How can the addition of extracellular polymeric substances (EPS)-based bioflocculant affect aerobic granular sludge (AGS)?

The ability of extracellular polymeric substances (EPS) recovered from aerobic granular sludge (AGS) to act as bioflocculant was tested in a pilot-scale sequencing batch reactor (SBR), fed with low-strength municipal wastewater. EPS were compared with the addition of Na-alginate as a standardized biopolymer. The optimal dosage of both biopolymers was determined through jar-test assays (400 mg L of Na-alginate in a 250 mg Ca L and 50 mg L of EPS in pH of 2 ± 0.2). The addition of Na-alginate (Operational Period I- OP-I) and EPS (Operational Period II - OP-II) led to increased adhesion of particles with 2.9 ± 0.45 and 1.3 ± 0.3 g TSS L during OP-I and OP-II, respectively, and fast settling biomass (SVI between 68 and 78 mL g). Granule predominance occurred at early stages of OP-I (day 37) and OP-II (day 44), presenting diameters mainly within the 212-600 μm range. The reactor showed removal efficiencies of 85% for biochemical oxygen demand (BOD) and above 50% for N-NH during the study periods. Furthermore, the addition of EPS as a bioflocculant promoted a substantial increase in polysaccharides (PS = 153.01 ± 121 mg g) and proteins (PN = 121.96 ± 69 mg g), while the addition of Na-alginate affected mostly the PS content (87 ± 24 mg g). The microbial community shifted mainly from Betaproteobacteria (45%) during OP-I to Alphaproteobacteria (64%) in OP-II. Therefore, EPS affected both physical-chemical and microbial features of the AGS biomass without any change in treatment efficiencies. EPS is a promising resource to be recovered from aerobic granular sludge and to be used as an alternative and sustainable bioflocculant.