Sewage treatment process

          How to deal with water pollution has always been a very complex environmental issue, among which the oily wastewater generated by the petroleum industry and other industries in daily production is an important source of water pollution, and the treatment of oily wastewater is more difficult. Therefore, oily wastewater is a key direction for water pollution control. This article provides a comprehensive overview of the sources and hazards of oily wastewater, and analyzes in detail several effective oily wastewater treatment processes currently available.

          With the increasing emphasis on environmental protection in social development, the maturity of oily wastewater treatment technology has greatly constrained the survival and development of oil fields. From the overall situation of oil fields in China, the water content in the produced fluids of various oil fields is as high as 80%. The cost of treating oily wastewater in oil fields has far exceeded the cost of investing in oil and gas treatment in the past. To some extent, the focus of oil field work has shifted from oil and gas treatment to the treatment of oily wastewater. In the process of treating oily wastewater, in order to better control costs, oil fields will not increase investment in it. However, the relevant laws and regulations on pollution control in the country are becoming increasingly strict, and the treatment process of oily wastewater is facing significant challenges.

1. Analysis of the source of oily wastewater

The current sources of oily wastewater that cause water pollution are very complex, and it is precisely because of the complexity of the sources that it is difficult to control. In addition to the oil extraction industry, oily wastewater is also generated in industrial production processes such as steel, pharmaceuticals, and food processing. These oily wastewater can be divided into four types: emulsified oil, dispersed oil, dissolved oil, and floating oil.

1.1 Petrochemical industry

In the petrochemical industry, in order to effectively improve oil recovery, a large amount of water is injected into the ground during the process of extracting crude oil, enhancing the pressure on the bottom layer. In the transportation and consumption of oil, the participation of water is also indispensable. It can be said that there is the shadow of oily wastewater in the entire oil production chain. Although various oilfield enterprises have been continuously optimizing and upgrading their oil extraction and transportation technologies in response to the national call for sustainable and green development, and have achieved certain results, at the same time, more complex oily wastewater has emerged, making treatment more difficult.

1.2 Chemical and pharmaceutical industry

Because the chemical and pharmaceutical industry requires high concentration process production, in the specific manufacturing process, in order to react and pretreat the raw materials, and selectively separate the produced products, a large amount of lubricating oil and water are needed, resulting in a large amount of oily wastewater in production.

2. Overview of the hazards of oily wastewater

2.1 Hazards of oily wastewater to drinking water sources

There are more opportunities for the growth of bacteria and other microorganisms in oily wastewater, and even carcinogenic substances may be present. When oily wastewater infiltrates into our daily drinking water sources, there may be many harmful substances present, which will directly enter the human body. Whether it is humans or livestock, drinking contaminated water may increase the risk of drinking water poisoning or illness, posing a great threat to life.

2.2 Harm of oily sewage to Rivers and Lakes

Generally speaking, the density of oily sewage is relatively small. After flowing into Rivers and Lakes, it will float on the water surface, making it impossible for the air to exchange effectively with the gas in the water body, reducing the oxygen content in the water body. For a long time, the growth of animals and plants in the water body will be affected, the overall quality of the water body will be damaged, and the use value of water resources will be greatly reduced.

2.3 Hazards of oily wastewater to soil

When oily wastewater is used for agricultural soil irrigation, oil stains will deposit on the soil surface, thereby preventing gas exchange between the soil and air. The normal growth and metabolism of plants will also be affected, and even death may occur. Even if plants survive, the cultivated crops will pose a serious threat to human health after being consumed and absorbed by humans.

3. Analysis of the process and key technologies for oily wastewater treatment

3.1 Process of oily wastewater treatment

When treating oily wastewater, it is mainly divided into the following steps:

① Oil water separation stage. This stage is mainly carried out to reduce the emulsification degree of oily wastewater in the preliminary oil-water separation process. In this process, different types of wastewater need to be treated in different ways. Usually, oily wastewater with small differences in oil-water density needs to be treated with a filtration device, while oily wastewater with large particle size and high freezing point needs to be treated by heating and insulation.

② On this basis, further oil-water separation treatment is commonly carried out through floating or coagulation methods, mainly by adding PAM and PAC to oily wastewater to enable sufficient coagulation and flocculation reactions to occur in the wastewater. This treatment process can not only effectively reduce the phenomenon of oil blocking the device, but also maximize the oil removal performance of the device. Normally, the removal of SS and oil from oily wastewater occurs before efficient combined air flotation. Before efficient combined air flotation occurs, we also need to conduct water quality testing. If the water quality does not meet the standards, further treatment is required until it meets the standards before it can be discharged to the outside world.

3.2 Treatment methods for oily wastewater

3.2.1 Salt precipitation method The salt precipitation method mainly compresses the thickness of the double layer at the interface between oil particles and the water surface, causing the oil particles to be in an unstable state and achieving the purpose of water oil separation. Although the salting out method requires a significant amount of medication, has a slow reaction rate, occupies a large area of equipment, and has poor treatment efficiency, it is often used for preliminary treatment of oily wastewater due to its simple operation and low cost.

3.2.2 Flocculation technology is currently the most widely used oily wastewater treatment technology, which not only has strong adaptability, but also can effectively treat emulsified oil, dissolved oil, and some organic compounds that are difficult to biodegrade. The commonly used coagulants are mainly divided into three categories: composite, organic, and inorganic. Among them, the use of inorganic flocculants has the best effect, requiring only a small amount of flocculant to effectively treat, but there is a defect of excessive floc residue in the subsequent process. Although organic polymer flocculants have good effects, they have the problem of being too expensive to be widely tested and promoted. The combination of inorganic and organic flocculants can achieve better results, making composite flocculants an important research direction in flocculation methods.

3.2.3 Air flotation method is mainly suitable for the treatment of dispersed oil without surface dispersants, but it can only separate suspended substances in oily wastewater. If dissolved substances and other colloids need to be treated, further follow-up work is needed. The main method is to pressurize and dissolve air in the sewage, generating air microbubbles that float up with oil droplets in the water. If flocculants are added, the speed of ascent can be accelerated.

3.2.4 Membrane Separation Method Membrane separation method is essentially a process of screening oily wastewater, mainly using ultrafiltration and microfiltration methods to treat oily wastewater. The key point of this treatment process is how to screen the membrane and related components.

3.2.5 Adsorption Method Adsorption method is an effective process for treating oily wastewater. Although activated carbon can not only effectively adsorb oil, but also other substances in the wastewater, it has limited adsorption capacity, high cost, and difficulty in regeneration. It is generally used for deep treatment of oily wastewater, so new methods are being sought Adsorption materials have been under research, and currently there is a type of oil absorbing agent composed of inorganic fillers with oil absorbing properties and crosslinking polymerization agents. After long-term contact with oily wastewater, it can achieve a large capacity adsorption effect. In addition, there are also oil absorbing materials made by mixing inorganic oil absorbing fillers containing C6-C60 aliphatic amines or other derivatives with organic polymers, adsorbents made by treating peat with substances with lipophilic and hydrophobic properties, etc. However, these adsorption materials all have different shortcomings and need further improvement.

3.2.6 Advanced Oxidation Process Advanced oxidation technology is currently a hot research topic in the field of wastewater treatment worldwide, mainly a process that generates a large amount of OH during treatment. Hydroxyl radicals are an unstable active substance with strong oxidative properties, typically formed during water photodissociation, ozone oxidation, and some chemical reactions. They are known as the strongest oxidant after fluorine. The use of hydroxyl radicals for wastewater treatment can effectively oxidize pollutants in water, reduce the degree of water pollution, and achieve the goal of wastewater purification. According to the different oxidants used and specific operating methods, oxidation methods can be divided into three types: semiconductor photocatalytic oxidation method, Fenton reagent method, and ozone oxidation method. Oily wastewater mainly originates from industrial production, especially in the entire petroleum production chain. It not only involves a wide range and large output, but also poses great difficulties in treatment. It is a common industrial pollution that has a great impact on the environment and needs to be taken seriously. Active technological innovation is needed to effectively treat oily wastewater and protect the ecological environment.