Efficiency of zero valent iron in the modified fenton process for the reduction of excess sludge and the key role of citric acid through deflocculation

The study aimed for the excess sludge reduction by modified Fenton process (MFP) using zero valent iron (ZVI, Fe⁰) as a catalyst under the influence of sludge deflocculation. Citric acid was employed as a cationic binding agent to deflocculate the sludge by eliminating extracellular polymeric substances to improve the efficiency of the treatment process. The optimization of operational parameters such as pH, initial dosages of Fe⁰ and H₂O₂ and time were performed elaborately for the MFP by response surface methodology. The optimum conditions have been found as Fe⁰ dosage of 0.006 g/g suspended solids (SS), H₂O₂ dosage of 0.5 g/g SS and pH value of 3.5 with 50 min oxidation time period. The benefit of deflocculation was evident from the study as deflocculated sludge undergoing MFP produced higher reduction percentage of chemical oxygen demand, SS and volatile suspended solids than the control. Kinetic analysis was carried out under optimal conditions which prove that the rate was four times higher in the deflocculated sludge. Cost analysis revealed that ZVI mediated Fenton process through deflocculation reduced the sludge disposal cost. The unsolved ZVI can be separated easily from the reaction mixture using magnet to avoid iron contamination in the treated sludge at the end of the process. It is evident from the overall result of the study that ZVI can be successfully replaced for ferrous salt as a catalyst to carry out Fenton reaction. Subsequently, deflocculation improves the process efficiency.