Treatment of industrial printing and dyeing wastewater

In the era of industrial development, the printing and dyeing industry uses a large amount of water and pollutes a lot. It is really a headache to treat these wastewater. If the dry printing process is adopted for synthetic fibers and fabrics containing more than 75% synthetic fibers, the printing and dyeing wastewater can be eliminated. In addition, using sodium nitrate or hydrogen peroxide instead of potassium dichromate as oxidant can effectively eliminate harmful substances in wastewater.

Textile printing and dyeing wastewater has great difference in water quality due to its different products and production processes. According to the difference of water quality, textile printing and dyeing wastewater can be divided into three categories with obvious characteristics.

(1) Wastewater from wool washing

(2) Wastewater from chemical fibre industry;

(3) Textile printing and dyeing wastewater and water wool spinning and finishing wastewater.

Chemical treatment of industrial printing and dyeing wastewater. There are neutralization, coagulation and oxidation methods. Neutralization is to adjust the acidity and alkalinity of wastewater, and also to reduce the chroma of wastewater, so that some soluble dyes in wastewater can be changed into precipitation-prone substances, so as to improve the biological treatment effect. Coagulation is a method of flocculating and precipitating by adding coagulant to remove disperse dyes and colloidal substances, which can reduce the chroma by 90%, and remove COD by 50-70%. The oxidation method can be divided into aeration oxidation method and oxidant oxidation method. Aeration oxidation process is to direct air into the wastewater tank to oxidize reducing substances in wastewater, so that sulfur dyes and vat dyes can be precipitated. This method is generally not used alone, but as a pretreatment of aerobic biological treatment and coagulation treatment. The oxidant oxidation process uses oxidants such as chlorine, hypochlorite, ozone and hydrogen peroxide to remove dyes from wastewater by oxidation, and to reduce BOD and COD values in wastewater. In recent years, ozone is a good oxidant. Ozone has strong oxidation ability, which can effectively destroy the chromogenic groups of some dyes and decolorize wastewater, and can oxidize and degrade some organic compounds which are difficult to biodegrade without increasing sludge volume.