Petrochemical wastewater treatment process
(1) Physical processing method
Gravity separation method
Gravity separation method utilizes the density difference between oil and water to make oil droplets in wastewater float to the surface under the action of gravity, thereby achieving oil-water separation. Common facilities include oil separators, such as horizontal flow oil separators, inclined plate oil separators, etc. The laminar flow oil separation tank has a simple structure and is easy to operate. By setting baffles in the tank, the retention time of wastewater is extended, allowing oil droplets to float and gather on the water surface for enough time. Then, the oil is collected by a scraping device. The inclined plate oil separator improves the oil-water separation efficiency and shortens the wastewater retention time by increasing the separation area. The gravity separation method has a significant effect on removing larger oil droplets from wastewater, but its removal effect on emulsified oil is limited.
Air flotation method
The air flotation method is to introduce air or other gases into wastewater to generate tiny bubbles, causing emulsified oil, fine suspended solids, and other substances in the wastewater to adhere to the bubbles and float to the surface with the bubbles to achieve separation. The air flotation method is divided into dissolved air flotation, dispersed air flotation, and electrolytic air flotation. Dissolved air flotation is the process of dissolving air in water under pressure, and then releasing tiny bubbles at atmospheric pressure. The bubbles have small particle size, uniform distribution, and good air flotation effect. The air flotation method can effectively remove emulsified oil and some dissolved organic matter, which plays an important role in improving the subsequent treatment effect of wastewater.
Filtering method
The filtration method is to intercept suspended solids, colloidal substances, etc. in wastewater through filtering media. The commonly used filter media include quartz sand, activated carbon, and filter mesh. For example, quartz sand filters can remove most of the solid particles in wastewater, reducing its turbidity. Activated carbon filters can not only remove suspended solids, but also adsorb some organic pollutants and odors in wastewater, further purifying the wastewater. The filtration method is often used as a part of pre-treatment or deep treatment processes, in combination with other treatment methods.
(2) Chemical treatment method
Coagulation sedimentation method
The coagulation precipitation method is to add coagulants to wastewater, causing the colloids and fine suspended solids in the wastewater to lose their stability, agglomerate with each other to form larger particles, and then separate them through precipitation. Common coagulants include aluminum salts (such as aluminum sulfate, polyaluminum chloride), iron salts (such as iron sulfate, polymeric iron sulfate), etc. The metal ions produced by the hydrolysis of coagulants in water undergo electrostatic neutralization, adsorption bridging, and other interactions with pollutants in wastewater, promoting the aggregation of pollutants. For example, polyaluminum chloride hydrolyzes in water to form aluminum hydroxide colloids, which can adsorb oil droplets, organic matter, and suspended solids in wastewater, forming larger floc precipitates. The coagulation sedimentation method has a good effect on removing suspended solids, some organic matter, and heavy metal ions in wastewater, but it will produce a large amount of sludge and requires proper treatment.
Chemical oxidation method
Chemical oxidation method is the use of oxidants to oxidize and decompose organic pollutants in wastewater into harmless or low toxicity substances. Common oxidants include ozone, hydrogen peroxide, chlorine based oxidants, etc. Ozone oxidation has strong oxidizing properties and can selectively oxidize various organic pollutants in wastewater, such as phenols, aldehydes, etc., decomposing them into small molecules such as carbon dioxide and water. Meanwhile, ozone can also play a role in decolorization and deodorization. Hydrogen peroxide can generate hydroxyl radicals in the presence of catalysts such as ferrous ions, which have stronger oxidation ability and can effectively treat some difficult to degrade organic compounds. The chemical oxidation method has a significant effect on improving the biodegradability of wastewater and removing recalcitrant organic matter, but the cost of oxidants is high and may cause secondary pollution.
Electrochemical method
Electrochemical method utilizes the principle of electrolysis to treat wastewater by undergoing redox reactions on the surface of electrodes. During the electrolysis process, pollutants in the wastewater are oxidized at the anode and may be reduced or undergo other reactions at the cathode. For example, strong oxidants such as hydroxyl radicals generated by the anode can oxidize organic pollutants, while the reduction effect of the cathode can remove heavy metal ions from wastewater. Electrochemical method has the advantages of high processing efficiency, no need to add a large amount of chemical agents, and simple operation, but it has high energy consumption and electrode materials are prone to wear and tear, which requires further optimization and improvement.
(3) Biological treatment method
Activated sludge process
Activated sludge process is a widely used biological treatment method. It utilizes the microbial community in activated sludge to adsorb, decompose, and metabolize organic matter in wastewater under aerobic conditions. Activated sludge is composed of various microorganisms such as bacteria, fungi, protozoa, and metazoa. They come into full contact with wastewater in the aeration tank and convert organic pollutants in the wastewater into carbon dioxide, water, and microbial cell substances. The activated sludge process can be divided into traditional activated sludge process, sequencing batch reactor (SBR), oxidation ditch process, etc. according to different operating modes. The traditional activated sludge method has a stable treatment effect, but there are problems such as sludge swelling; The SBR method has the advantages of simple process flow, good treatment effect, and strong adaptability to changes in water quality and quantity; Oxidation ditch has the characteristics of good treatment effect, low energy consumption, and low sludge production. The activated sludge process has a good effect on treating petrochemical wastewater with good biodegradability, but for wastewater containing difficult to degrade organic matter, pretreatment is needed to improve biodegradability.
Biofilm method
Biofilm method is to attach microorganisms to the surface of a solid carrier to form a biofilm. When wastewater flows through the biofilm, the pollutants in it are degraded by the microorganisms in the biofilm. Common biofilm methods include biofilters, biological turntables, and biological contact oxidation tanks. A biofilter forms a biofilm on the filter material, and wastewater passes through the filter material from top to bottom. Pollutants are adsorbed and degraded by the biofilm. The bio disc is composed of a series of discs, with a portion of the discs submerged in wastewater. Microorganisms grow on the surface of the discs to form biofilms. As the discs rotate, the biofilms alternately come into contact with wastewater and air to degrade pollutants. The biological contact oxidation tank is equipped with packing materials in the tank, and biofilm is attached to the packing materials. Through aeration, the wastewater is fully in contact with the biofilm. The biofilm method has the advantages of high microbial concentration, strong resistance to shock loads, and low sludge production, and has good application prospects for treating petrochemical wastewater.
Anaerobic biological treatment method
Anaerobic biological treatment method is the use of anaerobic microorganisms to decompose organic matter in wastewater into substances such as methane and carbon dioxide under anaerobic conditions. Common anaerobic biological treatment processes include Upflow Anaerobic Sludge Bed (UASB), Anaerobic Baffle Reactor (ABR), etc. In the UASB reactor, wastewater flows from bottom to top through the sludge bed. Anaerobic microorganisms in the sludge bed convert organic matter into methane and carbon dioxide, and the generated biogas drives the sludge and wastewater to flow upward, forming a good mass transfer effect. Anaerobic biological treatment method has a good treatment effect on high concentration organic wastewater, which can generate biogas for energy recovery and utilization. However, the organic matter content in the treated effluent is relatively high, and it often needs to be combined with aerobic biological treatment method.
(4) Deep processing method
Adsorption method
Adsorption method is the use of adsorbents to remove pollutants from wastewater through adsorption. Common adsorbents include activated carbon, resin, etc. Activated carbon has a developed pore structure and a large specific surface area, which can adsorb organic pollutants, heavy metal ions, etc. in wastewater. For example, powdered activated carbon can be directly added to wastewater for adsorption, while granular activated carbon can be used in fixed bed adsorption columns. Resin adsorbents have the characteristic of selective adsorption, which can adsorb specific pollutants as needed. For example, ion exchange resins can be used to remove heavy metal ions from wastewater. The adsorption method has good treatment effects, but the regeneration and replacement costs of the adsorbent are high, and it is necessary to design the adsorption process and regeneration plan reasonably.
Membrane separation method
Membrane separation method utilizes the selective permeability of membranes to separate pollutants from water in wastewater. The main membrane separation technologies include reverse osmosis (RO), ultrafiltration (UF), nanofiltration (NF), etc. Reverse osmosis membranes can block almost all ions and small molecule organic compounds, producing highly pure water suitable for applications that require high effluent quality, such as reuse treatment. Ultrafiltration membranes mainly remove large molecular organic compounds, colloids, and suspended solids from wastewater. Nanofiltration membranes are intermediate between ultrafiltration and reverse osmosis, with a certain ability to intercept divalent and multivalent ions, and can be used to remove some salts and organic matter from wastewater. Membrane separation method has good treatment effect and small footprint, but the membrane components have high cost and are prone to pollution, requiring regular cleaning and maintenance.