Probing the contribution of extracellular polymeric substance fractions to activated-sludge bioflocculation using particle image velocimetry in combination with extended DLVO analysis

Extracellular polymeric substances (EPS) play a crucial role in the flocculation of activated sludge during biological wastewater treatment. However, as EPS may be classified into sub-fractions, the exact roles of different EPS fractions in sludge flocculation remain to be further defined. This study investigated the contribution of EPS fractions to the sludge flocculation based on a refined EPS classification considering the function and composition of the EPS sub-fractions, including loosely-bound extra-microcolony polymers (LB-EMPs), tightly-bound extra-microcolony polymers (TB-EMPs) and extra-cellular polymers (ECPs). The flocculation ability of the sludge before and after the extraction of the EPS fractions was determined using the particle image velocimetry (PIV) technique. Combined with the flocculation test and sludge characterization, the effects of the EPS fractions on the interaction energy and attachment of microbial cells were evaluated by extended DLVO analysis. The PIV results indicate that sludge flocculation ability decreases with the extraction of EPS. The interaction-energy curves obtained from the sludge before and after the stepped EPS extraction suggest that different EPS fractions contribute to sludge flocculation to different degrees. The interaction energy of LB-EMPs and TB-EMPs is always negative, indicating their important and positive roles in sludge flocculation. In contrast, the contribution of ECPs to sludge flocculation is not always positive, depending on the separation distance between the sludge cells. The quantitative findings will help develop more effective approaches to enhance sludge flocculation by regulating the EPS contents.