Industrial wastewater refers to wastewater, sewage and waste liquids produced during industrial production, which contains industrial production materials, intermediate products and products lost with water, as well as pollutants produced in the production process. With the rapid development of industry, the types and quantities of waste water have increased rapidly, and the pollution of water bodies has become increasingly widespread and serious, threatening human health and safety. Therefore, for the protection of the environment, the treatment of industrial wastewater is more important than the treatment of municipal wastewater. Industrial wastewater contains many toxic and harmful substances, and its treatment has always been a hot topic of concern. Industrial wastewater is divided into wastewater containing phenol, wastewater containing mercury, wastewater containing oil, wastewater containing heavy metals, wastewater containing cyanide, wastewater from paper industry, wastewater from printing and dyeing, wastewater from chemical industry, metallurgical wastewater, acid and alkali wastewater, etc. The treatment methods for different industrial wastewater are also different.

What are the hazards of phenol-containing wastewater and how can it be treated?
Phenol-containing wastewater mainly comes from industrial sectors such as coking plants, gas plants, petrochemical plants, insulating material plants and the production processes of petroleum cracking for ethylene, synthetic phenol, polyamide fibres, synthetic dyes, organic pesticides and phenolic resins. Phenol-containing wastewater mainly contains phenol-based compounds such as phenol, cresol, dimethyl phenol and nitro cresol. Phenolic-based compounds are protoplasmic toxicants that can cause protein coagulation.

Phenol-containing wastewater with a mass concentration of 1000mg/L is usually referred to as high concentration phenol-containing wastewater, which must be recovered and then treated. Phenol-containing wastewater with a mass concentration of less than 1000mg/L is referred to as low concentration phenol-containing wastewater. This type of wastewater is usually recycled and the phenol is concentrated and recovered for treatment. The methods of recovering phenol are solvent extraction, steam blowing, adsorption, closed cycle, etc. Wastewater containing phenol at a mass concentration of 300mg/L or less can be treated by biological oxidation, chemical oxidation, physical-chemical oxidation and other methods and then discharged or recycled.

How is mercury-containing wastewater treated and what are the characteristics of mercury-containing compounds?
Mercury-containing wastewater mainly comes from non-ferrous metal smelters, chemical plants, pesticide plants, paper mills, dye plants and thermal instrumentation plants. Methods of removing inorganic mercury from wastewater include sulphide precipitation, chemical coagulation, activated carbon adsorption, metal reduction, ion exchange and microbiological methods. Generally alkaline mercury-containing wastewater is usually treated by chemical coagulation or sulphide precipitation. Acidic mercury-containing wastewater can be treated by the metal reduction method. Organic mercury wastewater is more difficult to treat and is usually treated by oxidising organic mercury to inorganic mercury first.

What are the characteristics of oily wastewater and how can it be treated?
Oily wastewater mainly comes from industrial sectors such as petroleum, petrochemical, iron and steel, coking, gas generating stations, machining and so on. The relative density of oil pollutants in wastewater is less than 1, except for heavy tar, which has a relative density of 1.1 or more. (1) Floating oil, oil droplets with a particle size greater than 100μm, which can be easily separated from the wastewater. (2) Dispersed oil. The oil droplet particle size is between 10 and 100μm, and is pleaded to float in water. (3) Emulsified oil, the particle size of oil droplets is less than 10μm, not easy to be separated from the wastewater. As the concentration of oil in the wastewater discharged from different industrial sectors varies greatly, such as the wastewater produced 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 tar content in the wastewater discharged from the gas generating station can be up to 2000-3000mg/L.

Therefore, the treatment of oily wastewater should first use the grease trap to recover floating oil or heavy oil, with a treatment efficiency of 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 more difficult to treat, so the emulsification phenomenon should be prevented or mitigated. One of the methods is to reduce the emulsification of oil in the wastewater during the production process: the second is to minimise the number of times the wastewater is lifted by pumps during the treatment process to avoid increasing the degree of emulsification. Treatment methods are usually air flotation and emulsion breaking.

What are the sources of heavy metal wastewater and their treatment principles?
Heavy metal wastewater mainly comes from wastewater discharged from mining, smelting, electrolysis, electroplating, pesticides, pharmaceuticals, paints, industrial waste water pigments and other enterprises. The type, content and presence of heavy metals in wastewater varies from one manufacturer to another. Since heavy metals cannot be destroyed by decomposition, they can only shift their location and transform their physical and chemical forms.

For example, after chemical precipitation treatment, the heavy metals in the wastewater are transformed from dissolved ionic form to insoluble chemical form and precipitated from the water into the sludge: after ion exchange treatment, the heavy metal ions in the wastewater are transferred to the ion exchange resin, which is regenerated and then transferred from the ion exchange resin to the regeneration waste solution. Therefore, the principles of heavy metal wastewater treatment are: firstly, the fundamental thing is to reform the production process and not to use or use less toxic heavy metals, and secondly, to use reasonable process flow, scientific management and operation to reduce the amount of heavy metals and the amount lost with the wastewater, and to minimise the amount of wastewater discharged.

Heavy metal wastewater should be treated in situ at the place of generation and not mixed with other wastewater to avoid complicating treatment. It should not be discharged directly into urban sewers without treatment to avoid expanding heavy metal pollution. The treatment of heavy metal wastewater can usually be divided into two categories: one is to convert the heavy metals in the wastewater in a dissolved state into insoluble metal compounds or elements, which are removed from the wastewater by precipitation and upwelling. Methods such as neutralisation precipitation, sulphide precipitation, up-floating separation, electrolytic precipitation (or up-floating), diaphragm electrolysis, etc. can be applied: the second is to concentrate and separate the heavy metals in the wastewater without changing their chemical form, and methods such as reverse osmosis, electrodialysis, evaporation and ion exchange can be applied. These methods should be used individually or in combination according to the quality and quantity of the wastewater.

How to treat cyanide-containing wastewater?
Cyanide-containing wastewater mainly comes from electroplating, gas, coking, metallurgy, metal processing, chemical fiber, plastic, pesticide, chemical and other sectors. Cyanide wastewater is a toxic industrial wastewater, unstable in water and easy to decompose, inorganic cyanide and organic cyanide are both highly toxic substances, which can cause acute poisoning when ingested by humans. Cyanide is lethal to humans at 0.18, potassium cyanide at 0.12g, and the mass concentration of cyanide in water bodies lethal to fish is 0.04-0.1mg/L.

Cyanide-containing wastewater treatment measures mainly include: (1) reform the process to reduce or eliminate the outgoing cyanide-containing wastewater, such as the adoption of cyanide-free plating method can eliminate the industrial wastewater from the plating workshop. (2) Wastewater with high cyanide content should be recycled, and wastewater with low cyanide content should be purified and treated before discharge. Recycling methods include acidified aeration-alkali absorption method, steam desorption method, etc. Treatment methods include alkaline chlorination, electrolytic oxidation, pressurised hydrolysis, biochemical method, bio-iron method, ferrous sulphate method, air blow-off method, etc. The alkaline chlorination method is widely used, the ferrous sulphate method is incomplete and unstable, and the air-blown method pollutes the atmosphere and the effluent does not meet the discharge standard, so it is seldom used.

What are the characteristics of pesticide wastewater and its treatment method?
There are many varieties of pesticides and the quality of pesticide wastewater is complex. Its main characteristics are:

(1) high concentration of pollutants, chemical oxygen demand (COD) can reach tens of thousands of mg per litre: (2) high toxicity, in addition to containing pesticides and intermediates, the wastewater also contains toxic substances such as phenol, arsenic, mercury and many biodegradable substances: (3) malodorous, irritating to human respiratory tract and mucous membranes: (4) unstable water quality and quantity.

Therefore, the pollution of pesticide wastewater to the environment is very serious. The purpose of pesticide wastewater treatment is to reduce the concentration of pollutants in pesticide production wastewater, improve the recycling rate and strive to achieve harmlessness.

The treatment methods for pesticide wastewater include activated carbon adsorption, wet oxidation, solvent extraction, distillation and activated sludge methods. However, the development of high efficiency, low toxicity, low residue of new pesticides, which is the direction of pesticide development. Some countries have banned the production of organochlorine and organo-mercury pesticides such as hexa-hexa, and actively research and use microbial pesticides, which is a new way to fundamentally prevent pesticide wastewater from polluting the environment.

How to treat chemical industrial wastewater?
Chemical industry wastewater mainly comes from petrochemical industry, coal chemical industry, acid and alkali industry, chemical fertilizer industry, plastic industry, pharmaceutical industry, dye industry, rubber industry and other production wastewater. The main measures for the prevention and control of chemical wastewater pollution are: firstly, the production process and equipment should be reformed to reduce pollutants, prevent wastewater from being discharged, and carry out comprehensive use and recycling; for wastewater that must be discharged, the degree of treatment should be chosen according to the water quality and requirements.

The primary treatment mainly separates suspended solids, colloidal matter, oil slick or heavy oil in water. Water quality and quantity adjustment, natural sedimentation, floating and oil separation methods can be used.

Secondary treatment is mainly to remove biodegradable organic dissolved substances and some colloidal substances, reduce the biochemical oxygen demand and some chemical oxygen demand in the wastewater, usually using biological treatment. After biological treatment, a considerable amount of COD remains in the wastewater, sometimes with high colour, smell and taste, or due to high environmental health standards, further purification is required by tertiary treatment methods.

Tertiary treatment is mainly for the removal of 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.

In this way, the contamination of pesticide wastewater to the climate is intense. The motivation behind pesticide wastewater treatment is to diminish the grouping of contaminations in pesticide creation wastewater, further develop the reusing rate and endeavor to accomplish innocuousness.

The treatment techniques for pesticide wastewater incorporate initiated carbon adsorption, wet oxidation, dissolvable extraction, refining and enacted muck strategies. Notwithstanding, the improvement of high effectiveness, low poisonousness, low buildup of new pesticides, which is the bearing of pesticide advancement. A few nations have restricted the development of organochlorine and organo-mercury pesticides, for example, hexa-hexa, and effectively examination and utilize microbial pesticides, which is a better approach to in a general sense keep pesticide wastewater from dirtying the climate.

How to treat substance modern wastewater?
Substance industry wastewater basically comes from petrochemical industry, coal synthetic industry, corrosive and antacid industry, compound manure industry, plastic industry, drug industry, color industry, elastic industry and other creation wastewater. The primary measures for the avoidance and control of substance wastewater contamination are: initially, the creation interaction and hardware ought to be changed to lessen poisons, keep wastewater from being released, and do exhaustive use and reusing; for wastewater that should be released, the level of treatment ought to be picked by the water quality and necessities.

The essential treatment primarily isolates suspended solids, colloidal matter, oil spill or weighty oil in water. Water quality and amount change, regular sedimentation, drifting and oil partition techniques can be utilized.

Auxiliary treatment is fundamentally to eliminate biodegradable natural broke down substances and a few colloidal substances, lessen the biochemical oxygen interest and some compound oxygen interest in the wastewater, ordinarily utilizing organic treatment. After natural treatment, a lot of COD remaining parts in the wastewater, here and there with high tone, smell and taste, or because of high ecological wellbeing principles, further decontamination is expected by tertiary treatment techniques.

Tertiary treatment is principally for the evacuation of natural toxins and disintegrated inorganic poisons that are hard to biodegrade in wastewater. Usually utilized strategies incorporate initiated carbon adsorption and ozone oxidation, and particle trade and film partition methods can likewise be utilized.

Different synthetic modern wastewater can be treated in various ways as per different water quality, water amount and the necessities of the treated release water quality.