According to our engineering summary for many years, the printing and dyeing industry consumes a large amount of water, usually 100-200 tons of water per ton of textiles. Among these, 80-90% are discharged from printing and dyeing wastewater. Commonly used treatment methods are recycling and harmless treatment.

Recycling and utilization: 

(1) Wastewater can be recycled according to the characteristics of water quality, such as the separation of bleaching and boiling wastewater and dyeing and printing wastewater, the former can be washed conversely. One water can be used for multiple purposes to reduce emissions; 

(2) Alkali recovery and utilization, usually using evaporation method, such as large amount of alkali, three-effect evaporation recovery, small amount of alkali, can be recovered by film evaporation; 

(3) Dye recovery, such as Shilin dyeing. The material can be acidified into cryptobaric acid, which is colloidal particles. It is suspended in the residual liquid and recycled after precipitation and filtration.

Harmless treatment can be divided into the following: 

(1) Physical treatment methods include precipitation method and adsorption method. Sedimentation method mainly removes suspended matter in waste-water, while adsorption method mainly removes dissolved pollutants and decolorization in wastewater;

(2) Chemical treatment methods include neutralization, coagulation and oxidation. Neutralization is to regulate the acidity and alkalinity of wastewater, and also to reduce the chroma of wastewater; coagulation is to remove disperse dyes and colloidal substances in wastewater; oxidation is to oxidize reducing substances in wastewater to precipitate sulfur dyes and reducing dyes;

(3) Biological treatment methods include activated sludge, biological rotary table, biological rotary drum and biological contact oxidation. In order to improve the quality of effluent and meet the discharge standards or recovery requirements, several methods are often used to treat the effluent jointly.

Due to the diversity of wastewater quality in different dyeing and printing plants, the main manifestations are as follows:

(1) Various chemicals are put into textile processing;

(2) Printing and dyeing wastewater mainly comes from sewage, grease, various kinds of added sizes, dyes, surfactants, auxiliaries, acids and bases;

(3) Wastewater characteristics: large amount of water, high color, large change in pH (common high alkalinity), poor biodegradability.

We mainly adopt the combination of physical and chemical treatment and biological treatment.
(1) Physical treatment: 

Activated carbon adsorption, Ultrafiltration, Nanofiltration, Reverse osmosis;

(2) Chemical treatment: 

Coagulation precipitation, Chemical oxidation (Fenton reagent, ozone oxidation, chlorine oxide), Micro-electrolysis, Photocatalytic oxidation (commonly used titanium dioxide as oxidant);

(3) Biological treatment: 

Anaerobic hydrolysis + Contact oxidation.