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Municipal sewage

Table of contents（Click to go to where you want to see）

6.1. Physical treatment methods ( Our company’s main products and cases)

6.2. Chemical method
6.3. Mechanical method
6.4. Biological treatment
6.5 supercritical water technology
6.6 Photocatalytic oxidation technology
6.7 Microelectrolysis
6.8 Membrane separation technology

How are oily and emulsified wastewater generated?

The sources of oily wastewater are extensive. Oily wastewater will be produced in oil field mining, natural gas extraction, daily chemical production, food processing, pharmaceutical production, and metal processing. The composition and properties of oily wastewater in different industries are different.

The industrial discharge of oily wastewater is a significant amount produced by food and beverages. Still, from the perspective of fats, hydrocarbons, and petroleum fractions such as diesel, gasoline, and kerosene, most of those dissolved in water come from the petrochemical and metal processing industries. These ingredients are present as an oil-in-water emulsion. Oily wastewater is carcinogenic and mutagenic to human health and inhibits plant growth.

Without proper treatment, the discharge of oily sewage will increase the biological oxygen demand and chemical oxygen demand of the water body and form a film on the water's surface that reduces the penetration of sunlight into the water body, thereby damaging the aquatic system.

What are the characteristics of oily wastewater ?

There are more than 230 types of organic species in oily wastewater, including volatile phenols, ammonia nitrogen, cyanides, organophosphates, phenols, organic acids, etc. Oils have complex compositions and may contain over 150 toxic and harmful substances, including naphthalene, pyrene, phenanthrene, anthracene, etc.

Often, oily wastewater can also cause severe damage to ecosystems at lower concentrations. There are many types of organic species in oily wastewater with complex components, and their morphological properties can change with changes in pH in the water environment.

Oil substances in oily wastewater usually exist in four forms: floating, dispersed, emulsified, and dissolved oil. Among them, floating and dispersed oil can be effectively removed through general physical methods, while emulsified and dissolved oils are more difficult to deal with.

The difficulty in treating emulsified oil is mainly reflected in the fact that there is a stable emulsified film on its surface, which hinders the coalescence of oil droplets and makes it more difficult to remove when entering the environment. It will seriously impact soil, water, and the entire ecosystem; the oil that dissolves oil The diameter of the beads is much smaller than that of emulsified oil, with the smallest being only a few nanometers. It is difficult to remove and can easily cause environmental pollution.

Therefore, emulsified and dissolved oil's harmless treatment and resource utilization are essential for sustainable industrial development.

What are the main sources of oily wastewater ?

Oil and natural gas extraction process

The global production of oily wastewater from oil and natural gas extraction is approximately 800 million barrels daily.
In addition to petroleum, these wastewaters contain complex components such as linear chain hydrocarbons, aromatic hydrocarbons (benzene, toluene, xylene), metals, natural radioactive substances, and corrosion inhibitors.
Food and dairy processing

In the food, beverage, and dairy industries, the main components of oily wastewater are water and oil. Most of the oils come from meat, poultry, seafood, and dairy products themselves, so the organic carbon content of wastewater is usually high. In addition, different production processes also bring in other additives.
Metal working process

Another primary source of oily wastewater is the metal processing industry, mainly from metal cutting fluids and workpiece flushing water that lose their performance during workpiece processing. Wastewater is composed chiefly of waste metal cutting fluids, mainly of oils, fatty acids, surfactants, heavy metals, and other additives. Generally, metal-cutting fluid wastewater has poor biodegradability, and some metal-cutting fluids are highly toxic and have been classified as hazardous waste. Category.

What are the hazards of oily wastewater ?

Impact on water bodies

When oil enters the water body, since the density of oil is lower than that of water and is immiscible with water, the oil will float on the surface of the water body and spread rapidly to form an oil film.

Studies have shown that 1 L of crude oil can cover 100 to 2,000 m2 of water surface. The oil film generated is detrimental to the photosynthesis of plants in the water. It hinders the heat transfer process between the water and the atmosphere, causing adverse effects on local areas' hydrological and meteorological conditions. Certain influence. At the same time, oil degradation requires a large amount of dissolved oxygen in the water. The complete degradation of 1 L of crude oil involves the consumption of 0.4 × 106 L of dissolved oxygen in the water. Excessive consumption of dissolved oxygen will cause hypoxia in the water body, which in turn will reduce the ability of the water body to purify itself and cause the water body to become depleted. Smell.
Effects on living things

Polycyclic aromatic hydrocarbons in oil will affect the vision and development of fish. High-viscosity crude oil will also adhere to fish eggs, causing the fish eggs to fail to hatch. Oil dispersed in the water is harmful to bird embryos and can also pollution of birds' feathers, affecting their ability to fly, making them unable to move and lose their ability to hunt, leading to death and a series of adverse effects;

Another characteristic of the biological hazards of oily wastewater is its accumulation. The accumulation of some toxic substances in the body of water will cause damage to the ecosystem. It may also be transmitted to the human body through the food chain, harming human health.
Impact on soil and groundwater

Experiments have shown that even adding a small amount of emulsified oil to domestic sewage will cause severe damage to the soil environment. The oily wastewater will cover the soil and hinder the normal respiration of various organisms, resulting in many organisms dying. At the same time, soil N will be reduced. , P content may also cause crop yield reduction.
Impact on the atmosphere

Volatile substances (benzene, toluene, chloroform, cyclohexane, etc.) in oily wastewater will emit unpleasant odors and affect the atmospheric environment. At the same time, such volatile substances dispersed in the air may also damage organisms, the liver, and the nervous system, and even cause cancer.
Additive toxicity

In addition to the harm caused by the oil, various pollutants dissolved in oily wastewater are also an essential reason for its profound impact.

Surveys have shown that among the various organic substances contained in industrial wastewater, except for phenolic substances that are soluble in water, other substances such as organic pesticides, polycyclic aromatic hydrocarbons, aromatic hydrocarbons, polychlorinated biphenyls, dyes, etc., are all oil-soluble. Therefore, when oil pollutants contain this type of organic matter, the toxicity of wastewater will be significantly enhanced. Most of these substances are artificial, have good chemical stability, and are difficult to degrade. Most of them have the risk of causing cancer, deformation, and neonatal malformation.

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What are the technical difficulties in treating wastewater containing emulsified oil?

Generally speaking, oil and water do not dissolve each other and are easier to separate. However, under surfactants, high-speed shear, and collision conditions, oil-water mixture systems can form a thermodynamically stable or metastable state.
Oil can be divided into four categories according to its state in water：

**The presence of oils in water**
Type	Particle size range	Features
Oil slick	＞150 μm	Will form a continuous oil film/layer on the water surface
Dispersion oil	20-150 μm	Unstable, easy to form oil slick under external influence
Emulsified oil	0.1-20 μm	Stable in water and not easy to coalesce
Dissolve oil	＜0.1 μm	Dissolved in water in a molecular state, accounting for a very small portion of the total oil

Among them, floating oil and dispersed oil have relatively large particle sizes of oil droplets. When the oil-water mixture is stationary/slowly flowing, the oil droplets coalesce and float to the water's surface to form a continuous oil layer. It is relatively easy to separate from water.

However, emulsified oil contains surfactants that promote emulsification, resulting in a smaller particle size of the emulsified oil, and the oil-water mixed system is remarkably stable. Usually, the treatment of wastewater containing emulsified oil requires two steps of "emulsification + oil-water separation" to completely purify the wastewater. In this process, the emulsified oil must be separated after demulsification. Otherwise, the demulsified oil droplets will be separated from the emulsified oil droplets. The surfactant contacts again and slowly coalesces and disperses in the water, with the risk of secondary emulsification.

Traditional oil-water separation technology makes it difficult to fully realize the simultaneous coordination of "emulsification" and "oil-water separation." Separation methods with differences in density between oil and water (such as sedimentation, flotation, and hydrocyclone) require very long residence times. In contrast, chemical treatments (such as the Fenton and coagulation methods) must simultaneously meet strong demulsification capabilities and sufficient addition amounts—an adequate residence time.

Compared with traditional methods, the membrane separation method can directly realize the connection between "emulsification" and "oil-water separation." It has the advantages of no need for additional chemicals, minimal secondary pollution and no oily sludge generation, low post-processing costs, recyclable oil products, etc., characteristics, so it has a competitive advantage in treating wastewater containing emulsified oil.

What are the treatment methods for oily wastewater?

Many countries and regions have formulated regional regulations to strengthen oily wastewater treatment before discharge. In the North Sea region, the Oslo-Paris (OSPAR) Convention stipulates that the upper limit of oil content in discharged wastewater is 30 mg L −1. The Paris Convention signed in 1883 stipulates that the upper limit of oil content in water discharged into the sea from offshore oil fields is 40 mg L− 1. Onshore oil fields are 5 mg L -1. China stipulates that oily wastewater's maximum allowable discharge concentration is 10 mg L-1. Norway has reduced the oil concentration limit allowed for discharge from offshore facilities to the Norwegian continental shelf from 40 mg L-1 to 30 mg L-1 since 2007. To achieve the discharge limits required by the above regulations, there is an urgent need to develop oily wastewater treatment technologies to achieve treatment targets at a lower cost-effectiveness.

Standard oily wastewater treatment methods currently include gravity sedimentation, flotation, coagulation, oxidation, biodegradation, adsorption, and membrane separation. Their characteristics and application range are shown in the figure.


Method	Processing principle	Features	Application
Gravity/mechanical separation	Oil-water density difference	Good adaptability to oil concentration
		Long separation time
		Not suitable for emulsifying/dissolving oils
Flotation	Highly dispersed tiny bubbles serve as carriers to adhere tiny oil droplets in water	Can separate larger oil particles in water
		Less sludge and high separation efficiency
		High energy consumption and long residence time
Concrete	Coagulants reduce the surface charge of oil droplets and promote their aggregation and sedimentation.	Large amount of pharmaceutical dosage
		Sludge production is too high
		High processing costs
Oxidation	Extremely oxidizing substances convert oil pollutants into small molecular substances	High efficiency and fast response
		Small footprint
		Requires a lot of chemicals and costs a lot
Adsorption	Porous adsorbents adsorb oil pollutants in water	Can remove most contaminants
		The oil content of the effluent can be reduced to 0.2 mg L^-1
		Limited adsorption capacity,High cost and difficult to regenerate
Biology	Microorganisms' own metabolism degrades oils	No secondary pollution, more environmentally friendly
		Suitable for low oil concentration
		Toxic substances in the water will affect the effect
Membrane separation	Screening or selective permeation of membranes	No chemical additives required
		It occupies less space and can be resourced
		There is a problem of pollution regeneration

Physical treatment methods(Our company’s main products and cases)

Physical methods for oily wastewater purification can be divided into gravity separation (GS) and dissolved air flotation (DAF). The GS system is based on the density difference between oil and water. A significant density difference is required between oil and water to achieve good separation.

Currently, GS is used as a first-level separation process for dispersed floating oil and is unsuitable for separating emulsified oil. GS is a straightforward system with many shortcomings, such as limited separation capacity, the need for a large setup area, and complex management and operation.

The principle of DAF is to introduce pressurized air at the bottom of an open basin. When the bubbles rise to the top of the basin, they will bring contaminants with them. The size of microbubbles produced by traditional DAF ranges from 20 to 100 µm. Microbubbles attach to the oil droplets, increasing the buoyancy of the oil droplets and causing them to move upward. During the DAF process, the pressure and saturation of air in the wastewater are two critical parameters to monitor. For microbubbles to arise and float to the system's surface, the pressure must be reduced to atmospheric conditions containing an excess of dissolved gases.

Application case(Preprocessing-physical method)

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Case Studies-Oil And Emulsion Project for A Fuel Recycling Enterprise in Zhejiang

This case introduces a fuel recycling enterprise in Zhejiang Province, which uses the high-efficiency dissolved air flotation produced by our factory to effectively separate oil and emulsion, providing good conditions for subsequent sewage treatment.

Case Studies-Electroplating Wastewater Project of A Jiangsu Enterprise

This case introduces a wastewater treatment project of an electroplating enterprise in Jiangsu. Our factory's high-efficiency dissolved air flotation technology was used to effectively treat the electroplating wastewater, providing good conditions for subsequent wastewater treatment.

Case Studies-Nanjing Steel Project

This case study introduces how Nanjing Iron and Steel Group uses our Lamella Plate Clarifier to remove fine particles from diluted tailings, achieving good separation results and clear effluent.

Case-Studies-Agricultural-machinery-production-and-processing-wastewater-treatment.jpg

Case Studies-Changzhou Dongfeng Agricultural Machinery Production Wastewater Treatment

This case study introduces how Changzhou Dongfeng Agricultural Machinery Group uses our factory's high-efficiency dissolved air flotation to treat oily wastewater, such as machining and cutting fluids. After treatment, the removal rate of oil, grease, suspended solids, etc., in the raw water is over 90%, significantly reducing the back-end processing load.

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Chemical method

Flocculation technology is a standard chemical method currently used for cleaning oily wastewater. The flocculation method adds a flocculant to the wastewater to neutralize the negative charge of the oil suspension or emulsion and bridges the particles together to form a floc. This method has been widely used in the treatment of palm oil wastewater. The effectiveness of this method depends mainly on the flocculant's type and dosage, the oil's initial concentration, and the wastewater's temperature and pH.

Compared with membrane filtration, DAF, and biotechnology methods, flocculation is simple and has lower capital and operating costs. However, the main disadvantage of this method is related to the flocculants. Inorganic flocculants such as aluminum sulfate, polymerized ferrous sulfate, and poly aluminum chloride are cheap and easy to use but have poor flocculation effects. When using inorganic flocculants, the pH needs to be adjusted. Organic polymer flocculants such as polyacrylamide have high flocculation capacity at low dosages and can be used in various pH ranges. However, because they are not biodegradable, they harm health and the environment.

Mechanical method

Mechanical coalescents (MC) are used in mechanical methods. In MC, tiny oil droplets collide and adhere to other substances in the coalescer. Larger droplets are formed, which can be separated by buoyancy due to differences in density.

Mechanical separation of emulsified oil using the MC method is effective, especially when the oil droplet size is less than 10 m. Due to space limitations, MC is often used to treat oily wastewater at sea. Coalescing agents are compact, long-lasting, effectively separate liquid-liquid phases, and require minimal additional chemicals.

Common coalescing agents include plate coalescing agents, filler coalescing agents, coalescing filter separators, and fiber coalescing agents. A new fiber coalescing agent is reported. The coalescent can reduce the oil content of offshore produced water from 1 200 mg/L to 25 mg/L with a residence time of 1 h—plate and packed coalescers separate emulsified oils with droplet sizes greater than 20 µm. In comparison, filter separators and fiber coalescers are used for emulsified oils with droplet sizes less than 10 µm.

Biological treatment

Standard biological treatment methods can be divided into aerobic and anaerobic treatment systems.

Anaerobic systems require less energy because they eliminate the aeration process, can convert organic pollutants into methane gas, require fewer nutrients, and produce less sludge. The method can also produce valuable by-products, such as biodegradable plastics.

The aerobic biological treatment method treats wastewater with high temperatures and pollutant concentrations due to its accelerated biodegradation kinetics. However, in such biological treatment systems, microbial cells can be affected by toxic chemicals and high-salinity wastewater, reducing the system's overall efficiency. To solve this problem, aerobic granulation technology and its application in aerobic granular-activated sludge reactors have been explored.

Aerobic granular activated sludge reactors are more stable in oily wastewater treatment due to their microbial diversity, compact particle structure, good settleability, reasonable biomass retention rate, and stability against toxic pollutants. These aerobic granules have more minor reactor volume requirements, lower investment costs, and the ability to remove nutrients instantly. Aerobic granulation technology can be used to treat dairy industry wastewater, sewage, and brewery wastewater.

supercritical water technology

Supercritical water oxidation (SCWO) and supercritical water gasification (SCWG) are used to treat heavy oily wastewater, such as oily sludge, as an alternative to incineration.

SCWO technology uses water above its thermodynamic critical point (374°C, 22.1 MPa) as the reaction medium to convert hydrocarbons into water, molecular nitrogen, and carbon dioxide quickly through an accelerated oxidation process. By-products such as chlorine, phosphorus, and sulfur are converted into corresponding inorganic acids or salts after neutralization with alkali. Liquid and gaseous products can be discharged into the environment without any post-treatment. SCWG exploits the ability of supercritical water to dissolve organic biomass components in wastewater and to break down polymeric biomass structures.

Its main advantage is the ability to generate energy by gasifying oily wastewater.

Microelectrolysis

Microelectrolysis treats high-concentration oily wastewater containing large amounts of organic polymers, salts, and chemical cleaning agents, such as pre-plating wastewater, acid mine drainage, and rainwater.

Microelectrolysis is a process that combines redox, electrochemistry, physical adsorption, flocculation, and other functions. This method can achieve treatment processes such as decolorization, improved flocculation, refractory organic oxidation, and improved biodegradability.

Membrane separation technology

In the past decade, membrane separation technology (MST) has removed most chemicals and inorganic and organic compounds from wastewater. MST requires a smaller land area compared to other traditional methods.

MST enables efficient, selective, and consistent contaminant separation. MST also has good productivity, stability, low failure rate, and economical use. Currently, polymer and ceramic membranes are widely studied for oily wastewater filtration.

MST can be divided into three categories based on the driving force for separation: pressure-driven, osmotic-driven, and thermal-driven. Among these three driving forces, pressure driving is the most commonly used in oily wastewater treatment. Pressure-driven membranes can be further classified into reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF). Microfiltration and ultrafiltration processes have been reported to treat oily wastewater. Ultrafiltration is a low-pressure operation that requires low capital and operating costs.

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