Treatment Of Finishing Wastewater In Dyeing And Printing By Combined Coagulation-nanofiltration Technology

Textile printing industry is an important part of our national economic system.The production process consumes enormous water and produces huge amount of wastewater which is of high biochemical oxygen demand(COD),high chromaticity and great change of pH value,causing serious threaten to the environment.The complex processing involving different chemical additives leads to the huge difficulty in settling the discharge water.Accordingly,taking different measures for specific process will ease the difficulty of treatment of wastewater.In this paper,the coagulation-nanofiltration combined technology was developed according to the properties of components in the printing wastewater such as fiber,softener,oil,resin,slurry and so on,and the operating parameters were studied systematically.The main conclusions can be summarized as follows:(1)The performances of four flocculants(poly aluminum chloride,polymerized ferric chloride,aluminum sulfate and ferric chloride)in removing COD and turbidity in the wastewater were examined with polyacrylamide as coagulant aid,revealing that the polyaluminum chloride presented the best result.Moreover,the optimum coagulation conditions were determined through orthogonal experiments and the removal rates of turbidity and COD were more than 90%and 85%respectively under the conditions.(2)To further ensue the treated wastewater to meet the discharge standards,four NF membranes were applied to remove the COD of the supernatant from flocculation process.The results showed that NF270 exhibited the best performance with COD<50 mg/L.Furthermore,the effect of operating conditions including flux,temperature,pH on the removal of COD in the supernatant were studied in detail.Then,the concentrate of NF process was treated by flocculation and the COD reduced to about 430mg/L,and it was lowered to around 30 mg/L after NF.(3)Based on the laboratory results,pilot experiment was conducted to examine the influence of temperature,TMP,cross-flow velocity,showing that higher temperature,higher cross-flow velocity,and lower TMP could effectively inhibit the membrane fouling with the final water meeting the discharge standards.(4)The coagulation-nanofiltration combined technology can be used to treat the wastewater to meet the industrial water reuse standard of GB/T19923-2005 " The reuse of urban recycling water--Water quality standard for industrial uses".