Industrial wastewater treatment equipment

Industrial wastewater treatment equipment

Industrial wastewater encompasses production effluents, process wastewater, and cooling water. It refers to the wastewater and waste liquids generated during industrial manufacturing processes, containing industrial raw materials, intermediate products, by‑products, and pollutants that are carried away with the water. Industrial wastewater is highly diverse in type and complex in composition. For example, electrolytic salt‑production wastewater may contain mercury; heavy‑metal smelting wastewater often contains lead, cadmium, and other metals; electroplating wastewater typically includes cyanide and various heavy metals such as chromium; petroleum‑refining wastewater contains phenols; and pesticide‑manufacturing wastewater may harbor a range of pesticides. Because industrial wastewater frequently harbors multiple toxic substances, it poses significant risks to the environment and human health. Therefore, it is essential to develop strategies for its comprehensive utilization—transforming potential harm into benefits—and to implement appropriate treatment measures tailored to the specific constituents and concentrations of pollutants before discharge.

Classification of Industrial Wastewater

There are typically three types:

The first classification method categorizes industrial wastewater based on the chemical nature of its predominant pollutants: wastewater containing mainly inorganic pollutants is classified as inorganic wastewater, while wastewater dominated by organic pollutants is classified as organic wastewater. For example, electroplating wastewater and wastewater from mineral processing are inorganic, whereas wastewater from food or petroleum processing is organic. Wastewater generated in the textile dyeing and printing industry is typically mixed. Moreover, the composition of effluent varies across different industries.

The second classification method categorizes industrial wastewater according to the products and processing materials of the enterprises, such as metallurgical wastewater, papermaking wastewater, coking‑gas wastewater, metal pickling wastewater, chemical fertilizer wastewater, textile dyeing and printing wastewater, dyestuff wastewater, tannery wastewater, pesticide wastewater, power‑plant wastewater, and so on.

The third classification method categorizes wastewater according to the predominant pollutants it contains, such as acidic wastewater, alkaline wastewater, cyanide‑containing wastewater, chromium‑containing wastewater, cadmium‑containing wastewater, mercury‑containing wastewater, phenol‑containing wastewater, aldehyde‑containing wastewater, oil‑containing wastewater, sulfur‑containing wastewater, organophosphorus‑containing wastewater, and radioactive wastewater, among others.

The first two classification schemes do not address the principal constituents of pollutants in wastewater, nor do they indicate the potential hazards posed by the wastewater.

Principles of Industrial Wastewater Treatment

(1) Prioritize the adoption of non-toxic production processes to replace or upgrade outdated technologies, and strive to eliminate or minimize the generation of toxic and hazardous wastewater during production.

(2) During the use of toxic raw materials and the production of toxic intermediates and products, operations must be strictly controlled and monitored; leaks and spills shall be eliminated, losses minimized, and rational processes and equipment employed whenever possible.

(3) Wastewater containing highly toxic substances—such as certain heavy metals, radioactive materials, and high‑concentration phenol or cyanide—should be segregated from other wastewater streams to facilitate treatment and the recovery of valuable resources.

(4) Wastewater with high flow rates but low pollutant concentrations should be appropriately treated and recycled, rather than discharged into the sewer system, so as not to increase the load on urban sewers and municipal wastewater treatment facilities.

(5) Organic wastewater similar to municipal sewage, such as food-processing wastewater, sugar-refining wastewater, and paper-making wastewater, may be discharged into the municipal sewage system for treatment.

(6) Certain toxic wastewater streams that are biodegradable, such as phenol‑ and cyanide‑containing effluents, shall first be treated to meet the permissible discharge standards before being discharged into the municipal sewer system, followed by further biochemical treatment.

(7) Wastewater containing toxic substances that are difficult to biodegrade shall be treated separately and must not be discharged into the municipal sewer system. The emerging trend in industrial wastewater treatment is to recover and reuse wastewater and pollutants as valuable resources or to implement closed-loop recycling.

Main pollutants in industrial wastewater

Pollution caused by industrial wastewater primarily includes: organic oxygen-demanding substance pollution, chemical toxicant pollution, inorganic suspended solids pollution, heavy metal contamination, acid pollution, alkaline pollution, nutrient‑related pollution, thermal pollution, and pathogen contamination. Many of these pollutants are colored, malodorous, or prone to foaming, giving industrial wastewater an unappealing appearance and leading to widespread waterbody contamination that directly threatens public health and safety. Consequently, controlling industrial wastewater is of paramount importance.

Characteristics of Industrial Wastewater

A defining characteristic of industrial wastewater is the wide variability in both water quality and quantity, which depends on the specific production processes and methods employed. For instance, wastewater from sectors such as power generation and mining typically contains predominantly inorganic pollutants, whereas industries like papermaking and food processing generate effluents with very high organic content; their BOD5 (five-day biochemical oxygen demand) often exceeds 2,000 mg/L, and in some cases can reach as high as 30,000 mg/L. Even within the same production process, water quality can fluctuate significantly: in oxygen‑top‑blown converter steelmaking, the pH of the wastewater may range from 4 to 13 across different stages of a single heat, while suspended solids can vary between 250 and 25,000 mg/L.

Another characteristic of industrial wastewater is that, with the exception of indirect cooling water, it typically contains a variety of substances associated with the raw materials used in the process, and these substances often exist in different forms within the effluent. For example, fluorine in glass‑industry and electroplating wastewaters generally occurs as hydrogen fluoride (HF) or fluoride ions (F⁻), whereas in phosphatic fertilizer plant effluents it is present as silicon tetrafluoride (SiF₄); similarly, nickel may exist in wastewater either in its ionic form or as a complex species. These features further complicate the treatment and purification of such wastewaters.

The volume of industrial wastewater depends on water consumption. Industries such as metallurgy, papermaking, petrochemicals, and power generation consume large amounts of water, resulting in correspondingly large volumes of wastewater; for example, some steel plants generate 200 to 250 tons of wastewater per ton of steel produced. However, the actual amount of wastewater discharged by each plant also depends on the rate of water recycling. For instance, steel plants with high recycling rates discharge only about 2 tons of wastewater per ton of steel.

Industrial Wastewater Treatment Methods

The primary methods for treating high-concentration, hardly degradable organic wastewater include chemical oxidation, extraction, adsorption, incineration, catalytic oxidation, and biochemical treatment. Among these, the biochemical process stands out as the most mature, with simple equipment, large treatment capacity, and low operating costs, making it the most widely used approach in wastewater treatment.

In wastewater treatment projects, conventional biochemical processes are predominantly employed, such as the A/O process, the A2/O process, or their modified variants. Among these biochemical methods, the activated sludge process is currently one of the most widely used biological treatments for organic wastewater. With its large specific surface area, high activity, and efficient mass transfer, the activated sludge process is a highly effective artificial biological treatment method.

The purpose of wastewater treatment is to separate pollutants from wastewater by means of specific processes or to break them down into harmless, stable substances, thereby purifying the effluent. Typically, this involves preventing the transmission of toxic substances and pathogenic microorganisms and eliminating visible contaminants that cause unpleasant odors or sensations, in order to meet the requirements of various applications.

Wastewater treatment is quite complex, and the choice of treatment method must be based on the wastewater’s quality and quantity, as well as the receiving water body into which it is discharged or its intended use. At the same time, considerations should also include the handling and utilization of sludge and residues generated during the treatment process, potential secondary pollution issues, and the recovery and reuse of flocculants.

The selection of wastewater treatment methods depends on the nature, composition, and physical state of the pollutants in the wastewater, as well as the desired water quality standards. In general, wastewater treatment can be broadly categorized into three main types: physical, chemical, and biological processes.

Physical methods: These employ physical processes to treat, separate, and recover pollutants from wastewater. For example, sedimentation is used to remove suspended particles with a specific gravity greater than 1 while recovering these solids; flotation (or air flotation) can eliminate emulsified oil droplets or suspended matter with a specific gravity close to 1; filtration removes suspended particles from water; and evaporation is employed to concentrate non-volatile, soluble substances in wastewater, among other applications.

Chemical methods: These employ chemical reactions or physicochemical processes to recover soluble wastes or colloidal substances. For example, neutralization is used to treat acidic or alkaline wastewater; extraction exploits the differential solubility of soluble contaminants between two phases to recover phenols, heavy metals, and other compounds; and redox processes are employed to remove reducible or oxidizable pollutants from wastewater and to inactivate pathogenic microorganisms in natural water bodies.

Biological treatment: This method utilizes the biochemical activities of microorganisms to degrade organic pollutants in wastewater. For example, biofiltration and the activated sludge process are employed to treat domestic sewage or organic industrial effluents, converting and breaking down organic matter into inorganic salts and thereby achieving purification.

Each of the aforementioned methods has its own scope of applicability; therefore, it is essential to leverage their respective strengths while compensating for their limitations, ensuring they complement one another. In practice, it is often difficult to achieve satisfactory treatment results by relying on a single approach. The choice of which treatment method to apply to a particular wastewater stream depends first on factors such as the wastewater’s quality and quantity, the effluent discharge requirements, the economic value of waste recovery, and the characteristics of the available treatment technologies. Subsequently, this decision is informed by thorough investigations, scientific testing, and consideration of the prescribed effluent standards, local conditions, and technical feasibility.

Industrial Wastewater Discharge Standards

GB3545-83 Emission Standard for Water Pollutants in the Sugar‑Processing Industry GB3546-83 Emission Standard for Water Pollutants in the Sugarcane‑Sugar‑Processing Industry GB3547-83 Emission Standard for Water Pollutants in the Synthetic Fatty Acid Industry GB3548-83 Emission Standard for Water Pollutants in the Synthetic Detergent Industry GB3549-83 Emission Standard for Water Pollutants in the Leather‑Tanning Industry GB3550-83 Emission Standard for Water Pollutants in the Petroleum‑Development Industry GB3551-83 Emission Standard for Water Pollutants in the Petroleum‑Refining Industry GB3553-83 Emission Standard for Water Pollutants in the Film‑Developing Industry GB4280-84 Emission Standard for Water Pollutants in the Chromium‑Salt Industry GB4281-84 Emission Standard for Water Pollutants in the Petrochemical Industry GB4282-84 Emission Standard for Water Pollutants in the Sulfuric‑Acid Industry GB4283-84 Emission Standard for Water Pollutants in the Yellow‑Phosphorus Industry GB4912-85 Emission Standard for Water Pollutants in the Light‑Metal Industry GB4913-85 Emission Standard for Water Pollutants in the Heavy Nonferrous‑Metal Industry GB4916-85 Emission Standard for Water Pollutants in the Asphalt Industry GB5469-85 Emission Standard for Wastewater from Railway Freight‑Car Washing