Treatment Processes for Cotton Textile Printing and Dyeing Wastewate

Textile dyeing and printing wastewater refers to the wastewater generated during the dyeing and printing processes in the textile industry. The wastewater is discharged from all four stages of the dyeing and printing process. In the pretreatment stage (including processes such as scouring, desizing, boiling, bleaching, and mercerization), desizing wastewater, boiling wastewater, bleaching wastewater, and mercerization wastewater are discharged. The dyeing process generates dyeing wastewater, while the printing process produces printing wastewater and soaping wastewater. The finishing process results in finishing wastewater. Each of these steps contributes to the diverse array of pollutants present in the wastewater. Currently, various methods are employed for the treatment of textile dyeing and printing wastewater, including physical-chemical, chemical, and bio-chemical approaches.

2.1 Physical-Chemical Methods

2.1.1 Adsorption Method

Among physical-chemical methods, the most commonly used is the adsorption method. The adsorption method utilizes porous solid materials to adsorb one or more substances from wastewater onto the surface of the solid, thereby removing them. Adsorbents include regenerable adsorbents such as activated carbon, ion exchange fibers, and non-regenerable adsorbents such as various natural minerals (bentonite, diatomaceous earth), industrial waste materials (coal slag, fly ash), and natural waste materials (charcoal, sawdust), etc.

① Adsorption with Activated Carbon

Activated carbon is widely used as an excellent adsorbent in the water treatment industry and is still the best adsorbent for decolorization of dyeing and printing wastewater. It is commonly used in the advanced treatment process of dyeing and printing wastewater and exhibits selective adsorption of dyes. The decolorization sequence is as follows: alkaline dyes, direct dyes, acid dyes, sulfur dyes. It has achieved achievements in the research on the adsorption of dyes in wastewater using newly developed activated carbon fibers in recent years.

② Ion Exchange Fibers

Cellulose-based adsorbents produced by cellulose modification have also been reported for decolorization treatment of dyeing and printing wastewater. This type of adsorbent is effective in decolorizing dye wastewater and is easy to regenerate. Research has shown that this fiber has the advantages of high-speed, high-efficiency, and deep decolorization of cationic dye wastewater, far superior to general activated carbon, and is easy to regenerate. A base-propylated cellulose fiber has a greater affinity than cellulose itself for reactive dyes, direct dyes, and complex reducing dyes, and has better decolorization effects on wastewater from other dyes except alkaline dyes.

③ Natural Mineral Adsorbents

Natural clay resources are abundant, inexpensive, and readily available, and research on using clays or modified clays and other natural minerals as adsorbents for dyeing and printing wastewater treatment is quite extensive. It has shown good results in treating dyeing and printing wastewater, with removal rates of over 74% and color removal rates of over 93%, superior to conventional water purifiers, and the waste residue can be comprehensively utilized without causing secondary pollution.

④ Coal and Coal Ash Adsorbents

Coal, after crushing, screening, soaking, inoculation, and other processing treatments, has a porous structure with a large surface area, well-developed mesopores, and few micropores, which is conducive to the adsorption of larger molecules. The use of fly ash adsorption for dyeing and printing wastewater and detergent wastewater treatment has shown good results. Experimental results show that the treated wastewater has good decolorization and CODCr removal effects.

2.1.2 Membrane Filtration Method

Membrane filtration is the process of using the micropores of a membrane to filter and separate suspended solids from dyeing and printing wastewater, purifying the water quality. Separation membranes are special thin-layer materials with selective permeability that allow certain substances in the fluid to pass through while blocking others, thus concentrating and purifying them. Currently, pressure-driven membrane separation techniques such as reverse osmosis (RO), ultrafiltration (UF), and nanofiltration (NF) are mainly used in dyeing and printing wastewater treatment. These techniques have good separation effects, with color removal rates exceeding 99% and CODCr removal rates above 92%, allowing the treated water to be reused.

2.1.3 Dissolved Air Flotation (DAF) Method

The dissolved air flotation (DAF) method, specifically the recirculation type, involves introducing air into water to generate fine bubbles while adding a coagulant. The small suspended particles in the water, such as fine fibers, attach to the air bubbles and rise to the water surface, forming a float scum. This method effectively removes suspended substances from the water, improving water quality.

2.1.4 Ultrasonic Cavitation Method

The treatment of dyeing and printing wastewater using ultrasonic waves is based on the generation of localized high temperatures, pressures, and shear forces in the solution. This induces the decomposition of water molecules and dye molecules into free radicals, triggering various reactions and promoting flocculation.

2.1.5 High-Energy Physical Methods

When a high-energy particle beam bombards a water solution, the water molecules become excited and ionized, generating ions, excited molecules, and secondary electrons. These radiative products interact with each other to produce substances with highly reactive capabilities before diffusing into the surrounding medium. However, the devices that produce high-energy particles are expensive, require advanced technology, and consume significant energy, making it difficult to implement them in practical operations.

2.2 Chemical Methods

2.2.1 Coagulation and Precipitation Method

The coagulation and precipitation method is a commonly used method for treating dyeing and printing wastewater. Coagulation and precipitation involve adding coagulants and flocculants to the wastewater, causing colloids to collide and aggregate under certain external forces, forming larger flocs that adsorb and remove pollutants. Coagulants used in the treatment of dyeing and printing wastewater include inorganic low-molecular-weight coagulants, inorganic high-molecular-weight coagulants, organic high-molecular-weight coagulants, and microbial coagulants.

① Inorganic Coagulants

Inorganic coagulants such as iron salts and aluminum salts are widely used in the decolorization treatment of dyeing and printing wastewater. When iron salts and aluminum salts are added to dyeing and printing wastewater, they exhibit good decolorization effects on dispersed dyes, sulfide dyes, oxidized and reduced dyes, and complex reactive dyes with larger molecular weights. However, their coagulation efficiency is not ideal for water-soluble dyes that do not easily form colloids. Poly aluminum chloride (PAC) and poly ferric sulfate (PFS) are examples of inorganic high-molecular-weight coagulants that have shown better coagulation effects than traditional iron and aluminum salts in many experimental studies.

② Organic High-Molecular-Weight Coagulants

Recent research has shown that organic high-molecular-weight coagulants, especially artificially synthesized ones, exhibit better decolorization effects on dyeing and printing wastewater. Organic high-molecular-weight coagulants used in dyeing and printing wastewater treatment can be categorized into surfactants, natural polymers and their derivatives, and synthetic organic high-molecular-weight coagulants. Natural polymer coagulants, due to their wide availability, low cost, non-toxicity, and biodegradability, show good prospects. Examples of natural polymer coagulants used in dyeing and printing wastewater treatment include natural starch and its derivatives, lignin and its derivatives, and chitosan and its derivatives.

③ Microbial Coagulants

Microbial coagulants are a new type of water treatment agent that is safe, efficient, and naturally degradable. They are obtained through the extraction and purification of microorganisms or their secretions using biotechnology. Compared to conventional coagulants, microbial coagulants have advantages such as easy solid-liquid separation, minimal sedimentation, and wide applicability. Research on microbial coagulants is an important topic in the field of coagulant research worldwide.

2.2.2 Electrochemical Method

The electrochemical method involves the electrochemical reactions of electrolytes under the action of direct current in wastewater treatment. Electrochemical methods can be further divided into internal electrolysis, electrocoagulation, electroflotation, electrooxidation, and microelectrolysis. Under optimal process conditions, the removal efficiency of CODcr can reach 90.40%, and the removal efficiency of color can reach 100%.

2.2.3 High-Temperature Deep Oxidation Method

The high-temperature deep oxidation method uses oxidants to oxidize dye molecules and other organic substances. This method not only removes pollutants from high-concentration organic wastewater but also utilizes the heat energy of high-concentration organic compounds in the wastewater as a heat source for treatment, demonstrating high efficiency, energy conservation, and no secondary pollution. In recent years, various high-temperature deep oxidation technologies have been developed, including wet air oxidation, supercritical water oxidation, and incineration techniques.

2.3 Biochemical Methods

2.3.1 Activated Sludge Method

The activated sludge method is currently the most widely used method and includes processes such as plug-flow activated sludge and surface aeration tanks. The activated sludge method has the advantages of relatively low investment and good effectiveness. However, the surface aeration tank has some disadvantages, such as short-circuiting issues, difficulty in adjusting the oxygen and reflux rates, and foam generation from excessive surfactants, which can affect the oxygenation efficiency. Consequently, the use of surface aeration tanks has decreased in recent years. On the other hand, the plug-flow activated sludge method has been adopted in larger-scale applications.

2.3.2 Biological Contact Oxidation Method

The biological contact oxidation method involves adding fillers to wastewater, allowing bacteria to attach to their surface and increasing the contact area between bacteria and organic substances in the wastewater. This method has the advantages of high volume load, small footprint, low sludge production, no filamentous bulking, no need for sludge reflux, easy management, and the presence of specialized microorganisms on the fillers that can degrade specific organic compounds. Therefore, it has been widely used in the treatment of dyeing and printing wastewater. The biological contact oxidation method can be quickly restarted after a shutdown and reduces the impact of production shutdowns without wastewater discharge on the biological treatment efficiency. Despite the relatively higher investment, it is increasingly applied due to its adaptability to situations where wastewater treatment management is less advanced and land is limited. It is particularly suitable for treating medium to small volumes of dyeing and printing wastewater.

2.3.3 Biological Rotating Discs and Tower Filter

Biological rotating discs, tower filters, and similar processes have been used in the treatment of dyeing and printing wastewater and have achieved good results. Some plants are still operating these processes. However, these methods require a larger footprint, have more environmental impacts, and generally have lower treatment efficiency compared to other processes. As a result, they are currently less commonly used.

2.3.4 Anaerobic Treatment

For concentrated and poorly biodegradable dyeing and printing wastewater, anaerobic treatment methods can significantly improve the removal efficiency of organic matter. Currently, there is limited engineering experience and few engineering examples in this area in China. Anaerobic reactions mainly involve three stages: liquefaction, acidogenesis, and methanogenesis, which are completed by fermentative bacteria, hydrogen-producing and acid-producing bacteria, and methane-producing bacteria, respectively. Anaerobic reactions can also effectively treat nitrogen-containing substances that cannot be treated by aerobic reactions and ultimately generate nitrogen gas through denitrification, reducing the pollution of nitrites in the natural environment.

2.3.5 Anoxic Hydrolysis and Aerobic Biological Treatment Process

Anoxic reactions can convert complex and difficult-to-degrade high-molecular-weight organic compounds into small molecular-weight organic compounds under the action of facultative microorganisms, thereby improving their bioavailability and subsequently enhancing the removal efficiency of organic matter in the subsequent aerobic biological treatment. In recent years, this process has been applied in the treatment of dyeing and printing wastewater with good results. The hydraulic retention time in the anoxic stage is generally determined based on the influent COD concentration. It has shown significant improvement in color removal compared to a single aerobic process. It not only treats high-concentration and high-color dye wastewater effectively but also greatly improves the biodegradability of refractory wastewater.