introduction of oil-water separation process

1 centrifugal separation method

centrifugal separation method is to make the container containing oily wastewater rotate at high speed to form a centrifugal force field. Because the density of solid particles and oil droplets is different from that of wastewater, the centrifugal force is different, so as to remove solid particles and oil droplets from wastewater. The commonly used equipment is hydraulic cyclone separator.

2 flotation

flotation, also known as air flotation, is a water treatment technology that is being deeply studied and continuously promoted at home and abroad. In this method, air or other gases are introduced into the water to generate fine bubbles, so that some fine suspended oil beads and solid particles in the water attach to the bubbles and float to the water surface with the bubbles to form scum (oily foam layer), and then the oil is skimmed by an appropriate skimmer. This method is mainly used to treat dispersed oil, emulsified oil and fine suspended solids with the particle size of 11 ~ 61 μ m remaining in water after the treatment of grease trap, and the oil content of the effluent can be reduced to 21 ~ 31 mg/L. According to the different ways of generating bubbles, air flotation can be divided into pressurized air flotation, aerated air flotation and electrolytic air flotation, among which pressurized dissolved air flotation is the most widely used.

3 biological oxidation method

biological oxidation method is a method to purify wastewater by biochemical action of microorganisms. Oil is a kind of hydrocarbon organic matter, which can be decomposed into carbon dioxide and water by microbial metabolism and other life activities. Organic matter in oily wastewater is mostly dissolved and emulsified, and BOD5 is high, which is beneficial to biological oxidation. For wastewater with oil concentration below 31 ~ 51 mg/L and other biodegradable harmful substances, biochemical treatment is often used, which is mainly used to remove dissolved oil from wastewater. The common biochemical treatment methods of oily wastewater include activated sludge method, biological filtration method and biological turntable method. Activated sludge process has a good treatment effect and is mainly used to treat wastewater with high requirements and stable water quality. Compared with activated sludge process, biofilm process adheres to the surface of filler carrier, so that microorganisms with slow reproduction speed can also exist, thus forming a stable ecosystem. However, because it is difficult to control the amount of microorganisms attached to the carrier surface, the flexibility in operation is poor and the volume load is limited.

4 gravity separation method

gravity separation method is a typical primary treatment method, which uses the density difference between oil and water and the incompatibility between oil and water to separate oil droplets, suspended solids and water in a static or flowing state. The oil droplets dispersed in water slowly float up and stratify under buoyancy, and the floating speed of oil droplets depends on the size of oil droplets, the density difference between oil and water, the flow state and the viscosity of fluid. 2 filtration method

filtration method is to remove harmful substances such as suspended solids and oil in wastewater by passing the wastewater through a device with holes or through a filter layer composed of a certain granular medium, using its interception, screening and inertial collision. There are three commonly used filtration methods: layered filtration, membrane filtration and fiber medium filtration. Membrane filtration method, also known as membrane separation method [5], uses microporous membrane to intercept oil beads and surfactants, which is mainly used to remove emulsified oil and some dissolved oil. Filter membrane includes ultrafiltration membrane, reverse osmosis membrane and mixed filter membrane. Membrane materials include organic membrane and inorganic membrane. Common organic membranes include cellulose acetate membrane, polysulfone membrane and polypropylene membrane, while common inorganic membranes include ceramic membrane, alumina, cobalt oxide and titanium oxide. Emulsified oil is in a stable state, and it is difficult to separate it by physical or chemical methods. With the rapid development of membrane science, the treatment of emulsified oil wastewater by membrane process has been gradually accepted and applied in industry.

5 chemical method

chemical method, also known as chemical method, is a method of adding chemicals to transform pollutants in wastewater into harmless substances by chemical action, so that wastewater can be purified. Commonly used chemical methods include neutralization, precipitation, coagulation, redox and so on. Coagulation is mainly used for oily wastewater. Coagulation method is to add a certain proportion of flocculant to oily wastewater, and after hydrolysis in water, positively charged micelles and negatively charged emulsified oil are electrically neutralized, oil particles are aggregated, and the particle size becomes larger. At the same time, flocs are generated to adsorb fine oil droplets, and then oil-water separation is realized by sedimentation or air flotation. The common flocculants are inorganic flocculants such as polyaluminum chloride (PAC), ferric chloride, aluminum sulfate and ferrous sulfate, and organic polymer flocculants such as acrylamide and polyacrylamide (PAM). The dosage and pH value of different flocculants are different. This method is suitable for emulsified oil droplets and other fine suspended substances that cannot be separated by gravity sedimentation.

6 adsorption method

adsorption method uses lipophilic materials to adsorb dissolved oil and other dissolved organic substances in wastewater. The most commonly used oil absorption material is activated carbon, which can absorb dispersed oil, emulsified oil and dissolved oil in wastewater. As the adsorption capacity of activated carbon is limited (generally 31-81 mg/g for oil), the cost is high, and the regeneration is difficult, it is generally only used as the last stage of multi-stage treatment of oily wastewater, and the oil concentration in the effluent can be reduced to 1. 1-1. 2 mg/L.. In 1976, Hunan Changling Refinery adopted activated carbon adsorption for advanced treatment in wastewater treatment. At home and abroad, some beneficial results have also been achieved in the development of new adsorbents. It is found that flake graphite can adsorb the heavy oil released by the oil spill of offshore oil tankers and easily separate from water. Adsorption resin is a new type of organic adsorption material developed in recent years, which has good adsorption performance and easy regeneration, and has the trend of gradually replacing activated carbon.