Electro-dewatering treatment of sludge: Assessment of the influence on relevant indicators for disposal in agriculture

Waste activated sludge requires effective dewatering, high biological stability and retention of nutrients prior to disposal for agricultural application. The study was conducted to evaluate the impact of pressure-driven electro-dewatering (EDW) on improving sludge characteristics related to disposal in agriculture, including biological stability, pathogen availability, heavy metals concentrations and nutrients content. Thickened conditioned and mechanically dewatered sludge samples were collected from two wastewater treatment plants (WWTPs), characterized by different stabilization processes, and treated by a lab-scale device at 5, 15 and 25 V. EDW increased significantly the dry solid (DS) content, up to 43–45%, starting from 2 to 3% of raw sludge. The endogenous value of specific oxygen uptake rate (SOUR), monitored as indicator of biological stability, increased up to 56% and 39% after EDW tests for sludge from two WWTPs. On the other hand, the exogenous SOUR decreased, indicating a significant drop in the active bacterial population. Likewise, a 1–2 log unit reduction was observed for E. coli after EDW tests at 15 and 25 V. However, no remarkable removal of heavy metals, namely chromium, nickel, lead, copper and zinc, was achieved. Finally, the concentration of nutrients for soil, such as carbon, nitrogen, phosphorus and sulfur, was not affected by the EDW process. In conclusion, EDW exerts considerable effects on the biological characteristics of sludge, which should be considered in a proper design of sludge management to ensure safe and sustainable resource recovery.