Characteristics and Challenges of Industrial Wastewater Treatment
This article introduces the treatment challenges of wastewater from the textile dyeing industry, hospitals, electroplating, paper mills, leather production, monosodium glutamate factories, pesticide production, electrophoresis, washing, power plants, printing, breweries, dairy products, circuit boards, starch production, slaughterhouses, and coke plants.
1. Phenolic Wastewater
When the phenol concentration in water reaches 0.1 to 0.2 mg/L, fish begin to have an off-flavor and are not edible. When the concentration increases to 1 mg/L, it affects fish spawning, and at concentrations of 5-10 mg/L, fish will die in large numbers. The presence of phenols in drinking water can affect human health; even at a concentration as low as 0.002 mg/L, chlorination will result in the formation of chlorophenols, which produce a foul odor.
Wastewater containing phenols with a concentration of 1,000 mg/L or higher is classified as high-concentration phenolic wastewater, which requires phenol recovery before further treatment. Wastewater with a phenol concentration of less than 1,000 mg/L is considered low-concentration phenolic wastewater. Such wastewater is usually recycled, with phenol concentrated and recovered for further treatment. Methods for recovering phenol include solvent extraction, steam stripping, adsorption, and closed-loop processes. Wastewater with a phenol concentration below 300 mg/L can be treated and discharged or recovered using biological oxidation, chemical oxidation, or physico-chemical oxidation methods.
2. Mercury-containing Wastewater
Alkaline mercury-containing wastewater is usually treated by chemical coagulation or sulfide precipitation methods. Acidic mercury-containing wastewater can be treated using metal reduction methods. Low-concentration mercury wastewater can be treated by activated carbon adsorption, chemical coagulation, or activated sludge methods. Organic mercury wastewater is more difficult to treat, and it is typically first oxidized to inorganic mercury before further treatment.
3. Oil-containing Wastewater
Oil substances in wastewater usually exist in three states:
a. Free Oil: Oil droplets with a particle size greater than 100 μm, which are easy to separate from the wastewater.
b. Dispersed Oil: Oil droplets with a particle size between 10 and 100 μm, suspended in the water.
c. Emulsified Oil: Oil droplets with a particle size smaller than 10 μm, which are difficult to separate from the wastewater.
The oil content in wastewater varies significantly across different industrial sectors. For example, wastewater from the refining process contains about 150-1,000 mg/L of oil, coke wastewater contains about 500-800 mg/L of tar, and wastewater from gasification plants may contain 2,000-3,000 mg/L of tar.
Therefore, the treatment of oil-containing wastewater should first use oil separation tanks to recover free oil or heavy oil, with a treatment efficiency of 60%-80%. The oil content in the effluent is about 100-200 mg/L. The emulsified oil and dispersed oil in the wastewater are more difficult to treat, so it is important to prevent or reduce emulsification. One approach is to reduce the emulsification of oil in the wastewater during the production process. Another is to minimize the use of pumps to lift the wastewater during treatment to avoid increasing the degree of emulsification. Common treatment methods include flotation and demulsification.
4. Heavy Metal Wastewater
Heavy metals cannot be degraded or destroyed; they can only be transferred to different locations or transformed into other physical and chemical forms.
For example, after chemical precipitation treatment, heavy metals in wastewater are transformed from dissolved ionic forms into insoluble compounds and precipitate out, transferring from the water to the sludge. After ion exchange treatment, heavy metal ions in wastewater transfer to ion exchange resins, and after regeneration, they are released from the resins into the regeneration waste liquid. Therefore, the principle for treating heavy metal wastewater is: First, the most fundamental approach is to reform production processes to reduce or eliminate the use of highly toxic heavy metals. Secondly, reasonable process flows, scientific management, and operation should be adopted to minimize the use and loss of heavy metals through wastewater and to reduce the discharge of wastewater as much as possible.
Heavy metal wastewater should be treated at the source, without mixing with other wastewater, to avoid complicating the treatment process. It should never be directly discharged into urban sewer systems without treatment, as this could exacerbate heavy metal pollution. The treatment of heavy metal wastewater can generally be divided into two categories: First, transforming dissolved heavy metals in wastewater into insoluble metal compounds or elements, which can then be removed from the water by precipitation or flotation. Methods for this include neutralization precipitation, sulfide precipitation, flotation separation, electrochemical precipitation (or flotation), and membrane electrolysis. Second, concentrating and separating the heavy metals in wastewater without altering their chemical forms. Methods for this include reverse osmosis, electrodialysis, evaporation, and ion exchange. These methods should be used individually or in combination, depending on the quality and quantity of the wastewater.
5. Cyanide-Containing Wastewater
The main treatment measures for cyanide-containing wastewater are:
a. Process Reform: Reduce or eliminate the discharge of cyanide-containing wastewater. For example, the use of cyanide-free electroplating methods can eliminate industrial wastewater from electroplating workshops.
b. Wastewater with High Cyanide Content: Should be recycled and reused. Wastewater with low cyanide content should be purified before discharge.
Methods of recycling include acidification aeration–alkaline solution absorption, and steam desorption. Treatment methods include alkaline chlorination, electrochemical oxidation, pressurized hydrolysis, biochemical methods, biological iron methods, ferrous sulfate methods, and air stripping. Among these, alkaline chlorination is more widely applied, while ferrous sulfate treatment is incomplete and unstable. Air stripping, which causes atmospheric pollution and does not meet discharge standards, is rarely used.
6. Pesticide Wastewater
The treatment methods for pesticide wastewater include activated carbon adsorption, wet oxidation, solvent extraction, distillation, and activated sludge methods. However, the development of highly efficient, low-toxicity, and low-residue new pesticides is the direction for pesticide development. Some countries have already banned the production of organochlorine pesticides such as DDT and organomercury pesticides, and are actively researching and using microbial pesticides. This represents a fundamentally new approach to preventing pesticide wastewater pollution in the environment.
7. Food Industry Wastewater
In addition to appropriate pretreatment based on water quality characteristics, biological treatment is generally recommended for food industry wastewater. If the effluent quality requirements are very high or the wastewater has a high organic content, a two-stage aeration tank or two-stage biofilter, or a multi-stage biological rotating disc, may be used. Additionally, a combination of two biological treatment devices or an anaerobic-aerobic series process can also be employed.
8. Paper Industry Wastewater
The treatment of paper industry wastewater should focus on improving water reuse rates, reducing water consumption and wastewater discharge, while also actively exploring various reliable, economical methods that can fully utilize the useful resources in wastewater. For example, flotation can recover fibrous solids from white water, with a recovery rate of up to 95%, and the clarified water can be reused. The combustion method can recover sodium hydroxide, sodium sulfide, sodium sulfate, and other sodium salts combined with organic materials from black liquor. Neutralization can adjust the wastewater pH value, coagulation and flotation can remove suspended solids from wastewater, chemical precipitation can remove color, biological treatment can reduce BOD and is particularly effective for kraft paper wastewater, wet oxidation is successful for treating sulfite pulp wastewater. Additionally, international methods such as reverse osmosis, ultrafiltration, and electrodialysis are also employed for treatment.
9. Textile Dyeing Industry Wastewater
The textile dyeing industry uses a large amount of water, typically consuming 100 to 200 tons of water for every ton of textile processed, with 80% to 90% of this water discharged as wastewater. Common treatment methods include recycling and harmless treatment.
Recycling:
a. Wastewater can be recycled separately based on water quality characteristics. For example, the separation of bleaching and scouring wastewater from dyeing and printing wastewater. The former can be reused for washing, reducing discharge.
b. Alkali liquid recycling is commonly done by evaporation. For large volumes of alkali liquid, triple-effect evaporation is used; for smaller volumes, thin-film evaporation is used.
c. Dye recovery, such as for solubilized dyes, can be acidified to form indigo, which becomes a colloidal particle suspended in the residual liquid. After precipitation and filtration, it can be recycled.
Harmless Treatment:
a. Physical treatment methods include precipitation and adsorption. Precipitation is mainly used to remove suspended solids from wastewater, while adsorption is used to remove dissolved pollutants and decolorize the water.
b. Chemical treatment methods include neutralization, coagulation, and oxidation. Neutralization adjusts the pH of the wastewater and can also reduce its color. Coagulation removes dispersed dyes and colloidal substances from wastewater. Oxidation oxidizes reductive substances in the wastewater, causing sulfur-based and reductive dyes to precipitate.
c. Biological treatment methods include activated sludge, biological disc, biological drum, and biological contact oxidation. To improve effluent quality and meet discharge standards or recycling requirements, multiple methods are often combined for treatment.
10. Dye Production Wastewater
The treatment of dye production wastewater should be selected based on the wastewater's characteristics and the discharge requirements. For example:
a. To remove solid impurities and inorganic substances, coagulation and filtration methods can be used.
b. To remove organic matter and toxic substances, chemical oxidation, biological methods, and reverse osmosis are commonly employed.
c. For decolorization, a treatment process combining coagulation and adsorption methods is generally used.
d. To remove heavy metals, ion exchange methods can be applied.
11. Chemical Industry Wastewater
The main measures for the prevention and control of chemical wastewater pollution are:
Firstly, production processes and equipment should be reformed to reduce pollutants, prevent wastewater discharge, and promote comprehensive utilization and recycling. For wastewater that must be discharged, its treatment level should be selected based on the water quality and discharge requirements.
a. Primary Treatment primarily separates suspended solids, colloidal substances, floating oils, or heavy oils from the water. Methods such as water quality and quantity regulation, natural sedimentation, flotation, and oil separation can be used.
b. Secondary Treatment mainly removes biodegradable organic dissolved substances and some colloidal matter, reducing biochemical oxygen demand (BOD) and part of the chemical oxygen demand (COD). This is typically achieved through biological treatment.
After biological treatment, the wastewater still contains a significant amount of COD, and sometimes exhibits high color, odor, or taste. If environmental sanitation standards are strict, tertiary treatment methods are required for further purification.
c. Tertiary Treatment mainly targets the removal of organic pollutants that are difficult to biodegrade and dissolved inorganic pollutants. Common methods include activated carbon adsorption, ozone oxidation, ion exchange, and membrane separation technologies.
Different chemical industrial wastewaters can be treated using various methods depending on their specific water quality, volume, and the discharge requirements for treated effluent.
12. Acid-Base Wastewater
Acid-base wastewater has strong corrosive properties and needs proper treatment before it can be discharged.
The principles for treating acid-base wastewater are:
a. High-concentration acid-base wastewater should prioritize recycling. Based on water quality, volume, and different process requirements, local or regional scheduling should be conducted to maximize reuse. If reuse is difficult, or if the concentration is low and the volume is large, concentration methods can be used to recover the acid and base.
b. Low-concentration acid-base wastewater, such as cleaning water from acid and alkali wash tanks, should undergo neutralization treatment. For neutralization, the principle of "treating waste with waste" should be considered first. For example, acid and alkali wastewater can neutralize each other, or waste alkali (slag) can be used to neutralize acidic wastewater, while waste acid can be used to neutralize alkaline wastewater. If these conditions are not met, neutralizing agents may be used for treatment.
