Agent-assisted electrokinetic treatment of sewage sludge: Heavy metal removal effectiveness and nutrient content characteristics

Sewage sludge (SS) is rich in nutrient elements such as phosphorus (P), nitrogen (N), and potassium (K), and therefore a candidate material for use in agriculture. But high content of heavy metals (HMs) can be a major obstacle to its further utilization. Therefore, an appropriate HM removal technology is required before its land application. In this study, an innovative biodegradable agent (citric acid, FeCl3, ammonium hydroxide, tetrasodium iminodisuccinate (IDS), and tea saponin) assisted electrokinetic treatment (EK) was performed to investigate the HM removal efficiency (RHMs) and nutrient transportation. Citric acid, IDS, and FeCl3-assisted EK showed a preferable average RHMs (Rave) reduction of 52.74-59.23%, with low energy consumption. After treatment, the content of Hg (0.51 mg kg-1), Ni (13.23 mg kg-1), and Pb (26.45 mg kg-1) elements met the criteria of national risk control standard, in all cases. Following the treatment, most HMs in SS had a reduced potential to be absorbed by plants or be leached into water systems. Risk assessment indicated that the Geoaccumulation index (Igeo) value of HMs has decreased by 0.28-2.40, and the risk of Pb (Igeo=-0.74) reduced to unpolluted potential. Meanwhile, no excessive nutrient loss in SS occurred as a result of the treatment, on the contrary, there was a slight increase in P content (18.17 mg g-1). These results indicate that agent-assisted EK treatment could be an environmentally-friendly method for RHMs and nutrient element recovery from SS, opening new opportunities for sustainable SS recycling and its inclusion into circular economy concepts.