Dynamics of exopolymer chemistry, rheology and anaerobic granule properties at various specific loadings with diverse wastewaters

Forming granular sludge with sufficient strength and settleability is crucial for successful operation of high-rate anaerobic reactors. The granule properties are influenced by reactor operational parameters such as sludge loading rate (SLR) and substrate composition, which in turn influence the extracellular polymeric substances (EPS). In this study, we investigated the dynamic response of granule and EPS characteristics to varied SLRs using three lab-scale reactors. The reactors were fed with different organic substrates: R Ac+Try (acetate and tryptone), R Glu+Try (glucose and tryptone), and R Glu+Sta (glucose and starch). At SLRs above 1 g COD gVSS⁻¹ d⁻¹, granules grown in reactors containing tryptone (R Ac+Try and R Glu+Try ) generally exhibited similar and consistent physical properties (e.g., diameter, settleability, and shear strength) across different SLRs, whereas granules in R Glu+Sta were different. Our results further illustrate that this similarity in R Ac+Try and R Glu+Try granules results from high protein content (70% ~ 90%) in their EPS, which confers the EPS with higher resistance to shear softening compared to EPS of R Glu+Sta . Interestingly, the relative abundance ( RA ) of β -sheets in the EPS remained stable in all reactors and SLRs. Contrary, the RA of α -helices, which are known to enhance hydrogel stiffness, increased with SLR in R Ac+Try and decreased with SLR in R Glu+Try . Overall, our results show steady operation of sugar-grown granules at an SLR of 1.4 g COD gVSS⁻¹ d⁻¹ with addition of tryptone, exceeding the typical limits observed in industrial practice. These findings indicate that the highest levels of SLRs currently implemented at full scale should be reconsidered. ● Proteinaceous organics ensure granule’s integrity at SLR up to 1.4 g COD gVSS⁻¹ d⁻¹. ● EPS of granules treating proteinaceous wastewater consist of 70-90% protein. ● Proteins in EPS lead to greater EPS-gel resistance to shear softening. ● Granule physical properties strongly depend on the viscoelasticity of their EPS.