Improving the dewaterability of citric acid wastewater sludge by Fenton treatment

The citric acid wastewater treatment produces a large quantity of sludge, which contains more organic matter and is more difficult to be dewatered by mechanical devices, compared to the municipal wastewater sludge. Fenton conditioning was a highly effective and affordable approach to improving the dewaterability of citric acid wastewater sludge while preserving the fertilizing properties. The optimal condition of Fenton treatment was pH 5.0, H2O2 dose of 20 mg g−1 dry solids, and Fe2+ dose of 80 mg g−1 dry solids. Capillary suction time was reduced from 19.3 s to 10.7 s, and the water content of sludge cake was reduced from 85.3% to 73.8% under this condition. There was no significant difference in capillary suction time and the water content of sludge cake reduction by Fenton treatment between the lab-scale and scale-up experiments. The mechanism investigations of extracellular polymeric substances, zeta-potential, particles size, and morphological changes of the sludge samples conditioned by Fenton treatment under the optimal condition demonstrated that the enhanced coagulation by Fenton treatment was the mechanism dominating the sludge dewaterability improvement. Most critically to the citric acid wastewater sludge, the majority of fertilizing properties of the sludge were preserved after Fenton treatment. The volatile suspended solids, total organic carbon, total nitrogen, potassium and phosphorus were mostly maintained in the solid phase. The cost of Fenton reagents under the optimal condition was approximately 49 US$/t dry solids, which made Fenton treatment a cost reasonable approach to citric acid wastewater sludge conditioning.