Using recycled chemical sludge as a coagulant aid in chemical wastewater treatment in Mobarakeh Steel Complex

Coagulation–flocculation is a process commonly applied to treat many types of industrial wastewater. However, chemicals used in a wastewater treatment plant, including coagulants/flocculants, are costly. This study evaluated the feasibility of recycling the chemical sludge of the Mobarakeh Steel Complex (MSC), and using it as a coagulant aid in the process of wastewater treatment. The experiments were conducted in a jar test apparatus in consecutive cycles by adding a fraction of the settled sludge to samples. The response surface methodology (RSM) was employed to design the experiments and optimize the process. The response in the RSM model was residual turbidity, and the factors affecting the response included the sludge recycle ratio and return sludge cycle number. The sludge volume index (SVI) of the chemical sludge was also examined in each cycle of the tests to assess the sludge quality. The results revealed that recycling the sludge reduced the use of chemicals up to 60%. Finally, it was concluded that flocculation sludge could be considered as an alternative to chemicals, without adversely affecting the removal efficiency. Furthermore, SVI results showed that an increase in the number of cycles led to a decrease in SVI in each cycle, resulting in sludge volume reduction, better dewatering, and decreased sludge treatment costs.