Unravelling the influences of sewer-dosed iron salts on activated sludge properties with implications on settleability, dewaterability and sludge rheology

Although the beneficial impacts of iron dosing to sewer on activated sludge unit's performance, especially in relation to phosphate removal, have been reported, the extent of impacts on different sludge properties affecting the operation and performance of the activated sludge unit are not fully understood. In this study, we investigated the influences of iron salt dosing to sewer on both settleability and dewaterability of downstream activated sludge unit. We also examined, based on the comparative assessment of different key activated sludge properties, possible underlying factors responsible for the changes in sludge settleability and dewaterability. For this, iron chloride was dosed to a sewer reactor of integrated laboratory sewer-bioreactor system. The activated sludge samples were obtained from two separate reactors, an experimental sequencing batch reactor (SBR-E) downstream of sewer reactor receiving iron dosing and a control SBR (SBR-C) downstream of a sewer reactor without any iron dosing. Iron-conditioned sludge showed improved settleability and dewaterability over the unconditioned activated sludge. Mean differences in settleability and dewaterability between two sludges were 22.5 ± 7.8 mL/g (p < 0.05) and 7.8 ± 1.2% (p < 0.05), respectively. Iron-conditioned sludge showed lower contents of soluble extracellular polymeric substances (EPS) fractions, protein and polysaccharide contents, and monovalent-to-divalent (M⁺/D⁺⁺) cations ratio, but higher humification index as compared to the unconditioned sludge. Iron-conditioned sludge exhibited marginal increment in mean particle size (D_v50) and settleable particle size classes (100–400 μm) but reduction in supracolloidal particle size classes (1–100 μm). In terms of sludge rheology, iron-conditioned sludge exhibited relatively lower relative sludge network strength, viscosity, yield stress, elastic/viscous/complex moduli (G′/G′′/G*), and damping factor tan(δ) but increased shear compliance (J) and shear strain (%) with time.The iron-conditioned sludge therefore exhibited relatively weaker deformation resistance and sludge elasticity. Based on the foregoing results, we posit the combined synergistic effect of favourable changes to the key sludge properties, might be responsible for the observed improvement in settleability and dewaterability of iron-conditioned sludge.