I. Basic Process of Industrial Wastewater Treatment
The industrial wastewater treatment process is usually designed based on the quality, quantity and treatment goals of the wastewater, and generally includes the following stages:
Preprocessing stage
Regulating tank: Balances the water quality (such as pH value, pollutant concentration) and water volume of the wastewater to prevent subsequent treatment equipment from being impacted.
Grating and grit chamber: Remove larger suspended solids (such as fibers, debris) and sand particles in the wastewater to prevent pipe blockage.
Neutralization treatment: By adding acid-base regulators (such as lime, sulfuric acid), the pH value of the wastewater is adjusted to an appropriate range (usually 6-9), creating conditions for subsequent treatment.
Main processing stage
Physical, chemical or biological treatment methods should be selected based on the types of pollutants in the wastewater (such as organic matter, heavy metals, suspended solids, etc.).
Deep processing stage
The effluent after the main treatment is further purified to remove the residual trace pollutants, so that the water quality meets the reuse standards or more stringent discharge standards.
Sludge treatment stage
Concentrate, dewater and stabilize the sludge produced during the treatment process to prevent secondary pollution.
Ii. Common Methods for Industrial Wastewater Treatment
(1) Physical treatment method
Precipitation method
Principle: Utilize the force of gravity to make suspended solids in wastewater settle and separate.
Application: It is often used to remove inorganic particles (such as sand grains, metal oxides) and some organic substances (such as fibers, oil droplets) from wastewater. For instance, the precipitation of heavy metal hydroxides in electroplating wastewater.
Filtration method
Principle: Through the retention effect of filter materials (such as quartz sand, activated carbon, membranes), suspended solids and colloids in wastewater are removed.
Application: Membrane filtration (such as ultrafiltration and reverse osmosis) is commonly used in advanced treatment to remove dissolved organic matter and heavy metal ions, such as in the reuse treatment of wastewater from the electronics industry.
Air flotation method
Principle: Air is introduced into the wastewater or bubbles are generated to make pollutants adhere to the bubbles and float to the water surface, thereby separating and removing them (such as emulsified oil and fine suspended solids).
Application: Oil removal treatment in petrochemical wastewater.
(2) Chemical treatment method
Coagulation method
Principle: Coagulants (such as polyaluminium chloride and polyacrylamide) are added to cause colloids and fine suspended solids in the wastewater to aggregate into large particles and settle.
Application: Removing color and suspended solids from printing and dyeing wastewater, and treating fibrous pollutants from papermaking wastewater.
REDOX method
Principle: Pollutants are oxidized or reduced into harmless substances through oxidants (such as ozone, hydrogen peroxide, sodium hypochlorite) or reductants (such as ferrous salts).
Application: Cyanide wastewater is oxidized and decomposed by sodium hypochlorite into non-toxic nitrogen and carbonate. The chromium-containing wastewater is treated by reducing hexavalent chromium to trivalent chromium with ferrous salts and then precipitated and removed.
Chemical precipitation method
Principle: Chemical agents (such as sulfides and hydroxides) are added to react with heavy metal ions in the wastewater to form insoluble precipitates.
Application: Removal of heavy metal ions such as copper, nickel and zinc from electroplating wastewater.
(3) Biological treatment method
Aerobic biological treatment
Principle: By taking advantage of the metabolic activities of aerobic microorganisms (such as bacteria and fungi), organic matter is decomposed into carbon dioxide and water.
Common techniques:
Activated sludge process: Through aeration, microorganisms form flocculent sludge, which adsorbs and degrades organic matter, such as the AAO process commonly used in urban sewage treatment plants.
Biofilm method: Microorganisms attach to the surface of the filler to form a biofilm. When wastewater flows through, it is purified, such as the biological contact oxidation method and the biological aerated filter (BAF).
Anaerobic biological treatment
Principle: Under anaerobic conditions, anaerobic microorganisms decompose organic matter into gases such as methane and carbon dioxide, while removing pollutants.
Application: Pretreatment of high-concentration organic wastewater (such as food processing wastewater and brewing wastewater), which can reduce the load of subsequent aerobic treatment.
Iii. Characteristics of Industrial Wastewater Treatment in Typical Industries
Characteristics of industrial wastewater, difficulties and key points in treatment, and common treatment methods
The chemical industry contains organic substances (such as benzene and phenol), heavy metals, and salts, which are highly toxic, have high color intensity, and poor biodegradability. Remove toxic organic substances and heavy metals to improve biodegradability. Pretreatment (coagulation + oxidation) + anaerobic + aerobic combined process.
The electroplating industry contains heavy metals (chromium, cadmium, nickel) and cyanide, and the pH value fluctuates greatly. Therefore, the emission standards must be strictly controlled. Classification treatment and recovery of heavy metals, and complete oxidation of cyanide. Chemical precipitation method (hydroxide/sulfide) + membrane treatment.
The printing and dyeing industry contains dyes, sizing agents, and surfactants, featuring high color intensity, high organic matter concentration, and strong alkalinity. Decolorization and removal of organic matter to reduce COD (Chemical Oxygen Demand). Coagulation sedimentation + anaerobic biological treatment + aerobic biological treatment.
The papermaking industry contains lignin, fibers, bleaching agents, a large amount of suspended solids, high COD and deep color. Remove suspended solids and organic matter to reduce chroma. Air flotation method + anaerobic + aerobic treatment + deep filtration.
Food processing contains protein, oil and sugar, has a high concentration of organic matter, is prone to spoilage and has good biodegradability. Reduce the concentration of organic matter and prevent excessive reproduction of microorganisms. Pretreatment (oil separation + sedimentation) + aerobic biological treatment.
Iv. Development Trends of Industrial Wastewater Treatment
Resource utilization
The shift from "meeting discharge standards" to "wastewater reuse", such as recovering water resources from wastewater through membrane technology or extracting valuable substances (such as heavy metals and chemical raw materials) from wastewater.
Intelligence and Automation
By applying Internet of Things (iot) and big data technologies to monitor water quality and the operational status of treatment equipment in real time, precise dosing and intelligent regulation can be achieved, thereby enhancing treatment efficiency and reducing costs.
Efficient and low-cost technology research and development
Develop new types of catalysts (such as photocatalysis and electrocatalysis) and highly efficient biological strains to enhance the removal efficiency of refractory pollutants and reduce treatment energy consumption at the same time.
Zero emission target
Some highly polluting industries (such as coal chemical engineering and electroplating) are promoting "zero-emission" technologies, treating wastewater through processes like evaporation and crystallization to a state where there is no liquid discharge and only solid waste is produced.
V. Precautions
Water quality monitoring: Real-time monitoring of wastewater quality, adjusting treatment processes based on changes in pollutants to prevent fluctuations in treatment effects.
Compliant discharge: Strictly comply with national and local discharge standards (such as "Integrated Wastewater Discharge Standard" GB 8978-1996), and different industries have their own exclusive discharge standards (such as "Discharge Standard of Pollutants for Electroplating Industry" GB 21900-2008).
Safe operation: When treating toxic wastewater (such as cyanide and heavy metals), it is necessary to take proper protective measures for the operators to prevent the leakage of chemicals and secondary pollution.
Industrial wastewater treatment