Revealing the variations in physicochemical, morphological, fractal, and rheological properties of digestate during the mesophilic anaerobic digestion of iron-rich waste activated sludge

Abstract Dosing of iron (Fe)-salts in sewers to control odour and corrosion problems have proven to be effective on phosphate and sulfide removal in downstream treatment units. However, the interaction of Fe with sludge may impact the sludge properties during wastewater treatment and sludge digestion. Herein, we investigated the downstream impacts of sewer-dosed Fe-salt on key digestate properties including digestate dewaterability. For this, Fe-salt was dosed to a sewer reactor and resultant iron-rich waste activated sludge (Fe-WAS) was digested in an anaerobic digester (AD) in the experimental line of integrated laboratory system running in parallel to a control system. Iron containing and non-iron containing digestates were sourced from the respective AD reactors of experimental and control lines. Results showed improved dewaterability in iron containing digestate than non-iron containing digestate, which was attributed to the variations in key digestate properties. Compared to non-iron containing digestate, iron containing digestate exhibited the decreased contents of bound water, soluble extracellular polymeric substances (S-EPS), protein, polysaccharide, and monovalent-to-divalent (M+/D++) cations ratio. Likewise, we observed the increased mean particle size (Dv50) for iron containing digestate than the non-iron containing digestate, but fractal dimension (Df) values were comparable. Besides, iron containing digestate exhibited a reduced degree of thixotropy, relative sludge network strength, viscosity, yield stress, flow stress, and storage/loss/complex (G′/G′′/G*) moduli but increased creep compliance and shear strain (%) than non-iron containing digestate. The combined synergistic effects of such favorable changes amongst the key properties of iron containing digestate, might have been responsible for improving it's dewaterability.