Industrial wastewater refers to the wastewater, sewage, and waste liquids generated during industrial production processes. It contains industrial raw materials, intermediate products, and pollutants that are lost with the water during production. The common industrial wastewaters and their characteristics are classified into several categories:
1. Coal-Fired Power Plant Desulfurization Wastewater Characteristics and Treatment Process
Characteristics of Power Plant Desulfurization Wastewater:
Most power plants use a limestone-gypsum wet desulfurization process for flue gas desulfurization. This process mainly consists of a limestone slurry preparation system, gypsum dehydration system, and desulfurization wastewater treatment system. During the continuous circulation of water in the desulfurization slurry, heavy metals and chloride ions (Cl-) accumulate. This accelerates the corrosion of desulfurization equipment and affects the quality of gypsum, making it necessary to discharge the wastewater promptly.
Desulfurization Wastewater Treatment Process:
The treatment process includes the following steps:
Desulfurization Wastewater → Wastewater Tank → Wastewater Pump → pH Neutralization Tank → Settling Tank → Flocculation Tank → Clarifier → Discharge Tank → Discharge Pump → Standard Discharge.
The wastewater treatment system consists of three main components: wastewater treatment, dosing, and sludge treatment. The system is equipped with wastewater tanks, three-in-one tanks, clarifiers, sludge pumps, discharge tanks, clear water pumps, fans, and dewatering machines. The wastewater contains various impurities such as Cl-, Mg2+, fluorides, nitrites, heavy metals (e.g., Cu2+, Hg2+), and insoluble CaSO4 and fine dust. To meet discharge standards, appropriate treatment devices are required.
2. Chemical Industry Wastewater
Chemical industry wastewater mainly comes from industries such as petroleum, coal chemistry, acid-base manufacturing, fertilizers, plastics, pharmaceuticals, dyes, and rubber.
Main Measures for Pollution Prevention and Control:
The main measures to prevent pollution are to reform production processes and equipment, reduce pollutants, prevent wastewater discharge, and promote comprehensive utilization and recycling. For wastewater that must be discharged, the treatment degree should be chosen according to the water quality and requirements.
- Primary Treatment: This mainly involves separating suspended solids, colloids, floating oils, or heavy oils from the water. Methods such as water quality and quantity adjustment, natural sedimentation, flotation, and oil separation are commonly used.
- Secondary Treatment: This aims to remove biodegradable organic soluble substances and some colloids, reducing biochemical oxygen demand (BOD) and some chemical oxygen demand (COD). Biological treatment is typically used. After biological treatment, wastewater may still contain considerable COD and sometimes high color, odor, or taste. In such cases, tertiary treatment is required to further purify the water.
- Tertiary Treatment: This is used to remove non-biodegradable organic pollutants and dissolved inorganic pollutants. Common methods include activated carbon adsorption, ozone oxidation, ion exchange, and membrane separation technologies. Various chemical industrial wastewater can be treated using different methods based on water quality, volume, and the required discharge quality.
3. Textile Printing and Dyeing Industry Wastewater
The textile printing and dyeing industry consumes large amounts of water. Typically, for every ton of fabric processed, 100 to 200 tons of water is used, with 80% to 90% of that water discharged as wastewater. Common treatment methods include recycling and harmless treatment.
Recycling:
Wastewater can be recycled and reused based on its characteristics. For instance, wastewater from bleaching and scouring, and from dyeing and printing can be separately collected. The former can be used for countercurrent washing or multi-use to reduce discharge volume.
- Alkaline Liquor Recycling: This is usually done through evaporation methods. If the alkaline liquid volume is large, a triple-effect evaporator can be used. For smaller volumes, a thin-film evaporator is more efficient.
- Dye Recycling: Certain dyes, such as those from the Shilin dye, can be acidified into colloidal particles, which can then be suspended in the residual liquid. After precipitation and filtration, they can be recovered and reused.
Harmless Treatment:
- Physical Treatment: Methods include precipitation and adsorption. Precipitation is mainly used to remove suspended solids from the wastewater, while adsorption is employed to remove dissolved pollutants and decolorize the wastewater.
- Chemical Treatment: Methods such as neutralization, coagulation, and oxidation are common. Neutralization adjusts the pH level of the wastewater and can also reduce its color. Coagulation removes dispersed dyes and colloidal substances, while oxidation is used to treat reducible substances in the wastewater, causing sulfur-based and reducing dyes to precipitate.
- Biological Treatment: Methods like activated sludge, biological disc treatment, biological drum treatment, and biological contact oxidation are often used. To improve effluent quality and meet discharge or recycling standards, multiple treatment methods are often used in combination.
4 Paper Industry Wastewater
Paper industry wastewater mainly arises from the pulping and papermaking processes. Pulping involves separating fibers from plant materials and making the pulp, followed by bleaching. Papermaking involves diluting the pulp, shaping it, pressing, and drying to form paper. Both processes generate large amounts of wastewater.
- Pulping Wastewater: The most polluted, particularly black liquor produced during pulping. This wastewater is dark brown and has high pollutant concentrations, including BOD levels between 5-40 g/L, a large amount of fiber, inorganic salts, and pigments.
- Bleaching Wastewater: Contains a significant amount of acidic and alkaline substances.
- Papermaking Wastewater: Known as white water, which contains a lot of fiber and additives such as fillers and adhesives used in the process.
Paper Industry Wastewater Treatment:
The focus should be on improving water reuse rates, reducing water consumption, and minimizing wastewater discharge. Additionally, exploring reliable, cost-effective methods that can fully utilize valuable resources in the wastewater is important. For example:
- Froth Flotation: Can recover fibrous solids from white water with a recovery rate of up to 95%, with clarified water being reusable.
- Combustion: Can recover sodium hydroxide, sodium sulfide, sodium sulfate, and other sodium salts bound to organic substances in black water.
- Neutralization: Adjusts the pH of the wastewater.
- Coagulation and Flotation: Can remove suspended solids from the wastewater.
- Chemical Precipitation: Can be used for decolorization.
- Biological Treatment: Effective in removing BOD, especially for kraft paper mill wastewater.
- Wet Oxidation: Has been successful in treating sulfite pulp wastewater. Additionally, reverse osmosis, ultrafiltration, and electrodialysis are also used for wastewater treatment.
5 Dye Manufacturing Wastewater
Dye manufacturing wastewater contains various substances such as acids, alkalis, salts, halogens, hydrocarbons, amines, nitro-compounds, dyes, intermediates, and potentially hazardous substances like pyridine, cyanides, phenols, biphenylamine, and heavy metals (e.g., mercury, cadmium, chromium). This wastewater is complex and toxic, making it difficult to treat. Therefore, the treatment methods for dye manufacturing wastewater should be selected based on the characteristics of the wastewater and discharge requirements.
Treatment Methods:
- Removal of Solid Impurities and Inorganic Matter: Coagulation and filtration are commonly used.
- Removal of Organic and Toxic Substances: Chemical oxidation, biological methods, and reverse osmosis are typically employed.
- Decolorization: Coagulation and adsorption methods are often combined in the treatment process.
- Heavy Metal Removal: Ion exchange methods are commonly used to remove heavy metals from wastewater.
6 Food Industry Wastewater
The food industry generates a wide variety of wastewater due to its diverse raw materials and products, leading to significant differences in both the volume and quality of wastewater produced. Common pollutants in food industry wastewater include:
- Floating solids: Such as vegetable leaves, fruit peels, meat fragments, poultry feathers, etc.
- Suspended solids: These include fats, proteins, starch, colloidal substances, etc.
- Dissolved substances: Acids, alkalis, salts, sugars, etc.
- Organic contaminants: Including sand, mud, and other organic matter carried by raw materials.
- Pathogens: Bacteria, viruses, etc.
Food industry wastewater is characterized by high levels of organic material and suspended solids, which makes it prone to spoilage, although it generally lacks significant toxicity. Its main environmental harm is causing eutrophication of water bodies, which can lead to the death of aquatic animals and fish, promote the generation of odors from organic matter in the sediment, and degrade water quality.
Treatment Methods:
To treat food industry wastewater, biological treatment methods are generally the most suitable, following appropriate pre-treatment based on the specific characteristics of the water. If the effluent quality requirements are high, or if the organic content is very high, multi-stage biological treatments such as two-stage aeration tanks or multi-stage biological rotating discs, or combined treatments using anaerobic and aerobic systems can be applied.
7 Pesticide Wastewater
Pesticide wastewater is highly complex due to the variety of pesticides produced. The main characteristics of pesticide wastewater are:
- High pollutant concentration: The chemical oxygen demand (COD) can reach tens of thousands of mg/L.
- High toxicity: In addition to pesticides and their intermediates, the wastewater may contain toxic substances such as phenols, arsenic, mercury, and other compounds that are difficult for biological systems to degrade.
- Odor: The wastewater has a strong and unpleasant odor, which can irritate the respiratory system and mucous membranes.
- Unstable water quality and quantity: The water quality and volume fluctuate, adding complexity to treatment.
Pesticide wastewater poses a serious environmental threat. The goal of treatment is to reduce the concentration of pollutants, improve recycling efficiency, and ultimately achieve harmless discharge.
Treatment Methods:
- Activated carbon adsorption
- Wet oxidation
- Solvent extraction
- Distillation
- Activated sludge treatment
Additionally, developing new pesticides with high efficiency, low toxicity, and low residue is the future direction of pesticide production. Some countries have already banned the production of harmful organochlorine and organomercury pesticides like DDT and aldrin, and are actively researching and using microbial pesticides. This represents a fundamental approach to preventing pesticide wastewater from polluting the environment.
8 Cyanide-Containing Wastewater
Cyanide-containing wastewater primarily originates from industries such as electroplating, coal gasification, coking, metallurgy, metal processing, synthetic fibers, plastics, pesticides, and chemicals. Cyanide wastewater is a highly toxic industrial wastewater, unstable in water, and prone to decomposition. Both inorganic and organic cyanides are extremely toxic, and ingestion can cause acute poisoning. The lethal dose of cyanide for humans is approximately 0.18g, while for cyanide potassium, it is 0.12g. Cyanide in water can be lethal to fish at concentrations as low as 0.04-0.1 mg/L.
Treatment Measures for Cyanide-Containing Wastewater:
- Process Reform: One key measure is to reduce or eliminate the discharge of cyanide-containing wastewater, such as by adopting cyanide-free electroplating methods.
- Recovery of High-Cyanide Wastewater: High-concentration cyanide wastewater should undergo recovery processes, while low-concentration cyanide wastewater should be purified before discharge. Recovery methods include:
- Acidic aeration – Alkaline absorption method
- Steam desorption method
Treatment Methods:
- Alkaline chlorination: Widely used for treating cyanide wastewater.
- Electrolytic oxidation: Effective but not as widely applied.
- Pressurized hydrolysis: Also used for breaking down cyanide compounds.
- Biochemical methods: Use of biological processes to degrade cyanide, though less common.
- Iron-based methods: Use of iron to treat cyanide wastewater.
- Ferrous sulfate method: Often ineffective and unstable.
- Air stripping: This method is less commonly used due to the risk of atmospheric pollution and inability to meet discharge standards.
9 Phenol-Containing Wastewater
Phenol-containing wastewater mainly originates from industries such as coking plants, gas works, petrochemical plants, insulation material manufacturers, as well as from the production processes of ethylene from petroleum cracking, synthetic phenol, polyamide fibers, synthetic dyes, organic pesticides, and phenolic resins. The wastewater contains phenolic compounds, which are highly toxic and can denature proteins.
Treatment Methods:
- Physical Methods: Adsorption, activated carbon, or other adsorbents are commonly used for phenol removal.
- Chemical Methods: Oxidation, especially using ozone or chlorine, can effectively degrade phenolic compounds.
- Biological Methods: Aerobic biological treatment, such as activated sludge or biofilm reactors, can break down phenolic compounds in the wastewater.
- Advanced Oxidation Processes (AOPs): These processes, such as UV-H2O2 or Fenton’s reagent, can also be effective for treating phenolic wastewater.
10 Mercury-Containing Wastewater
Mercury-containing wastewater originates mainly from non-ferrous metal smelting, chemical plants, pesticide factories, paper mills, dye factories, and thermocouple and instrumentation manufacturers. Mercury compounds vary in toxicity, with methylmercury being particularly dangerous. It is easily absorbed into the human body, difficult to degrade, and accumulates in the brain, posing serious long-term health risks.
Treatment Methods:
- Chemical Precipitation: Using agents like sulfides to form insoluble mercury compounds that can then be removed by settling or filtration.
- Activated Carbon Adsorption: Mercury can be adsorbed onto activated carbon or other adsorbents.
- Ion Exchange: This process can selectively remove mercury ions from the wastewater.
- Electrochemical Methods: Electrolysis can help precipitate mercury as an insoluble form.
- Bioremediation: Certain bacteria can be used to reduce mercury to less toxic forms.
11 Heavy Metal Wastewater
Heavy metal wastewater is generated from industries such as mining, metallurgy, electroplating, pharmaceuticals, pesticides, paints, and pigments. The types, concentrations, and chemical forms of heavy metals in wastewater vary across different industries. Common heavy metals in such wastewater include lead, cadmium, copper, nickel, chromium, and zinc.
Treatment Principles:
- Source Reduction: The most fundamental measure is to reform production processes to reduce or eliminate the use of toxic heavy metals. Proper production techniques and scientific management can also reduce heavy metal consumption and loss in wastewater.
- On-site Treatment: Heavy metal wastewater should be treated at the point of generation to prevent mixing with other wastewater and complicating the treatment process. It should never be discharged untreated into municipal sewers, as this would worsen heavy metal pollution.
Treatment Methods:
- Precipitation Methods: Convert dissolved heavy metals into insoluble metal compounds, which can be removed by settling or flotation. Common techniques include:
- Neutralization precipitation
- Sulfide precipitation
- Electrolytic precipitation or flotation
- Membrane electrolysis
- Concentration and Separation Methods: These methods concentrate heavy metals without altering their chemical form and include techniques such as:
- Reverse osmosis
- Electrodialysis
- Evaporation
- Ion exchange
These methods should be selected based on the characteristics of the wastewater, such as metal types, concentration, and volume.
12 Metallurgical Wastewater
Metallurgical wastewater is characterized by large volumes, diverse types, and complex and variable water quality. The main sources of metallurgical wastewater include cooling water, acid washing wastewater, washing wastewater (from dust removal, coal gas, or flue gas treatment), slag washing wastewater, coking wastewater, and wastewater generated from production condensate, separation, or spillage.
Development Trends in Metallurgical Wastewater Treatment:
- Waterless or Low-Water Processes: The adoption of new technologies and processes that reduce or eliminate water usage and pollution, such as dry quenching of coke, preheating of coking coal, and desulfurization and cyanide removal from coke oven gas directly.
- Resource Recovery: The development of comprehensive utilization technologies to recover useful substances and thermal energy from wastewater and flue gas, minimizing the loss of materials and fuels.
- Water Recycling: Developing new treatment technologies that improve water recycling rates and optimize water quality stabilization measures. This includes the balanced use of water for different processes to improve the efficiency of wastewater treatment.
- Magnetic Treatment: Magnetic methods for treating steel plant wastewater are efficient, require less space, and are easy to manage, making them an attractive option for the future of metallurgical wastewater treatment.
In addition to traditional treatments such as filtration, flotation, and chemical precipitation, new technologies like membrane filtration (e.g., reverse osmosis) and advanced oxidation processes are being explored to handle more complex and contaminated metallurgical wastewater.
13 Acid-Base Wastewater
Acidic Wastewater:
Acidic wastewater mainly comes from industries such as steel mills, chemical factories, dyeing plants, electroplating plants, and mines. It contains various harmful substances or heavy metal salts. The concentration of acid can vary significantly, ranging from less than 1% to over 10%.
Alkaline Wastewater:
Alkaline wastewater is commonly produced in industries like textile dyeing, leather processing, papermaking, and oil refining. The concentration of alkaline substances can vary, with some being higher than 5% and others lower than 1%. Besides acids and bases, these wastewaters often contain acid salts, basic salts, and other inorganic and organic substances.
Treatment Principles:
- High Concentration Acid-Base Wastewater: The priority should be recovery and reuse. Depending on the quality and quantity of the wastewater and the process requirements, wastewater should be recycled or concentrated.
- Low Concentration Acid-Base Wastewater: For wastewater such as acid wash tank cleaning water or alkaline rinse tank water, neutralization treatment should be employed.
- Neutralization: The principle of “waste for waste” should be prioritized. For example, acid waste can be neutralized by using alkaline waste (sludge), or waste acid can neutralize alkaline wastewater. In the absence of these conditions, neutralizing agents may be used.
14 Ore Dressing Wastewater
Ore dressing (mineral processing) wastewater is characterized by large volumes, high suspended solids, and the presence of harmful substances such as heavy metal ions and flotation reagents. Heavy metals commonly found include copper, zinc, lead, nickel, barium, cadmium, arsenic, and rare elements.
Flotation Reagents:
- Collectors: Such as yellow drugs (RocssMe), black drugs [(RO)2PSSMe], and white drugs [CS(NHC6H5)2].
- Inhibitors: Such as cyanides (KCN, NaCN) and water glass (Na2SiO3).
- Frothers: Such as turpentine oil and phenolic compounds (C6H4CH3OH).
- Activators: Such as copper sulfate (CuSO4) and heavy metal salts.
- Sulfurizing Agents: Such as sodium sulfide.
- Pulp Regulators: Such as sulfuric acid, lime, etc.
Treatment Methods:
- Tailings Dam: The first step in treating ore dressing wastewater is usually to use a tailings dam to remove suspended solids, and reduce heavy metals and flotation reagent levels.
- Heavy Metal Removal: Can be achieved through methods like lime neutralization or calcination of dolomite for adsorption.
- Flotation Reagents Removal: Adsorption methods using ore or activated carbon can be effective for removing flotation reagents.
- Cyanide-containing Wastewater: Chemical oxidation methods are commonly used to treat cyanide-contaminated wastewater.
15 Oil-Containing Wastewater
Oil-containing wastewater mainly comes from industries such as petroleum, petrochemical, steel, coking, coal gasification, and mechanical processing. The oil pollutants in wastewater, except for heavy tar with a density greater than 1.1, usually have a density of less than 1. Oil can exist in three forms in wastewater:
- Floating Oil: Oil droplets larger than 100 μm, which are easier to separate from wastewater.
- Dispersed Oil: Oil droplets ranging between 10–100 μm, suspended in the water.
- Emulsified Oil: Oil droplets smaller than 10 μm, which are difficult to separate from wastewater.
The oil concentrations vary significantly depending on the industry:
- Oil Refining: Wastewater may contain 150–1000 mg/L of oil.
- Coking: Wastewater can contain 500–800 mg/L of tar.
- Coal Gasification: Wastewater can contain 2000–3000 mg/L of tar.
Treatment Methods:
- Oil Skimming: The first step is to use an oil separation tank to recover floating oils or heavy oils. The efficiency is typically 60%–80%, with effluent oil content around 100–200 mg/L.
- Emulsified Oil: Emulsified oil is more difficult to treat. Preventing or reducing emulsification during production and minimizing the number of times wastewater is pumped are key strategies.
- Treatment Methods:
- Dissolved Air Flotation (DAF): A common method to separate suspended oils.
- Demulsification: This method is used to break emulsions and separate emulsified oils. Various demulsifiers can be used depending on the type of emulsion.
By addressing both floating and emulsified oils, these methods effectively reduce oil content in wastewater, ensuring compliance with discharge standards.