**3. Current oil–water separation techniques**

Typically, OWS is made up of three segments, which are separator unit, filter unit, and oil content monitor and control unit. The separator and filter units are included as treatment units, where many designs and different principles are applied. The gravity and centrifugal separators are commonly used as the first stage of the treatment, followed by other separation techniques, which is called as polishing treatment. The examples of the polishing unit are flotation, coagulation and flocculation, filtration, biological treatment, as well as absorption and adsorption [16]. Normal techniques, such as gravitational and centrifugation, are used for oily wastewater that has two distinct phases; meanwhile, addition of chemical or biological de-emulsification is required for separation of emulsified oily bilge water [17].

#### **3.1 Gravitational method**

Typically, oily bilge water treatment onboard starts with a gravitational method in order to remove heavy fractions and lighter fractions based on density difference. In this method, coalescing materials made of oleophilic polymer in the form of loose-packed media or parallel plate are used to attract the oil droplets to adhere to the plate [16]. Examples of oleophilic polymer used as the coalescing plate separators are polyethylene, fiberglass, and nylon [18].

The free-moving dispersed oil droplets continue to adhere to the plate or media until it can break from the coalescing material and float up to the surface of the tank. The presence of the oil detected by the sensors then automatically triggers the OWS to remove the collected oil to a waste oil tank. However, this method can only be effective when the phase of the oil and water is separated distinctively [19]. In other words, in some instances, the gravitational method is not suitable since the bilge water typically consists of emulsified oil formed due to the chemical emulsifiers (solvents and cleaning agents) as well as mechanical means such as ship's motion and transfer system pump [16]. **Figure 1** shows the gravitational separator process.

service life might last long, reducing the cost of maintenance and repair. Centrifugal separators are also more compact and require smaller bilge water holding tanks [16]. However, high capital cost is needed for the centrifuges, and regular maintenance must be done since large horsepower motors are used during the process. **Figure 2** shows an example of manufactured separator (PureBilge by Alfa

In the inlet stream of the unit, the bilge water is accelerated by the XLrator with less shearing and foaming in order to prevent the oil drops from separating and further emulsion formation. Then, it flows into the separator, in which coalescence occurs due to high centrifugal force. Flocculation of small oil drops takes place and

Separation of oil by flotation occurs due to the difference in density of oil and water, where water is denser than oil, forming a scum layer on top of the water. Floatation technique can be divided into many different techniques, including

Electroflotation separates oil from water through electrochemical reactions by electrolysis, where tiny bubbles produced from electrolysis will cause the pollutants to

flocculants is added to promote bigger flocs for easier separation [21].

electroflotation, froth flotation [22], and dissolved air flotation (DAF).

Laval).

**Figure 2.**

**Figure 1.**

*Gravitational separator [20].*

*Oil–Water Separation Techniques for Bilge Water Treatment*

*DOI: http://dx.doi.org/10.5772/intechopen.91409*

*Patented Alfa Laval XLrator [21].*

**3.3 Flotation**

**151**

As can be seen from the figure above, as the oily bilge water flows through the parallel plate, oil globules are formed and float up to the surface to form oil layer. Oil skimmer is used to skim off the oil layer. Then, oil discharge valve and purge water valve are opened, where the oil is removed from the unit by the purge water.

### **3.2 Centrifugation method**

Centrifugal separators are the alternative option for the gravity separators. The same principle is applied, in which the oil is separated based on the different density of oil and water as well as coalescence of the oil droplets. Somehow, the centrifugal acceleration causes the gravity to increase more, and the coagulation and flocculation processes are enhanced in order to separate the emulsified oil. This type of separator has many advantages as compared to gravity OWS. Since it can separate more oil from the bilge water, including emulsified oil, less oil content is loaded to the next treatment step, which is usually called a polishing unit. Thus, the polisher's *Oil–Water Separation Techniques for Bilge Water Treatment DOI: http://dx.doi.org/10.5772/intechopen.91409*

**Figure 1.** *Gravitational separator [20].*

Other than that, surfactant is one of the significant chemicals contained in the bilge water. The mixtures of oil and surfactants may cause higher toxicity since the oil and surfactants alone are toxic themselves. This may be due to the synergistic effects or the crude oil that has been dissolved, causing it to be consumable for the

Typically, OWS is made up of three segments, which are separator unit, filter unit, and oil content monitor and control unit. The separator and filter units are included as treatment units, where many designs and different principles are applied. The gravity and centrifugal separators are commonly used as the first stage of the treatment, followed by other separation techniques, which is called as polishing treatment. The examples of the polishing unit are flotation, coagulation and flocculation, filtration, biological treatment, as well as absorption and adsorption [16]. Normal techniques, such as gravitational and centrifugation, are used for oily wastewater that has two distinct phases; meanwhile, addition of chemical or biological de-emulsification is required for separation of emulsified oily bilge

Typically, oily bilge water treatment onboard starts with a gravitational method in order to remove heavy fractions and lighter fractions based on density difference. In this method, coalescing materials made of oleophilic polymer in the form of loose-packed media or parallel plate are used to attract the oil droplets to adhere to the plate [16]. Examples of oleophilic polymer used as the coalescing plate

The free-moving dispersed oil droplets continue to adhere to the plate or media until it can break from the coalescing material and float up to the surface of the tank. The presence of the oil detected by the sensors then automatically triggers the OWS to remove the collected oil to a waste oil tank. However, this method can only be effective when the phase of the oil and water is separated distinctively [19]. In other words, in some instances, the gravitational method is not suitable since the bilge water typically consists of emulsified oil formed due to the chemical emulsifiers (solvents and cleaning agents) as well as mechanical means such as ship's motion and transfer system pump [16]. **Figure 1** shows the gravitational separator process.

As can be seen from the figure above, as the oily bilge water flows through the parallel plate, oil globules are formed and float up to the surface to form oil layer. Oil skimmer is used to skim off the oil layer. Then, oil discharge valve and purge water valve are opened, where the oil is removed from the unit by the purge water.

Centrifugal separators are the alternative option for the gravity separators. The same principle is applied, in which the oil is separated based on the different density of oil and water as well as coalescence of the oil droplets. Somehow, the centrifugal acceleration causes the gravity to increase more, and the coagulation and flocculation processes are enhanced in order to separate the emulsified oil. This type of separator has many advantages as compared to gravity OWS. Since it can separate more oil from the bilge water, including emulsified oil, less oil content is loaded to the next treatment step, which is usually called a polishing unit. Thus, the polisher's

exposed organisms [12–15].

*Resources of Water*

water [17].

**3.1 Gravitational method**

**3.2 Centrifugation method**

**150**

**3. Current oil–water separation techniques**

separators are polyethylene, fiberglass, and nylon [18].

service life might last long, reducing the cost of maintenance and repair. Centrifugal separators are also more compact and require smaller bilge water holding tanks [16]. However, high capital cost is needed for the centrifuges, and regular maintenance must be done since large horsepower motors are used during the process.

**Figure 2** shows an example of manufactured separator (PureBilge by Alfa Laval).

In the inlet stream of the unit, the bilge water is accelerated by the XLrator with less shearing and foaming in order to prevent the oil drops from separating and further emulsion formation. Then, it flows into the separator, in which coalescence occurs due to high centrifugal force. Flocculation of small oil drops takes place and flocculants is added to promote bigger flocs for easier separation [21].

## **3.3 Flotation**

Separation of oil by flotation occurs due to the difference in density of oil and water, where water is denser than oil, forming a scum layer on top of the water. Floatation technique can be divided into many different techniques, including electroflotation, froth flotation [22], and dissolved air flotation (DAF). Electroflotation separates oil from water through electrochemical reactions by electrolysis, where tiny bubbles produced from electrolysis will cause the pollutants to

float to the water body surface [23]. In froth flotation, the separation takes place when the oil adheres onto the fine bubbles generated when air is introduced into the system. Surfactant is added to adsorb the air or water interface of the bubbles of air with the head groups (hydrophilic) in the water and the tail groups (hydrophobic) in the air. Hence, when the bubbles rise through the solution, the oil will concentrate on the bubble surfaces and foam is formed [24].

Suspended growth process maintains the microorganisms in liquid suspension by proper mixing methods. Meanwhile, attached growth attach the microorganisms to an inert packing material, where the wastewater will flow past the biofilm to remove the organic materials [29]. For OWS onboard, biofilm is used, in which the bacteria are attached to a synthetic support media. In this bioreactor, aerators are installed under the media to supply oxygen to the bacteria for bacterial growth as well as for oxidation of the organic contaminants to take place. Other than that, a clarifier is needed in order to remove the biomass formed at the end of the

Biological treatment, with flexible operation, simple maintenance and management, as well as stable effluent quality [26], is indeed a suitable method since no waste oil is produced by the process [16]. Small oil droplets of emulsified oil, which are hardly removed by physical and chemical treatment, can be degraded easily by the bacteria [31]. However, CO2 will be produced from the process, resulting in increment in greenhouse gases. Even though the operating cost is low, the capital

**Techniques Advantages Disadvantages References**

oils from water

problem

device

operator 2. High operating cost

operator

Not effective to separate emulsified

1. Use huge horsepower motors 2. Require frequent maintenance 3. High capital cost for centrifuges

1. Repairing and maintenance

2. Issue in manufacturing of the

1. Need to be operated by skillful

3. Produce high amount of sludge which then needs to be disposed 4. A lot of experiments might be needed due to complexity of wastewater composition

1. Loading spikes can occur 2. High capital cost

1. Need to replace the media frequently (if necessary)

3. Need to be operated by skillful

[16, 19]

[26, 27]

[16, 27, 28, 33]

[16, 27]

[16]

[16]

processes [16].

Coagulation and flocculation

Absorption and adsorption

**Table 1.**

**153**

Gravitational Effective for discrete phases of oil and water

waste

Flotation 1. Less investment needed

Biological 1. Able to effectively degrade

emulsified oil 2.Can remove other organic

pollutants 3. No waste oil produced 4.Mechanically simple 5. Low operating cost

vessels 2. Relatively compact 3. Low capital and operation

costs

*Comparison of the separation techniques.*

2. Do not require large bilge water holding tanks 3. Produce small amount of

*Oil–Water Separation Techniques for Bilge Water Treatment*

*DOI: http://dx.doi.org/10.5772/intechopen.91409*

2. Low energy consumption 3. Easy to maintain 4. Produce less sludge

1.Can remove emulsified oil and dissolved oil 2. High adaptability

organic pollutants including

1. Suitable for less than 400 GT

4. Require low maintenance

4.Can be run without continuous man-hours operation and supervision

Centrifugation 1.More compact

Meanwhile, dissolved air flotation (DAF) introduces micro gas bubbles into the flotation chamber that has been formed when water is saturated with gas under pressure [25]. The oil droplet will spread around the gas, and conglomerate will continue to rise to the surface of the solution. The advantages of flotation treatment are the following: less investment needed, low energy consumption, and easy to maintain [26]. However, the statement contradicts with Yu et al. [27] who stated that flotation requires high energy consumption and has repairing and maintenance problem as well as issue in manufacturing of the device.

### **3.4 Coagulation and flocculation**

According to Yu et al. [27], coagulation process is a robust oil–water separation technology because it is able to separate dissolved and emulsified oil; hence, it is vastly applied in the latest oily wastewater treatment method [28]. In the coagulation process, coagulant, a chemical substance, is added to the wastewater to destabilize the charge of colloidal particles in the solution [29] which is too tiny for gravitational settling. When the particles are destabilized, larger flocs are formed making it easier to settle and then are skimmed off to the clarifier or sludge thickener.

Anyhow, many experiments might be needed if the coagulation method is going to be used in treating oily wastewater. This is because of the complexity of oily wastewater, making it hard to choose the most suitable coagulants for effective separation of oil and water to take place [27]. In a study done by Zeng et al. [30], oil removal efficiency is improved up to 99% when aggregation of poly-zinc silicate (PZSS) with anionic polyacrylamide (A-PAM) is used as the coagulating and flocculating chemicals. Somehow, higher costs are needed, and it could cause water bodies'secondary pollution and difficulties to the next process [27].
