Oily Water Separator (OWS) is a piece of special equipment for shipping or the marine industry. It is used to separate oil and water mixtures into their separate components. They are found on board ships where they are used to separate oil from oily wastes such as bilge before draining sewage into the environment. These sewage discharges must comply with the requirements of Marpol 78/73.

Bilge water is an almost inevitable product of ship operations. Oil leaks from running machines such as diesel generators, air compressors, and the main propulsion engine. Modern OWS has alarms and automatic shut-off devices that when the amount of wastewater oil storage exceeds a certain limit, activate.

Target

The main purpose of ship oily water separators (OWS) is to remove oil and other pollutants that can be harmful to the oceans. The International Maritime Organization (IMO) publishes the regulations through the Marine Environment Protection Committee (MEPC).On July 18, 2003, the MEPC issued new regulations that each ship must follow after this date. This document is known as MEPC 107 and provides details on the guidelines and specifications of pollution prevention equipment for ship machinery space. Each OWS must be able to access clean water below 15 ppm of type C oil or heavily emulsified oil and any other contaminants that may be found. All OCM monitors must be anti-tampered with. Also, OCM must be enabled whenever OWS is cleared. OWS must be able to remove contaminants as well as oil. Some of these contaminants include lubrication, crop cleaning, combustion soot, fuel oil, rust, sewage, and several others that can be harmful to the ocean environment.

The water content of the bottom of the ship

The Bilge area is the lowest area on the ship. Bilge water collected here includes wastewater or water left over from boilers, water collection, drinking water, and other places where water cannot overflow. However, bilge water does not only involve drainage. Another system that is discharged to the Bilge system comes from the propulsion zone of the ship. Fuels, lubricants, hydraulic fluid, antifreeze, solvents, and cleaning chemicals are discharged in small amounts into the engine room. OWS is intended to eliminate a large proportion of these pollutants before being discharged into the environment (beyond the sea).

Design and operation

All OWS equipment, new or old, can separate oil and water automatically, and produce clean water for over-discharge containing no more than 15 parts per million oil. OWS equipment is approved by testing it with special cocktails mixed with oil and water. At first, these compounds were very simple, basically more than a mixture of clean water and diesel fuel, but have become more advanced under the Marpol MEPC 107 (49). A large number of these equipment models, manufacturers, and equipment types begin with a kind of gravity separation from Bilgewater. Decanting is referred to the sitting of oil and water. It should meet the 15 ppm standard, but it does not always. So, manufacturers add some features to their products to ensure meeting this criterion. The separation that takes place inside the OWS allows the oil that floats upwards to be automatically trapped in a sludge tank or dirty oil storage tank.

OWS must be equipped with an oil content meter (OCM) that samples the water before discharging to assess the oil amount. If the oil amount is below 15 ppm, OCM lets water discharge. If it is more, an alarm will be activated by OCM which moves a three-way valve. This valve recirculates the water to a tank on the OWS suction side.

It uses a light sensor for its function. The OCM takes a little sample of the OWS overboard discharge line and shines a small ray of light through the sample to the light sensor. The result from light analysis indicates if the OWS overboard water is dischargeable or not. Since the oil amount in the sample scatters the light, the change in the light signal in the sensor shows the existence of oil in the sample. When the signal indicates the approximate amount of 15 ppm oil, it will lead the water to the OWS cycle again. The sensor calibration is usually tested in the laboratory, but it can be done on the vessel via a three-sample liquid. If the OCM ends up sampling a certain amount of heavy oil, the OCM is neutralized and needs to be washed or cleaned.

The OCM cleaning process is done in two ways:

1. Running fresh water through the OCM via a permanent connection
2. Opening the OCM sample area, scrubbing it with a bottle brush.

At the end of the process, the removed water from OWS flows to collecting spaces and follows two stages:

1. Physical impurities and particles are eliminated via the first-stage filter.
2. Then, final oil separation is done by a coalescer filter in the second-stage filter.

In the coalescence process, every oil droplet comes together and makes a single, big droplet. Therefore, two phases of water and oil are created. In the next step, the oil is drained away automatically or manually. In the vast majority of modern ships, this process is automated.

Oil record book

One of the essences of all cargo vessels is preparing an oil record book. Based on MARPOL Convention, the chief engineer should record all oil or sludge transfers and discharges within the vessel. The reason for this record is security requirements. The authorities check if the crew in a vessel has performed ani illegal oil discharges at sea.

In the first part of this book, you should enter the date, operational code, and item number in the appropriate columns. It is crucial to enter each operation record without any delay.

Types of OWS

• Gravity plate separator

In this type of OWS, there are sets of plates with an affinity for oil, not water. The water flows through the plates and the oil content coalesces on the underside of the plates. Then it flows off the plates and comes together at the top of the chamber. The separated oil will be transferred to the water-oil tank and then discharged to a treatment device on the sea shore. This type of Oily Water Separator use is common in ships, but it has some problems which do not let a complete oil and water separation.

If an oil droplet diameter is more than 20 micrometers, it will pass through the plates and cannot be removed from the water. Various oil types in bilge wastes can make limitations the separation efficiency. Especially when high viscous oils such as bunker which is the residual fuel oil used in steam power plants exist, the efficiency will be lower. The fouled plates should be removed immediately.

• Electrochemical

The purification of wastewater from oil and contaminants with this type of separator is still developing. The process is called electrochemical emulsification. In this method, electrolytic bubbles are created and attract pollutants such as sludge. Then, the combination of bubbles and pollutants is carried out to the treatment chamber. At the top of this chamber, oil and other particles are transferred to a waste oil tank. 

• Bioremediation

In this type of oily water separation device, microorganisms treat contaminated water. The need for these organisms is a nutrient-rich environment including hydrocarbons, contaminants, and oxygen.

Some pilot-scale studies use these remedies in one stage of the purification. There was a plate separator that removed the huge popularity of contaminants. It treated low concentrations o pollutants with organic contaminants like glycerol, solvents, jet fuel, detergents, and phosphates. When the process was done, carbon dioxide, water, and organic sludge remained.

• Centrifugal

In this type, the separation process is done by centrifugation. This device is called, a centrifugal water-oil separator, an oil-centrifugal oil separator, or a liquid-centrifugal liquid separator. A cylindrical container is fixed into another container, it rotates and the cylinder inside rotates with it. There are two different liquid phases in this process: low density, and high density. The low density is usually oil, and the high one is water. Water is collected at the edge of the rotating vessel, and the oil collects at the axis of rotation. Oil-water centrifuge separators are used to filter diesel and lubricating oils by removing waste particles and impurities.