1. What is Oily Wastewater?

Oily wastewater refers to water containing fats (fatty acids, soaps, fats, waxes, etc.) and various oils (mineral oils, animal and plant oils). Its characteristics include high COD (Chemical Oxygen Demand) and BOD (Biochemical Oxygen Demand) levels, a distinct odor, color, flammability, and susceptibility to oxidation. Oily wastewater is generally lighter than water and poorly soluble in it. It is a significant industrial wastewater with widespread impact and severe harm, mainly causing:

- Water quality deterioration and harm to aquatic life

- Risks to human health

- Air pollution

- Negative effects on crop production

- Damage to natural landscapes and clean water sources

Due to its pollution potential, China has set the maximum allowable discharge concentration for oily wastewater at 1 mg/L.

2. Forms of Oil in Water

Suspended Oil: Particle size ≥100 μm; it floats quickly to the surface when left undisturbed, forming a continuous oil film on the water's surface.

Dispersed Oil: Particle size 10-100 μm; suspended and dispersed in the water phase. With sufficient time or external force, it can coalesce into larger droplets and float to the surface, or it may further reduce in size and turn into emulsified oil.

Emulsified Oil: Particle size 0.1-10 μm (extremely fine oil droplets); stabilized in a water-in-oil form due to the influence of surfactants at the oil-water interface, making oil-water separation difficult with simple settling.

Not all three forms of oil are necessarily present in all oily wastewater. However, in representative industries such as electroplating wastewater, all three are present, with oil concentrations typically ranging from 300-500 mg/L, where emulsified oil forms the largest proportion.

3. Common Oily Wastewater Treatment Processes and Methods

The common treatment methods for oily wastewater include the following 11 processes:

3.1 CDFU (Cyclonic Dissolved Gas Flotation Unit)

Developed by the Shenzhen Clear Science & Technology Co., Ltd (SINOKLE) company, this new oily wastewater treatment technology uses CDFU (Cyclonic Dissolved Gas Flotation Unit), fine filtration, and KHC High-Efficiency Coalescing Pressure Degreaser as core technologies. The device can remove emulsified oil with precision down to 0.1 µm, achieving effluent oil content of less than 10 ppm. This technology is feasible, reliable, and offers strong environmental and economic benefits. Compared to traditional technologies, it is at the international leading level.

3.2 Oil-Water Separation by Sedimentation

The oil-water sedimentation separation method utilizes the density difference between the oil and water phases and their immiscibility to achieve separation, and it is considered primary treatment. Sedimentation separation occurs in oil separation tanks, commonly using horizontal flow, parallel plate, or corrugated plate designs. The design of horizontal flow oil separators is mainly based on Stokes' law, which can be used to calculate the minimum oil droplet diameter that can be removed by an oil separator with a given surface area. The flow conditions within the separator also greatly impact its oil removal capacity and effectiveness, with the ideal flow state being laminar flow, which facilitates the rise of oil droplets and the settling of solid particles.

3.3 Coarse-Particle Coalescence Method for Oily Wastewater

The coarse-particle coalescence method for oily wastewater utilizes the different affinities between the oil-water phases and coalescing materials for separation. As the oily wastewater passes through the coalescing material, fine oil droplets coalesce into larger oil particles, increasing their rising speed. This is considered as secondary treatment.

In this method, the coalescing materials are placed in a coalescing device. As the wastewater flows through, it removes the dispersed oil. The key to this technology is the coalescing material, which can be either fibrous or granular. Common hydrophilic materials are fibers such as polyamide, polyvinyl alcohol, and vinylon, with acid groups (e.g., sulfonic or phosphoric groups) and salts incorporated. Hydrophobic materials include wax-like spheres, polyolefin or polystyrene spheres, foam bodies, and polyurethane foam. Some researchers believe the optimal contact angle for these materials should be less than 7°.

The oil removal efficiency and feasibility of the process are determined by observing changes in the oil droplet size distribution before and after coalescence. The main evaluation indicators are oil removal rate and effluent oil content.

The coarse-particle coalescence method does not require chemical additives, produces no secondary pollution, has a small footprint, and has low construction costs.

However, this method requires the influent concentration to be relatively low. Therefore, oily wastewater must undergo pre-treatment before entering the equipment; otherwise, the effluent oil concentration will be high (typically above 10 mg/L), often requiring further advanced treatment.

3.4 Filtration Method for Oily Wastewater

The filtration method for oily wastewater utilizes mechanisms such as particle media filtration, interception, inertial collision, screening, surface adhesion, and coalescence to remove oil from the water. It is typically used for secondary or advanced treatment. Common particle media filters include quartz sand, anthracite, glass fibers, and polymeric materials.

For a locomotive factory's oily wastewater, the treatment process involves oil separation, flocculation and sedimentation, followed by filtration, with effluent quality meeting discharge standards. The oil removal rate can reach 95%, making it fully suitable for use in related production workshops.

Filtration equipment is simple, easy to operate, and has low investment costs. However, as the system operates, the pressure drop gradually increases, requiring frequent backwashing to ensure normal operation.

3.5 Membrane Separation Method for Oily Wastewater

The membrane separation method for oily wastewater has rapidly developed over the past two decades. This method uses porous membranes as the separation medium to retain oil and surfactants in the oily wastewater, allowing water molecules to pass through and achieving oil-water separation. The key to membrane separation technology lies in the selection of membranes and components.

Membrane materials can be categorized into polymeric membranes and inorganic membranes. Common polymeric membranes include cellulose acetate, polyphenylsulfone, polypropylene, and polyvinylidene fluoride. Common inorganic membrane materials include alumina, zirconia, and titania. Based on pore size, membranes can be classified into microfiltration, ultrafiltration, and reverse osmosis.

This method is most suitable for oily wastewater with high discharge requirements and moderate treatment volumes.

3.6 Flotation Method for Oily Wastewater

The flotation method for oily wastewater relies on oil droplets adhering to microbubbles in the water, which increases buoyancy and causes the oil to float for separation. This method is mainly used to treat dispersed oils, emulsified oils, and fine suspended solids in oily wastewater, which are difficult to remove through natural gravity separation.

Since the microbubbles are composed of non-polar molecules, they can combine with water-soluble oils and carry the oil droplets upward, increasing the rising speed by nearly a thousand times, resulting in high oil-water separation efficiency. Depending on the method of bubble generation, flotation can be categorized into pressurized dissolved air flotation, impeller flotation, and aeration flotation.

To enhance flotation efficiency, inorganic or organic polymer flocculants can be added to the wastewater, known as flocculation flotation, further improving oil-water separation. This method is now widely applied in treating oilfield wastewater, petrochemical wastewater, food oil production wastewater, and other industrial processes, and has become a relatively mature technology.

3.7 Adsorption Method for Oily Wastewater

The adsorption method for oily wastewater utilizes porous solid adsorbents to adsorb dissolved oils and other soluble organic substances in the wastewater. Common adsorbents include activated carbon, which has excellent adsorption properties for oil and can also effectively adsorb other organic substances in wastewater. However, its adsorption capacity is limited (typically 30-80 mg/g for oil), and it is costly with difficult regeneration, which limits its application.

After treatment using adsorption, the effluent oil content can be reduced to below 5 mg/L, making this method typically used for advanced treatment of oily wastewater. Some scholars treated oily wastewater from a shipyard, achieving effluent oil content below 5 mg/L, with most results under 1 mg/L. The adsorbents used were modified bentonite, sulfonated coal, waste activated carbon, crushed coke, and organic fibers, all of which are readily available materials.

3.8 Coagulation Method for Oily Wastewater

The coagulation method for oily wastewater involves adding a specific amount of flocculant to the wastewater, which generates oil-attracting flocs. These flocs adsorb the fine oil droplets, and then the oil is removed by sedimentation or flotation methods.

Common flocculants include aluminum sulfate, ferrous sulfate, ferric chloride, polymeric aluminum chloride, and polymeric aluminum chloride, which are inorganic flocculants, as well as polyacrylamide and acrylamide, which are organic flocculants. The pH range for each flocculant varies.

To enhance coagulation, it is often beneficial to combine two flocculants. This method requires a large amount of chemicals and results in a significant volume of sludge. It is suitable for treating large volumes of wastewater with low oil content, such as emulsified oil or other fine suspended solids.

3.9 Salting-Out Method for Oily Wastewater

The salting-out method for oily wastewater involves adding inorganic salt electrolytes to the wastewater. The electrolytes drive the cations from the oil droplet's diffusion layer into the adsorption layer, causing the destruction of the double electric layer. This results in the oil droplets becoming neutral, restoring the attractive forces between them, which leads to coalescence and achieves demulsification.

Common electrolytes used are salts of calcium, magnesium, and aluminum, which can neutralize charges and alter the properties of surfactants, improving treatment effectiveness. The typical salt dosage is between 1% and 5%. After treatment, the effluent oil content is generally greater than 10 mg/L.

However, the salting-out method has the disadvantages of slow aggregation, long sedimentation times, large equipment footprint, and poor performance when treating oil emulsions stabilized by surfactants.

3.10 Electrolytic Method for Oily Wastewater

The electrolytic method for oily wastewater includes electrolytic coagulation adsorption and electrolytic flotation. Electrolytic coagulation adsorption involves using a soluble electrode to electrolyze emulsified oil wastewater. Metal ions are dissolved from the soluble anode (Fe or Al), which then hydrolyze to form hydroxides that adsorb and coagulate the emulsified and dissolved oils. These are then removed by sedimentation.

This method is mainly used for secondary treatment of cooling lubricants in the machining industry after chemical flocculation. The electrolytic coagulation adsorption method offers advantages such as small footprint, simple operation, good treatment performance, and relatively low sludge volume.

However, it also has drawbacks, including high anode metal consumption, the need for large amounts of salts as auxiliary agents, high electricity consumption, and high operating costs. Despite extensive research on the issue of anode passivation, it has not yet been fundamentally solved.

3.11 Biochemical Method for Oily Wastewater

The biochemical method for oily wastewater is highly effective in removing dissolved oils from water. However, it cannot remove dispersed oil or suspended oil. If these two types of oil enter the system, they can severely impact the entire biochemical system. Therefore, when applying the biochemical method to treat oily wastewater, it is essential to carry out proper pre-treatment to remove suspended and dispersed oils from the water beforehand.