The discharge of industrial wastewater is one of the important factors that cause water quality safety problems. Failure to meet the standards for industrial wastewater treatment will lead to serious consequences. The following is a brief introduction to some industrial wastewater treatment methods.

Phenolic wastewater

Phenol-containing wastewater mainly comes from coking plants, gas plants, petrochemical plants, insulation material plants and other industrial sectors, as well as the production process of petroleum cracking to ethylene, synthetic phenol, polyamide fiber, synthetic dyes, organic pesticides and phenolic resin. Phenol-containing wastewater mainly contains phenol-based compounds, such as phenol, cresol, xylenol, and nitrocresol. Phenolic compounds are protoplasmic poisons that coagulate proteins.

When the mass concentration of phenol in the water reaches 0.1-0.2mg /L , the fish will have a peculiar smell and cannot be eaten ; if the mass concentration increases to 1mg /L , it will affect fish spawning. die. Phenols in drinking water can affect human health. Even if the concentration of phenols in water is only 0.002mg/L , disinfection with chlorine will produce chlorophenols stench. Usually, the phenol -containing wastewater with a mass concentration of 1000mg/L is called high-concentration phenol-containing wastewater, and this wastewater must be recovered after phenol is recovered before treatment.

Phenol-containing wastewater with a mass concentration of less than 1000 mg/L is called low-concentration phenol-containing wastewater. Usually, this kind of waste water is recycled, and the phenol is concentrated and recovered for post-treatment. Methods for recovering phenol include solvent extraction, steam stripping, adsorption, and closed cycle methods. Wastewater with a concentration of phenol below 300mg/L can be treated by biological oxidation, chemical oxidation, physical chemical oxidation and other methods before being discharged or recycled.

Mercury wastewater

Mercury-containing wastewater mainly comes from non-ferrous metal smelters, chemical plants, pesticide plants, paper mills, dye plants, and thermal instrumentation plants. Methods for removing inorganic mercury from wastewater include sulfide precipitation, chemical coagulation, activated carbon adsorption, metal reduction, ion exchange, and microbial methods. Generally, alkaline mercury-containing wastewater is usually treated by chemical coagulation or sulfide precipitation. Acidic mercury-containing wastewater can be treated by metal reduction. Low-concentration mercury-containing wastewater can be treated by activated carbon adsorption, chemical coagulation or activated sludge. Organic mercury wastewater is difficult to treat. Usually, organic mercury is first oxidized to inorganic mercury and then treated.

The toxicity of various mercury compounds varies widely. Elemental mercury is basically non-toxic ; mercury chloride in inorganic mercury is a highly toxic substance, and phenylmercury in organic mercury decomposes quickly and is not very toxic; methylmercury is easily absorbed when it enters the human body, not easy to degrade, and excreted very slowly. It is easy to accumulate in the brain. High toxicity, such as Minamata disease is caused by methylmercury poisoning.

Oily Wastewater Treatment

Oily wastewater mainly comes from petroleum, petrochemical, steel, coking, gas generation stations, mechanical processing and other industrial sectors. The relative density of oil pollutants in waste water is less than 1 except heavy tar whose relative density is above 1.1 . Oil substances usually exist in three states in wastewater. (1) Floating oil, the particle size of oil droplets is greater than 100μm , which is easy to separate from wastewater. (2) Dispersed oil . The particle size of oil droplets is between 10 and 100 μm , and they float in water. (3) Emulsified oil, the particle size of oil droplets is less than 10μm , and it is not easy to separate from wastewater.

Due to the great difference in the oil concentration in the wastewater discharged from different industrial sectors, such as the wastewater generated in the oil refining process, the oil content is about 150-1000mg / L , the tar content in the coking wastewater is about 500-800mg /L , and the wastewater discharged from the gas generation station The tar content can reach 2000-3000mg / L .

Therefore, the treatment of oily wastewater should first use the grease trap to recover floating oil or heavy oil, the treatment efficiency is 60% to 80% , and the oil content in the effluent is about 100 to 200mg/L; the emulsified oil and dispersed oil in the wastewater are difficult Therefore, emulsification should be prevented or reduced. One of the methods is to pay attention to reducing the emulsification of oil in the wastewater during the production process ; the second is to minimize the number of times the wastewater is lifted by the pump during the treatment process so as not to increase the degree of emulsification. Treatment methods usually adopt air flotation and demulsification.

Heavy metal wastewater

Heavy metal wastewater mainly comes from wastewater discharged from mining, smelting, electrolysis, electroplating, pesticides, medicines, paints, pigments and other enterprises. The type, content and existing form of heavy metals in wastewater vary with different production enterprises. Since heavy metals cannot be decomposed and destroyed, they can only transfer their existing positions and change their physical and chemical forms.

For example, after chemical precipitation treatment, heavy metals in wastewater are transformed from dissolved ions into insoluble compounds and precipitated, and transferred from water to sludge; after ion exchange treatment, heavy metal ions in wastewater are transferred to ion exchange resins After regeneration, it is transferred from the ion exchange resin to the regeneration waste liquid. Therefore, the principle of heavy metal wastewater treatment is: First, the fundamental thing is to reform the production process . Do not use or use less toxic heavy metals ; secondly, use reasonable process flow, scientific management and operation to reduce the amount of heavy metals and the amount of waste water loss, as much as possible Reduce the amount of waste water discharged outside.

Heavy metal wastewater should be treated at the place where it is generated, and not mixed with other wastewater, so as not to complicate the treatment. It should not be directly discharged into urban sewers without treatment, so as not to expand heavy metal pollution.

The treatment of heavy metal wastewater can usually be divided into two categories ; one is to convert the dissolved heavy metals in the wastewater into insoluble metal compounds or elements, which are removed from the wastewater by precipitation and flotation . Applicable methods such as neutralization precipitation, Sulfide precipitation method, floating separation method, electrolytic precipitation ( or floating ) method, diaphragm electrolysis method, etc.; the second is to concentrate and separate the heavy metals in the wastewater without changing their chemical form. The applicable methods include reverse osmosis method , electrodialysis, evaporation and ion exchange. These methods should be used alone or in combination according to the wastewater quality, water quantity, etc.

Cyanide Wastewater

Cyanide-containing wastewater mainly comes from electroplating, gas, coking, metallurgy, metal processing, chemical fiber, plastic, pesticide, chemical and other departments. Cyanide-containing wastewater is a kind of highly toxic industrial wastewater. It is unstable in water and easy to decompose. Both inorganic cyanide and organic cyanide are highly toxic substances, and human ingestion can cause acute poisoning. The lethal dose of cyanide to human body is 0.18 , potassium cyanide is 0.12g , and the lethal mass concentration of cyanide to fish in water is 0.04-0.1mg /L .

The treatment measures for cyanide-containing wastewater mainly include: (1) Reform the process to reduce or eliminate the discharge of cyanide-containing wastewater. For example, the use of cyanide-free electroplating can eliminate the industrial wastewater in the electroplating workshop. (2) Wastewater with high cyanide content should be recycled, and waste water with low cyanide content should be purified before discharge. Recovery methods include acidification aeration, lye absorption, steam desorption, etc.

Treatment methods include alkaline chlorination method, electrolytic oxidation method, pressurized hydrolysis method, biochemical method, biological iron method, ferrous sulfate method, air blowing method, etc. Among them, the alkaline chlorination method is widely used, the ferrous sulfate method is not thorough and unstable, and the air stripping method pollutes the atmosphere, and the effluent does not meet the discharge standard . It is rarely used.

Paper Industry Wastewater

Papermaking wastewater mainly comes from the two production processes of pulping and papermaking in the papermaking industry. Pulping is to separate the fibers from plant raw materials, make pulp, and then bleach it ; papermaking is to dilute, shape, squeeze, and dry the pulp to make paper. Both processes discharge large volumes of wastewater. The waste water produced by pulping is seriously polluted. The wastewater discharged during pulp washing is dark brown, which is called black water. The concentration of pollutants in black water is very high, BOD is as high as 5-40g/L , and it contains a lot of fibers, inorganic salts and pigments.

The wastewater discharged from the bleaching process also contains a large amount of acid and alkali substances. The waste water discharged from the paper machine is called white water, which contains a lot of fibers and fillers and sizing materials added during the production process. The treatment of paper industry wastewater should focus on increasing the recycling water rate, reducing water consumption and wastewater discharge, and actively exploring various reliable, economical and fully utilizing useful resources in wastewater treatment methods.

For example, the flotation method can recover fibrous solid matter in white water, the recovery rate can reach 95% , and the clarified water can be reused ; the combustion method can recover sodium hydroxide, sodium sulfide, sodium sulfate and other sodium salts combined with organic matter in black water. Neutralization method can adjust the pH value of wastewater ; coagulation sedimentation or flotation method can remove suspended solids in wastewater ; chemical precipitation method can decolorize ; biological treatment method can remove BOD , which is more effective for kraft paper wastewater ; wet oxidation method is more effective in treating sulfurous acid pulp wastewater success. In addition, there are also treatment methods such as reverse osmosis, ultrafiltration, and electrodialysis at home and abroad.

Printing and dyeing industrial wastewater

The printing and dyeing industry consumes a lot of water , usually 100-200t of water is consumed per ton of textiles for printing and dyeing . 80% -90 % of it is discharged as printing and dyeing wastewater. Common treatment methods include recycling and harmless treatment.

Recycling: (1) Wastewater can be recycled according to water quality characteristics, such as the diversion of bleaching and cooking wastewater and dyeing and printing wastewater. The former can be washed by convection. One water can be used for multiple purposes to reduce emissions ; Evaporation method recovery, if the amount of lye is large, it can be recovered by three-effect evaporation, if the amount of lye is small, it can be recovered by thin film evaporation; ( 3) dye recovery . For example, Shihlin dye can be acidified into cryptobaic acid, which is colloidal particles . Suspended in residual In the liquid, it is recycled after precipitation and filtration.

Harmless treatment can be divided into: (1) Physical treatment methods include precipitation and adsorption. The precipitation method mainly removes suspended solids in wastewater ; the adsorption method mainly removes dissolved pollutants and decolorization in wastewater. (2) Chemical treatment methods include neutralization, coagulation and oxidation. The neutralization method is to adjust the pH in the wastewater, and it can also reduce the chroma of the wastewater ; the coagulation method is to remove the disperse dyes and colloidal substances in the wastewater ; the oxidation method is to oxidize the reducing substances in the wastewater, so that the sulfur dyes and vat dyes are precipitated. (3) Biological treatment methods include activated sludge, biological turntable, biological drum, and biological contact oxidation.

In order to improve the quality of effluent water and meet discharge standards or recycling requirements , it is often necessary to adopt several methods for combined treatment.

Dye Production Wastewater

Dye production wastewater contains acids, alkalis, salts, halogens, hydrocarbons, amines, nitro substances, dyes and their intermediates, and some also contain pyridine, cyanide, phenol, benzidine, and heavy metals such as mercury, cadmium, and chromium. These wastewater components are complex . They are toxic and difficult to handle. Therefore, the treatment of dye production wastewater should select appropriate treatment methods according to the characteristics of wastewater and its discharge requirements .

For example: to remove solid impurities and inorganic substances, coagulation and filtration methods can be used ; to remove organic substances and toxic substances, chemical oxidation, biological and reverse osmosis methods are mainly used ; decolorization generally adopts a process composed of coagulation and adsorption methods Process, to remove heavy metals can use ion exchange method and so on.

Chemical Industry Wastewater

Chemical industry wastewater mainly comes from production wastewater discharged from petrochemical industry, coal chemical industry, acid-base industry, fertilizer industry, plastics industry, pharmaceutical industry, dye industry, rubber industry, etc.

The main measures for the prevention and control of chemical wastewater pollution are: first, the production process and equipment should be reformed to reduce pollutants, prevent wastewater from being discharged, and carry out comprehensive utilization and recycling; the degree of treatment of wastewater that must be discharged should be selected according to water quality and requirements. Primary treatment mainly separates suspended solids, colloids, slick oil or heavy oil in water. Methods such as water quality and quantity adjustment, natural sedimentation, flotation and oil separation can be used. The secondary treatment is mainly to remove biodegradable organic dissolved substances and some colloids, and reduce the biochemical oxygen demand and part of the chemical oxygen demand in the wastewater. Biological treatment is usually used.

A considerable amount of COD remains in the wastewater after biological treatment, sometimes with high color, smell, and taste, or because of high environmental sanitation standards, it needs to be further purified by tertiary treatment methods. Tertiary treatment is mainly to remove organic pollutants and dissolved inorganic pollutants that are difficult to biodegrade in wastewater. Commonly used methods include activated carbon adsorption and ozone oxidation, and ion exchange and membrane separation techniques can also be used.

Different treatment methods can be selected for various chemical industrial wastewater according to different water quality, water volume and the requirements of the treated effluent quality.

Acid-base wastewater

Acid wastewater mainly comes from iron and steel factories, chemical factories, dye factories, electroplating factories and mines, etc., which contain various harmful substances or heavy metal salts. The mass fraction of acid varies greatly, ranging from low less than 1% to high greater than 10% . Alkaline wastewater mainly comes from printing and dyeing factories, leather factories, paper mills, oil refineries, etc. Some of them contain organic or inorganic bases.

The mass fraction of alkali is higher than 5% and lower than 1% . Acid-base wastewater often contains acid salts, basic salts, and other inorganic and organic substances in addition to acids and bases. Acid-base wastewater is highly corrosive and must be properly treated before it can be discharged.

The general principles of treating acid-base wastewater are: (1) High-concentration acid-base wastewater should be recycled and reused first. According to water quality, water volume and different process requirements, the plant or regional dispatch should be carried out, and reuse should be possible: if it is difficult to reuse, Or if the concentration is low and the water volume is large, the acid and alkali can be recovered by concentration and membrane method. (2) Low-concentration acid-base wastewater, such as the cleaning water of the pickling tank and the rinsing water of the alkali washing tank, should be neutralized. For neutralization treatment, the principle of treating waste with waste should be considered first. For example, acid and alkali wastewater are mutually neutralized, or waste alkali ( slag ) is used to neutralize acid wastewater, and waste acid is used to neutralize alkaline wastewater. In the absence of these conditions, neutralizer treatment can be used.

Metallurgical wastewater

The main characteristics of metallurgical wastewater are large water volume, various types, and complex and changeable water quality. According to the source and characteristics of wastewater, it mainly includes cooling water, pickling wastewater, washing wastewater ( dust removal, gas or flue gas ) , slag washing wastewater, coking wastewater, and wastewater condensed, separated or overflowed from production.

The development trend of metallurgical wastewater treatment is: (1) Develop and adopt new processes and technologies that do not use or use less water and have no or less pollution, such as dry coke quenching, coking coal preheating, and desulfurization directly from coke oven gas decyanation , etc.; (2) develop comprehensive utilization technologies, such as recovering useful substances and heat energy from waste water and waste gas, and reduce the loss of materials and fuels; Improve the recycling rate of water ; (4) develop new treatment processes and technologies suitable for the characteristics of metallurgical wastewater, such as the use of magnetic methods to treat steel wastewater. It has the advantages of high efficiency, less land occupation, and convenient operation and management.