Development and Application of a Polycationic Soybean Protein-Based Flocculant for Enhanced Flocculation and Dewatering of Dairy Manure

In agricultural and waste management systems, dairy manure wastewater is often recycled for irrigation. However, a key challenge lies in handling suspended solids (SS) and effectively dewatering sludge. To address this, an innovative polycationic soybean protein-based flocculant (SPI+) was developed and applied to enhance flocculation and sludge dewatering efficiency. The polycationic protein chains of SPI+ were synthesized by grafting 2-(methacryloyloxy)ethyl trimethylammonium chloride (META) monomers onto soybean protein isolate (SPI) chains using an energy-efficient thiol-ene photografting method. This approach achieved a grafting ratio of 85%, endowing the SPI+ with a stable and strong positive zeta-potential (+30 mV) across a range of pH conditions. The SPI+ exhibited exceptional flocculation performance, achieving a 96% flocculation efficiency, reducing sludge filtration resistance by 55%, and lowering filter cake moisture content by 10%. The SPI+ flocculation and dewatering performance is comparable with synthetic-based commercial flocculant. This remarkable performance of SPI+ is attributed to its ability to effectively neutralize charges, form robust inter-particle bridges, and interact strongly with extracellular polymeric substances (EPS), particularly their protein components, within the sludge matrix. These properties significantly enhance both sludge aggregation and dewaterability. The underlying mechanisms of flocculation and dewatering were further elucidated using confocal imaging, surface morphology analysis of flocs, and quantification of EPS protein and polysaccharide content, providing valuable insights into its functional efficacy.