How to Treat Chemical Wastewater? Common Process Methods
The chemical industry comprises two major categories—organic chemicals and inorganic chemicals. [1] Chemical products are diverse and complex in composition. Wastewater discharged from chemical plants is called chemical wastewater. Chemical wastewater varies widely; most is highly toxic, difficult to purify, tends to accumulate in organisms, exhibits pronounced oxygen-consuming properties in water bodies, and easily degrades water quality.
1. Complex water-quality composition with many by-products. The raw materials for reactions are often solvents or cyclic-structure compounds, which increases the difficulty of treatment;
2. High concentration of pollutants in the wastewater, caused by incomplete raw-material conversion and the entry of large amounts of solvent media used in raw materials or production into the wastewater system;
3. Many toxic and hazardous substances—wastewater from fine-chemical production contains many organic pollutants that are toxic and harmful to microorganisms, such as halogenated compounds, nitro compounds, and dispersants or surfactants with bactericidal effects;
4. Many poorly biodegradable substances; the B/C ratio is low and biodegradability is poor;
Inorganic chemical wastewater comes from industries that produce basic chemical raw materials such as acids, alkalis, and salts from inorganic minerals. In such production the main use is cooling water, and the discharged wastewater contains acids, alkalis, large amounts of salts, and suspended solids, and sometimes sulfides and toxic substances. Organic chemical wastewater is more diverse in composition, including wastewater from the production of synthetic rubber, synthetic plastics, man-made fibers, synthetic dyes, paints and coatings, and pharmaceuticals. It has strongly oxygen-consuming properties, high toxicity, and—because most of it consists of artificially synthesized organic compounds—strong polluting potential and poor degradability.
1. Industrial wastewater flowing directly into channels, rivers, and lakes pollutes surface water; if highly toxic, it can cause the death or even extinction of aquatic flora and fauna;
2. Industrial wastewater may also seep into groundwater and pollute it;
3. If nearby residents use the polluted surface or groundwater as domestic water, it endangers their health, and in severe cases causes death;
4. Industrial wastewater seeping into soil causes soil pollution, affecting the growth of plants and soil microorganisms.
5. Some industrial wastewater also carries an unpleasant stench, polluting the air.
6. Toxic and hazardous substances in industrial wastewater are ingested and absorbed by animals and plants and remain in their bodies, then reach the human body through the food chain, causing harm.
For example, the water-pollution problem in the roadside ditch along Nonggu Avenue in Jiawang District, Xuzhou, Jiangsu, has persisted for years. Local sheep-hoof workshops illegally discharged a mixture of chemical wastewater containing caustic soda and hydrogen peroxide together with blood water, fat, and other organic pollutants, forming a 12-km-long black, malodorous waterbody that prevents irrigation and seriously affects the lives of nearby villagers. Over the past two years, the workshops discharged approximately 100 tons of caustic soda and 140 tons of hydrogen peroxide. Experts point out that such direct discharge of industrial wastewater has a serious impact on environmental media including air, soil, and groundwater. [7]
1. Strictly control the total volume of pollutant discharge and continue to reduce industrial pollution; promote zero-discharge systems with closed-loop wastewater recycling for key polluting industries such as steel, power, chemicals, and coal; and effectively strengthen the review and supervision of discharging units.
2. Accelerate the building of water-saving industry and a water-saving society. Further study management measures for industrial water conservation, standardize the scope of enterprise water conservation, and strictly enforce water-use quotas and water-conservation standards for key pollution-discharging industries.
As an emerging treatment process developed in recent years, micro-electrolysis has been widely applied. Micro-electrolysis filler produced by existing processes has overcome the drawback of hardening and passivation, and the filler can operate continuously and efficiently.
4. Strictly control persistent organic pollutants. Chemical wastewater has a relatively high proportion of persistent organic pollutants, which must be strictly controlled.
Chemical wastewater is discharged from every chemical-product production process (including process wastewater, cooling water, exhaust-gas scrubbing water, and equipment and tank-yard washing water).
Chemical-industry wastewater is generated during chemical production. Different industries, enterprises, raw materials, production methods, and equipment types all have a great influence on the quantity of wastewater generated and the types and concentrations of pollutants.
The characteristics of chemical wastewater are as follows:
1. Toxicity and irritancy
Chemical wastewater contains many pollutants, some of which are toxic or highly toxic substances—such as cyanide, phenol, arsenic, mercury, cadmium, and lead. Some substances are difficult to degrade and, through long-term accumulation in organisms, cause poisoning—such as organochlorine compounds. Some are claimed to be carcinogenic, such as polycyclic aromatic hydrocarbons (PAHs). In addition, there are irritating and corrosive substances such as inorganic acids and alkalis.
2. Both Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) are high
Chemical wastewater (especially petrochemical production wastewater) contains various organic acids, alcohols, aldehydes, ketones, ethers, and epoxides, characterized by high BOD and COD. Once discharged into a water body, such wastewater further oxidizes and decomposes in the water, consuming large amounts of dissolved oxygen and directly threatening the survival of aquatic organisms.
Wastewater discharged from chemical production sometimes shows strong acidity and sometimes strong alkalinity; pH is unstable and greatly harms aquatic organisms, structures, and crops.
4. Many eutrophying substances
Some chemical-production wastewater contains excessively high levels of phosphorus and nitrogen, causing eutrophication of water areas and massive proliferation of algae and microorganisms; in severe cases it forms “red tides,” causing large die-offs of fish.
Because chemical reactions often occur at high temperatures, the discharged wastewater has a high temperature. When such high-temperature wastewater is discharged into water areas, it causes thermal pollution of the water body, reduces dissolved oxygen, and thereby destroys the living conditions of aquatic organisms.
6. Oil pollution is fairly common
Petrochemical wastewater generally contains oil, which not only harms the survival of aquatic organisms but also increases the complexity of wastewater treatment.
Water areas polluted by harmful chemical substances, even if pollutant discharge is reduced or stopped, still take a very long time to recover to their original state—especially for heavy-metal pollutants that can be biologically enriched; even after discharge stops, the pollution state is difficult to eliminate.
1. In the production process, process water and cooling water consumption is very large; production processes are backward and equipment obsolete; clean and dirty flows are difficult to separate; water consumption is high; and the rate of water recycling and reuse is low.
2. Chemical wastewater has complex water quality and high pollutant content.
3. Most pollutants in the wastewater are toxic, such as heavy metals, lead, cadmium, etc.
From the above characteristics it can be seen that chemical wastewater is complex and difficult to treat.
The pollutants in chemical wastewater are diverse, so it is often impossible to remove all pollutants with a single treatment unit. Generally, a given wastewater must pass through a treatment system composed of several methods and several treatment units before it can meet discharge requirements.
According to the characteristics of different pollutants, various chemical-wastewater treatment methods have been developed. Based on their action principles, these methods are divided into four major categories: physical treatment, chemical treatment, physicochemical treatment, and biological treatment.
A wastewater-treatment method that uses physical action to separate and recover pollutants (including oil films and oil droplets) present in a suspended state in the wastewater. Depending on the physical action, it can be further divided into gravity separation, centrifugal separation, and screening/interception methods.
Compared with other methods, physical methods offer advantages such as simple equipment, low cost, convenient management, and stable effect, and are mainly used to remove floating matter, suspended solids, sand, oil, and similar substances from wastewater.
Physical methods include filtration, gravity separation, and centrifugal separation.
A wastewater-treatment method that uses chemical reactions and mass-transfer action to separate and remove dissolved or colloidal pollutants from wastewater, or to convert them into harmless substances. It can be used to remove metal ions, fine colloidal organic matter, inorganic matter, plant nutrients (nitrogen, phosphorus), emulsified oil, color, odor, and acids/alkalis from wastewater.
Chemical methods include neutralization, coagulation, redox, and electrochemical methods. [1]
In chemical and oil-refining enterprises, low-concentration acid- and alkali-containing wastewater, when there is no recovery or comprehensive-utilization value, is often treated by neutralization. Neutralization is also frequently used as a pretreatment step to adjust the pH of wastewater.
The coagulation method adds a coagulant to the wastewater; because the coagulant is an electrolyte, it forms micelles in the wastewater and undergoes electric neutralization with colloidal matter, forming flocs that settle. Flocculation and sedimentation can remove fine suspended particles of 10⁻³–10⁻⁶ size, as well as color, oil, microorganisms, nitrogen and phosphorus and other eutrophic substances, heavy metals, and organic matter.
After redox treatment, the organic and inorganic substances in the wastewater are transformed into non-toxic or low-toxicity substances, thereby achieving the purpose of wastewater treatment. Commonly used oxidation methods include air oxidation, chlorine oxidation, ozone oxidation, and wet oxidation.
Electrolysis is a process that uses direct current to carry out dissolution redox reactions. Generally, according to the purification mechanism of pollutants, it can be divided into electrolytic oxidation, electrolytic reduction, electrolytic coagulation, and electrolytic flotation.
Use physicochemical action to remove pollutants from wastewater. After physical treatment, wastewater still contains certain fine suspended matter and dissolved organic matter; to further remove residual pollutants in the water, physicochemical methods can be applied.
Mainly include adsorption, ion exchange, membrane separation, extraction, steam stripping, and air stripping.
A wastewater-treatment method that uses the metabolic action of microorganisms to convert dissolved, colloidal, and finely suspended organic pollutants in wastewater into stable, harmless substances.
The essence of the biological treatment process is an organic-matter decomposition process involving microorganisms; the microorganisms that decompose organic matter are mainly bacteria, while other microorganisms such as algae and protozoa also participate, but to a lesser extent.
3. Vigorously promote urban sewage treatment and reuse. Fundamentally avoid further deterioration of the urban water environment; in addition, improve the urban drainage system and raise the technical level of urban sewage treatment. Water-scarce cities should arrange the construction of reuse facilities while planning sewage-treatment facilities, and carry out advanced treatment of sewage.
Especially for wastewater with high organic concentration, high toxicity, high color, and poor biodegradability, it can greatly reduce the color and COD of the wastewater and raise the B/C ratio—that is, improve the biodegradability of the wastewater. It can be widely applied to the treatment and reuse of wastewater from printing and dyeing, chemicals, electroplating, pulp and paper, pharmaceuticals, wool washing, pesticides, soy-sauce, and alcohol industries, among others.
1. Dye and printing-dyeing wastewater; coking wastewater; petrochemical water—for the above wastewaters, the B/C value increases significantly while decoloring.
2. Petroleum wastewater; leather wastewater; pulp and paper wastewater, wood-processing wastewater—for the above wastewaters, the BOD/COD value increases substantially after treatment.
3. Electroplating wastewater; printing wastewater; mining wastewater; other wastewater containing heavy metals—heavy metals can be removed from the above wastewaters.
4. Organic-phosphorus agricultural wastewater; organic-chlorine agricultural wastewater—the biodegradability of the above wastewaters is greatly improved, and phosphorus and sulfides can be removed.
