Investigating the Reuse of the Return Sludge of the Clarifier unit to Improve the Efficiency of the Coagulation and Flocculation Process in the Water Treatment Plant

Document Type : Research Paper


1 PhD. Student of Environmental Pollution, Dept. of Environmental Sciences and Engineering, Lahijan Campus, Islamic Azad University, Lahijan, Iran

2 Assoc. Prof., Dept. of Environmental Sciences and Engineering, Lahijan Campus, Islamic Azad University, Lahijan, Iran

3 Assist. Prof., Dept. of Environmental Sciences and Engineering, Lahijan Campus, Islamic Azad University, Lahijan, Iran


To optimize the use of coagulants and coagulant aids, it is important to understand the structure of the sludge and its effect on parameters such as turbidity and color. In this study, the levels of total solids, fixed solids, and volatile solids were measured in the return sludge from a water treatment plants coagulation unit. The impact of these parameters on the consumption of ferric chloride and polyelectrolyte, as well as on the reduction of turbidity and color, was investigated. Thirty samples were taken from the inlet water, outlet water, and return sludge to measure each parameter. The highest levels of TS, FS and VS were found to be 8.4%, 96.2%, and 15.4%, respectively. In this case, the consumption of ferric chloride and polyelectrolyte was lowest, at 3 and 0.03 mg/L, respectively. The highest levels of turbidity and color in the return sludge were found to come from the inlet water, at 55 NTU and 19 TCU, respectively. The Kolmogorov-Smirnov test showed that an increase in TS, FS and VS in the return sludge reduced the consumption of ferric chloride and polyelectrolyte by p < 0.05. Additionally, an increase in return sludge solids led to a higher removal rate of turbidity and color, with a confidence level of p < 0.05. Overall, this study highlights the importance of understanding the structure of chemical sludge and its impact on the effectiveness of coagulants and coagulant aids in water treatment plants. By optimizing the use of these chemicals, water treatment plants can operate more efficiently and effectively, resulting in cleaner and safer drinking water for communities.


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