Synergetic conditioning of sewage sludge via Fe2+/persulfate and skeleton builder: Effect on sludge characteristics and dewaterability

• Fe2+/S2O82−−phosphogypsum for sludge conditioning is firstly proposed.
• SO4− was identified as the dominant radical by radical quenching experiment.
• CaSO4·2H2O could be newly generated and acted as an effective skeleton builder.
• The transforms of proteins and polysaccharides in floc were determined.
• The bound water was released and reduced significantly.

High water affinity of extracellular polymeric substances (EPS) and high compressibility of sewage sludge solids have negative impacts on sludge dewatering. In this study, a composite conditioner, Fe2+–activated sodium persulfate (denoted as Fe2+/SPS) combined with thermal–pretreated phosphogypsum (PG), was used to improve the sludge dewaterability. The mechanism of the composite conditioning of sewage sludge was elucidated: the proteins in tightly bound EPS were transferred into the filtrate and loosely bound EPS, and the polysaccharides in loosely and tightly bound EPS were transferred into the filtrate; the bound water was released and reduced from 2.60 g/g DS (dry solid) initially to 0.81 g/g DS; specific resistance to filtration and capillary suction time were reduced by 91.6% and 88.4%, respectively. Radical quenching experiment indicated that sulfate radical (SO4−) is the dominant free radical and plays an important role in determining the oxidation–reduction potential during conditioning. Moreover, both the XRD and SEM results clearly showed that Fe2+/SPS combined with PG promoted the generation of column–shaped dihydrate gypsum in the conditioned sludge. Thus, the dihydrate gypsum crystals could act as skeleton builders, which create a more permeable and rigid lattice structure of the sludge cake. The improvement of sludge dewatering was confirmed by diaphragm filter press dewatering process, which yielded 45.7 wt% cake moisture content and 91.7% dewatering efficiency.

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