Recent Development of Tannic Acid-Based Flocculants in Sewage and Sludge Treatments: A Crucial Review

Flocculation is a prevalent technique in solid–liquid separation treatment processes. However, the limited applicability and potential for secondary pollution associated with traditional flocculants have prompted increasing interest in natural flocculants in recent years. Among these, tannic acid (TA)-based flocculants have shown promising application prospects with eco-friendly and cost-effective characteristics. This paper presents a comprehensive review of the recent research on the use of TA and its modified forms in sewage handling and sludge dewatering. It examined the various methods of TA modification, efficacy of TA-based flocculants, regulation of process conditions, and the use of chemical reagents in conjunction with TA-based flocculants. Especially, the combination of chemical reagents and modified TA-based flocculants could achieve a reduction rate of over 90% in COD, turbidity, and chromaticity of sewage, and deep dewatering of sludge (moisture of dewatered sludge cake < 60%). It was found that charge neutralization, hydrogen bond, and hydrophobic interaction play a dominant role in TA-based flocculation process. Through the analysis of scale-up aspects and economic feasibility of TA-based flocculants, underscoring the importance of developing sustainable and effective TA-based flocculants for more environmentally friendly sewage and sludge treatment solutions.