Treatment of highly concentrated wastewater containing multiple synthetic dyes by a combined process of coagulation/flocculation and nanofiltration

• The wastewaters were treated by coagulation/flocculation (CF), nanofiltration (NF) and CF–NF.
• ~90% dyes removal was achieved by CF treatment for 1000 ppm multiple dye wastewater.
• A NF hollow fiber membrane was fabricated, with which ~100% dyes were rejected.
• The combined CF–NF process can improve overall performance for the wastewater treatment.

The treatments of dyes (acid, basic and reactive dyes) wastewater were studied by applying individual coagulation/flocculation (CF) and nanofiltration (NF) processes as well as their combination (referred as CF–NF). For the treatment of highly concentrated multiple dyes wastewater (MDW, 1000 ppm), polyaluminum chloride (PAC) and polydiallyldimethyl ammonium chloride (PDDA) were found to be the most effective coagulant and flocculant, respectively. The CF process can achieve about 90% of dye removal at the optimal dosage of PAC/PDDA=400/200 ppm, and the MDW with pH>3 is favorable for the CF treatment. A positively charged NF hollow fiber membrane was fabricated and used for NF treatment. It is able to remove almost 100% dyes with a permeate flux of about 1.0 L m−2 h−1 under an operating pressure of 1 bar. The combination of CF and NF can complement each other׳s strengths and overcome their individual limitations. The NF treatment can completely remove the strong color left in CF treated dye solutions, while the efficiency of coagulant/flocculant is improved by treating NF concentrated streams and subsequently results in much less sludge. In addition, membrane fouling is abated and NF permeate flux is increased by applying the CF process as a pretreatment. Thus, the combination of CF–NF improves the overall performance for the dyes wastewater treatment.

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