important piece of equipment carried on board. It is used to ensure that water is discharged overboard within legal limits. The OWS must be maintained in full working order and operated according to MARPOL regulations. International legislation relating to oil pollution is becoming more and more stringent in the limits set for oil discharge.

One of the main causes of oily water filtering equipment malfunction is the effect of bilge water containing cleaning agents. Detergent-based cleaning fluids can produce chemically stabilised oil emulsions which cannot be separated on board ship by the gravity alone. The best way to improve the performance of oily bilge water separating equipment is to avoid the use of surfactant-based cleaning materials.

There are various other types of separators including oil cleaning, waste fuel recovery, bilge water treatment, crankcase gas cleaning, exhaust gas Recirculation (EGR) water treatment and exhaust gas cleaning.