Decontamination of an Industrial Cotton Dyeing Wastewater by Chemical and Biological Processes

The organic matter mineralization, decolorization, biodegradability improvement, and toxicity reduction of an industrial cotton dyeing wastewater were investigated. Different treatment approaches, including single and integrated processes, were studied, namely, coagulation/flocculation per se and its combination with Fenton oxidation (approach 1), the Fenton process alone (approach 2) and its integration with either coagulation/flocculation (approach 3) or biological oxidation in a sequential batch reactor (approach 4). All approaches provided a wastewater that meets the Portuguese legislated discharge limits; however, approaches 1 and 3 require smaller operating costs (1.0 €/m³) with global removal efficiencies for these two strategies of 70.8% for chemical oxygen demand (COD), 66.1% for dissolved organic carbon (DOC), and 47.6% for biological oxygen demand (BOD₅) in approach 1 and 69.2% for COD, 60.4% for BOD₅, and 72.4% for DOC in approach 3, along with almost complete color removal in both strategies and toxicity reduction (0.0% inhibition of Vibrio fischeri in the effluent from approach 1). A slight increase of the effluent biodegradability was also observed (BOD₅:COD increased from 0.26 to 0.33–0.46, and specific oxygen uptake rate (SOUR) increased from 8.85 to 9.3–11.3 mgO₂/(g_VSS h)), depending on the treatment approach. The use of a cheap process (coagulation/flocculation) as pretreatment allowed reducing the amount of reagents needed in the Fenton process, making the latter treatment slightly cheaper and thereby reducing the overall costs