Why is Printing and Dyeing Wastewater Difficult to Treat? Summary of Treatment Technologies

Printing and dyeing wastewater refers to the wastewater discharged during the pretreatment, dyeing, printing, and finishing processes of cotton, wool, hemp, silk, chemical fiber, or blended products. The printing and dyeing industry is a major water-consuming sector in the textile industry, with water acting as a medium throughout the entire printing and dyeing process.

 

I. Water Quality and Quantity Characteristics of Printing and Dyeing Wastewater

 

Due to the complex printing and dyeing processes and the use of a wide variety of chemicals such as dyes, sizes, and auxiliaries, printing and dyeing wastewater is characterized by large volume, high organic pollutant content, deep color, high alkalinity, and large water quality fluctuations, making it a type of industrial wastewater that is difficult to treat.

 

1. Main Pollutants and Composition of Printing and Dyeing Wastewater

Printing and dyeing wastewater contains dyes, sizes, auxiliaries, oils, acids and alkalis, fiber impurities, sand, and inorganic salts. Among them, dyes contain nitro, amino compounds, and heavy metals such as copper, chromium, zinc, and arsenic, which are highly biologically toxic and cause severe pollution.

Dyes are the main source of pollutants in wastewater, with diverse types and varying biodegradability.

Auxiliaries are another major source, including surfactants, metal complexing agents, etc., which are classified into wetting and penetrating agents, emulsifying and dispersing agents, defoamers, fixing agents, resin finishing agents, flame retardant and antistatic agents, etc.

A small amount of oil from scouring and finishing processes is also present, with low content and minimal impact on water quality.

 

2. Sources and Water Quality/Quantity Characteristics of Printing and Dyeing Wastewater

Printing and dyeing wastewater is a mixed stream from the above processes, containing contaminants such as raw material dirt, grease, added sizes, dyes, surfactants, auxiliaries, and acids/alkalis.Wastewater is generated in every stage of printing and dyeing:

Pretreatment (singeing, desizing, scouring, bleaching, mercerizing): desizing wastewater, scouring wastewater, bleaching wastewater, mercerizing wastewater

Dyeing: dyeing wastewater

Printing: printing wastewater and soaping wastewater

Finishing: finishing wastewater

 

3. Water Quality Characteristics of Wastewater from Different Printing and Dyeing Products

Wastewater quality varies with fiber raw materials, product types, production processes, dye/auxiliary types, processing methods, and rinsing frequency.Based on fiber raw materials, textile printing and dyeing is divided into cotton dyeing, wool dyeing, silk dyeing, and hemp dyeing.

 

II. Discharge Standards for Printing and Dyeing Wastewater

 

The discharge standard for printing and dyeing wastewater is Discharge Standard of Water Pollutants for Textile Dyeing and Finishing Industry (GB4287-1992).

Category I: Harmful substances that accumulate in the environment or organisms and exert long-term impacts on human health (with specified maximum allowable discharge concentrations).

 

 

Category II: Harmful substances with less long-term impacts (with specified maximum allowable discharge concentrations).

 

 

III. Treatment Technologies for Printing and Dyeing Wastewater

 

The main treatment methods include physical treatment, chemical treatment, physicochemical treatment, biological treatment, and alkali reduction treatment. Biological treatment is the primary method, often combined with other pretreatment technologies.

 

1. Physical Treatment

 

(1) Membrane Separation

As a high-efficiency separation technology, membrane separation uses the selective permeability of biological membranes to separate, concentrate, and recover pollutants from wastewater.

Advantages: no chemical addition, no secondary pollution, simple operation, low energy consumption, recyclable salts and dyes, reusable treated water.

Applications: ultrafiltration (UF) and reverse osmosis (RO) for dye wastewater; decolorization rate 95%–98%, COD₍Cr₎ removal 60%–90%, dye recovery >95%.

Nanofiltration (NF): high energy consumption and membrane fouling under high pressure (>1.0 MPa); submerged filtration improves efficiency and saves energy.

 

High-Energy Physical Treatment

High-energy particle beams bombard water to generate highly reactive ·OH radicals and H atoms, which decompose organic matter.

Advantages: high organic removal rate, small footprint, simple operation.

Disadvantages: expensive equipment, high technical requirements, high energy consumption, low energy efficiency.

Breakthrough: The first electron beam irradiation demonstration project for printing and dyeing wastewater was built in Jinhua, Zhejiang in March 2017.

 

Ultrasonic Technology

Ultrasound degrades refractory organic pollutants by combining advanced oxidation, incineration, and supercritical water oxidation features.

Mechanism: Ultrasonic cavitation breaks organic bonds and accelerates flocculation, reducing color, COD, and aniline.

Status: Most research remains at the laboratory stage.

 

2. Physicochemical Treatment

 

(1) Adsorption

Suitable for advanced treatment of low-concentration printing and dyeing wastewater, with low investment and simple operation.Adsorbents include activated carbon, macroporous adsorption resin, kaolin, diatomite, and coal cinder.

Activated carbon: strong adsorption for water-soluble dyes but expensive and hard to regenerate.

Macroporous resin: good stability, easy regeneration, effective for aromatic sulfonates and naphthols.

Low-cost adsorbents (kaolin, coal cinder): good decolorization but large sludge production.

 

 

Coagulation

Including precipitation and air flotation, widely used in small and medium-sized enterprises for low cost, large capacity, and high decolorization.Common coagulants: aluminum sulfate, aluminum chloride, ferrous sulfate, polyaluminum chloride (PAC), polyferric sulfate (PFS), polyacrylamide (PAM).

Effective for hydrophobic dyes (sulfur, vat, disperse dyes) but poor for hydrophilic dyes.

Disadvantages: sensitive to water quality changes, low COD removal, large and hard-to-dewater sludge.

 

3. Chemical Treatment

 

(1) Chemical Oxidation

Destroys dye chromophores using ozone, Fenton reagent, chlorine, or sodium hypochlorite.

Ozonation: good decolorization, no sludge, but high cost, poor for insoluble dyes, low COD removal.

Fenton oxidation: ·OH from H₂O₂/Fe²⁺ breaks dye chains; combined with coagulation; enhanced by UV/oxalate.

 

Photochemical Oxidation

Including photolysis, photosensitized oxidation, photo-initiated oxidation, and photocatalytic oxidation (most widely studied).

Catalysts: TiO₂, CdS, Fe₂O₃, WO₃; TiO₂ is ideal for stability, non-toxicity, and low cost.

Advantages: mild conditions, strong oxidation, complete mineralization; disadvantages: high investment and energy consumption, poor for high-concentration wastewater.

 

Wet Air Oxidation (WAO)

Oxidizes organics at high temperature (125–320 °C) and high pressure (0.5–20 MPa).Supercritical Water Oxidation (SCWO): above 374 °C and 22.05 MPa, homogeneous oxidation, >99% organic removal in 60 s, fast and efficient.

 

(2) Electrolysis

Converts pollutants into harmless substances via electrode reactions; cost reduced with power development.

Iron-carbon internal electrolysis: forms galvanic cells to generate Fe²⁺/Fe³⁺ flocs and active [H]/[O], decolorizes and improves biodegradability.

Electrocatalytic oxidation: generates ·OH, O₃, H₂O₂ to completely mineralize organics, suitable for high-concentration wastewater pretreatment.

 

4. Biological Treatment

 

Divided into aerobic, anaerobic, and anaerobic-aerobic combined processes; aerobic treatment dominates in China.

Aerobic: high COD/BOD₅ removal, poor decolorization.

Anaerobic: high decolorization, low sludge yield, recoverable methane.

 

--Aerobic Biological Treatment

1. Activated Sludge Process: low investment, good organic removal, partial decolorization.
2. SBR: time-based plug flow and spatial complete mixing, potential for refractory organics.
3. Biofilm Process: higher decolorization than activated sludge; includes contact oxidation and biological filters.
4. Biological Contact Oxidation: combines activated sludge and biofilm advantages, low sludge, easy operation.
5. MBR: integrates activated sludge and membrane separation, retains refractory organics, reusable water.

 

--Anaerobic Biological Treatment

1. Anaerobic Biofilter: high microbial concentration, long sludge retention, sensitive to temperature.
2. UASB: high-efficiency reactor with three-phase separator, >90% COD/color removal.
3. ABR: baffle structure, multi-stage anaerobic, no clogging, easy startup.
4. Anaerobic Fluidized Bed: short HRT, high load, small footprint.
5. IC Reactor: double UASB structure, high volume load, strong shock resistance.
6. Hydrolysis Acidification: stops at hydrolysis/acidification, improves biodegradability, low COD removal (40%–50%).

 

--Anaerobic-Aerobic Combined Processes

1. Combines advantages of anaerobic and aerobic treatment; typical processes:
2. Anaerobic-aerobic-biological carbon contact
3. Anaerobic-aerobic biological rotating disc
4. Hydrolysis acidification-aerobic

 

IV. New Biological Treatment Technologies for Printing and Dyeing Wastewater

 

1. Bioaugmentation Technology

Adds highly efficient degrading strains (e.g., white-rot fungi) to enhance pollutant removal. White-rot fungi produce lignin peroxidase and manganese peroxidase for broad-spectrum dye decolorization.

 

2. Immobilized Microorganism Technology

Fixes microbes on carriers for high activity and stability, higher efficiency than suspended systems, less sludge.

 

3. Difficulty: Treatment of Alkali Reduction Wastewater

Alkali reduction hydrolyzes polyester fibers to simulate silk; wastewater contains high concentrations of terephthalic acid, ethylene glycol, and oligomers.

Characteristics: high COD₍Cr₎, high alkalinity, poor biodegradability.

Treatment: acid precipitation + electrocatalytic oxidation + salt-tolerant bacteria degradation + multi-effect catalytic oxidation.

 

4. Typical Printing and Dyeing Wastewater Treatment Process

Grid → Regulation tank (aeration for homogenization) → Facultative/hydrolysis acidification tank (improves biodegradability) → Contact oxidation tank → Flocculation tank → Sedimentation tank → Disinfection → Discharge; sludge is thickened and dewatered.

 

5. Process Cases by Product Type

 

Cotton Woven/Knitted Dyeing Wastewater

Cotton woven: longer process, higher load; regulation (6–8 h), hydrolysis acidification (4–10 h), contact oxidation (8–10 h).

Cotton knitted: no sizing, lower organic load, shorter process.

Silk Dyeing Wastewater

Natural silk degumming: high-concentration biodegradable wastewater; treated by UASB + aerobic + coagulation.

 

 

Natural silk dyeing: similar to wool dyeing; biological treatment is effective.

Polyester simulated silk: alkali reduction wastewater requires separate pretreatment.

 

Wool Scouring Wastewater

High-concentration organic wastewater with wool grease; process: grid → grit chamber → regulation → coagulation air flotation → hydrolysis acidification → anaerobic fermentation → aerobic → coagulation; wool grease is recyclable.

 

Wool Textile Dyeing Wastewater

Good biodegradability (B/C ≈ 0.3–0.4), water-soluble dyes; process: grid → regulation → hydrolysis acidification → contact oxidation → BAF → coagulation/photochemical oxidation.

 

Hemp Dyeing Wastewater

Hemp degumming: high-concentration alkaline organic wastewater; treated by anaerobic + aerobic + coagulation/photochemical oxidation.

Hemp dyeing: similar to cotton dyeing, adjustable parameters.