Dewaterability and heavy metals stability of aerobically digested sludge improved by citric acid assisting...

Citric acid assisting Fe²⁺-activated sodium percarbonate (CA/Fe²⁺/SPC) process was investigated to simultaneously improve dewaterability and heavy metals (HMs) stability of aerobically digested sludge (AeDS). The results showed that CA/Fe²⁺/SPC process was more conducive to enhance the dewatering performance of sludge than Fe²⁺/SPC process due to the chelating effect of CA, and the normalized capillary suction time and the water content of sludge cake decreased to 2.2 s L/g total suspended solids and 71.4 %, respectively. After conditioning, the enhanced hydrophobicity, the looser structure, and the neutralized negative charge of sludge were in favor of improving sludge dewaterability. Mechanistic research suggested that hydroxyl radicals played the dominant role in CA/Fe²⁺/SPC and Fe²⁺/SPC processes, which could effectively degrade extracellular polymeric substances (EPS) (especially tightly bound-EPS) and destruct α-helix structure of protein, leading to release a considerable amount of EPS-bound water and further improve dewaterability. In addition, CA/Fe²⁺/SPC and Fe²⁺/SPC processes were both helpful to strengthen the stability of HMs in sludge and decrease its ecological risk. The obtained findings not only are conducive for the understanding of CA/Fe²⁺/SPC process for sludge conditioning, but also develop a potential alternative for the harmless disposal of sludge. However, deeper studies about the optimization and adaptability of this process in actual situation should be carried out in the future.