Increasing environmental awareness coupled with more stringent regulation standards has triggered various industries to challenge themselves in seeking appropriate wastewater treatment technologies. Coagulation−flocculation process is regarded as one of the most important and widely used treatment processes of industrial wastewaters due to its simplicity and effectiveness. This paper provides a critical review on recent studies of coagulation−flocculation treatment processes of various industrial wastewaters. The limitations and challenges for the coagulation−flocculation process such as the toxicity and health hazard posed by inorganic coagulants, production of large amount of toxic sludge, ineffectiveness in removing heavy metals and emerging contaminants, increase in effluent color, inefficient pollutant removal using natural coagulants, and complexity of scaling up procedure are presented. In addition, an overview on the influence of process parameters on treatment efficiency is included in this review. Finally, this review concludes with recommendations for improvements and new directions for this long-established process.

The textile industry presents a global pollution problem owing to the dumping or accidental discharge of dye waste-water into waterways, which is having a major impact on the quality and aesthetics of water resources. The World Bank estimates that 17 to 20% of industrial water pollution comes from textile dyeing and treatment. This percentage represents an appalling environmental challenge for clothing designers and other textile manufacturers. Recently, the application of metal oxide semiconductors in the advanced oxidation process (AOP) has gained wide interest for the treatment of dye waste-water owing to its good degradation efficiency, low toxicity and physical and chemical properties. AOP refers to a set of chemical treatment procedures designed to remove organic and inorganic materials from waste-water by oxidation. In this paper, recently developed metal oxide semiconductors are discussed, in which the semiconductors are generally divided into three categories: (i) titanium dioxide; (ii) zinc oxide; and (iii) other metal oxides (such as vanadium oxide, tungsten oxide, molybdenum oxide, indium oxide and cerium oxide). The syntheses and modification methods as well as the efficiency of each category are discussed and analyzed.

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