Effect of COD/SO₄²⁻ ratios on the extracellular polymer properties of granular sludge from ABR treatment of sulfate organic wastewater

To investigate the effect of COD/SO₄²⁻ ratios on the characteristics of granular sludge extracellular polymer substances (EPS) in anaerobic baffled reactor (ABR) treating sulfate organic wastewater, the COD and SO₄²⁻ removal effect, EPS content variation and the spatial distribution of each compartment were investigated by reducing COD/SO₄²⁻ ratios in stages after each stage of stable operation based on the successful start-up of the ABR. The results showed that during the reduction of COD/SO₄²⁻ ratio from 12.5 to 4.0, the total removal rate of COD and SO₄²⁻ was around 95%, and the EPS content of anaerobic granular sludge in each compartment showed a trend of first increasing and then decreasing, with the highest EPS content in compartment 3 at a COD/SO₄²⁻ ratio of 6.9, and the highest EPS content in compartments 1, 2, 4 and 5 at a COD/SO₄²⁻ ratio of 4.0. When the COD/SO₄²⁻ ratio decreased to 3.0, the total removal rate of COD and SO₄²⁻ decreased to 60% and 90%, respectively, and the EPS content of each compartment decreased significantly, and the life activity of microorganisms was inhibited. The results of in situ fluorescence staining showed that when the COD/SO₄²⁻ ratio was 3.0, the granular sludge was loose and the fluorescence intensity was reduced, and the protein (PN) was distributed throughout the granular sludge, but mainly on the outer side, and its distribution was less affected by the change of COD/SO₄²⁻ ratio, while the distribution of polysaccharide (PS) was more affected, resulting in the decrease of sludge structural stability, in which the PS in compartments 2,3 and 4 changed to be distributed to the outside of the granular sludge, and the PS in compartment 5 was concentrated from uniform distribution to the inside.