This article introduces the water quality characteristics 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. Textile Dyeing Wastewater

Textile dyeing wastewater is characterized by large water volume, high organic pollutant content, deep color, high alkalinity, and significant variations in water quality, making it a difficult industrial wastewater to treat. Four main processes in textile processing produce wastewater: the pretreatment stage (including processes such as scouring, desizing, boiling, bleaching, and mercerizing) generates desizing wastewater, boiling wastewater, bleaching wastewater, and mercerizing wastewater; the dyeing process produces dyeing wastewater; the printing process generates printing wastewater and soap solution wastewater; and the finishing process produces finishing wastewater. Textile dyeing wastewater is a mixture of all these types of wastewater, or a composite wastewater excluding the bleaching wastewater.

2. Hospital Wastewater

Hospital wastewater refers to the wastewater discharged by hospitals into the natural environment or urban sewage systems. The water quality varies depending on the nature, scale, and location of the hospital. The daily wastewater discharge per bed is approximately 200 to 1,000 liters. The main pollutants in hospital wastewater include pathogens (parasite eggs, pathogenic bacteria, viruses, etc.), organic matter, floating and suspended solids, and radioactive pollutants. In untreated raw wastewater, the total bacterial count can exceed 10^8 bacteria/mL.

3. Electroplating Wastewater

Electroplating wastewater is very complex in composition. In addition to cyanide (CN-) wastewater and acidic/alkaline wastewater, heavy metal wastewater is one of the most hazardous categories in the electroplating industry. Based on the heavy metal elements contained in the wastewater, it can generally be classified into the following types: chromium (Cr) wastewater, nickel (Ni) wastewater, cadmium (Cd) wastewater, copper (Cu) wastewater, zinc (Zn) wastewater, gold (Au) wastewater, silver (Ag) wastewater, etc. 

4. Paper Industry Wastewater

The paper industry is one of the sectors with high energy consumption and material consumption, causing significant environmental pollution. The main pollution characteristics of paper wastewater are its large discharge volume, high COD (Chemical Oxygen Demand) content, high levels of suspended solids (SS), and severe color.

The main issues to be addressed in wastewater treatment are: the high concentrations of SS and COD in paper wastewater. COD consists of both non-dissolved and dissolved parts, with non-dissolved COD typically making up the majority of the total COD. When SS is removed from the wastewater, the vast majority of non-dissolved COD is also removed. Therefore, the primary challenges in treating waste paper mill wastewater are the removal of SS and COD.

5. Tanning Wastewater

Currently, the leather industry production generally includes several stages such as degreasing, dehairing, softening, tanning, dyeing, drying, and finishing. Various chemicals are added during the process, resulting in wastewater containing oil, collagen, animal and plant fibers, organic and inorganic solids, sulfides, chromium, salts, surfactants, dyes, and other pollutants and toxic substances. The water quality characteristics of comprehensive tanning wastewater are: COD (Chemical Oxygen Demand) 3000–4000 mg/L, BOD₅ (Biochemical Oxygen Demand) 1000–2000 mg/L, SS (Suspended Solids) 2000–4000 mg/L, and pH value between 8 and 11.

The wastewater mainly originates from pre-tanning preparations, tanning, and other wet processing stages. The most polluted are the degreasing wastewater, dehairing wastewater, and chromium-tanning wastewater, which together account for about 50% of the total wastewater but contain the majority of pollutants. The mass fraction of various pollutants in the total volume of wastewater is: COD 80%, BOD₅ 75%, SS 70%, sulfides 93%, sodium chloride 50%, and chromium compounds 95%.

The characteristics of tanning wastewater are reflected in the following aspects:

a. Large fluctuations in water quality and quantity;

b. Good biodegradability;

c. High concentration of suspended solids, which are prone to decay and generate large amounts of pollution;

d. Contains toxic compounds such as S^2- and chromium.

6. Monosodium Glutamate (MSG) Wastewater

The high-concentration organic wastewater generated during monosodium glutamate (MSG) production mainly refers to the mother liquor discharged after extracting glutamic acid from the fermentation broth. This type of wastewater has the following characteristics: high COD (Chemical Oxygen Demand), high BOD (Biochemical Oxygen Demand), high sulfate content, high NH₃^-N (ammonia nitrogen), high microbial content, low temperature, and low pH. MSG wastewater is one of the most difficult industrial wastewaters to treat.

7. Pesticide Wastewater

Pesticide wastewater has a high concentration of pollutants, with COD (Chemical Oxygen Demand) potentially reaching tens of thousands of milligrams per liter. It is highly toxic, containing not only pesticides and intermediates, but also toxic substances such as phenols, arsenic, mercury, and many other compounds that are difficult for organisms to degrade. The wastewater has an unpleasant odor and can irritate the respiratory system and mucous membranes. Additionally, both the water quality and quantity are unstable.

8. Electrophoretic Wastewater

The reason water-based resins in electrophoretic coatings can be diluted and dispersed with water is primarily due to the hydrophilic groups present on the polymer molecular chains. For example, these resins may contain carboxyl groups (—COOH), hydroxyl groups (—OH), ether groups (—O—), amino groups (—NH₂), etc. Depending on the charge carried by the water-dispersible resin, it can be classified into anodic electrophoretic paint (also called anionic electrophoretic paint) if it contains carboxyl groups (—COOH), or cathodic electrophoretic paint (also called cationic electrophoretic paint) if it contains amino groups (—NH₂). Electrophoretic mobility and coulomb efficiency are the two most important electrophoretic characteristics in cathodic electrophoretic coating.

9. Laundry Wastewater

The main pollutants in laundry wastewater are anionic surfactants, which, after entering the water, combine with other pollutants to form certain dispersed colloidal particles. Surfactants like LAS (Linear Alkylbenzene Sulfonate) in the wastewater exist in the form of dispersions and colloidal particles, and the organic components are mainly surfactants. The wastewater has the following characteristics:

a. Complex Composition

The primary pollutant in the wastewater, anionic surfactants, combines with other pollutants after entering the water, forming dispersed colloidal particles. Surfactants like LAS exist as dispersions and adsorbed colloidal particles, which significantly impact the physicochemical and biochemical properties of the wastewater.

b. Large Fluctuation in Water Quality and Irregular Discharge Patterns

Surfactant wastewater is typically alkaline, with a pH between 8 and 11. The LAS content in wastewater can range from several milligrams per liter, such as in wool washing wastewater, to a few tens of milligrams per liter, as in bathing wastewater. In China, synthetic detergent manufacturing plants typically discharge wastewater with LAS and other anionic surfactants at loads ranging from 10 to 60 mg/L, with higher values reaching up to 135 mg/L. COD (Chemical Oxygen Demand) can vary significantly, from hundreds to tens of thousands or even over one hundred thousand.

c. Toxicity and Harm to Aquatic Systems

LAS itself has certain toxicity and can cause chronic toxicity to animals and humans. It can reduce the oxygen transfer rate in water, and in severe cases, lead to hypoxia, water stagnation, and hinder the self-purification of the water body. High phosphate content can potentially cause water bodies to become eutrophic, leading to the growth of floating algae.

10. Power Plant Wastewater

Power plant wastewater mainly includes the following types: washing water, ash flushing water, domestic sewage, circulating water concentrate, and resin regeneration wastewater. These wastewaters primarily contain oils, suspended solids, scale inhibitors, biocides, hardness, ash content, high levels of salts, some organic compounds, high concentrations of sulfites, sulfates, fluorides, heavy metals, and certain thermal energy sources. 

11. Printing and Dyeing Wastewater

Printing and dyeing wastewater contains a large amount of acrylic macromolecules. If not treated and directly discharged into the urban sewage network, it will have a significant impact on the wastewater treatment process, damaging the biochemical treatment system and polluting the water environment. The volume of wastewater is relatively small, but the CODcr is very high (up to 20,000), with a certain amount of suspended solids, bacteria, and dissolved substances. The turbidity and color are also high.

12. Beer Wastewater

Beer wastewater primarily comes from various production areas, including the malt workshop (mashing wastewater), saccharification workshop (saccharification and filtration wash wastewater), fermentation workshop (fermentation tank wash and filtration wash wastewater), bottling workshop (bottle washing, sterilization wastewater, and beer spilled during bottle breaking), as well as cooling wastewater used in production.

Beer industry wastewater mainly contains organic compounds such as sugars and alcohols, with relatively high concentrations of organic matter. Although it is non-toxic, it is prone to decomposition and, when discharged into water bodies, consumes a large amount of dissolved oxygen, causing serious harm to the water environment. Water quality and quantity vary depending on the season, with peak organic matter concentrations during periods of high flow. In Chinese beer factories, the wastewater typically has a CODcr content ranging from 1,000 to 2,500 mg/L and a BOD₅ content of 600 to 1,500 mg/L. This wastewater has a high biodegradability and contains a certain amount of Kjeldahl nitrogen and phosphorus.

13. Dairy Wastewater

Dairy wastewater is produced during the processing of condensed milk, cheese, cream, dairy beverages, ice cream, and dairy snacks. The wastewater primarily comes from cleaning containers and equipment, and its main components include the raw materials used in dairy products.

In milk processing factories, the wastewater from raw milk treatment contains 0.2% BOD (20-300 mg/L), indicating relatively low pollution. However, wastewater from cheese and cream processing is more polluted, with COD levels reaching 3,000 mg/L and BOD levels as high as 2,400 mg/L. The total nitrogen (TN) content can reach 90 mg/L, total phosphorus (TP) is around 16 mg/L, oil content may be as high as 200 mg/L, and suspended solids can reach 600 mg/L. Raw materials such as cream and condensed milk should be recovered as by-products, and efforts should be made to reduce their loss during production.

The high-concentration wastewater in dairy processing comes from cleaning and sterilizing containers, equipment, and pipelines, and can have a COD value exceeding 20,000 mg/L. Typically, it is above 5,000 mg/L. The wastewater volume is approximately 1.0 m³ for every ton of raw milk processed. The volume may vary depending on product types, production volume, and factory management. The wastewater from cleaning workshops, floor wash water, and other water usage (such as office and domestic water) creates low-concentration wastewater. The COD value of this wastewater is usually below 1,000 mg/L, and about 3-4 m³ of low-concentration wastewater is generated per ton of raw milk processed.

Generally, wastewater from liquid milk and powdered milk production has a COD of about 1,500-3,000 mg/L, while wastewater from yogurt, cream, ice cream, soft serve, cheese, and other dairy products typically has a COD of 4,000-7,000 mg/L.

The main pollutants in dairy wastewater include milk proteins (such as casein, whey proteins), lactose, milk fat, various minerals found in raw milk, and acids and alkalis used for cleaning equipment, pipelines, and containers. The pH of the wastewater generally ranges from 6.5 to 7.0.

14. Circuit Board Wastewater

Circuit board wastewater can be categorized into several types: general cleaning water, ink wastewater, EDTA complexed copper wastewater, copper-ammonia complexed wastewater, organic wastewater, fluoride-containing wastewater, high copper wastewater, concentrated acid wastewater, concentrated alkaline wastewater, and other discarded replacement liquid. It contains heavy metal ions such as copper, nickel, lead, and tin, as well as high molecular organic compounds, complexing agents, and other pollutants. The heavy metal ions, COD, SS, pH, and other parameters often exceed the standards, with a variety of types and complex pollutants.

15. Starch Wastewater

Starch wastewater mainly comes from the washing, pressing, filtration, and concentration processes during corn starch production. The wastewater contains a large amount of dissolved organic pollutants, such as proteins, sugars, carbohydrates, fats, and amino acids. It also contains inorganic compounds with nitrogen (N) and phosphorus (P), as well as certain amounts of volatile acids and ash. This type of wastewater is considered a high-concentration organic wastewater with good biodegradability, making it easily degradable. The COD of starch wastewater typically ranges from 2500 to 6000 mg/L, and the SS content ranges from 800 to 1200 mg/L.

16. Slaughterhouse Wastewater

Slaughterhouse wastewater contains a large amount of contaminants, including blood, grease, hair, internal organs, undigested food, and feces. It also has an unpleasant reddish color and a strong blood-like odor, along with harmful pathogens such as Escherichia coli and Streptococcus faecalis, which pose risks to human health. These wastewaters are characterized by significant variations in concentration and high organic content, and direct discharge into the environment will severely pollute water bodies.

17. Coke Wastewater

Coke wastewater is generated from the production processes in coke plants and gas plants, including washing water, gas washing water, separation water from steam diversion, and tank drainage. It contains dozens of inorganic and organic compounds. The inorganic compounds primarily include large amounts of ammonia salts, thiocyanates, sulfides, cyanides, etc. The organic compounds, in addition to phenols, include both monocyclic and polycyclic aromatic compounds, as well as nitrogen, sulfur, and oxygen-containing heterocyclic compounds. It is heavily polluted and represents a major environmental issue in industrial wastewater discharge. With complex components, high pollutant concentrations, high color, and significant toxicity, it is highly stable and a typical example of hard-to-degrade organic wastewater.