Deep Treatment and Reuse Process for Wastewater in the Textile Dyeing and Printing Industry

Textile dyeing and printing wastewater is characterized by large volumes, high color intensity, significant pH fluctuations, and the presence of refractory dyes and auxiliaries. The cost of end-of-pipe treatment alone is high, and today, more and more enterprises are adopting deep treatment and reuse processes to recover both water and thermal energy.

The conventional process begins with a fine screen to intercept fibers and other debris. The influent then flows into an equalization tank, where water quality is equalized. Subsequently, the wastewater enters a coagulation reaction tank, where we dose polyaluminum chloride or polyacrylamide to destabilize and coagulate the dye colloids. After sedimentation, the supernatant enters a hydrolysis acidification tank, where large-molecule dyes undergo chain scission and ring opening, improving biodegradability. The wastewater then proceeds to an aerobic contact oxidation tank or an activated sludge tank to remove the vast majority of COD and ammonia nitrogen. The effluent from the secondary clarifier is further treated by sand filtration or fiber ball filtration, further reducing the suspended solids content.

To meet reuse standards, the subsequent treatment must incorporate membrane separation technology. The ultrafiltration system can reject residual colloids and microorganisms, and its permeate turbidity is generally below 0.1 NTU. The UF permeate serves as feed water for the reverse osmosis system. The RO membrane achieves a very high removal rate for both dye hydrolysis products and inorganic salts. The permeate is colorless and transparent, with extremely low conductivity, and can be reused as soft water for processes such as dyeing and rinsing, with the system achieving a salt rejection rate of over 97%. For workshops with more stringent requirements, a mixed bed or EDI can be added. A small volume of RO concentrate is either recirculated to the front-end equalization unit or discharged after decolorization via advanced oxidation. Drawing on our understanding of textile wastewater characteristics, we have designed fouling-resistant ultrafiltration membrane modules and RO systems capable of handling wide variations in feed water quality, equipped with automated chemical cleaning, to help the textile dyeing and printing industry achieve a sustainable water cycle.