Disintegration of activated sludge flocs in presence of sulfide

Addition of sulfide to activated sludge led to a change of floc structure due to specific reduction of Fe(III) to FeS. The change was observed as a weakening of the floc strength leading to an increased shear sensitivity of the flocs. The degree of disintegration thus depended on the shear rate in the system. The floc disintegration led to the dissolution of up to 10% of the total organic matter of the flocs when all Fe(III) was reduced. Bacteria, organic colloids and dissolved extracellular polymeric substances (EPS) were released from the activated sludge flocs. Laboratory-grown microbial flocs, grown without Fe(III), did not disintegrate upon addition of sulfide, which indicates that no direct bacterial response to sulfide occurred. Laboratory-grown microbial flocs grown in presence of Fe(III) showed some disintegration in the presence of sulfide, but not as much as was observed for activated sludge from a treatment plant. From these data and microscopical investigations it appears that the matter released was mainly EPS, colloids and loosely adhered bacteria, probably arising from the wastewater, while the bacterial microcolonies, particularly typical of the laboratory-grown flocs, were not affected by the sulfide treatment. The changes observed in the floc structure and floc stability have important implications for the solid-liquid separation at activated sludge treatment plants.